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Sample records for radical cation formation

  1. The formation of DNA sugar radicals from photoexcitation of guanine cation radicals.

    PubMed

    Shukla, Lata I; Pazdro, Robert; Huang, James; DeVreugd, Christopher; Becker, David; Sevilla, Michael D

    2004-05-01

    In this investigation of radical formation and reaction in gamma- irradiated DNA and model compounds, we report the conversion of the guanine cation radical (one-electron oxidized guanine, G(.+)) to the C1' sugar radical and another sugar radical at the C3' or C4' position (designated C3'(.)/C4'(.)) by visible and UV photolysis. Electron spin resonance (ESR) spectroscopic investigations were performed on salmon testes DNA as well as 5'-dGMP, 3'-dGMP, 2'-deoxyguanosine and other nucleosides/nucleotides as model systems. DNA samples (25- 150 mg/ml D(2)O) were prepared with Tl(3+) or Fe(CN)(3-)(6) as electron scavengers. Upon gamma irradiation of such samples at 77 K, the electron-gain path in the DNA is strongly suppressed and predominantly G(.+) is found; after UV or visible photolysis, the fraction of the C1' sugar radical increases with a concomitant reduction in the fraction of G(.+). In model systems, 3'- dGMP(+.) and 5'-dGMP(+.) were produced by attack of Cl(.-)(2) on the parent nucleotide in 7 M LiCl glass. Subsequent visible photolysis of the 3'-dGMP(+.) (77 K) results predominantly in formation of C1'(.) whereas photolysis of 5'-dGMP(+.) results predominantly in formation of C3'(.)/C4'(.). We propose that sugar radical formation is a result of delocalization of the hole in the electronically excited base cation radical into the sugar ring, followed by deprotonation at specific sites on the sugar. PMID:15161365

  2. Zeaxanthin Radical Cation Formation in Minor Light-Harvesting Complexes of Higher Plant Antenna

    SciTech Connect

    Avenson, Thomas H.; Ahn, Tae Kyu; Zigmantas, Donatas; Niyogi, Krishna K.; Li, Zhirong; Ballottari, Matteo; Bassi, Roberto; Fleming, Graham R.

    2008-01-31

    Previous work on intact thylakoid membranes showed that transient formation of a zeaxanthin radical cation was correlated with regulation of photosynthetic light-harvesting via energy-dependent quenching. A molecular mechanism for such quenching was proposed to involve charge transfer within a chlorophyll-zeaxanthin heterodimer. Using near infrared (880-1100 nm) transient absorption spectroscopy, we demonstrate that carotenoid (mainly zeaxanthin) radical cation generation occurs solely in isolated minor light-harvesting complexes that bind zeaxanthin, consistent with the engagement of charge transfer quenching therein. We estimated that less than 0.5percent of the isolated minor complexes undergo charge transfer quenching in vitro, whereas the fraction of minor complexes estimated to be engaged in charge transfer quenching in isolated thylakoids was more than 80 times higher. We conclude that minor complexes which bind zeaxanthin are sites of charge transfer quenching in vivo and that they can assume Non-quenching and Quenching conformations, the equilibrium LHC(N)<--> LHC(Q) of which is modulated by the transthylakoid pH gradient, the PsbS protein, and protein-protein interactions.

  3. Halogenated silanes, radicals, and cations

    NASA Astrophysics Data System (ADS)

    Wang, Liming; He, Yi-Liang

    2008-09-01

    Quantum chemistry study has been carried out on the structure and energetics of halogenated silanes, radicals, and cations (SiHxXy0,+1, X = F, Cl, Br; x + y = 1-4). The geometries are optimized at B3LYP/6-31+G(2df,p) level. The adiabatic ionization energiess (IEas), relative energetics of cations, proton affinities (PAs) of silanes, and the enthalpies of formation are predicted using G3(CC) model chemistry. Non-classical ion complex structures are found for hydrogenated cations and transition states connecting classical and non-classical structures are also located. The most stable cations for silylene and silyl radicals have their classical divalent and trivalent structures, and those for silanes have non-classical structures except for SiH3Br+ and SiH2Br2+. The non-classical structures for halosilane cations imply difficulty in experimentally measurement of the adiabatic ionization energies using photoionization or photoelectron studies. For SiH3X, SiH2X2, and SiHX3, the G3(CC) adiabatic IEas to classical ionic structures closest to their neutrals agree better with the photoelectron spectroscopic measurements. The transition states between classical and non-classical structures also hamper the photoionization determination of the appearance energies for silylene cations from silanes. The G3(CC) results for SiHx0,+1 agree excellently with the photoionization mass spectrometric study, and the results for fluorinated and chlorinated species also agree with the previous theoretical predictions at correlation levels from BAC-MP4 to CCSD(T)/CBS. The predicted enthalpy differences between SiH2Cl+, SiHCl2+, and SiCl3+ are also in accordance with previous kinetics study. The G3(CC) results show large discrepancies to the collision-induced charge transfer and/or dissociation reactions involving SiFx+ and SiClx+ ions, for which the G3(CC) enthalpies of formation are also significantly differed from the previous theoretical predictions, especially on SiFx+ (x = 2-4). The G3

  4. Radiolytic generation of radical cations in xenon matrices. Tetramethylcyclopropane radical cation and its transformations

    SciTech Connect

    Qin, X.Z.; Trifunac, A.D. )

    1990-04-05

    Radiolytic generation of radical cations in xenon matrices containing electron scavengers is illustrated by studying the 1,1,2,2-tetramethylcyclopropane radical cation. Dilute and concentrated solutions of tetramethylcyclopropane in xenon without electron scavengers and neat tetramethylcyclopropane yielded neutral radicals upon {gamma}-irradiation. Speculation on the mechanisms of radical formation is presented. The radical species observed in the {gamma}-irradiation of neat tetramethylcyclopropane appears to be identical with the paramagnetic species observed in CF{sub 2}ClCFCl{sub 2} above 120 K, suggesting that a neutral radical rather than the ring-opened distonic radical cation is observed in the CF{sub 2}ClCFCl{sub 2} matrix.

  5. Formation of environmentally persistent free radical (EPFR) in iron(III) cation-exchanged smectite clay.

    PubMed

    Nwosu, Ugwumsinachi G; Roy, Amitava; dela Cruz, Albert Leo N; Dellinger, Barry; Cook, Robert

    2016-01-01

    Environmentally persistent free radicals (EPFRs) have been found at a number of Superfund sites, with EPFRs being formed via a proposed redox process at ambient environmental conditions. The possibility of such a redox process taking place at ambient environmental conditions is studied utilizing a surrogate soil system of phenol and iron(III)-exchanged calcium montmorillonite clay, Fe(III)CaM. Sorption of phenol by the Fe(III)CaM is demonstrated by Fourier-transformed infra-red (FT-IR) spectroscopy, as evidenced by the peaks between 1345 cm(-1) and 1595 cm(-1), and at lower frequencies between 694 cm(-1) and 806 cm(-1), as well as X-ray diffraction (XRD) spectroscopy, as shown by an increase in interlayer spacing within Fe(III)CaM. The formation and characterization of the EPFRs is determined by electron paramagnetic resonance (EPR) spectroscopy, showing phenoxyl-type radical with a g-factor of 2.0034 and ΔHP-P of 6.1 G at an average concentration of 7.5 × 10(17) spins per g. EPFRs lifetime data are indicative of oxygen and water molecules being responsible for EPFR decay. The change in the oxidation state of the iron redox center is studied by X-ray absorption near-edge structure (XANES) spectroscopy, showing that 23% of the Fe(III) is reduced to Fe(II). X-ray photoemission spectroscopy (XPS) results confirm the XANES results. These findings, when combined with the EPFR concentration data, demonstrate that the stoichiometry of the EPFR formation under the conditions of this study is 1.5 × 10(-2) spins per Fe(II) atom. PMID:26647158

  6. Analysis of diarylmethylamine compounds using electrospray mass spectrometry: formation mechanisms of radical ions and dehydro cations.

    PubMed

    Cai, Tian; Xu, Xiao-Ying; Wu, Zhi-Jun

    2015-12-01

    A series of diarylmethylamine compounds were analyzed using electrospray ionization quadrupole time-of-flight mass spectrometry (ESI-QTOF-MS). [M](+)˙ and [M - H](+) were both observed, but showed different abundances. A possible mechanism for the formation of [M](+)˙ and [M - H](+) was proposed to explicate the rule for the ratio change of I([M](+)˙)/I([M-H](+)). The [M](+)˙ has two structures, which can interconvert into each other in the gas phase. The substituted groups on the benzene rings play a crucial role in the transfer between the two structures. Electron withdrawing groups can prevent the formation of carbocations, thus nitro-containing diarylmethylamines remained mainly as structure I and were detected as [M](+)˙. On the contrary, electron donating groups help to stabilize carbocations. This makes structure I transfer to structure II, and structure II prefers to further generate [M - H](+) by loss of an H radical. Nuclear magnetic resonance and D-labelled MS experiments indicate that the 1-C-H bond has strong activity. PMID:26465612

  7. Pentachlorophenol radical cations generated on Fe(III)-montmorillonite initiate octachlorodibenzo-p-dioxin formation in clays: DFT and FTIR studies

    PubMed Central

    Gu, Cheng; Liu, Cun; Johnston, Cliff T.; Teppen, Brian J.; Li, Hui; Boyd, Stephen A.

    2011-01-01

    Octachlorodibenzodioxin (OCDD) forms spontaneously from pentachlorophenol (PCP) on the surfaces of Fe(III)-saturated smectite clay (1). Here, we used in situ FTIR methods and quantum mechanical calculations to determine the mechanism by which this reaction is initiated. As the clay was dehydrated, vibrational spectra showed new peaks that grew and then reversibly disappeared as the clay rehydrated. First principle DFT calculations of hydrated Fe-PCP clusters reproduced these transient FTIR peaks when inner-sphere complexation and concomitant electron transfer produced Fe(II) and PCP radical cations. Thus, our experimental (FTIR) and theoretical (quantum mechanical) results mutually support the hypothesis that OCDD formation on Fe-smectite surfaces is initiated by the reversible formation of metastable PCP radical cations via single electron transfer from PCP to Fe(III). The negatively charged clay surface apparently selects for this reaction mechanism by stabilizing PCP radical cations. PMID:21254769

  8. Dynamics of poly(4-hydroxystyrene) radical cation

    NASA Astrophysics Data System (ADS)

    Okamoto, Kazumasa; Kozawa, Takahiro; Tagawa, Seiichi

    2008-03-01

    Poly(4-hydroxystyrene) (PHS) has been used in KrF excimer laser (248 nm) lithography as a backbone polymer, and is also a promising material for EUV or electron beam lithography. Analysis of the intermediate species is important for the control of reactions in resist materials. Since the size of integrated circuits fabricated for mass production will decrease below 30 nm and the size error must also be decreased to the molecular level, the elucidation of proton dynamics at the molecular level is also important for reducing the deviation of the resist pattern size. In this study, the dynamics of PHS radical cations were studied, because PHS radical cation is main source of proton. The transient absorption of PHS was observed in the near-infrared region (NIR) in p-dioxane solutions by pulse radiolysis. The intramolecular PHS dimer radical cation (M2 +.) were observed, whereas p-cresol shows no distinct CR band. Although the radical cations of phenol derivatives are known to be easily deprotonated, it was found that M2 +. formation prevents deprotonation by its charge resonance stabilization.

  9. Cations or Radicals? Inherent Reactivity of Biosynthetic Intermediates in the B-Ring Formation of Rotenoid Natural Products.

    PubMed

    Kirkpatrick, Adam K; Siebert, Matthew R

    2016-04-21

    Compounds of the rotenoid class are naturally occurring in the Leguminosae and Nyctaginacae families. Rotenoids have found a myriad of uses, for example, in the agricultural industry as an insecticide and piscicide, and as an anticancer therapeutic. The scientific literature questions whether cyclization of the rotenoid B-ring occurs via a pathway containing either cationic or free-radical intermediates. In this work, both propositions are analyzed using DFT (B3LYP and M06-2X) and the G3 composite method in gas- and (implicit) solution-phase. The accuracy of these methods is compared to several experimental C-H bond dissociation energies (BDEs). We find that of the methods surveyed M06-2X provides the most accurate BDEs. Further, there is a clear thermodynamic preference for the free-radical pathway. PMID:27014924

  10. Polymerization of ionized acetylene clusters into covalent bonded ions: evidence for the formation of benzene radical cation.

    PubMed

    Momoh, Paul O; Abrash, Samuel A; Mabrouki, Ridha; El-Shall, M Samy

    2006-09-27

    Since the discovery of acetylene and benzene in protoplanetary nebulae under powerful ultraviolet ionizing radiation, efforts have been made to investigate the polymerization of ionized acetylene. Here we report the efficient formation of benzene ions within gas-phase ionized acetylene clusters (C2H2)n+ with n = 3-60. The results from experiments, which use mass-selected ion mobility techniques, indicate that the (C2H2)3+ ion has unusual stability similar to that of the benzene cation; its primary fragment ions are similar to those reported from the benzene cation, and it has a collision cross section of 47.4 A2 in helium at 300 K, similar to the value of 47.9 A2 reported for the benzene cation. In other words, (C2H2)3+ structurally looks like benzene, it has stability similar to that of benzene, it fragments such as benzene, therefore, it must be benzene! PMID:16984178

  11. Investigation of Fragmentation of Tryptophan Nitrogen Radical Cation

    NASA Astrophysics Data System (ADS)

    Piatkivskyi, Andrii; Happ, Marshall; Lau, Justin Kai-Chi; Siu, K. W. Michael; Hopkinson, Alan C.; Ryzhov, Victor

    2015-08-01

    This work describes investigation of the fragmentation mechanism of tryptophan N-indolyl radical cation, H3N+-TrpN• ( m/ z 204) studied via DFT calculations and several gas-phase experimental techniques. The main fragment ion at m/ z 131, shown to be a mixture of up to four isomers including 3-methylindole (3MI) π-radical cation, was found to undergo further loss of an H atom to yield one of the two isomeric m/ z 130 ions. 3-Methylindole radical cation generated independently (via CID of [CuII(terpy)3MI]•2+) displayed gas-phase reactivity partially similar to that of the m/ z 131 fragment, further confirming our proposed mechanism. CID of deuterated tryptophan N-indolyl radical cation ( m/ z 208) suggested that up to six H atoms are involved in the pathway to formation of the m/ z 131 ion, consistent with hydrogen atom scrambling during CID of protonated Trp.

  12. Terthiophene radical cations end-capped by bicyclo[2.2.2]octene units: formation of bent pi-dimers mutually attracted at the central position.

    PubMed

    Yamazaki, Daisuke; Nishinaga, Tohru; Tanino, Nobuhide; Komatsu, Koichi

    2006-11-15

    A terthiophene fused with bicyclo[2.2.2]octene units only at both ends was newly synthesized. Since there is no steric hindrance at the central position, this terthiophene has a possibility to interact only at the central position. One-electron oxidation of this terthiophene afforded a highly stable radical-cation salt as deep blue crystals. The result of X-ray crystal structural analysis demonstrated a characteristically bent pi-dimereric structure, which is formed by mutual attraction of single radical-cation species at the central position to minimize the steric repulsion. Remarkably short intermolecular distances between the central thiophene rings of each unit of the dimeric pair, that is, 2.976(10) A for Cbeta-Cbeta, 3.091(10) A for Calpha-Calpha, and 3.779(3) A for S-S, are good indication of the existence of attracting interaction, which was confirmed by theoretical calculations. This interaction was experimentally demonstrated by the reversible formation of the pi-dimer in CH2Cl2 solution using ESR and UV-vis-NIR spectroscopy. The crystal of the pi-dimer is in its singlet state and ESR silent in the solid state at 300 K, but the signal of a triplet state of the pi-dimer was observed by heating the solid at 400 K. This indicates that this pi-dimer has a quite small triplet-singlet enegy gap and the triplet state is thermally accessible. PMID:17090025

  13. Electronic spectrum of 9-methylanthracenium radical cation

    NASA Astrophysics Data System (ADS)

    O'Connor, Gerard D.; Sanelli, Julian A.; Dryza, Vik; Bieske, Evan J.; Schmidt, Timothy W.

    2016-04-01

    The predissociation spectrum of the cold, argon-tagged, 9-methylanthracenium radical cation is reported from 8000 cm-1 to 44 500 cm-1. The reported spectrum contains bands corresponding to at least eight electronic transitions ranging from the near infrared to the ultraviolet. These electronic transitions are assigned through comparison with ab initio energies and intensities. The infrared D1←D0 transitions exhibit significant vibronic activity, which is assigned through comparison with TD-B3LYP excited state frequencies and intensities, as well as modelled vibronic interactions. Dissociation of 9-methylanthracenium is also observed at high visible-photon energies, resulting in the loss of either CH2 or CH3. The relevance of these spectra, and the spectra of other polycyclic aromatic hydrocarbon radical cations, to the largely unassigned diffuse interstellar bands, is discussed.

  14. Structure and reactions of cation-radicals of esters in freon matrices

    SciTech Connect

    Belevskii, V.N.; Belopushkin, S.I.; Fel'dman, V.I.

    1987-11-01

    In CFCl/sub 3/ matrices the cation-radicals of methyl and ethyl formates, formed in ..gamma..-irradiated solutions, at 77 K efficiently undergo intramolecular H atom transfer to form the secondary cation-radicals HC(OH)OCH/sub 2/CH/sub 2/ and DC(OH)OCD/sub 2/CH/sub 2/. This process does not occur in the deuteroformate cation-radical DCOOCH/sub 2/CD/sub 3//sup +./, which is observed in the ESR spectra in different conformations, depending on the temperature. Ion-molecule reactions involving cation-radicals are indicated

  15. Fragmentation Pathways in the Uracil Radical Cation

    SciTech Connect

    Zhou, Congyi; Matsika, Spiridoula; Kotur, Marija; Weinacht, Thomas C.

    2012-08-24

    We investigate pathways for fragmentation in the uracil radical cation using ab initio electronic structure calculations. We focus on the main fragments produced in pump–probe dissociative ionization experiments. These are fragments with mass to charge ratios (m/z) of 69, 28, 41, and 42. Barriers to dissociation along the ground ionic surface are reported, which provide an estimate of the energetic requirements for the production of the main fragments. Finally, direct and sequential fragmentation mechanisms have been analyzed, and it is concluded that sequential fragmentation after production of fragment with m/z 69 is the dominant mechanism for the production of the smaller fragments.

  16. Electron Spin Resonance Spectroscopic Studies of Radical Cation Reactions.

    NASA Astrophysics Data System (ADS)

    Dai, Sheng

    1990-01-01

    A spin Hamiltonian suitable for theoretical analyses of ESR spectra in this work is derived by using the general effective Hamiltonian theory in the usual Schrodinger representation. The Permutation Indices method is extended to obtain the dynamic exchange equations used in ESR lineshape simulation. The correlation between beta-hydrogen coupling constants and their geometric orientations is derived through the use of a perturbation method. The three electron bond model is extended to rationalize unimolecular rearrangements of radical cations. The ring-closed radical cations of 9,10-octalin oxide and syn-sesquinorbornene oxide have been characterized by ESR spectroscopy in the CFCl_3 matrix at low temperature. The ESR spectra of the former radical cation exhibit a novel alternating linewidth effect arising from an internal relation between the coupling constants for the four equivalent pairs of hydrogens. The self-electron-transfer rate constants between the methyl viologen dication and cation have been determined by dynamic ESR lineshape simulations at room temperature in allyl alcohol, water, methanol and propargyl alcohol solvents. The radical cation formed by the radiolytic oxidation of allylamine in Freon matrices at 77 K is shown to be the 3-iminiopropyl distonic species(3-iminium-1-propyl radical) resulting from a symmetry-allowed 1,2-hydrogen shift in the parent radical cation. The nucleophilic endocyclization of the but-3-en-1-ol radical cation to the protonated tetrahydrofuran -3-yl radical was observed in the radiolytic oxidation of but-3-en-1-ol in Freon matrices. ESR studies of the radiolytic oxidation of 1,5-hexadiyne have resulted in the first spectroscopic characterization of the radical cation Cope rearrangement, the 1,5-hexadiyne radical cation isomerizing to the 1,2,4,5 -hexatetraene radical cation. ESR studies show that the symmetric(C_{rm 2v}) bicyclo (3.3.0) -octa-2,6-diene-4,8-diyl(a bridged 1,4 -bishomobenzene species) radical cation is

  17. ESR study of the aziridine and azetidine radical cations: evidence for the C. C ring-opened aziridine radical cation

    SciTech Connect

    Qin, X.Z.; Williams, F.

    1986-05-22

    The radical cations from aziridine and azetidine have been characterized by ESR spectroscopy following their generation in the solid state by ..gamma.. irradiation of dilute solutions of the parent compounds in the CFCl/sub 3/ matrix at 77 K. The ESR parameters of the azetidine radical cation are typical of those for nitrogen-centered amine radical cations such as Me/sub 2/NH*/sup +/. On the other hand, the radical cation formed from aziridine has very different ESR parameters that compare closely to those for the isoelectronic C...C ring-opened form of the oxirane radical cation and the allyl radical. The radical cation formed from azetidine is therefore assigned a ring-closed structure with the unpaired electron in a 2p/sub z/ orbital on nitrogen perpendicular to the ring plane, whereas the cation from aziridine is an allylic C...C ring-opened planar isomer with the unpaired electron in a nonbonding ..pi.. orbital centered mainly on the two end carbon atoms. The neutral 1-aziridinyl and 1-azetidinyl radicals have been detected as radical products following the ..gamma.. irradiation of the parent compounds in the CFCl/sub 2/CF/sub 2/Cl and CF/sub 3/CCl/sub 3/ matrices. In particular, the 1-azetidinyl radical is produced cleanly from the azetidine radical cation in the CFCl/sub 2/CF/sub 2/Cl matrix at ca. 100 K.

  18. Catalytic Carbocation Generation Enabled by the Mesolytic Cleavage of Alkoxyamine Radical Cations.

    PubMed

    Zhu, Qilei; Gentry, Emily C; Knowles, Robert R

    2016-08-16

    A new catalytic method is described to access carbocation intermediates via the mesolytic cleavage of alkoxyamine radical cations. In this process, electron transfer between an excited state oxidant and a TEMPO-derived alkoxyamine substrate gives rise to a radical cation with a remarkably weak C-O bond. Spontaneous scission results in the formation of the stable nitroxyl radical TEMPO(.) as well as a reactive carbocation intermediate that can be intercepted by a wide range of nucleophiles. Notably, this process occurs under neutral conditions and at comparatively mild potentials, enabling catalytic cation generation in the presence of both acid sensitive and easily oxidized nucleophilic partners. PMID:27403637

  19. Aryl sulfoxide radical cations. Generation, spectral properties, and theoretical calculations.

    PubMed

    Baciocchi, Enrico; Del Giacco, Tiziana; Gerini, Maria Francesca; Lanzalunga, Osvaldo

    2006-08-17

    Aromatic sulfoxide radical cations have been generated by pulse radiolysis and laser flash photolysis techniques. In water (pulse radiolysis) the radical cations showed an intense absorption band in the UV region (ca. 300 nm) and a broad less intense band in the visible region (from 500 to 1000 nm) whose position depends on the nature of the ring substituent. At very low pulse energy, the radical cations decayed by first-order kinetics, the decay rate increasing as the pH increases. It is suggested that the decay involves a nucleophilic attack of H(2)O or OH(-) (in basic solutions) to the positively charged sulfur atom to give the radical ArSO(OH)CH(3)(*). By sensitized [N-methylquinolinium tetrafluoborate (NMQ(+))] laser flash photolysis (LFP) the aromatic sulfoxide radical cations were generated in acetonitrile. In these experiments, however, only the band of the radical cation in the visible region could be observed, the UV band being covered by the UV absorption of NMQ(+). The lambda(max) values of the bands in the visible region resulted almost identical to those observed in water for the same radical cations. In the LFP experiments the sulfoxide radical cations decayed by second-order kinetics at a diffusion-controlled rate, and the decay is attributed to the back electron transfer between the radical cation and NMQ(*). DFT calculations were also carried out for a number of 4-X ring substituted (X = H, Me, Br, OMe, CN) aromatic sulfoxide radical cations (and their neutral parents). In all radical cations, the conformation with the S-O bond almost coplanar with the aromatic ring is the only one corresponding to the energy minimum. The maximum of energy corresponds to the conformation where the S-O bond is perpendicular to the aromatic ring. The rotational energy barriers are not very high, ranging from 3.9 to 6.9 kcal/mol. In all radical cations, the major fraction of charge and spin density is localized on the SOMe group. However, a substantial delocalization

  20. Photochemical generation, isomerization, and oxygenation of stilbene cation radicals

    SciTech Connect

    Lewis, F.D.; Bedell, A.M.; Dykstra, R.E.; Elbert, J.E. ); Gould, I.R.; Farid, S. )

    1990-10-24

    The cation radicals of cis- and trans-stilbene and several of their ring-substituted derivatives have been generated in solution directly by means of pulsed-laser-induced electron transfer to singlet cyanoanthracenes or indirectly via electron transfer from biphenyl to the singlet cyanoanthracene followed by secondary electron transfer from the stilbenes to the biphenyl cation radical. Transient absorption spectra of the cis- and trans-stilbene cation radicals generated by secondary electron transfer are similar to those previously obtained in 77 K matrices. Quantum yields for radical ion-pair cage escape have been measured for direct electron transfer from the stilbenes to three neutral and one charged singlet acceptor. These values increase as the ion-pair energy increases due to decreased rate constants for radical ion-pair return electron transfer, in accord with the predictions of Marcus theory for highly exergonic electron transfer. Cage-escape efficiencies are larger for trans- vs cis-stilbene cation radicals, possibly due to the greater extent of charge delocalization in the planar trans vs nonpolar cis cation radicals. Cage-escape stilbene cation radicals can initiate a concentration-dependent one way cis- {yields} trans-stilbene isomerization reaction.

  1. UVA-visible photo-excitation of guanine radical cations produces sugar radicals in DNA and model structures

    PubMed Central

    Adhikary, Amitava; Malkhasian, Aramice Y. S.; Collins, Sean; Koppen, Jessica; Becker, David; Sevilla, Michael D.

    2005-01-01

    This work presents evidence that photo-excitation of guanine radical cations results in high yields of deoxyribose sugar radicals in DNA, guanine deoxyribonucleosides and deoxyribonucleotides. In dsDNA at low temperatures, formation of C1′• is observed from photo-excitation of G•+ in the 310–480 nm range with no C1′• formation observed ≥520 nm. Illumination of guanine radical cations in 2′dG, 3′-dGMP and 5′-dGMP in aqueous LiCl glasses at 143 K is found to result in remarkably high yields (∼85–95%) of sugar radicals, namely C1′•, C3′• and C5′•. The amount of each of the sugar radicals formed varies dramatically with compound structure and temperature of illumination. Radical assignments were confirmed using selective deuteration at C5′ or C3′ in 2′-dG and at C8 in all the guanine nucleosides/tides. Studies of the effect of temperature, pH, and wavelength of excitation provide important information about the mechanism of formation of these sugar radicals. Time-dependent density functional theory calculations verify that specific excited states in G•+ show considerable hole delocalization into the sugar structure, in accord with our proposed mechanism of action, namely deprotonation from the sugar moiety of the excited molecular radical cation. PMID:16204456

  2. Radical-radical interactions among oxidized guanine bases including guanine radical cation and dehydrogenated guanine radicals.

    PubMed

    Zhao, Jing; Wang, Mei; Yang, Hongfang; Zhang, Meng; Liu, Ping; Bu, Yuxiang

    2013-09-19

    We present here a theoretical investigation of the structural and electronic properties of di-ionized GG base pairs (G(•+)G(•+),G(-H1)(•)G(•+), and G(-H1)(•)G(-H1)(•)) consisting of the guanine cation radical (G(•+)) and/or dehydrogenated guanine radical (G(-H1)(•)) using density functional theory calculations. Different coupling modes (Watson-Crick/WC, Hoogsteen/Hoog, and minor groove/min hydrogen bonding, and π-π stacking modes) are considered. We infer that a series of G(•+)G(•+) complexes can be formed by the high-energy radiation. On the basis of density functional theory and complete active space self-consistent (CASSCF) calculations, we reveal that in the H-bonded and N-N cross-linked modes, (G(•+)G(•+))WC, (G(-H1)(•)G(-H1)(•))WC, (G(-H1)(•)G(-H1)(•))minI, and (G(-H1)(•)G(-H1)(•))minIII have the triplet ground states; (G(•+)G(•+))HoogI, (G(-H1)(•)G(•+))WC, (G(-H1)(•)G(•+))HoogI, (G(-H1)(•)G(•+))minI, (G(-H1)(•)G(•+))minII, and (G(-H1)(•)G(-H1)(•))minII possess open-shell broken-symmetry diradical-characterized singlet ground states; and (G(•+)G(•+))HoogII, (G(•+)G(•+))minI, (G(•+)G(•+))minII, (G(•+)G(•+))minIII, (G(•+)G(•+))HoHo, (G(-H1)(•)G(•+))minIII, (G(-H1)(•)G(•+))HoHo, and (G(-H1)(•)G(-H1)(•))HoHo are the closed-shell systems. For these H-bonded diradical complexes, the magnetic interactions are weak, especially in the diradical G(•+)G(•+) series and G(-H1)(•)G(-H1)(•) series. The magnetic coupling interactions of the diradical systems are controlled by intermolecular interactions (H-bond, electrostatic repulsion, and radical coupling). The radical-radical interaction in the π-π stacked di-ionized GG base pairs ((G(•+)G(•+))ππ, (G(-H1)(•)G(•+))ππ, and (G(-H1)(•)G(-H1)(•))ππ) are also considered, and the magnetic coupling interactions in these π-π stacked base pairs are large. This is the first theoretical prediction that some di

  3. DFT study on the cycloreversion of thietane radical cations.

    PubMed

    Domingo, Luis R; Pérez-Ruiz, Raúl; Argüello, Juan E; Miranda, Miguel A

    2011-06-01

    The molecular mechanism of the cycloreversion (CR) of thietane radical cations has been analyzed in detail at the UB3LYP/6-31G* level of theory. Results have shown that the process takes place via a stepwise mechanism leading to alkenes and thiobenzophenone; alternatively, formal [4+2] cycloadducts are obtained. Thus, the CR of radical cations 1a,b(•+) is initiated by C2-C3 bond breaking, giving common intermediates INa,b. At this stage, two reaction pathways are feasible involving ion molecule complexes IMCa,b (i) or radical cations 4a,b(•+) (ii). Calculations support that 1a(•+) follows reaction pathway ii (leading to the formal [4+2] cycloadducts 5a). By contrast, 1b(•+) follows pathway i, leading to trans-stilbene radical cation (2b(•+)) and thiobenzophenone. PMID:21561127

  4. Mechanism for radical cation transport in duplex DNA oligonucleotides.

    PubMed

    Liu, Chu-Sheng; Hernandez, Rigoberto; Schuster, Gary B

    2004-03-10

    We investigated the photoinduced one-electron oxidation of a series of DNA oligomers having a covalently linked anthraquinone group (AQ) and containing [(A)(n)GG](m) or [(T)(n)GG](m) segments. These oligomers have m GG steps, where m = 4 or 6, separated by (A)(n) or (T)(n) segments, where n = 1-7 for the (A)(n) set and 1-5 for the (T)(n) set. Irradiation with UV light that is absorbed by the AQ causes injection of a radical cation into the DNA. The radical cation migrates through the DNA, causing chemical reaction, primarily at GG steps, that leads to strand cleavage after piperidine treatment. The uniform, systematic structure of the DNA oligonucleotides investigated permits the numerical solution of a kinetic scheme that models these reactions. This analysis yields two rate constants, k(hop), for hopping of the radical cation from one site to adjacent sites, and k(trap), for irreversible reaction of the radical cation with H(2)O or O(2). Analysis of these findings indicates that radical cation hopping in these duplex DNA oligomers is a process that occurs on a microsecond time scale. The value of k(hop) depends on the number of base pairs in the (A)(n) and (T)(n) segments in a systematic way. We interpret these results in terms of a thermally activated adiabatic mechanism for radical cation hopping that we identify as phonon-assisted polaron hopping. PMID:14995205

  5. Sugar radicals formed by photo-excitation of guanine cation radical in oligonucleotides

    PubMed Central

    Adhikary, Amitava; Collins, Sean; Khanduri, Deepti; Sevilla, Michael D.

    2008-01-01

    This work presents evidence that photo-excitation of guanine cation radical (G•+) in dGpdG and DNA-oligonucleotides: TGT, TGGT, TGGGT, TTGTT, TTGGTT, TTGGTTGGTT, AGA and AGGGA in frozen glassy aqueous solutions at low temperatures leads to hole transfer to the sugar phosphate backbone and results in high yields of deoxyribose radicals. In this series of oligonucleotides we find that, G•+ on photo-excitation, at 143 K leads to the formation of predominantly C5′• and C1′• with small amounts of C3′•. Photo-conversion yields of G•+ to sugar radicals in oligonucleotides decreased as the overall chain length increased. However, for high molecular weight dsDNA (salmon testes) in frozen aqueous solutions substantial conversion of G•+ to C1′• (only) sugar radical is still found (ca. 50%). Within the cohort of sugar radicals formed we find a relative increase in the formation of C1′• with length of the oligonucleotide along with decreases in C3′• and C5′• For dsDNA in frozen solutions, only the formation of C1′• is found via photo-excitation of G•+ without a significant temperature dependence (77 K to 180 K). Long wavelength visible light (>540 nm) is observed to be about as effective as light under 540 nm for photoconversion of G•+ to sugar radicals for short oligonucleotides but gradually loses effectiveness with chain length. This wavelength dependence is attributed to base-to-base hole transfer for wavelengths >540 nm. Base-to-sugar hole transfer is suggested to dominate under 540 nm. These results may have implications for a number of investigations of hole transfer through DNA in which DNA-holes are subjected to continuous visible illumination. PMID:17547448

  6. A Supramolecularly Activated Radical Cation for Accelerated Catalytic Oxidation.

    PubMed

    Jiao, Yang; Li, Wan-Lu; Xu, Jiang-Fei; Wang, Guangtong; Li, Jun; Wang, Zhiqiang; Zhang, Xi

    2016-07-25

    Tuning the activity of radicals is crucial for radical reactions and radical-based materials. Herein, we report a supramolecular strategy to accelerate the Fenton reaction through the construction of supramolecularly activated radical cations. As a proof of the concept, cucurbit[7]uril (CB[7]) was introduced, through host-guest interactions, onto each side of a derivative of 1,4-diketopyrrolo[3,4-c]pyrrole (DPP), a model dye for Fenton oxidation. The DPP radical cation, the key intermediate in the oxidation process, was activated by the electrostatically negative carbonyl groups of CB[7]. The activation induced a drastic decrease in the apparent activation energy and greatly increased the reaction rate. This facile supramolecular strategy is a promising method for promoting radical reactions. It may also open up a new route for the catalytic oxidation of organic pollutants for water purification and widen the realm of supramolecular catalysis. PMID:27273046

  7. Magnetic Resonance Studies of Proton Loss from Carotenoid Radical Cations

    SciTech Connect

    Kispert, Lowell D; Focsan, A Ligia; Konovalova, Tatyana A; Lawrence, Jesse; Bowman, Michael K; Dixon, David A; Molnar, Peter; Deli, Jozsef

    2007-06-11

    Carotenoids, intrinsic components of reaction centers and pigment-protein complexes in photosynthetic membranes, play a photoprotective role and serve as a secondary electron donor. Before optimum use of carotenoids can be made in artificial photosynthetic systems, their robust nature in living materials requires extensive characterization of their electron transfer, radical trapping ability, stability, structure in and on various hosts, and photochemical behavior. Pulsed ENDOR and 2D-HYSCORE studies combined with DFT calculations reveal that photo-oxidation of natural zeaxanthin (I) and violaxanthin (II) on silica-alumina produces not only the carotenoid radical cations (Car•+) but also neutral radicals (#Car•) by proton loss from the methyl groups at positions 5 or 5', and possibly 9 or 9' and 13 or 13'. Notably, the proton loss favored in I at the 5 position by DFT calculations, is unfavorable in II due to the epoxide at the 5, 6 position. DFT calculations predict the isotropic methyl proton couplings of 8-10 MHz for Car•+ which agree with the ENDOR for carotenoid α-conjugated radical cations. Large α-proton hyperfine coupling constants (>10 MHz) determined from HYSCORE are assigned from the DFT calculations to neutral carotenoid radicals. Proton loss upon photolysis was also examined as a function of carotenoid polarity [Lycopene (III) versus 8'-apo-β-caroten-8'-al (IV)]; hydrogen bonding [Lutein (V) versus III]; host [silica-alumina versus MCM-41 molecular sieve]; and substituted metal in MCM-41. Loss of H+ from the 5(5'), 9(9') or 13(13') methyl positions has importance in photoprotection. Photoprotection involves nonphotochemical quenching (NPQ) in which 1Ch1* decays via energy transfer to the carotenoid which returns to the ground state by thermal dissipation; or via electron transfer to form a charge transfer state (I •+…Chl•-), lower in energy than 1Chl*. Formation of I •+ results in bond lengthening, a mechanism for nonradiative energy

  8. Formation of ions and radicals from icy grains in comets

    NASA Technical Reports Server (NTRS)

    Jackson, William M.

    1992-01-01

    Two theoretical models for the formation of radicals from ice grains are examined to determine if this can explain the jets in comets. It is shown that the production rates for these radicals by the photolysis of molecules in the icy grains are not high enough to explain the jets. A new mechanism is proposed involving the release of cations and anions in the gas phase as the icy mantle surrounding the grains is evaporated. Solar visible radiation can then form radicals by photodetachment of the electrons from these anions. The production rate of radicals formed in this manner is in accord with the production rates of the observed radicals.

  9. Pyridine radical cation and its fluorine substituted derivatives

    USGS Publications Warehouse

    Bondybey, V.E.; English, J.H.; Shiley, R.H.

    1982-01-01

    The spectra and relaxation of the pyridine cation and of several of its fluorinated derivatives are studied in low temperature Ne matrices. The ions are generated by direct photoionization of the parent compounds. Of the compounds studied, laser induced → and → fluorescence is observed only for the 2, 6‐difluoropyridine cation. The analysis of the spectrum indicates that the ion is planar both in the and states. The large variety in the spectroscopic and relaxation behavior of fluoropyridine radical cations is explained in terms of their electronic structure and of the differential shifts of the individual electronic states caused by the fluorine substitution.

  10. Photoinactivation of PS2 secondary donors by PS2 cation radicals and superoxide radicals

    SciTech Connect

    Chen, G.X.; Cheniae, G.M.; Blubaugh, D.J.; Golbeck, J.H.

    1991-12-31

    Illumination of Mn- and Cl-depleted PS2 causes rapid irreversible inactivation of specific redox-active components on the donor side of the PS2 Reaction Center (RC). Under aerobic conditions, weak light preillumination of NH{sub 2}OH-PS2 causes rapid loss of Y{sub Z}{sup {plus_minus}} formation, Y{sub Z} {yields} P{sub 680}{sup +}, the A{sub T}-band thermoluminescence emission, the Y{sub Z}{sup +}-dependent (Site 1) photooxidation of exogenous e{sup {minus}} donors, and the capability to photoligate Mn{sup 2+} into the water oxidizing enzyme (photoactivation), all without significantly affecting P{sub 680}{sup +}/Q{sub A}{sup {minus}} charge separation. In contrast, aerobic high light preillumination of Mn-depleted PS2 promotes very rapid and parallel loss of photoactivation and A{sub T}-band emission capabilities significantly than loss of either Y{sub Z}{sup +}-formation or P{sub 680}{sup +}/Q{sub A}{sup {minus}} charge separation capabilities. These photodamages and those to Cl-depleted thylakoids (4,5) generally are believed to be caused by reactions between the highly oxidizing cation radicals (P{sub 680}{sup +}/Chl{sup +}) and nearby amino acid residues of D{sub 1}>D{sub 2}. The reported promotion of the photodamages by e{sup {minus}} acceptors of Q{sub A}{sup {minus}}/Q{sub B}{sup {minus}} their inhibition by e{sup {minus}} donors to Y{sub Z}{sup +} and their occurrence under strict anaerobic conditions all tend to support the idea of direct damage by P{sub 680}{sup +}/Chl{sup +}. Our studies lead us to conclude that the photodamages to the donor side components are caused minimally by a rapid mechanism requiring both superoxide and PS2 cation radicals; and by a slower mechanism driven by the PS2 cation radicals only.

  11. Carotenoid cation radicals: electrochemical, optical, and EPR study

    SciTech Connect

    Grant, J.L.; Kramer, V.J.; Ding, R.; Kispert, L.D.

    1988-03-30

    The general aim of this investigation is to determine whether carotenoid cation radicals can be produced, and stabilized, electrochemically. Hence, the authors have undertaken a detailed study of the electrooxidation of various carotenoids (..beta..-carotene (I), ..beta..-apo-8'-carotenal (II), and canthaxanthin (III) using the techniques of cyclic voltammetry, controlled-potential electrolysis (cpe) in conjunction with optical spectroscopy, and EPR spectroscopy coupled with in situ electrolysis. They report the successful generation of carotenoid cation radicals via electrochemical oxidation and, furthermore, the stabilization of these radicals for several minutes in CH/sub 2/Cl/sub 2/ and C/sub 2/H/sub 4/Cl/sub 2/ solvents.

  12. p53 Mutagenesis by benzo[a]pyrene derived radical cations.

    PubMed

    Sen, Sushmita; Bhojnagarwala, Pratik; Francey, Lauren; Lu, Ding; Penning, Trevor M; Field, Jeffrey

    2012-10-15

    Benzo[a]pyrene (B[a]P), a major human carcinogen in combustion products such as cigarette smoke and diesel exhaust, is metabolically activated into DNA-reactive metabolites via three different enzymatic pathways. The pathways are the anti-(+)-benzo[a]pyrene 7,8-diol 9,10-epoxide pathway (P450/epoxide hydrolase catalyzed) (B[a]PDE), the benzo[a]pyrene o-quinone pathway (aldo ketose reductase (AKR) catalyzed) and the B[a]P radical cation pathway (P450 peroxidase catalyzed). We used a yeast p53 mutagenesis system to assess mutagenesis by B[a]P radical cations. Because radical cations are short-lived, they were generated in situ by reacting B[a]P with cumene hydroperoxide (CuOOH) and horse radish peroxidase (HRP) and then monitoring the generation of the more stable downstream products, B[a]P-1,6-dione and B[a]P-3,6-dione. On the basis of B[a]P-1,6 and 3,6-dione formation, approximately 4 μM of radical cation was generated. In the mutagenesis assays, the radical cations produced in situ showed a dose-dependent increase in mutagenicity from 0.25 μM to 10 μM B[a]P with no significant increase seen with further escalation to 50 μM B[a]P. However, mutagenesis was 200-fold less than with the AKR pathway derived B[a]P, 7-8-dione. Mutant p53 plasmids, which yield red colonies, were recovered from the yeast to study the pattern and spectrum of mutations. The mutation pattern observed was G to T (31%) > G to C (29%) > G to A (14%). The frequency of codons mutated by the B[a]P radical cations was essentially random and not enriched at known cancer hotspots. The quinone products of radical cations, B[a]P-1,6-dione and B[a]P-3,6-dione were more mutagenic than the radical cation reactions, but still less mutagenic than AKR derived B[a]P-7,8-dione. We conclude that B[a]P radical cations and their quinone products are weakly mutagenic in this yeast-based system compared to redox cycling PAH o-quinones. PMID:22768918

  13. Absorption and electroabsorption spectra of carotenoid cation radical and dication

    NASA Astrophysics Data System (ADS)

    Krawczyk, Stanisław

    1998-05-01

    Radical cations and dications of two carotenoids astaxanthin and canthaxanthin were prepared by oxidation with FeCl 3 in fluorinated alcohols at room temperature. Absorption and electroabsorption (Stark effect) spectra were recorded for astaxanthin cations in mixed frozen matrices at temperatures about 160 K. The D 0→D 2 transition in cation radical is at 835 nm. The electroabsorption spectrum for the D 0→D 2 transition exhibits a negative change of molecular polarizability, Δ α=-1.2·10 -38 C·m 2/V (-105 A 3), which seems to originate from the change in bond order alternation in the ground state rather than from the electric field-induced interaction of D 1 and D 2 excited states. Absorption spectrum of astaxanthin dication is located at 715-717 nm, between those of D 0→D 2 in cation radical and S 0→S 2 in neutral carotenoid. Its shape reflects a short vibronic progression and strong inhomogeneous broadening. The polarizability change on electronic excitation, Δ α=2.89·10 -38 C·m 2/V (260 A 3), is five times smaller than in neutral astaxanthin. This value reflects the larger energetic distance from the lowest excited state to the higher excited states than in the neutral molecule.

  14. Electron transfer reactions within zeolites: Radical cation from benzonorbornadiene

    SciTech Connect

    Pitchumani, K.; Ramamurthy, V.; Corbin, D.R.

    1996-08-28

    Zeolites are being used as solid acid catalysts in a number of commercial processes. Occasionally zeolites are also reported to perform as electron transfer agents. Recently, we observed that radical cations of certain olefins and thiophene oligomers can be generated spontaneously within ZSM-5 zeolites. We noticed that these radical cations generated from diphenyl polyenes and thiophene oligomers were remarkably stable (at room temperature) within ZSM-5 and can be characterized spectroscopically at leisure. We have initiated a program on electron transfer processes within large pore zeolites. The basis of this approach is that once a cation radical is generated within a large pore zeolite, it will have sufficient room to undergo a molecular transformation. Our aim is to identify a condition under which electron transfer can be routinely and reliably carried out within large pore zeolites such as faujasites. To our great surprise, when benzonorbornadiene A and a number of olefins were included in divalent cation exchanged faujasites. they were transformed into products very quickly (<15 min). This observation allowed us to explore the use of zeolites as oxidants. Results of our studies on benzonorbornadiene are presented in this communication. 16 refs., 1 fig.

  15. Gas-phase structure and reactivity of the keto tautomer of the deoxyguanosine radical cation.

    PubMed

    Feketeová, Linda; Chan, Bun; Khairallah, George N; Steinmetz, Vincent; Maître, Philippe; Radom, Leo; O'Hair, Richard A J

    2015-10-21

    Guanine radical cations are formed upon oxidation of DNA. Deoxyguanosine (dG) is used as a model, and the gas-phase infrared (IR) spectroscopic signature and gas-phase unimolecular and bimolecular chemistry of its radical cation, dG˙(+), A, which is formed via direct electrospray ionisation (ESI/MS) of a methanolic solution of Cu(NO3)2 and dG, are examined. Quantum chemistry calculations have been carried out on 28 isomers and comparisons between their calculated IR spectra and the experimentally-measured spectra suggest that A exists as the ground-state keto tautomer. Collision-induced dissociation (CID) of A proceeds via cleavage of the glycosidic bond, while its ion–molecule reactions with amine bases occur via a number of pathways including hydrogen-atom abstraction, proton transfer and adduct formation. A hidden channel, involving isomerisation of the radical cation via adduct formation, is revealed through the use of two stages of CID, with the final stage of CID showing the loss of CH2O as a major fragmentation pathway from the reformed radical cation, dG˙(+). Quantum chemistry calculations on the unimolecular and bimolecular reactivity are also consistent with A being present as a ground-state keto tautomer. PMID:25942055

  16. ESR of the cation radicals from γ-irradiated ketenes in haloalkane and SF 6 matrices

    NASA Astrophysics Data System (ADS)

    Shimokoshi, Kazuo; Fujisawa, Jun; Nakamura, Kazutaka; Sato, Shin; Shida, Tadamasa

    1983-08-01

    The cation radicals of ketene and its methyl and ethyl derivatives have been produced in γ-irradiated solid solutions of haloalkanes and SF 6. From the observed isotropic proton hyperfine parameters of these cation radicals and UHF MO (INDO) calculations, the cations are found to be characteristic of ordinary π-radicals.

  17. Oxoferryl porphyrin cation radicals in model systems: Evidence for variable metal-radical spin coupling

    NASA Astrophysics Data System (ADS)

    Bill, E.; Bominaar, E. L.; Ding, X.-Q.; Trautwein, A. X.; Winkler, H.; Mandon, D.; Weiss, R.; Gold, A.; Jayaraj, K.; Toney, G. E.

    1990-07-01

    Magnetic properties of frozen solutions of highly oxidized iron porphyrin complexes were investigated by EPR and Mössbauer spectroscopy. The Mössbauer spectra, recorded at low temperatures in various magnetic fields, were analyzed on the basis of spin Hamiltonian simulations. Spin coupling between ferryl iron (FeIV) and porphyrin cation radical was taken into account explicitly. Hyperfine and spin-coupling parameters are given for several complexes, together with zero-field parameters. One of the complexes exhibits weak spin coupling, it is the first model system exhibiting properties comparable to those of the oxoferryl cation radical enzyme Horse Radish Peroxidase I.

  18. Polyoxometalate salts of cationic nitronyl nitroxide free radicals

    NASA Astrophysics Data System (ADS)

    Coronado, Eugenio; Giménez-Saiz, Carlos; Gómez-García, Carlos J.; Romero, Francisco M.

    2008-12-01

    The cationic nitronyl nitroxide free radical of the N-methylpyridinium type p-MepyNN + has been combined with [Mo 8O 26] 4- and Keggin [SiW 12O 40] 4- polyanions to afford salts ( p-MepyNN) 4[Mo 8O 26]·DMSO (DMSO = dimethylsulfoxide) ( 1) and ( p-MepyNN) 4[SiW 12O 40]·6DMF (DMF = dimethylformamide) ( 2). Herein, their structural and magnetic properties are described.

  19. Ab initio study of the phenol-water cation radical

    NASA Astrophysics Data System (ADS)

    Hobza, Pavel; Burcl, Rudolf; Špirko, Vladimír; Dopfer, Otto; Müller-Dethlefs, Klaus; Schlag, Edward W.

    1994-07-01

    The phenol-water cation radical has been investigated by ab initio theory using the spin-restricted open-shell Hartree-Fock and spin-restricted open-shell second-order Møller-Plesset theories with 3-21G*(O) and 6-31G* basis sets. The full geometrical optimization was performed for several hydrogen-bonded structures and one hemibonded structure. Clearly, the most stable structure has been found for Cs symmetry with the linear hydrogen bond between the proton of the OH group of the phenol cation radical and the oxygen of the water, and the water hydrogens pointing away from the phenyl ring. For this structure harmonic (and for some intermolecular modes anharmonic) vibrational frequencies have been computed for various isotopic complexes. The computed shifts of phenol-localized intramolecular modes on complexation and on deuteration as well as the calculated intermolecular frequencies of the different isotopic complexes allow for an assignment of vibrational frequencies observed in the experimental zero-kinetic-energy (ZEKE) photoelectron spectra. Five out of a possible six intermolecular vibrations and several intramolecular modes have been assigned, including the 18b vibration which shows a strong blue shift in frequency upon complexation. Structure and properties of the phenol-water cation radical are compared with those of the corresponding neutral complex.

  20. Electron exchange involving a sulfur-stabilized ruthenium radical cation.

    PubMed

    Shaw, Anthony P; Ryland, Bradford L; Norton, Jack R; Buccella, Daniela; Moscatelli, Alberto

    2007-07-01

    Half-sandwich Ru(II) amine, thiol, and thiolate complexes were prepared and characterized by X-ray crystallography. The thiol and amine complexes react slowly with acetonitrile to give free thiol or amine and the acetonitrile complex. With the thiol complex, the reaction is dissociative. The thiolate complex has been oxidized to its Ru(III) radical cation and the solution EPR spectrum of that radical cation recorded. Cobaltocene reduces the thiol complex to the thiolate complex. The 1H and 31P NMR signals of the thiolate complex in acetonitrile become very broad whenever the thiolate and thiol complexes are present simultaneously. The line broadening is primarily due to electron exchange between the thiolate complex and its radical cation; the latter is generated by an unfavorable redox equilibrium between the thiol and thiolate complexes. Pyramidal inversion of sulfur in the thiol complex is fast at room temperature but slow at lower temperatures; major and minor conformers of the thiol complex were observed by 31P NMR at -98 degrees C in CD2Cl2. PMID:17569530

  1. Formation of a stable radical by oxidation of a tetraorganoborate.

    PubMed

    Braunschweig, Holger; Krummenacher, Ivo; Mailänder, Lisa; Pentecost, Leanne; Vargas, Alfredo

    2016-05-19

    Herein, we describe the selective formation of a stable neutral spiroborate radical by one-electron oxidation of the corresponding tetraorganoborate salt Li[B(C4Ph4)2], formally containing a tetrahedral borate centre and a s-cis-butadiene radical cation as the spin-bearing site. Spectroscopic and computational methods have been used to determine the spin distribution and the chromism observed in the solid state. PMID:27157624

  2. Dynamics of radical cations of poly(4-hydroxystyrene) in the presence and absence of triphenylsulfonium triflate as determined by pulse radiolysis of its highly concentrated solution

    NASA Astrophysics Data System (ADS)

    Okamoto, Kazumasa; Ishida, Takuya; Yamamoto, Hiroki; Kozawa, Takahiro; Fujiyoshi, Ryoko; Umegaki, Kikuo

    2016-07-01

    Pulse radiolysis of highly concentrated poly(4-hydroxystyrene) (PHS) solutions in cyclohexanone and p-dioxane was performed both with and without an onium-type photoacid generator (PAG). With increasing PHS concentration, the rate constant of deprotonation of PHS radical cations was found to decrease. In the presence of PAG, the yield of the multimer radical cation of PHS was shown to decrease. We found that pairing between the anions produced by the attachment of dissociative electrons of PAGs and the monomer PHS radical cations restrict local molecular motions, leading to the formation of the multimer PHS radical cations.

  3. Multi-State Vibronic Interactions in Fluorinated Benzene Radical Cations.

    NASA Astrophysics Data System (ADS)

    Faraji, S.; Köppel, H.

    2009-06-01

    Conical intersections of potential energy surfaces have emerged as paradigms for signalling strong nonadiabatic coupling effects. An important class of systems where some of these effects have been analyzed in the literature, are the benzene and benzenoid cations, where the electronic structure, spectroscopy, and dynamics have received great attention in the literature. In the present work a brief overview is given over our theoretical treatments of multi-mode and multi-state vibronic interactions in the benzene radical cation and some of its fluorinated derivatives. The fluorobenzene derivatives are of systematic interest for at least two different reasons. (1) The reduction of symmetry by incomplete fluorination leads to a disappearance of the Jahn-Teller effect present in the parent cation. (2) A specific, more chemical effect of fluorination consists in the energetic increase of the lowest σ-type electronic states of the radical cations. The multi-mode multi-state vibronic interactions between the five lowest electronic states of the fluorobenzene radical cations are investigated theoretically, based on ab initio electronic structure data, and employing the well-established linear vibronic coupling model, augmented by quadratic coupling terms for the totally symmetric vibrational modes. Low-energy conical intersections, and strong vibronic couplings are found to prevail within the set of tilde{X}-tilde{A} and tilde{B}-tilde{C}-tilde{D} cationic states, while the interactions between these two sets of states are found to be weaker and depend on the particular isomer. This is attributed to the different location of the minima of the various conical intersections occurring in these systems. Wave-packet dynamical simulations for these coupled potential energy surfaces, utilizing the powerful multi-configuration time-dependent Hartree method are performed. Ultrafast internal conversion processes and the analysis of the MATI and photo-electron spectra shed new light

  4. Theoretical studies on the dimerization of substituted paraphenylenediamine radical cations

    NASA Astrophysics Data System (ADS)

    Punyain, Kraiwan; Kelterer, Anne-Marie; Grampp, Günter

    2011-12-01

    Organic radical cations form dicationic dimers in solution, observed experimentally as diamagnetic species in temperature-dependent EPR and low temperature UV/Vis spectroscopy. Dimerization of paraphenylenediamine, N,N-dimethyl-paraphenylenediamine and 2,3,5,6-tetramethyl-paraphenylenediamine radical cation in ethanol/diethylether mixture was investigated theoretically according to geometry, energetics and UV/Vis spectroscopy. Density Functional Theory including dispersion correction describes stable dimers after geometry optimization with conductor-like screening model of solvation and inclusion of the counter-ion. Energy corrections were done on double-hybrid Density Functional Theory with perturbative second-order correlation (B2PLYP-D) including basis set superposition error (BSSE), and multireference Møller-Plesset second-order perturbation theory method (MRMP2) based on complete active space method (CASSCF(2,2)) single point calculation, respectively. All three dication π-dimers exhibit long multicenter π-bonds around 2.9 ± 0.1 Å with strongly interacting orbitals. Substitution with methyl groups does not influence the dimerization process substantially. Dispersion interaction and electrostatic attraction from counter-ion play an important role to stabilize the dication dimers in solution. Dispersion-corrected double hybrid functional B2PLYP-D and CASSCF(2,2) can describe the interaction energetics properly. Vertical excitations were computed with Tamm-Dancoff approximation for time-dependent Density Functional Theory (TDA-DFT) at the B3LYP level with the cc-pVTZ basis set including ethanol solvent molecules explicitly. A strong interaction of the counter-ion and the solvent ethanol with the monomeric species is observed, whereas in the dimers the strong interaction of both radical cation species is the dominating factor for the additional peak in UV/Vis spectra.

  5. Understanding electrochromic processes initiated by dithienylcyclopentene cation-radicals.

    PubMed

    Guirado, Gonzalo; Coudret, Christophe; Hliwa, Mohamed; Launay, Jean-Pierre

    2005-09-22

    Simple photochromic dithienylethylenes with either a perfluoro or a perhydro cyclopentene ring, and a variety of substituents (chlorine, iodine, trimethylsilyl, phenylthio, aldehyde, carboxylic acid, and ethynylanisyl), have been prepared and their electrochemical behavior was explored by cyclic voltammetry. All dithienylethylenes present two-electron irreversible oxidation waves in their open form, but the cation-radical of the open isomers can follow two different reaction pathways: dimerization or ring closure, whereas the halogen derivatives follow a dimerization mechanism, the presence of donor groups, such as the phenylthio-substituted compound, promote an efficient oxidative ring closure following an ECE/DISP mechanism. Electrochromic properties are also found in the corresponding ring-closed isomers. Depending on the substituents on the thiophene ring, and the perfluro or perhydro cyclopentene ring, open isomers can be obtained from oxidation (chemical or electrochemical) of the corresponding ring-closed isomers via an EC mechanism. This reaction pathway is favored by the presence of electron-withdrawing groups in the molecule. For all these compounds, closed or open, the oxidation lies between 0.8 and 1.5 V vs SCE, and provokes a permanent modification of the color, even after an oxidation-reduction cycle. This could be qualified as "electrochromism with memory". On the other hand, the ring-closed electron-rich isomers (E degrees < 0.8 V), which show reversible waves at the cation-radical or even dication level, give rise to "true electrochromism", for which no structural changes are observed. The experimental study was completed by theoretical calculations at the DFT level, using B3LYP density functional, which gave information on the total energy, the geometry, and the electronic structures of several representative compounds, either in the neutral form or in the cation-radical state. These results are important for the potential design of photochromic

  6. Excited state dynamics of the astaxanthin radical cation

    NASA Astrophysics Data System (ADS)

    Amarie, Sergiu; Förster, Ute; Gildenhoff, Nina; Dreuw, Andreas; Wachtveitl, Josef

    2010-07-01

    Femtosecond transient absorption spectroscopy in the visible and NIR and ultrafast fluorescence spectroscopy were used to examine the excited state dynamics of astaxanthin and its radical cation. For neutral astaxanthin, two kinetic components corresponding to time constants of 130 fs (decay of the S 2 excited state) and 5.2 ps (nonradiative decay of the S 1 excited state) were sufficient to describe the data. The dynamics of the radical cation proved to be more complex. The main absorption band was shifted to 880 nm (D 0 → D 3 transition), showing a weak additional band at 1320 nm (D 0 → D 1 transition). We found, that D 3 decays to the lower-lying D 2 within 100 fs, followed by a decay to D 1 with a time constant of 0.9 ps. The D 1 state itself exhibited a dual behavior, the majority of the population is transferred to the ground state in 4.9 ps, while a small population decays on a longer timescale of 40 ps. Both transitions from D 1 were found to be fluorescent.

  7. Structure and Reactivity of the N-Acetyl-Cysteine Radical Cation and Anion: Does Radical Migration Occur?

    NASA Astrophysics Data System (ADS)

    Osburn, Sandra; Berden, Giel; Oomens, Jos; O'Hair, Richard A. J.; Ryzhov, Victor

    2011-10-01

    The structure and reactivity of the N-acetyl-cysteine radical cation and anion were studied using ion-molecule reactions, infrared multi-photon dissociation (IRMPD) spectroscopy, and density functional theory (DFT) calculations. The radical cation was generated by first nitrosylating the thiol of N-acetyl-cysteine followed by the homolytic cleavage of the S-NO bond in the gas phase. IRMPD spectroscopy coupled with DFT calculations revealed that for the radical cation the radical migrates from its initial position on the sulfur atom to the α-carbon position, which is 2.5 kJ mol-1 lower in energy. The radical migration was confirmed by time-resolved ion-molecule reactions. These results are in contrast with our previous study on cysteine methyl ester radical cation (Osburn et al., Chem. Eur. J. 2011, 17, 873-879) and the study by Sinha et al. for cysteine radical cation ( Phys. Chem. Chem. Phys. 2010, 12, 9794-9800) where the radical was found to stay on the sulfur atom as formed. A similar approach allowed us to form a hydrogen-deficient radical anion of N-acetyl-cysteine, (M - 2H) •- . IRMPD studies and ion-molecule reactions performed on the radical anion showed that the radical remains on the sulfur, which is the initial and more stable (by 63.6 kJ mol-1) position, and does not rearrange.

  8. Electrochemical and electron paramagnetic resonance studies of a carotenoid cation radicals and dications: Effect of deuteration

    SciTech Connect

    Khaled, M.; Hadjipetrou, A.; Kispert, L. )

    1990-06-14

    The oxidation process involving the transfer of two electrons for {beta}-carotene is confirmed by bulk electrolysis in a CH{sub 2}Cl{sub 2} solvent and the observation of {Delta}E = 42 mV from cyclic voltammetric measurements. A similar process is also found to occur for {beta}-apo-8{prime}-carotenal and canthaxanthin. An additional cathodic peak between 0.2 0.5 relative to SCE is shown to be dependent on the initial formation of dications followed by the loss of H{sup +} as evidenced by a large isotope effect and most likely due to the reduction of a carotenoid cation. EPR evidence exists for the formation of radical cations by the reaction of diffusing carotenoid dictations with neutral carotenoids. The rate of formation is consistent with the differences in the diffusion coefficients of the carotenoids deduced by chronocoulometric measurements, being fastest for canthaxanthin.

  9. Reactions and structural investigation of chlorpromazine radical cation

    NASA Astrophysics Data System (ADS)

    Joshi, Ravi; Ghanty, Tapan K.; Mukherjee, T.

    2008-10-01

    Experimental and theoretical studies have been carried out to understand pro-oxidant behaviour of chlorpromazine radical cation (CPZ rad + ). Pulse radiolysis studies have shown that CPZ rad + oxidizes physiological antioxidants (uric acid and bilirubin), and biomolecules like, tyrosine and proteins (bovine serum albumin and casein), thereby acting as a pro-oxidant. Ab-initio quantum chemical calculations suggest structural and electronic changes on oxidation of CPZ. The calculations with Hartree-Fock and density functional methods show that ring nitrogen atom is the site of electron removal from CPZ and sulfur atom is the site of maximum spin in CPZ rad + . The calculations also suggest that oxidation of CPZ leads to increase in planarity of the tricyclic ring as well as tilting of alkyl side chain towards chlorine containing ring. The structural changes on oxidation of CPZ and spin delocalization in CPZ rad + fairly explain the pro-oxidant activity of CPZ.

  10. Spectroscopy of free-base N-confused tetraphenylporphyrin radical anion and radical cation.

    PubMed

    Alemán, Elvin A; Manríquez Rocha, Juan; Wongwitwichote, Wongwit; Godínez Mora-Tovar, Luis Arturo; Modarelli, David A

    2011-06-23

    The radical anions and radical cations of the two tautomers (1e and 1i) of 5,10,15,20-tetraphenyl N-confused free-base porphyrin have been studied using a combination of cyclic voltammetry, steady state absorption spectroscopy, and computational chemistry. N-Confused porphyrins (NCPs), alternatively called 2-aza-21-carba-5,10,15,20-tetraphenylporphyrins or inverted porphyrins, are of great interest for their potential as building blocks in assemblies designed for artificial photosynthesis, and understanding the absorption spectra of the corresponding radical ions is paramount to future studies in multicomponent arrays where electron-transfer reactions are involved. NCP 1e was shown to oxidize at a potential of E(ox) 0.65 V vs Fc(+)|Fc in DMF and reduce at E(red) -1.42 V, while the corresponding values for 1i in toluene were E(ox) 0.60 V and E(red) -1.64 V. The geometries of these radical ions were computed at the B3LYP/6-31+G(d)//B3LYP/6-31G(d) level in the gas phase and in solution using the polarizable continuum model (PCM). From these structures and that of H(2)TPP and its corresponding radical ions, the computed redox potentials for 1e and 1i were calculated using the Born-Haber cycle. While the computed reduction potentials and electron affinities were in excellent agreement with the experimental reduction potentials, the calculated oxidation potentials displayed a somewhat less ideal relationship with experiment. The absorption spectra of the four radical ions were also measured experimentally, with radical cations 1e(•+) and 1i(•+) displaying significant changes in the Soret and Q-band regions as well as new low energy absorption bands in the near-IR region. The changes in the absorption spectra of radical anions 1e(•-) and 1i(•-) were not as dramatic, with the changes occurring only in the Soret and Q-band regions. These results were favorably modeled using time-dependent density functional calculations at the TD-B3LYP/6-31+G(d)//B3LYP/6-31G

  11. Electronic spectra of the tetraphenylcyclobutadienecyclopentadienylnickel(II) cation and radical

    DOE PAGESBeta

    Peter R. Craig; Miller, John R.; Havlas, Zdenek; Trujillo, Marianela; Rempala, Pawel; Kirby, James P.; Noll, Bruce C.; Michl, Josef

    2016-05-02

    In this study, properties of the tetraphenylcyclobutadienecyclopentadienylnickel(II) cation 1 and its tetra-o-fluoro derivative 1a have been measured and calculated. The B3LYP/TZP optimized geometry of the free cation 1 agrees with a single-crystal X-ray diffraction structure except that in the crystal one of the phenyl substituents is strongly twisted to permit a close-packing interaction of two of its hydrogens with a nearby BF–4 anion. The low-energy parts of the solution electronic absorption and magnetic circular dichroism (MCD) spectra of 1 and 1a have been interpreted by comparison with TD-DFT (B3LYP/TZP) results. Reduction or pulse radiolysis lead to a neutral 19-electron radical,more » whose visible absorption and MCD spectra have been recorded and interpreted as well. The reduction is facilitated by ~0.1 V upon going from 1 to 1a« less

  12. Electronic Spectra of the Tetraphenylcyclobutadienecyclopentadienylnickel(II) Cation and Radical.

    PubMed

    Craig, Peter R; Havlas, Zdeněk; Trujillo, Marianela; Rempala, Pawel; Kirby, James P; Miller, John R; Noll, Bruce C; Michl, Josef

    2016-05-26

    Properties of the tetraphenylcyclobutadienecyclopentadienylnickel(II) cation 1 and its tetra-o-fluoro derivative 1a have been measured and calculated. The B3LYP/TZP optimized geometry of the free cation 1 agrees with a single-crystal X-ray diffraction structure except that in the crystal one of the phenyl substituents is strongly twisted to permit a close-packing interaction of two of its hydrogens with a nearby BF4(-) anion. The low-energy parts of the solution electronic absorption and magnetic circular dichroism (MCD) spectra of 1 and 1a have been interpreted by comparison with TD-DFT (B3LYP/TZP) results. Reduction or pulse radiolysis lead to a neutral 19-electron radical, whose visible absorption and MCD spectra have been recorded and interpreted as well. The reduction is facilitated by ∼0.1 V upon going from 1 to 1a. Unsuccessful attempts to prepare several other aryl substituted derivatives of 1 by the classical synthetic route are described in the Supporting Information . PMID:27136127

  13. Observation of radical cations by swiftness or by stealth.

    SciTech Connect

    Werst, D. W.; Trifunac, A. D.; Chemistry

    1998-01-01

    David W. Werst was born in Missouri and educated at the University of Missouri (B.S.) and the University of Minnesota (Ph.D.). His thesis work was carried out under the supervision of Paul Barbara and Ronald Gentry. He joined the Chemistry Division of Argonne National Laboratory in 1985 to conduct postdoctoral research in the group of Alexander Trifunac and has been a staff member of the Radiation and Photochemistry Group since 1989. His primary research interests are in the study of radical cation structure and reactivity and ionization processes in ordered and amorphous solids. Alexander D. Trifunac was born in Yugoslavia. He was educated at Columbia University (B.A.) and the University of Chicago (Ph.D.). His thesis work was on CIDNP, radical pair theory and experiments, with Gerhard L. Closs. He joined the Chemistry Division of Argonne National Laboratory in 1972 in a postdoctoral position. He is a senior scientist and is (since 1982) a group leader of the Radiation and Photochemistry Group. His research interests are in the study of chemistry of transient paramagnetic species involved in energy and charge-transfer processes occurring in radiolysis and in photoionization in liquids, glasses, and amorphous solids.

  14. Transient formation of the oxo-iron(IV) porphyrin radical cation during the reaction of iron(III) tetrakis-5,10,15,20-(N-methyl-4-pyridyl)porphyrin with hydrogen peroxide in aqueous solution.

    PubMed

    Saha, Tapan Kumar; Karmaker, Subarna; Tamagake, Keietsu

    2003-01-01

    The reaction of iron(III) tetrakis-5,10,15,20-(N-methyl-4-pyridyl)porphyrin (Fe(III)TMPyP) with hydrogen peroxide (H(2)O(2)) and the catalytic activity of the reaction intermediates on the luminescent peroxidation of luminol in aqueous solution were studied by using a double-mixing stopped-flow system. The observed luminescence intensities showed biphasic decay depending on the conditions. The initial flashlight decayed within <1 s followed by a sustained emission for more than 30 s. Computer deconvolution of the time-resolved absorption spectra under the same conditions revealed that the initial flashlight appeared during the formation of the oxo-iron(IV) porphyrin, TMPyPFe(IV) = O, which is responsible for the sustained emission. The absorption spectra 0.0-0.5 s did not reproduce well by a simple combination of the two spectra of Fe(III)TMPyP and TMPyPFe(IV) = O, indicating that transient species was formed at the initial stage. Addition of uric acid (UA) caused a significant delay in the initiation of the luminol emission as well as in the formation of the TMPyPFe(IV) = O. Both of them were completely diminished in the presence of UA equimolar with H(2)O(2), while mannitol had no effect at all. The delay of the light emission as well as the appearance of TMPyPFe(IV) = O was directly proportional to the [UA](0) but other kinetic profiles were not changed significantly. Based on these observations and the kinetic analysis, we confirmed the involvement of the oxo-iron(IV) porphyrin radical cation, (TMPyP)(.+)Fe(IV) = O, as an obligatory intermediate in the rate-determining step of the overall reaction, Fe(III)TMPyP + H(2)O(2) --> TMPyPFe(IV) = O, with a rate constant of k = 4.3 x 10(4)/mol/L/s. The rate constants for the reaction between the (TMPyP)(.+)Fe(IV) = O and luminol, and between the TMPyPFe(IV) = O and luminol were estimated to be 3.6 x 10(6)/mol/L/s and 1.31 x 10(4)/mol/L/s, respectively. PMID:12701092

  15. The chemistry of amine radical cations produced by visible light photoredox catalysis

    PubMed Central

    Hu, Jie; Wang, Jiang; Nguyen, Theresa H

    2013-01-01

    Summary Amine radical cations are highly useful reactive intermediates in amine synthesis. They have displayed several modes of reactivity leading to some highly sought-after synthetic intermediates including iminium ions, α-amino radicals, and distonic ions. One appealing method to access amine radical cations is through one-electron oxidation of the corresponding amines under visible light photoredox conditions. This approach and subsequent chemistries are emerging as a powerful tool in amine synthesis. This article reviews synthetic applications of amine radical cations produced by visible light photocatalysis. PMID:24204409

  16. Halogenated benzene radical cations and ground state degeneracy splitting by asymmetric substitution

    USGS Publications Warehouse

    Bondybey, V.E.; Vaughn, C.R.; Miller, T.A.; English, J.H.; Shiley, R.H.

    1981-01-01

    The absorption and laser induced fluorescence of several halogenated benzene radical cations were studied in solid Ne matrices. The spectra of 1,2,4-trifluorobenzene, l,3-dichloro-5-fluorobenzene, and l-chloro-3,5- difluorobenzene radical cations are observed and analyzed. Studies of fluorescence polarization and a photoselection technique were used to examine the splitting of the degeneracy of the benzene cation ground state by asymmetric subsitution. ?? 1981 American Institute of Physics.

  17. Novel Cβ-Cγ Bond Cleavages of Tryptophan-Containing Peptide Radical Cations

    NASA Astrophysics Data System (ADS)

    Song, Tao; Hao, Qiang; Law, Chun-Hin; Siu, Chi-Kit; Chu, Ivan K.

    2012-02-01

    In this study, we observed unprecedented cleavages of the Cβ-Cγ bonds of tryptophan residue side chains in a series of hydrogen-deficient tryptophan-containing peptide radical cations (M•+) during low-energy collision-induced dissociation (CID). We used CID experiments and theoretical density functional theory (DFT) calculations to study the mechanism of this bond cleavage, which forms [M - 116]+ ions. The formation of an α-carbon radical intermediate at the tryptophan residue for the subsequent Cβ-Cγ bond cleavage is analogous to that occurring at leucine residues, producing the same product ions; this hypothesis was supported by the identical product ion spectra of [LGGGH - 43]+ and [WGGGH - 116]+, obtained from the CID of [LGGGH]•+ and [WGGGH]•+, respectively. Elimination of the neutral 116-Da radical requires inevitable dehydrogenation of the indole nitrogen atom, leaving the radical centered formally on the indole nitrogen atom ([Ind]•-2), in agreement with the CID data for [WGGGH]•+ and [W1-CH3GGGH]•+; replacing the tryptophan residue with a 1-methyltryptophan residue results in a change of the base peak from that arising from a neutral radical loss (116 Da) to that arising from a molecule loss (131 Da), both originating from Cβ-Cγ bond cleavage. Hydrogen atom transfer or proton transfer to the γ-carbon atom of the tryptophan residue weakens the Cβ-Cγ bond and, therefore, decreases the dissociation energy barrier dramatically.

  18. High-level ab initio predictions for the ionization energy, electron affinity, and heats of formation of cyclopentadienyl radical, cation, and anion, C5H5/C5H5+/C5H5-.

    PubMed

    Lo, Po-Kam; Lau, Kai-Chung

    2014-04-01

    The ionization energy (IE), electron affinity (EA), and heats of formation (ΔH°f0/ΔH°f298) for cyclopentadienyl radical, cation, and anion, C5H5/C5H5(+)/C5H5(-), have been calculated by wave function-based ab initio CCSDT/CBS approach, which involves approximation to complete basis set (CBS) limit at coupled-cluster level with up to full triple excitations (CCSDT). The zero-point vibrational energy correction, core-valence electronic correction, scalar relativistic effect, and higher-order corrections beyond the CCSD(T) wave function are included in these calculations. The allylic [C5H5((2)A2)] and dienylic [C5H5((2)B1)] forms of cyclopentadienyl radical are considered: the ground state structure exists in the dienyl form and it is about 30 meV more stable than the allylic structure. Both structures are lying closely and are interconvertible along the normal mode of b2 in-plane vibration. The CCSDT/CBS predictions (in eV) for IE[C5H5(+)((3)A1')←C5H5((2)B1)] = 8.443, IE[C5H5(+)((1)A1)←C5H5((2)B1)] = 8.634 and EA[C5H5(-)((1)A1')←C5H5((2)B1)] = 1.785 are consistent with the respective experimental values of 8.4268 ± 0.0005, 8.6170 ± 0.0005, and 1.808 ± 0.006, obtained from photoelectron spectroscopic measurements. The ΔH°f0/ΔH°f298's (in kJ/mol) for C5H5/C5H5(+)/C5H5(-) have also been predicted by the CCSDT/CBS method: ΔH°f0/ΔH°f298[C5H5((2)B1)] = 283.6/272.0, ΔH°f0/ΔH°f298[C5H5(+)((3)A1')] = 1098.2/1086.9, ΔH°f0/ΔH°f298[C5H5(+)((1)A1)] = 1116.6/1106.0, and ΔH°f0/ΔH°f298[C5H5(-)((1)A1')] = 111.4/100.0. The comparisons between the CCSDT/CBS predictions and the experimental values suggest that the CCSDT/CBS procedure is capable of predicting reliable IE(C5H5)'s and EA(C5H5) with uncertainties of ± 17 and ± 23 meV, respectively. PMID:24621131

  19. Gas-phase electronic spectrum of the indole radical cation

    NASA Astrophysics Data System (ADS)

    Chalyavi, N.; Catani, K. J.; Sanelli, J. A.; Dryza, V.; Bieske, E. J.

    2015-08-01

    The visible and near-UV electronic spectrum of the indole radical cation is measured in the gas phase by photodissociation of indole+-Ar and indole+-He complexes in a tandem mass spectrometer. A series of resolved vibronic transitions extending from 610 to 460 nm are assigned to the D2 ← D0 band system, while weak transitions between 390 and 360 nm are assigned to the D3 ← D0 system, and a stronger, broad, unresolved absorption between 350 and 300 nm is attributed to the D4 ← D0 system. Time-dependent density functional theory calculations are used to assign vibronic structure of the D2 ← D0 band system, and show that the main active vibrational modes correspond to in-plane ring deformations. The strongest D2 ← D0 vibronic transitions of indole+-He do not correspond with any catalogued diffuse interstellar bands, even considering band displacements of up to 50 cm-1possibly caused by the attached He atom.

  20. Structure and Reactivity of the Distonic and Aromatic Radical Cations of Tryptophan

    NASA Astrophysics Data System (ADS)

    Piatkivskyi, Andrii; Osburn, Sandra; Jaderberg, Kendall; Grzetic, Josipa; Steill, Jeffrey D.; Oomens, Jos; Zhao, Junfang; Lau, Justin Kai-Chi; Verkerk, Udo H.; Hopkinson, Alan C.; Siu, K. W. Michael; Ryzhov, Victor

    2013-04-01

    In this work, we regiospecifically generate and compare the gas-phase properties of two isomeric forms of tryptophan radical cations—a distonic indolyl N-radical (H3N+ - TrpN•) and a canonical aromatic π (Trp•+) radical cation. The distonic radical cation was generated by nitrosylating the indole nitrogen of tryptophan in solution followed by collision-induced dissociation (CID) of the resulting protonated N-nitroso tryptophan. The π-radical cation was produced via CID of the ternary [CuII(terpy)(Trp)] •2+ complex. CID spectra of the two isomeric species were found to be very different, suggesting no interconversion between the isomers. In gas-phase ion-molecule reactions, the distonic radical cation was unreactive towards n-propylsulfide, whereas the π radical cation reacted by hydrogen atom abstraction. DFT calculations revealed that the distonic indolyl radical cation is about 82 kJ/mol higher in energy than the π radical cation of tryptophan. The low reactivity of the distonic nitrogen radical cation was explained by spin delocalization of the radical over the aromatic ring and the remote, localized charge (at the amino nitrogen). The lack of interconversion between the isomers under both trapping and CID conditions was explained by the high rearrangement barrier of ca.137 kJ/mol. Finally, the two isomers were characterized by infrared multiple-photon dissociation (IRMPD) spectroscopy in the ~1000-1800 cm-1 region. It was found that some of the main experimental IR features overlap between the two species, making their distinction by IRMPD spectroscopy in this region problematic. In addition, DFT theoretical calculations showed that the IR spectra are strongly conformation-dependent.

  1. Electronic absorption spectroscopy of polycyclic aromatic hydrocarbons (PAHs) radical cations generated in oleum: A superacid medium

    NASA Astrophysics Data System (ADS)

    Cataldo, Franco; Iglesias-Groth, Susana; Manchado, Arturo

    2010-12-01

    Oleum (fuming sulphuric acid), a well known superacid, was used as medium for the generation of the radical cation of a series of selected PAHs. The resulting radical cation spectra were studied by electronic absorption spectroscopy. Not only common PAHs like naphthalene, anthracene, tetracene, pentacene, perylene, pyrene, benzo[ a]pyrene, phenanthrene and picene were studied but also the less common and very large PAHs relevant also for the astrochemical research, like coronene, hexabenzocoronene, quaterrylene, dicoronylene and a coronene oligomer. A correlation between the first ionization potential ( IP1) of the PAHs studied and the energy to the so-called A-type band of the radical cations observed in oleum has led to the equation IP1 = 1.30 EA + 4.39 (in eV) which permits to estimate the energy of the PAHs radical cation transition ( EA) in the VIS-NIR knowing the relative ionization potential or vice versa.

  2. Mechanistic Investigation of Phosphate Ester Bond Cleavages of Glycylphosphoserinyltryptophan Radical Cations under Low-Energy Collision-Induced Dissociation

    NASA Astrophysics Data System (ADS)

    Quan, Quan; Hao, Qiang; Song, Tao; Siu, Chi-Kit; Chu, Ivan K.

    2013-04-01

    Under the conditions of low-energy collision-induced dissociation (CID), the canonical glycylphosphoserinyltryptophan radical cation having its radical located on the side chain of the tryptophan residue ([G p SW]•+) fragments differently from its tautomer with the radical initially generated on the α-carbon atom of the glycine residue ([G• p SW]+). The dissociation of [G• p SW]+ is dominated by the neutral loss of H3PO4 (98 Da), with backbone cleavage forming the [b2 - H]•+/y1 + pair as the minor products. In contrast, for [G p SW]•+, competitive cleavages along the peptide backbone, such as the formation of [G p SW - CO2]•+ and the [c2 + 2H]+/[z1 - H]•+ pair, significantly suppress the loss of neutral H3PO4. In this study, we used density functional theory (DFT) to examine the mechanisms for the tautomerizations of [G• p SW]+ and [G p SW]•+ and their dissociation pathways. Our results suggest that the dissociation reactions of these two peptide radical cations are more efficient than their tautomerizations, as supported by Rice-Ramsperger-Kassel-Marcus (RRKM) modeling. We also propose that the loss of H3PO4 from both of these two radical cationic tautomers is preferentially charge-driven, similar to the analogous dissociations of even-electron protonated peptides. The distonic radical cationic character of [G• p SW]+ results in its charge being more mobile, thereby favoring charge-driven loss of H3PO4; in contrast, radical-driven pathways are more competitive during the CID of [G p SW]•+.

  3. Novel Cβ-Cγ bond cleavages of tryptophan-containing peptide radical cations.

    PubMed

    Song, Tao; Hao, Qiang; Law, Chun-Hin; Siu, Chi-Kit; Chu, Ivan K

    2012-02-01

    In this study, we observed unprecedented cleavages of the C(β)-C(γ) bonds of tryptophan residue side chains in a series of hydrogen-deficient tryptophan-containing peptide radical cations (M(•+)) during low-energy collision-induced dissociation (CID). We used CID experiments and theoretical density functional theory (DFT) calculations to study the mechanism of this bond cleavage, which forms [M - 116](+) ions. The formation of an α-carbon radical intermediate at the tryptophan residue for the subsequent C(β)-C(γ) bond cleavage is analogous to that occurring at leucine residues, producing the same product ions; this hypothesis was supported by the identical product ion spectra of [LGGGH - 43](+) and [WGGGH - 116](+), obtained from the CID of [LGGGH](•+) and [WGGGH](•+), respectively. Elimination of the neutral 116-Da radical requires inevitable dehydrogenation of the indole nitrogen atom, leaving the radical centered formally on the indole nitrogen atom ([Ind](•)-2), in agreement with the CID data for [WGGGH](•+) and [W(1-CH3)GGGH](•+); replacing the tryptophan residue with a 1-methyltryptophan residue results in a change of the base peak from that arising from a neutral radical loss (116 Da) to that arising from a molecule loss (131 Da), both originating from C(β)-C(γ) bond cleavage. Hydrogen atom transfer or proton transfer to the γ-carbon atom of the tryptophan residue weakens the C(β)-C(γ) bond and, therefore, decreases the dissociation energy barrier dramatically. PMID:22135037

  4. Radical cations of sulfides and disulfides: An ESR study

    SciTech Connect

    Bonazzola, L.; Michaut, J.P.; Roncin, J.

    1985-09-15

    Exposure of dilute solutions of dimethylsulfide, methanethiol, tetrahydrothiophene, terbutyl and diterbutyl-sulfides, dimethyl-disulfide, and diterbutyldisulfide, in freon at 77 K to /sup 60/Co ..gamma.. rays gave the corresponding cations. From the reported ESR spectra, g tensors were obtained. It was found that both sulfide and disulfide cations exhibit the same g tensor: (g/sub max/ = 2.034 +- 0.002, g/sub int/ = 2.017 +- 0.001, g/sub min/ = 2.001 +- 0.005). From this result it has been shown that the disulfide cation is planar. This finding was supported by fully optimized geometry ab initio calculations.

  5. UV/Vis Action Spectroscopy and Structures of Tyrosine Peptide Cation Radicals in the Gas Phase.

    PubMed

    Viglino, Emilie; Shaffer, Christopher J; Tureček, František

    2016-06-20

    We report the first application of UV/Vis photodissociation action spectroscopy for the structure elucidation of tyrosine peptide cation radicals produced by oxidative intramolecular electron transfer in gas-phase metal complexes. Oxidation of Tyr-Ala-Ala-Ala-Arg (YAAAR) produces Tyr-O radicals by combined electron and proton transfer involving the phenol and carboxyl groups. Oxidation of Ala-Ala-Ala-Tyr-Arg (AAAYR) produces a mixture of cation radicals involving electron abstraction from the Tyr phenol ring and N-terminal amino group in combination with hydrogen-atom transfer from the Cα positions of the peptide backbone. PMID:27159034

  6. Low-temperature EPR and quantum chemical study of lactone radical cations and their transformations

    NASA Astrophysics Data System (ADS)

    Naumov, S.; Janovský, I.; Knolle, W.; Mehnert, R.; Turin, D. A.

    2005-07-01

    Radical cations of a number of lactones ( β-butyro-, γ-butyro-, γ-valero-, δ-hexano-, δ-valero- and ɛ-capro-) were radiolytically generated in CF 3CCl 3 matrix and investigated by EPR spectroscopy. The primary radical cation of the 4-membered ring β-butyrolactone is unstable even at 77 K and undergoes spontaneous ring opening and fragmentation, leading to the deprotonated neutral (CH 2CHCH 2) rad radical. The stability of the primary carbonyl-centred radical cations of the 5-, 6- and 7-membered lactone rings towards intramolecular H-shift from the C1 in α-position to carbonyl oxygen depends primarily on the ring size, which determines the activation energy of the transformation and distance L(H-O) of the carbonyl oxygen to the nearest H-atom on the ring. The larger the ring, the smaller the L(H-O) and also activation energy of the H-shift, making the transformation of the primary radical cation more feasible. The quantum chemical calculations facilitated the interpretation of the EPR spectra of the secondary radical cations.

  7. Comparing the gas-phase fragmentation reactions of protonated and radical cations of the tripeptides GXR

    NASA Astrophysics Data System (ADS)

    Wee, Sheena; O'Hair, Richard A. J.; McFadyen, W. David

    2004-05-01

    Electrospray ionization (ESI) mass spectrometry of methanolic solutions of mixtures of the copper salt (2,2':6',2''-terpyridine)copper(II) nitrate monohydrate ([Cu(II)(tpy)(NO3)2].H2O) and a tripeptide GXR (where X = 1 of the 20 naturally occurring amino acids) yielded [Cu(II)(tpy)(GXR)][radical sign]2+ ions, which were then subjected to collision induced dissociation (CID). In all but one case (GRR), these [Cu(II)(tpy)(GXR)][radical sign]2+ ions fragment to form odd electron GXR[radical sign]+ radical cations with sufficient abundance to examine their gas-phase fragmentation reactions. The GXR[radical sign]+ radical cations undergo a diverse range of fragmentation reactions which depend on the nature of the side chain of X. Many of these reactions can be rationalized as arising from the intermediacy of isomeric distonic ions in which the charge (i.e. proton) is sequestered by the highly basic arginine side chain and the radical site is located at various positions on the tripeptide including the peptide back bone and side chains. The radical sites in these distonic ions often direct the fragmentation reactions via the expulsion of small radicals (to yield even electron ions) or small neutrals (to form radical cations). Both classes of reaction can yield useful structural information, allowing for example, distinction between leucine and isoleucine residues. The gas-phase fragmentation reactions of the GXR[radical sign]+ radical cations are also compared to their even electron [GXR+H]+ and [GXR+2H]2+ counterparts. The [GXR+H]+ ions give fewer sequence ions and more small molecule losses while the [GXR+2H]2+ ions yield more sequence information, consistent with the [`]mobile proton model' described in previous studies. In general, all three classes of ions give complementary structural information, but the GXR[radical sign]+ radical cations exhibit a more diverse loss of small species (radicals and neutrals). Finally, links between these gas-phase results and key

  8. Generation and characterization of 1,2-diaryl-1,1,2,2-tetramethyldisilane cation radicals.

    PubMed

    Guirado, Gonzalo; Haze, Olesya; Dinnocenzo, Joseph P

    2010-05-21

    Nanosecond laser flash photolysis methods were used to generate and spectrally characterize the cation radicals of 1,2-diaryl-1,1,2,2,-tetramethyldisilanes (Ar = p-X-Ph, X = H, CH(3), OCH(3)) in hexafluoroisopropanol (HFIP) at room temperature. The disilane cation radicals rapidly reacted with methanol, with bimolecular rate constants ranging from 0.63 to 2.1 x 10(8) M(-1) s(-1). The cation radicals were found to react with tert-butanol 4-5 times more slowly than methanol, consistent with a small steric effect for nucleophile-assisted fragmentation of the Si-Si bond. The standard potentials for oxidation of the disilanes in HFIP were determined by two different methods: first, by measuring equilibrium constants for electron exchange between the disilanes and the cation radical of hexaethylbenzene and, second, by combining electrochemical data from cyclic voltammetry with the lifetimes of the disilane cation radicals measured by laser flash photolysis in the same media. Agreement between the two methods was excellent (

  9. Multistep π dimerization of tetrakis(n-decyl)heptathienoacene radical cations: a combined experimental and theoretical study.

    PubMed

    Ferrón, Cristina Capel; Capdevila-Cortada, Marçal; Balster, Russell; Hartl, František; Niu, Weijun; He, Mingqian; Novoa, Juan J; López Navarrete, Juan T; Hernández, Víctor; Ruiz Delgado, M Carmen

    2014-08-11

    Radical cations of a heptathienoacene α,β-substituted with four n-decyl side groups (D4T7(.) (+) ) form exceptionally stable π-dimer dications already at ambient temperature (Chem. Comm. 2011, 47, 12622). This extraordinary π-dimerization process is investigated here with a focus on the ultimate [D4T7(.) (+) ]2 π-dimer dication and yet-unreported transitory species formed during and after the oxidation. To this end, we use a joint experimental and theoretical approach that combines cyclic voltammetry, in situ spectrochemistry and spectroelectrochemistry, EPR spectroscopy, and DFT calculations. The impact of temperature, thienoacene concentration, and the nature and concentration of counteranions on the π-dimerization process is also investigated in detail. Two different transitory species were detected in the course of the one-electron oxidation: 1) a different transient conformation of the ultimate [D4T7(.) (+) ]2 π-dimer dications, the stability of which is strongly affected by the applied experimental conditions, and 2) intermediate [D4T7]2 (.) (+) π-dimer radical cations formed prior to the fully oxidized [D4T7]2 (.) (+) π-dimer dications. Thus, this comprehensive work demonstrates the formation of peculiar supramolecular species of heptathienoacene radical cations, the stability, nature, and structure of which have been successfully analyzed. We therefore believe that this study leads to a deeper fundamental understanding of the mechanism of dimer formation between conjugated aromatic systems. PMID:25043826

  10. Carbon-hydrogen vs. carbon-carbon bond cleavage of 1,2-diarylethane radical cations in acetonitrile-water

    SciTech Connect

    Camaioni, D.M.; Franz, J.A.

    1984-05-04

    Radical cations of 1,2-diarylethanes and 1-phenyl-2-arylethanes (Ar = phenyl, p-tolyl, p-anisyl) were generated in acidic 70% acetonitrile-water by Cu/sup 2 +/-catalyzed peroxydisulfate oxidation. The radical cations fragment mainly by loss of benzylic protons (C-H cleavage) rather than by alkyl C-C bond cleavage. The 1,2-diarylethanol products undergo further selective oxidation to aryl aldehydes and arylmethanols via rapid equilibration of diarylethane and diarylethanol radical cations. The radical cation of 2,3-dimethyl-2,3-diphenylbutane fragments efficiently by C-C cleavage, forming cumyl radical and cumyl cation. Oxidations of bibenzyl-bicumyl mixtures show selective oxidation of bicumyl dependent on total substrate concentration, providing evidence of equilibrating radical cations and showing that bicumyl fragments faster than bibenzyl loses protons. The effects of reaction conditions and substrate structure on reactivity are discussed.

  11. Calculation of structures and bond dissociation energies of radical cations: The importance of through-bond delocalization in bibenzylic systems

    SciTech Connect

    Camaioni, D.M. )

    1990-12-19

    Structures ad energies ({Delta}H{degree}{sub f}) of radical cations and their radical and cationic fragments have been calculated by use of AM1 semiempirical molecular orbital theory and compared with experimental data in the literature. Experimental {Delta}H{degree}{sub f} correlate linearly with calculated heats giving nonzero intercepts and nonunit slopes. The best correlations as judged by the variance of the fit are obtained when performed according to structure types, i.e., aromatic radical cations, alkane radical cations, radicals, and cations. These correlations enable corrections to AM1 values that allow prediction of experimental {Delta}H{degree}{sub f} with uncertainties that approach experimental uncertainties. Used in this way, AM1 can augment experimental thermochemical data and enable confident predictions of reaction enthalpies. Bibenzylic radical cations are calculated to have charge and sin localized in only one of the aromatic rings ether through space or through the ethylenic bond are found.

  12. Theoretical study of second-order hyperpolarizability for nitrogen radical cation

    NASA Astrophysics Data System (ADS)

    Tarazkar, Maryam; Romanov, Dmitri A.; Levis, Robert J.

    2015-05-01

    We report calculations of the static and dynamic hyperpolarizabilities of the nitrogen radical cation in doublet state. The electronic contributions were computed analytically using density functional theory and multi-configurational self-consistent field method with extended basis sets for non-resonant excitation. The open-shell electronic system of nitrogen radical cation provides negative second-order optical nonlinearity, suggesting that the hyperpolarizability coefficient, {{γ }(2)}, in the non-resonant regime is mainly composed of combinations of virtual one-photon transitions rather than two-photon transitions. The second-order optical properties of nitrogen radical cation have been calculated as a function of bond length starting with the neutral molecular geometry (S0 minimum) and stretching the N-N triple bond, reaching the ionic D0 relaxed geometry all the way toward dissociation limit, to investigate the effect of internuclear bond distance on second-order hyperpolarizability.

  13. Stability of phenol and thiophenol radical cations - interpretation by comparative quantum chemical approaches

    NASA Astrophysics Data System (ADS)

    Hermann, R.; Naumov, S.; Mahalaxmi, G. R.; Brede, O.

    2000-07-01

    The deprotonation kinetics of phenol-type radical cations, formed via a very efficient electron transfer in the pulse radiolysis of non-polar solutions, for example n-chlorobutane, is governed mainly by electronic effects due to the nature of the phenol substituents, whereas steric effects are of minor importance; thiophenols, which are sulphur analogues of phenols, exhibit a similar behavior. Comparative quantum chemical calculations show that the calculated spin densities at the hetero atoms correlate well with the experimentally determined radical cation lifetimes. Not only the Density Functional Theory (DTF) B3LYP but also the semiempirical quantum chemical model PM3 can be applied for the open shell systems mentioned.

  14. Spin Manipulation by Creation of Single-Molecule Radical Cations

    NASA Astrophysics Data System (ADS)

    Karan, Sujoy; Li, Na; Zhang, Yajie; He, Yang; Hong, I.-Po; Song, Huanjun; Lü, Jing-Tao; Wang, Yongfeng; Peng, Lianmao; Wu, Kai; Michelitsch, Georg S.; Maurer, Reinhard J.; Diller, Katharina; Reuter, Karsten; Weismann, Alexander; Berndt, Richard

    2016-01-01

    All-trans-retinoic acid (ReA), a closed-shell organic molecule comprising only C, H, and O atoms, is investigated on a Au(111) substrate using scanning tunneling microscopy and spectroscopy. In dense arrays single ReA molecules are switched to a number of states, three of which carry a localized spin as evidenced by conductance spectroscopy in high magnetic fields. The spin of a single molecule may be reversibly switched on and off without affecting its neighbors. We suggest that ReA on Au is readily converted to a radical by the abstraction of an electron.

  15. On the time behaviour of the concentration of pyrazinium radical cations in the early stage of the Maillard reaction

    NASA Astrophysics Data System (ADS)

    Stoesser, Reinhard; Klein, Jeannette; Peschke, Simone; Zehl, Andrea; Cämmerer, Bettina; Kroh, Lothar W.

    2007-08-01

    During the early stage of the Maillard reaction pyrazinium radical cations were detected by ESR within the reaction system D-glucose/glycine. The spectra were characterized by completely resolved hyperfine structure. The partial pressure of oxygen and the radical concentrations were measured directly in the reaction mixture by ESR using solutions of the spin probe TEMPOL and of DPPH, respectively. There are quantitative and qualitative relations of the actual concentration of the radical ions to the partial pressure of oxygen, the temperature-time regime and the mechanical mixing of the reaction system. These macroscopic parameters significantly affect both the induction period and the velocity of the time-dependent formation of free radicals. From in situ variations of p(O 2) and p(Ar) including the connected mixing effects caused by the passing the gases through the reaction mixture, steric and chemical effects of the stabilization of the radical ions were established. The determination of suitable and relevant conditions for stabilization and subsequent radical reactions contributes to the elucidation of the macroscopically known antioxidant activity of Maillard products.

  16. Spectroscopy and decay dynamics of several methyl-and fluorine-substituted benzene radical cations

    USGS Publications Warehouse

    Bondybey, V.E.; Vaughn, C.; Miller, T.A.; English, J.H.; Shiley, R.H.

    1981-01-01

    Spectra of several fluorobenzene cation radicals containing 1-3 methyl substituents were observed in solid Ne matrix and analyzed. Comparisons between these compounds and other fluorobenzenes studied previously as well as comparisons between the B?? state lifetimes in the gas phase and in the matrix are used to gain a deeper insight into the B?? state decay dynamics. ?? 1981 American Chemical Society.

  17. Isomerization of 4-vinylcyclohexene radical cation. A tandem mass spectrometry study

    SciTech Connect

    Vollmer, D.; Rempel, D.L.; Gross, M. L. ); Williams, F. )

    1995-02-08

    Investigation by matrix-isolation ESR has shown that 4-vinylcyclohexene, 1, surprisingly undergoes isomerization to the bicyclo[3.2.1]oct-2-ene ion, 3. Here we demonstrate the occurrence of this isomerization in the gas phase by use of tandem (MS/MS) sector and Fourier transform (FT) mass spectrometries. The radical cations of 4-vinylcyclohexene (IE = 8.93 eV) or bicyclo[3.2.1]oct-2-ene (approximately 14 kcal/mol more stable than that of 4-vinylcyclohexene) were formed, in separate trials, in a chemical ionization (CI) source by electron ionization (EI). The radical cations were then studied by obtaining their collisionally activated decomposition (CAD) spectra. The CAD spectra are similar, indicating that the isomerization has occurred. Both the sector and the FT mass spectrometer results reflect those obtained in the matrix-isolation ESR investigation. That is isomerizes to 3 at high internal energy, but is stable at low internal energy. Two mechanisms explain this rearrangement. The second mechanism is questionable because the most stable olefin radical cation formed from 5 is that of bicyclo[2.2.2]-2-octene, which gives different ESR and CAD spectra than those of 1 or 3. The CAD spectrum of bicyclo[2.2.2]-2-octene radical cation indicates that the retro-Diels-Alder loss of ethylene is more facile than that from 1 or 3. 18 refs., 3 figs.

  18. Gas phase regioselectivity in the deprotonation of p-cresol radical cation

    NASA Astrophysics Data System (ADS)

    Chiavarino, B.; Crestoni, M. E.; Fornarini, S.

    2003-04-01

    The deprotonation of the radical cation of p-cresol, a model of tyrosine residues, has been studied kinetically in the gas phase. The reaction has revealed the operation of competitive deprotonation sites depending on the strength of the base, as shown by an FT-ICR study using d-labelling.

  19. Participation of cationic intermediates in radical-induced homopolymerization of maleic anhydride

    SciTech Connect

    Gaylord, N.G.; Koo, J.Y.

    1981-03-01

    Since the failure to promote MAH polymerization in the presence of amine-containing redox catalyst systems suggested the presence of cationic intermediates, the radical-induced polymerization of MAH was carried out in the absence and in the presence of N,N-dimethylformamide (DMF) and N, N-dimethylaniline (DMA).

  20. Chronoamperometric study of the films formed by 4,4'-bipyridyl cation radical salts on mercury in the presence of iodide ions: consecutive two-dimensional phase transitions.

    PubMed

    Gómez, L; Ruiz, J J; Camacho, L; Rodríguez-Amaro, R

    2005-01-01

    This paper reports a new mathematical model for consecutive two-dimensional phase transitions that accounts for the chronoamperometric behavior observed in the formation of electrochemical phases by 4,4'-bipyridyl cation radical (BpyH(2)(*)(+)) on mercury in aqueous iodide solutions. Also, a new interpretation for the induction time is proposed. PMID:15620326

  1. Isomerization and dissociation of n-butylbenzene radical cation.

    PubMed

    Halbert, Stéphanie; Bouchoux, Guy

    2012-02-01

    Fragmentation mechanisms of ionized butylbenzene to give m/z 91 and m/z 92 fragment ions have been examined at the G3B3 and G3MP2B3 levels of theory. It is shown that the energetically favored pathways lead to tropylium, Tr(+), and methylene-2,4-cyclohexadiene, MCD(•+), ions. Formation of m/z 91 benzyl ions, Bz(+), by a simple bond fission (SBF) process, needs about 30 kJ/mol more energy than Tr(+). Possible formation of C(7)H(8)(•+) ions of structures different from the retro-ene rearrangement (RER) product, MCD(•+), has been also considered. Comparison with experimental data of this "thermometer" system is done through a kinetic modeling using Rice-Ramsperger-Kassel-Marcus (RRKM) and orbiting transition state (OTS) rate constant calculations on the G3MP2B3 0 K energy surface. The results agree with previous experimental observation if (i) the competitive formation of Tr(+) and Bz(+) is taken into account in the m/z 91 pathway, and (ii) the stepwise character of the RER fragmentation is introduced in the m/z 92 fragmentation route. PMID:22229805

  2. Ground and Excited-Electronic-State Dissociations of Hydrogen-Rich and Hydrogen-Deficient Tyrosine Peptide Cation Radicals.

    PubMed

    Viglino, Emilie; Lai, Cheuk Kuen; Mu, Xiaoyan; Chu, Ivan K; Tureček, František

    2016-09-01

    We report a comprehensive study of collision-induced dissociation (CID) and near-UV photodissociation (UVPD) of a series of tyrosine-containing peptide cation radicals of the hydrogen-rich and hydrogen-deficient types. Stable, long-lived, hydrogen-rich peptide cation radicals, such as [AAAYR + 2H](+●) and several of its sequence and homology variants, were generated by electron transfer dissociation (ETD) of peptide-crown-ether complexes, and their CID-MS(3) dissociations were found to be dramatically different from those upon ETD of the respective peptide dications. All of the hydrogen-rich peptide cation radicals contained major (77%-94%) fractions of species having radical chromophores created by ETD that underwent photodissociation at 355 nm. Analysis of the CID and UVPD spectra pointed to arginine guanidinium radicals as the major components of the hydrogen-rich peptide cation radical population. Hydrogen-deficient peptide cation radicals were generated by intramolecular electron transfer in Cu(II)(2,2':6',2″-terpyridine) complexes and shown to contain chromophores absorbing at 355 nm and undergoing photodissociation. The CID and UVPD spectra showed major differences in fragmentation for [AAAYR](+●) that diminished as the Tyr residue was moved along the peptide chain. UVPD was found to be superior to CID in localizing Cα-radical positions in peptide cation radical intermediates. Graphical Abstract ᅟ. PMID:27278824

  3. Ground and Excited-Electronic-State Dissociations of Hydrogen-Rich and Hydrogen-Deficient Tyrosine Peptide Cation Radicals

    NASA Astrophysics Data System (ADS)

    Viglino, Emilie; Lai, Cheuk Kuen; Mu, Xiaoyan; Chu, Ivan K.; Tureček, František

    2016-06-01

    We report a comprehensive study of collision-induced dissociation (CID) and near-UV photodissociation (UVPD) of a series of tyrosine-containing peptide cation radicals of the hydrogen-rich and hydrogen-deficient types. Stable, long-lived, hydrogen-rich peptide cation radicals, such as [AAAYR + 2H]+● and several of its sequence and homology variants, were generated by electron transfer dissociation (ETD) of peptide-crown-ether complexes, and their CID-MS3 dissociations were found to be dramatically different from those upon ETD of the respective peptide dications. All of the hydrogen-rich peptide cation radicals contained major (77%-94%) fractions of species having radical chromophores created by ETD that underwent photodissociation at 355 nm. Analysis of the CID and UVPD spectra pointed to arginine guanidinium radicals as the major components of the hydrogen-rich peptide cation radical population. Hydrogen-deficient peptide cation radicals were generated by intramolecular electron transfer in CuII(2,2':6',2″-terpyridine) complexes and shown to contain chromophores absorbing at 355 nm and undergoing photodissociation. The CID and UVPD spectra showed major differences in fragmentation for [AAAYR]+● that diminished as the Tyr residue was moved along the peptide chain. UVPD was found to be superior to CID in localizing Cα-radical positions in peptide cation radical intermediates.

  4. Ground and Excited-Electronic-State Dissociations of Hydrogen-Rich and Hydrogen-Deficient Tyrosine Peptide Cation Radicals

    NASA Astrophysics Data System (ADS)

    Viglino, Emilie; Lai, Cheuk Kuen; Mu, Xiaoyan; Chu, Ivan K.; Tureček, František

    2016-09-01

    We report a comprehensive study of collision-induced dissociation (CID) and near-UV photodissociation (UVPD) of a series of tyrosine-containing peptide cation radicals of the hydrogen-rich and hydrogen-deficient types. Stable, long-lived, hydrogen-rich peptide cation radicals, such as [AAAYR + 2H]+● and several of its sequence and homology variants, were generated by electron transfer dissociation (ETD) of peptide-crown-ether complexes, and their CID-MS3 dissociations were found to be dramatically different from those upon ETD of the respective peptide dications. All of the hydrogen-rich peptide cation radicals contained major (77%-94%) fractions of species having radical chromophores created by ETD that underwent photodissociation at 355 nm. Analysis of the CID and UVPD spectra pointed to arginine guanidinium radicals as the major components of the hydrogen-rich peptide cation radical population. Hydrogen-deficient peptide cation radicals were generated by intramolecular electron transfer in CuII(2,2 ':6 ',2 ″-terpyridine) complexes and shown to contain chromophores absorbing at 355 nm and undergoing photodissociation. The CID and UVPD spectra showed major differences in fragmentation for [AAAYR]+● that diminished as the Tyr residue was moved along the peptide chain. UVPD was found to be superior to CID in localizing Cα-radical positions in peptide cation radical intermediates.

  5. Photo-excitation of adenine cation radical [A•+] in the near UV-vis region produces sugar radicals in Adenosine and in its nucleotides

    PubMed Central

    Adhikary, Amitava; Khanduri, Deepti; Kumar, Anil; Sevilla, Michael D.

    2011-01-01

    In this study, we report the formation of ribose sugar radicals in high yields (85 – 100%) via photo-excitation of adenine cation radical (A•+) in Ado and its ribonucleotides. Photo-excitation of A•+ at low temperatures in homogenous aqueous glassy samples of Ado, 2′-AMP, 3′-AMP and 5′-AMP forms sugar radicals predominantly at C5′- and also at C3′-sites. The C5′• and C3′• sugar radicals were identified employing Ado deuterated at specific carbon sites: C1′, C2′, and at C5′. Phosphate substitution is found to deactivate sugar radical formation at the site of substitution. Thus, in 5′-AMP, C3′• is observed to be the main radical formed via photo-excitation at ca. 143 K whereas in 3′-AMP, C5′• is the only species found. These results were supported by results obtained employing 5′-AMP with specific deuteration at C5′-site (i.e., 5′,5′-D,D-5′-AMP). Moreover, contrary to the C5′• observed in 3′-dAMP, we find that C5′• in 3′-AMP shows a clear pH dependent conformational change as evidenced by a large increase in the C4′ β–hyperfine coupling on increasing the pH from 6 to 9. Calculations performed employing DFT (B3LYP/6-31G*) for C5′• in 3′-AMP show that the two conformations of C5′• result from strong hydrogen bond formation between the O5′-H and the 3′-phosphate dianion at higher pHs. Employing time-dependent density functional theory [TD-DFT, B3LYP/6-31G(d)] we show that in the excited state, the hole transfers to the sugar moiety and has significant hole localization at the C5′-site in a number of allowed transitions. This hole localization is proposed to lead to the formation of the neutral C5′-radical (C5′•) via deprotonation. PMID:19367991

  6. Multi-state vibronic interactions in the 1,2,3-trifluorobenzene radical cation.

    PubMed

    Faraji, Shirin; Köppel, Horst

    2012-12-14

    The multi-state and multi-mode vibronic interactions between the five lowest electronic states of the title compound are investigated theoretically by an ab initio quantum dynamical approach. The well-established linear vibronic coupling scheme is adopted, augmented by quadratic coupling terms for the totally symmetric modes. The pertinent system parameters are obtained from outer valence Greens function and equation-of-motion coupled-cluster ab initio calculations. Large-scale quantum dynamical simulations are performed employing a powerful wavepacket propagation scheme. The band shapes and line structures (as far as available) of the experimental photoelectron spectra are well reproduced. Time-dependent electronic populations reveal ultrafast internal conversion processes and allow for important insight into the fluorescence properties of the radical cation. The relation to other fluoro derivatives of the benzene radical cation is discussed. PMID:23249068

  7. Efficient scavenging of β-carotene radical cations by antiinflammatory salicylates.

    PubMed

    Cheng, Hong; Liang, Ran; Han, Rui-Min; Zhang, Jian-Ping; Skibsted, Leif H

    2014-02-01

    The radical cation generated during photobleaching of β-carotene is scavenged efficiently by the anion of methyl salicylate from wintergreen oil in a second-order reaction approaching the diffusion limit with k2 = 3.2 × 10(9) L mol(-1) s(-1) in 9 : 1 v/v chloroform-methanol at 23 °C, less efficiently by the anion of salicylic acid with 2.2 × 10(8) L mol(-1) s(-1), but still of possible importance for light-exposed tissue. Surprisingly, acetylsalicylate, the aspirin anion, reacts with an intermediate rate in a reaction assigned to the anion of the mixed acetic-salicylic acid anhydride formed through base induced rearrangements. The relative scavenging rate of the β-carotene radical cation by the three salicylates is supported by DFT-calculations. PMID:24336797

  8. Hydroxyl radical formation during peroxynitrous acid decomposition

    SciTech Connect

    Coddington, J.W.; Hurst, J.K.; Lymar, S.V.

    1999-03-24

    Yields of O{sub 2} formed during decomposition of peroxynitrous acid (ONOOH) under widely varying medium conditions are compared to predictions based upon the assumption that the reaction involves formation of discrete {sm{underscore}bullet}OH and {sm{underscore}bullet}NO{sub 2} radicals as oxidizing intermediates. The kinetic model used includes all reactions of {sm{underscore}bullet}OH, {sm{underscore}bullet}O{sub 2}{sup {minus}}, and reactive nitrogen species known to be important under the prevailing conditions; because the rate constants for all of these reactions have been independently measured, the calculations contain no adjustable fitting parameters. The model quantitatively accounts for (1) the complex pH dependence of the O{sub 2} yields and (2) the unusual effects of NO{sub 2} {sup {minus}}, which inhibits O{sub 2} formation in neutral, but not alkaline, solutions and also reverses inhibition by organic {sm{underscore}bullet}OH scavengers in alkaline media. Other observations, including quenching of O{sub 2} yields by ferrocyanide and bicarbonate, the pressure dependence of the decomposition rate, and the reported dynamic behavior for O{sub 2} generation in the presence of H{sub 2}O{sub 2}, also appear to be in accord with the suggested mechanism. Overall, the close correspondence between observed and calculated O{sub 2} yields provides strong support for decomposition via homolysis of the ONOOH peroxo bond.

  9. Electronic structures, vibrational spectra, and revised assignment of aniline and its radical cation: Theoretical study

    NASA Astrophysics Data System (ADS)

    Wojciechowski, Piotr M.; Zierkiewicz, Wiktor; Michalska, Danuta; Hobza, Pavel

    2003-06-01

    Comprehensive studies of the molecular and electronic structures, vibrational frequencies, and infrared and Raman intensities of the aniline radical cation, C6H5NH2+ have been performed by using the unrestricted density functional (UB3LYP) and second-order Møller-Plesset (UMP2) methods with the extended 6-311++G(df,pd) basis set. For comparison, analogous calculations were carried out for the closed-shell neutral aniline. The studies provided detailed insight into the bonding changes that take place in aniline upon ionization. The natural bond orbital (NBO) analysis has revealed that the pπ-radical conjugative interactions are of prime importance in stabilizing the planar, quinoid-type structure of the aniline radical cation. It is shown that the natural charges calculated for aniline are consistent with the chemical properties of this molecule (an ortho- and para-directing power of the NH2 group in electrophilic substitutions), whereas Mulliken charges are not reliable. The theoretical vibrational frequencies of aniline, calculated by the B3LYP method, show excellent agreement with the available experimental data. In contrast, the MP2 method is deficient in predicting the frequencies of several modes in aniline, despite the use of the extended basis set in calculations. The frequencies of aniline radical cation, calculated at the UB3LYP/6-311++G(df,pd) level, are in very good agreement with the recently reported experimental data from zero kinetic energy photoelectron and infrared depletion spectroscopic studies. The clear- cut assignment of the IR and Raman spectra of the investigated molecules has been made on the basis of the calculated potential energy distributions. Several bands in the spectra have been reassigned. It is shown that ionization of aniline can be easily identified by the appearance of the very strong band at about 1490 cm-1, in the Raman spectrum. The redshift of the N-H stretching frequencies and the blueshift of the C-H stretching

  10. Mechanistic Examination of Cβ–Cγ Bond Cleavages of Tryptophan Residues during Dissociations of Molecular Peptide Radical Cations

    SciTech Connect

    Song, Tao; Ma, Ching-Yung; Chu, Ivan K.; Siu, Chi-Kit; Laskin, Julia

    2013-02-14

    In this study, we used collision-induced dissociation (CID) to examine the gas-phase fragmentations of [GnW]•+ (n = 2-4) and [GXW]•+ (X = C, S, L, F, Y, Q) species. The Cβ–Cγ bond cleavage of a C-terminal decarboxylated tryptophan residue ([M - CO2]•+) can generate [M - CO2 - 116]+, [M - CO2 - 117]•+, and [1H-indole]•+ (m/z 117) species as possible product ions. Competition between the formation of [M - CO2 - 116]+ and [1H-indole]•+ systems implies the existence of a proton-bound dimer formed between the indole ring and peptide backbone. Formation of such a proton-bound dimer is facile via a protonation of the tryptophan γ-carbon atom as suggested by density functional theory (DFT) calculations. DFT calculations also suggested the initially formed ion 2--the decarboxylated species that is active against Cβ–Cγ bond cleavage -can efficiently isomerize to form a more-stable -radical isomer (ion 9) as supported by Rice-Ramsperger-Kassel-Marcus (RRKM) modeling. The Cβ–Cγ bond cleavage of a tryptophan residue also can occur directly from peptide radical cations containing a basic residue. CID of [WGnR]•+ (n = 1-3) radical cations consistently resulted in predominant formation of [M-116]+ product ions. It appears that the basic arginine residue tightly sequesters the proton and allows the charge-remote Cβ–Cγ bond cleavage to prevail over the charge-directed one. DFT calculations predicted the barrier for the former is 6.2 kcal mol -1 lower than that of the latter. Furthermore, the pathway involving a salt-bridge intermediate also was accessible during such a bond cleavage event.

  11. Triarylporphyrin meso-Oxy Radicals: Remarkable Chemical Stabilities and Oxidation to Oxophlorin π-Cations.

    PubMed

    Shimizu, Daiki; Oh, Juwon; Furukawa, Ko; Kim, Dongho; Osuka, Atsuhiro

    2015-12-16

    5-Hydroxy-10,15,20-triarylporphyrin (oxophlorin) and its Ni(II) and Zn(II) complexes were oxidized with PbO2 to give the corresponding porphyrin meso-oxy radicals as remarkably stable species. These radicals were fully characterized with X-ray diffraction analysis, UV/vis/NIR absorption and ESR spectroscopies, magnetic susceptibility measurement, electrochemical studies, and theoretical calculations. Free-base radical and its Ni(II) complex have been shown to exist as a monoradical in solution, while the Zn(II) complex exists in an equilibrium between monomer (doublet monoradical) and dimer (a non-Kekulé singlet biradicaloid) with a dimerization constant of KD = 3.0 × 10(5) M(-1) in noncoordinating CH2Cl2 but becomes a pyridine-coordinated monoradical upon addition of pyridine. Variable temperature magnetic susceptibility measurements of these radicals revealed different magnetic interactions in the solid-states, which has been interpreted in terms of their different packing structures in a microscopic sense. These radicals undergo one-electron oxidation and reduction in a reversible manner within narrow potential windows of 0.57-0.82 V. Finally, one-electron oxidation of Ni(II) and Zn(II) porphyrin meso-oxy radicals with tris(4-bromophenyl)aminium hexachloroantimonate furnished oxophlorin π-cations, which displayed nonaromatic closed-shell character, NIR absorption, and significant double bond character of the C-O bond. PMID:26609815

  12. Radical formation and radiation damage in adamantane

    SciTech Connect

    Lloyd, R.V.; DiGregorio, S.; DiMauro, L.; Wood, D.E.

    1980-10-30

    Unequivocal samples of the 1-adamantyl (1-Ad) and 2-Ad radicals have been prepared in a matrix of adamantane (Ad) by the simultaneous deposition of atomic sodium, 1- or 2-bromoadamantane, and adamantance at 77 K. The EPR spectrum of the 1-Ad radical contrary to previous reports has a clearly resolved hyperfine structure that can be analyzed in terms of the solution parameters of Krusic et al., and the spectrum of the 2-Ad radical is identical with that previously reported by Ferrell et al. It is also shown that conditions of purification and irradiation can greatly affect the spectra obtained upon X irradiation of Ad itself. Depending upon conditions, alicyclic radicals that are primary products of ring-opening reactions or benzylic-type radicals that are probably secondary reaction products can also be obtained in addition to 1-Ad and 2-Ad radicals.

  13. Solvent effects on the resonance Raman spectra of bacteriochlorophyll a cation radical

    NASA Astrophysics Data System (ADS)

    Misono, Yasuhito; Nishizawa, Ei-ichi; Limantara, Leenawaty; Koyama, Yasushi; Itoh, Koichi

    1995-04-01

    Resonance Raman (RR) spectra were measured for the cation radical of bacteriochlorophyll a in acetone, methanol, dichloromethane and mixed solvents of acetone and methanol. The ring-breathing (C a-C m stretching) frequency of the radical (abbreviated as vr+) was observed at 1601 cm -1 in acetone (forming a penta-coordinated monomer), at 1587 cm -1 in a methanol (forming a hexa-coordinated monomer) and at 1600 cm -1 in dichloromethane (forming a penta-coordinated aggregate). The RR spectrum of the radical in dichloromethane is almost identical to the transient RR spectrum ascribed to 'the aggregated T 1 species of Bchl a' formed in the particular solvent by Nishizawa, Limantara, Nanjou, Nagae, Kakuno and Koyama, indicating that their interpretation needs to be revised.

  14. External electric field promotes proton transfer in the radical cation of adenine-thymine

    NASA Astrophysics Data System (ADS)

    Zhang, Guiqing; Xie, Shijie

    2016-07-01

    According to pKa measurements, it has been predicted that proton transfer would not occur in the radical cation of adenine-thymine (A:T). However, recent theoretical calculations indicate that proton transfer takes place in the base pair in water below the room temperature. We have performed simulations of proton transfer in the cation of B-DNA stack composed of 10 A:T base pairs in water from 20 K to 300 K. Proton transfer occurs below the room temperature, meanwhile it could also be observed at the room temperature under the external electric field. Another case that interests us is that proton transfer bounces back after ˜300 fs from the appearance of proton transfer at low temperatures.

  15. Spin-coupling in ferric metalloporphyrin radical cation complexes: Mössbauer and susceptibility studies

    NASA Astrophysics Data System (ADS)

    Lang, George; Boso, Brian; Erler, Brian S.; Reed, Christopher A.

    1986-03-01

    The ferric metalloporphyrin π-radical cation complexes Fe(III) (OClO3)2 (TPP.) and [Fe(III) Cl (TPP.)] [SbCl6] were examined in microcrystalline form by Mössbauer spectroscopy and magnetic susceptometry over a range of temperatures and applied fields. All measurements on the six-coordinate Fe(OClO3)2 (TPP.) were consistent with isolated molecules having an S=5/2 iron site with zero field splitting (12 cm-1) S2z that is ferromagnetically coupled to the S=1/2 porphyrin radical by an energy term (-110 cm-1) Sṡs. Thus the ground state is overall spin-3. In the five-coordinate [FeCl (TPP.)] [SbCl6] the susceptibility is in reasonable agreement with the results of a calculation based on zero field splitting (12 cm-1) S2z for the S=5/2 iron and antiferromagnetic coupling (200 cm-1) Sṡs with the radical to give an overall spin-2 ground state. However, the Mössbauer measurements require a more complicated model having the same large intramolecular iron-radical coupling, a smaller zero field splitting (3 cm-1) S2z, and weak intermolecular antiferromagnetic coupling between heme pairs given by (32 cm-1) s1ṡs2 or, equivalently, (0.65 cm-1) S1ṡS2. A slightly improved correspondence with the measured susceptibility results. The intermolecular antiferromagnetic coupling probably results from crystallization of the [FeCl (TPP.)]+ cations in face-to-face dimers as observed in other closely related five-coordinate iron (III) porphyrins.

  16. Calmodulin Methionine Residues are Targets For One-Electron Oxidation by Hydroxyl Radicals: Formation of S therefore N three-electron bonded Radical Complexes

    SciTech Connect

    Nauser, Thomas; Jacoby, Michael E.; Koppenol, Willem H.; Squier, Thomas C.; Schoneich, Christian

    2005-02-01

    The one-electron (1e) oxidation of organic sulfides and methionine (Met) constitutes an important reaction mechanism in vivo.1,2 Evidence for a Cu(II)-catalyzed oxidation of Met35 in the Alzheimer's disease -amyloid peptide was obtained,3 and, based on theoretical studies, Met radical cations were proposed as intermediates.4 In the structure of -amyloid peptide, the formation of Met radical cations appears to be facilitated by a preexisting close sulfur-oxygen (S-O) interaction between the Met35 sulfur and the carbonyl oxygen of the peptide bond C-terminal to Ile31.5 Substitution of Ile31 with Pro31 abolishes this S-O interaction,5 significantly reducing the ability of -amyloid to reduce Cu(II), and converts the neurotoxic wild-type -amyloid into a non-toxic peptide.6 The preexisting S-O bond characterized for wild-type -amyloid suggests that electron transfer from Met35 to Cu(II) is supported through stabilization of the Met radical cation by the electron-rich carbonyl oxygen, generating an SO-bonded7 sulfide radical cation (Scheme 1, reaction 1).5

  17. Topological and spectroscopic study of three-electron bonded compounds as models of radical cations of methionine-containing dipeptides

    NASA Astrophysics Data System (ADS)

    Fourré, Isabelle; Bergès, Jacqueline; Braïda, Benoît; Houée-Levin, Chantal

    2008-12-01

    Small models of radical cations of methionine-containing dipeptides, which are stabilized by formation of two-centre three-electron (2c-3e) S∴X bonds (X = S, N and O), were investigated at the BH&HLYP/6-31G(d) level and by means of topological tools. The SX distance is not so important for stability but the relative orientation of both fragments is. The AIM and ELF topological analyses shows that the nature of the S∴X bond varies with X, from purely 2c-3e in S∴S + entities to electrostatic in S∴O + ones. The σSX → σSX∗ wavelengths, obtained at the TD-BH&HLYP/cc-pVTZ level, strongly depend on X and on conformation.

  18. Efficient radical cation stabilization of PANI-ZnO and PANI-ZnO-GO composites and its optical activity

    NASA Astrophysics Data System (ADS)

    Mathavan, T.; Divya, A.; Archana, J.; Ramasubbu, A.; Benial, A. Milton Franklin; Jothirajan, M. A.

    2016-05-01

    Polyaniline (PANI) and its composites PANI-ZnO (Zinc oxide) and PANI-ZnO-GO (Graphene oxide) were successfully constructed. These materials were characterized by electron spin resonance (ESR) technique and ultraviolet visible spectrometry. The parameters such as line width, g-factor and spin concentration were deduced from ESR spectra, from the results the radical cation stabilization of PANI, PANI-ZnO and PANI-ZnO-GO composites were compared by the polaron and bipolaron formation. The absorption features obtained in the UV absorption spectra reveal the band gap of these modified PANI composites and also predicted the information of increasing and decreasing features of signal intensity and spin concentration.

  19. Hg(2+) -induced in situ generated radical cation of (S)-BINOL-based polymer for highly enantioselective recognition of phenylalaninol.

    PubMed

    Jiao, Jiemin; Li, Fei; Zhang, Shuwei; Quan, Yiwu; Zheng, Wenhua; Cheng, Yixiang; Zhu, Chengjian

    2014-08-01

    Phenylalaninol enantiomers are one of the most important chiral compounds due to its presence in biologically active molecules and pharmaceutical products. In this paper, a novel chiral fluorescence polymer sensor incorporating (S)-BINOL and oligomeric aniline via a nucleophilic addition-elimination reaction is designed and synthesized. Polymer sensor exhibits "turn-off" fluorescence quenching response upon the addition of Hg(2+) , and "turn-on" moderate fluorescence enhancement behavior towards phenylalaninol enantiomers. Meanwhile, this kind of (S)-BINOL-based polymer sensor can exhibit highly selective enantioselective recognition response towards (L)-phenylalaninol upon the addition of Hg(2+) and the value of ef can reach as high as 5.4, which can be attributed to the formation of in situ generated radical cation arisen from oligomeric aniline moiety by Hg(2+) induction. PMID:25048009

  20. Characterizing radiation-induced oxidation of DNA by way of the monohydrated guanine-cytosine radical cation.

    PubMed

    Jaeger, Heather M; Schaefer, Henry F

    2009-06-11

    The interaction of one water molecule with the guanine-cytosine radical cation has been studied with ab initio and density functional methods in order to help elucidate the nature of oxidized aqueous DNA. The theoretical spin density of [GC]*(+) reveals that the radical center is localized on guanine. The adiabatic ionization potential lowers from 7.63 to 6.71 eV in concurrence with the formation of the Watson-Crick base pair and hydration by one water molecule. A natural bond orbital analysis of partial charges shows that approximately 80% of the positive charge persists on guanine upon hydration and formation of the Watson-Crick base pair with cytosine. Hydration energies were computed with second-order Z-averaged perturbation theory (ZAPT2) using the aug-cc-pVDZ basis set at 11 stationary points on the B3LYP/DZP++ potential energy surface. The hydration energy at the global minimum is 14.2 kcal mol(-1). The lowest energy structures correspond to hydration near the glycosidic bond sites. Structural changes in the Watson-Crick base pair are predominantly seen for monohydration in the groove regions of double-helix DNA. PMID:19445496

  1. Oxygen-18 tracer studies of enzyme reactions with radical/cation diagnostic probes

    SciTech Connect

    Moe, Luke A.; Fox, Brian G. . E-mail: bgfox@biochem.wisc.edu

    2005-12-09

    This review considers reactions of enzymes with the cyclopropanoid radical/cation diagnostic probes norcarane, 1,1-dimethylcyclopropane, and 1,1-diethylcyclopropane as elaborated by the use of {sup 18}O{sub 2} and {sup 18}OH{sub 2} to trace the origin of O-atoms incorporated during catalysis. The reactions of soluble and integral membrane diiron enzymes are summarized and compared to results obtained from cytochrome P450 studies. Norcarane proved to be an excellent substrate for the diiron enzyme toluene 4-monooxygenase and its engineered isoforms, with k {sub cat} and coupling between NADH utilization and total hydroxylated products comparable to that determined for toluene, the natural substrate. Results obtained with toluene 4-monooxygenase show that the un-rearranged and radical-rearranged alcohol products have a high percentage of O-atom incorporation (>80-95%) from O{sub 2}, while the cation-derived ring-expansion products have O-atom incorporation primarily derived from solvent water. Mechanistic possibilities accounting for this difference are discussed.

  2. Internal rotation of the methyl group in the radical cation of dimethyl ether

    NASA Astrophysics Data System (ADS)

    Matsushita, Michio; Momose, Takamasa; Shida, Tadamasa

    1990-04-01

    The radical cation of dimethyl ether has been studied by ESR in the temperature region of 6-140 K for focusing on the internal rotation of the methyl groups. The methyl groups rotate almost freely at above 70 K to give a septet ESR spectrum. At temperatures below 40 K there emerge extra lines due to the tunneling rotation of the methyl groups. From the analysis of the line shape, the interaction potential for the rotation of the two methyl groups, if any, should be approximated as proportional to cos 3theta1 cos 3theta2, where theta1 and theta2 denote the rotational angles of the methyl groups measured from the potential minima of the internal rotation of the methyl groups. The activation energy for the thermally induced internal rotation is determined to be about 100 cal/mol at temperatures above 25 K, whereas at lower temperatures the apparent activation energy drops sharply, which is consistent with the quantum tunneling of the methyl protons. The small activation energy of 100 cal/mol for the radical cation is compatible with the result of ab initio MO calculation for the potential barrier.

  3. Collision cross-sections of [C,H,O] cations and radical cations from aliphatic [C,H,O] compounds

    NASA Astrophysics Data System (ADS)

    van Houte, J. J.; van Thuijl, J.

    1994-05-01

    Over 260 collision cross-section [sigma]ot, expressed in »ngströms squared, have been determined for the studied ions at 20 and 70 eV by extrapolation of [sigma]t to zero target gas pressure, and these yield two types of structural information. The first type concerns occurrence and detection of cyclic ions, the second isomerization of parent molecular ions and different product ion distributions at 20 and 70 eV. In addition, examples of two distinct fragmentation mechanisms operative in the formation of identical daughter ions from a given precursor could be traced. Formation of cyclic daughter ions is, for instance, observed for C2H3O+ from oxirane, C3H5O+ from oxetane, C4H7O+2 from 4-methyl-1,3-dioxolane. Cyclic molecular ions are formed in varying proportions from oxirane, tetrahydrofuran, 2- and 4-methyl-1,3-dioxolane but not from porpylene oxide, oxetane and 1,3-dioxolane. Isomerization of the parent molecular ion is proposed for the following fragmentations: CH2 from allyl alcohol, CHO2+ from formic acid, C2H2O·+ from oxirane, and C3H6O·+ from 3-methyl butanal and 2-methyl pentanal. Different product ion distributions at 20 and 70 eV were found for C3H5O+ from ethyl propionate and 2-pentanone, C2H4O·+ and C4H8O·+ from butane-1,3-diol, and C3H6O·+ from 2- and 4-methyl-1,3-dioxolane. Two distinct fragmentation mechanisms were traced for the following processes: CH2OH, C2H2O·+ and C2H3O+ from methyl vinyl ether, CH2 and C2H5O+ from butane-1,3-diol and C2H2O·+ from butanone. Self protonation of acetaldehyde also appears to take place by two mechanisms. Energy partitioning is evident in the formation of formyl cations HCO+ but wears off for processes in which larger daughter ions are formed. For formyl cations from straight chain aldehydes, the 70 eV collision cross-section is linearly related to the logarithm of the reciprocal of the number of degrees of freedom in the parent molcule, log (1/DFp). One example of a proton-bound dimer is given, that of

  4. Gas-phase reactions of aryl radicals with 2-butyne: experimental and theoretical investigation employing the N-methyl-pyridinium-4-yl radical cation.

    PubMed

    Lam, A K Y; Li, C; Khairallah, G; Kirk, B B; Blanksby, S J; Trevitt, A J; Wille, U; O'Hair, R A J; da Silva, G

    2012-02-21

    Aromatic radicals form in a variety of reacting gas-phase systems, where their molecular weight growth reactions with unsaturated hydrocarbons are of considerable importance. We have investigated the ion-molecule reaction of the aromatic distonic N-methyl-pyridinium-4-yl (NMP) radical cation with 2-butyne (CH(3)C≡CCH(3)) using ion trap mass spectrometry. Comparison is made to high-level ab initio energy surfaces for the reaction of NMP and for the neutral phenyl radical system. The NMP radical cation reacts rapidly with 2-butyne at ambient temperature, due to the apparent absence of any barrier. The activated vinyl radical adduct predominantly dissociates via loss of a H atom, with lesser amounts of CH(3) loss. High-resolution Fourier transform ion cyclotron resonance (FT-ICR) mass spectrometry allows us to identify small quantities of the collisionally deactivated reaction adduct. Statistical reaction rate theory calculations (master equation/RRKM theory) on the NMP+2-butyne system support our experimental findings, and indicate a mechanism that predominantly involves an allylic resonance-stabilized radical formed via H atom shuttling between the aromatic ring and the C(4) side-chain, followed by cyclization and/or low-energy H atom β-scission reactions. A similar mechanism is demonstrated for the neutral phenyl radical (Ph˙)+2-butyne reaction, forming products that include 3-methylindene. The collisionally deactivated reaction adduct is predicted to be quenched in the form of a resonance-stabilized methylphenylallyl radical. Experiments using a 2,5-dichloro substituted methyl-pyridiniumyl radical cation revealed that in this case CH(3) loss from the 2-butyne adduct is favoured over H atom loss, verifying the key role of ortho H atoms, and the shuttling mechanism, in the reactions of aromatic radicals with alkynes. As well as being useful phenyl radical analogues, pyridiniumyl radical cations may form in the ionosphere of Titan, where they could undergo rapid

  5. COMPUTATIONAL STUDY OF INTERSTELLAR GLYCINE FORMATION OCCURRING AT RADICAL SURFACES OF WATER-ICE DUST PARTICLES

    SciTech Connect

    Rimola, Albert; Sodupe, Mariona; Ugliengo, Piero

    2012-07-20

    Glycine is the simplest amino acid, and due to the significant astrobiological implications that suppose its detection, the search for it in the interstellar medium (ISM), meteorites, and comets is intensively investigated. In the present work, quantum mechanical calculations based on density functional theory have been used to model the glycine formation on water-ice clusters present in the ISM. The removal of either one H atom or one electron from the water-ice cluster has been considered to simulate the effect of photolytic radiation and of ionizing particles, respectively, which lead to the formation of OH{sup .} radical and H{sub 3}O{sup +} surface defects. The coupling of incoming CO molecules with the surface OH{sup .} radicals on the ice clusters yields the formation of the COOH{sup .} radicals via ZPE-corrected energy barriers and reaction energies of about 4-5 kcal mol{sup -1} and -22 kcal mol{sup -1}, respectively. The COOH{sup .} radicals couple with incoming NH=CH{sub 2} molecules (experimentally detected in the ISM) to form the NHCH{sub 2}COOH{sup .} radical glycine through energy barriers of 12 kcal mol{sup -1}, exceedingly high at ISM cryogenic temperatures. Nonetheless, when H{sub 3}O{sup +} is present, one proton may be barrierless transferred to NH=CH{sub 2} to give NH{sub 2}=CH{sub 2}{sup +}. This latter may react with the COOH{sup .} radical to give the NH{sub 2}CH{sub 2}COOH{sup +.} glycine radical cation which can then be transformed into the NH{sub 2}CHC(OH){sub 2}{sup +.} species (the most stable form of glycine in its radical cation state) or into the NH{sub 2}CHCOOH{sup .} neutral radical glycine. Estimated rate constants of these events suggest that they are kinetically feasible at temperatures of 100-200 K, which indicate that their occurrence may take place in hot molecular cores or in comets exposed to warmer regions of solar systems. Present results provide quantum chemical evidence that defects formed on water ices due to the harsh

  6. Cytochrome c catalyses the formation of pentyl radical and octanoic acid radical from linoleic acid hydroperoxide.

    PubMed Central

    Iwahashi, Hideo; Nishizaki, Koji; Takagi, Ichiro

    2002-01-01

    A reaction of 13-hydroperoxide octadecadienoic acid (13-HPODE) with cytochrome c was analysed using ESR, HPLC-ESR and HPLC-ESR-MS by the combined use of the spin-trapping technique. The ESR, HPLC-ESR and HPLC-ESR-MS analyses showed that cytochrome c catalyses formation of pentyl and octanoic acid radicals from 13-HPODE. On the other hand, only the alpha-(4-pyridyl-1-oxide)-N-t-butylnitrone/octanoic acid radical adduct was detected in the elution profile of HPLC-ESR for a mixture of 13-HPODE with haematin, indicating that haematin catalyses the formation of octanoic acid radical. In addition, the reaction of 13-HPODE with cytochrome c was inhibited by chlorogenic acid, caffeic acid and ferulic acid via two possible mechanisms, i.e. reducing cytochrome c (chlorogenic acid and caffeic acid) and scavenging the radical intermediates (chlorogenic acid, caffeic acid and ferulic acid). PMID:11742529

  7. Single-Crystal X-ray Structures of conductive π-Stacking Dimers of Tetrakis(alkylthio)benzene Radical Cations.

    PubMed

    Chen, Xiaoyu; Gao, Feng; Yang, Wuqin

    2016-01-01

    Salts containing radical cations of 1,2,4,5-tetrakis(isopropylthio)benzene (TPB) and 1,2,4,5-tetrakis(ethylthio) benzene (TEB) have been successfully synthesized with . These newly synthesized salts have been characterized by UV-Vis absorption, EPR spectroscopy, conductivity measurement, single crystal X-ray diffraction analysis as well as DFT calculation. This study raises the first crystal structure of conductive π-stacking radical cation with single phenyl ring and reveals their conductivity has relationship with the stack structure which affected by the substituent. PMID:27403720

  8. Single-Crystal X-ray Structures of conductive π-Stacking Dimers of Tetrakis(alkylthio)benzene Radical Cations

    PubMed Central

    Chen, Xiaoyu; Gao, Feng; Yang, Wuqin

    2016-01-01

    Salts containing radical cations of 1,2,4,5-tetrakis(isopropylthio)benzene (TPB) and 1,2,4,5-tetrakis(ethylthio) benzene (TEB) have been successfully synthesized with . These newly synthesized salts have been characterized by UV-Vis absorption, EPR spectroscopy, conductivity measurement, single crystal X-ray diffraction analysis as well as DFT calculation. This study raises the first crystal structure of conductive π-stacking radical cation with single phenyl ring and reveals their conductivity has relationship with the stack structure which affected by the substituent. PMID:27403720

  9. Single-Crystal X-ray Structures of conductive π-Stacking Dimers of Tetrakis(alkylthio)benzene Radical Cations

    NASA Astrophysics Data System (ADS)

    Chen, Xiaoyu; Gao, Feng; Yang, Wuqin

    2016-07-01

    Salts containing radical cations of 1,2,4,5-tetrakis(isopropylthio)benzene (TPB) and 1,2,4,5-tetrakis(ethylthio) benzene (TEB) have been successfully synthesized with . These newly synthesized salts have been characterized by UV-Vis absorption, EPR spectroscopy, conductivity measurement, single crystal X-ray diffraction analysis as well as DFT calculation. This study raises the first crystal structure of conductive π-stacking radical cation with single phenyl ring and reveals their conductivity has relationship with the stack structure which affected by the substituent.

  10. Evidence for radical formation at Tyr-353 in Mycobacterium tuberculosis catalase-peroxidase (KatG).

    PubMed

    Zhao, Xiangbo; Girotto, Stefania; Yu, Shengwei; Magliozzo, Richard S

    2004-02-27

    Mycobacterium tuberculosis KatG is a heme-containing catalase-peroxidase responsible for activation, through its peroxidase cycle, of the front line antituberculosis antibiotic isoniazid (isonicotinic acid hydrazide). Formation of Compound I (oxyferryl heme-porphyrin pi-cation radical), the classical peroxidase intermediate generated when the resting enzyme turns over with alkyl peroxides, is rapidly followed by production of a protein-centered tyrosyl radical in this enzyme. In our efforts to identify the residue at which this radical is formed, nitric oxide was used as a radical scavenging reagent. Quenching of the tyrosyl radical generated in the presence of NO was shown using electron paramagnetic resonance spectroscopy, and formation of nitrotyrosine was confirmed by proteolytic digestion followed by high performance liquid chromatography analysis of the NO-treated enzyme. These results are consistent with formation of nitrosyltyrosine by addition of NO to tyrosyl radical and oxidation of this intermediate to nitrotyrosine. Two predominant nitrotyrosine-containing peptides were identified that were purified and sequenced by Edman degradation. Both peptides were derived from the same M. tuberculosis KatG sequence spanning residues 346-356 with the amino acid sequence SPAGAWQYTAK, and both peptides contained nitrotyrosine at residue 353. Some modification of Trp-351 most probably into nitrosotryptophan was also found in one of the two peptides. Control experiments using denatured KatG or carried out in the absence of peroxide did not produce nitrotyrosine. In the mutant enzyme KatG(Y353F), which was constructed using site-directed mutagenesis, a tyrosyl radical was also formed upon turnover with peroxide but in poor yield compared with wild-type KatG. Residue Tyr-353 is unique to M. tuberculosis KatG and may play a special role in the function of this enzyme. PMID:14665627

  11. Spontaneous Superlattice Formation in Nanorods through PartialCation Exchange

    SciTech Connect

    Robinson, Richard D.; Sadtler, Bryce; Demchenko, Denis O.; Erdonmez, Can K.; Wang, Lin-Wang; Alivisatos, A. Paul

    2007-03-14

    Lattice mismatch strains are widely known to controlnanoscale pattern formation in heteroepitaxy, but such effects have notbeen exploited in colloidal nanocrystal growth. We demonstrate acolloidal route to synthesizing CdS-Ag2S nanorod superlattices throughpartial cation exchange. Strain induces the spontaneous formation ofperiodic structures. Ab initio calculations of the interfacial energy andmodeling of strain energies show that these forces drive theself-organization. The nanorod superlattices exhibit high stabilityagainst ripening and phase mixing. These materials are tunablenear-infrared emitters with potential applications as nanometer-scaleoptoelectronic devices.

  12. Excited state properties of the astaxanthin radical cation: A quantum chemical study

    NASA Astrophysics Data System (ADS)

    Dreuw, Andreas; Starcke, Jan Hendrik; Wachtveitl, Josef

    2010-07-01

    Using time-dependent density functional theory, the excited electronic states of the astaxanthin radical cation (AXT rad + ) are investigated. While the optically allowed excited D 1 and D 3 states are typical ππ∗ excited states, the D 2 and D 4 states are nπ∗ states. Special emphasis is put onto the influence of the carbonyl groups onto the excited states. For this objective, the excited states of four hypothetical carotenoids and zeaxanthin have been computed. Addition of a carbonyl group to a conjugated carbon double bond system does essentially not change the vertical excitation energies of the optically allowed ππ∗ states due to two counter-acting effects: the excitation energy should increase due to the -M-effect of the carbonyl group and at the same time decrease owing to the elongation of the conjugated double bond system by the carbonyl group itself.

  13. Excited states and electrochromism of radical cation of the carotenoid astaxanthin

    NASA Astrophysics Data System (ADS)

    Krawczyk, Stanisław

    1998-09-01

    Radical cations of the carotenoid astaxanthin were generated by chemical oxidation with Fe(Cl) 3, and their absorption and electroabsorption (Stark) spectra at temperatures about 150 K were recorded in the spectral range from 5900 to 26000 cm -1 (380 to 1700 nm), covering two absorptive electronic transitions from D 0 (ground) to D 1 and D 2 excited states. The changes in static polarizability are negative and equal -40±10 A 3 for D 0→D 1 and -105±15 A 3 for D 0→D 2, pointing that dominant contribution to polarizabilities results from the coupling of D 1 and D 2 with the ground state. An approximate localization of the next excited state with ground-state parity is estimated based on arguments from perturbation theory.

  14. Aqueous oxidation of sulfonamide antibiotics: aromatic nucleophilic substitution of an aniline radical cation.

    PubMed

    Tentscher, Peter R; Eustis, Soren N; McNeill, Kristopher; Arey, J Samuel

    2013-08-19

    Sulfonamide antibiotics are an important class of organic micropollutants in the aquatic environment. For several, sulfur dioxide extrusion products have been previously reported upon photochemical or dark oxidation. Using quantum chemical modeling calculations and transient absorption spectroscopy, it is shown that single-electron oxidation from sulfadiazine produces the corresponding aniline radical cation. Density functional theory calculations indicate that this intermediate can exist in four protonation states. One species exhibits a low barrier for an intramolecular nucleophilic attack at the para position of the oxidized aniline ring, in which a pyrimidine nitrogen acts as a nucleophile. This attack can lead to a rearranged structure, which exhibits the same connectivity as the SO2 -extruded oxidation product that was previously observed in the aquatic environment and characterized by NMR spectroscopy. We report a detailed reaction mechanism for this intramolecular aromatic nucleophilic substitution, and we discuss the possibility of this reaction pathway for other sulfonamide drugs. PMID:23828254

  15. Jahn-Teller and pseudo-Jahn-Teller effects in trifluoromethane radical cation

    NASA Astrophysics Data System (ADS)

    Ghanta, Susanta

    2016-08-01

    Jahn-Teller (JT) and pseudo-Jahn-Teller (PJT) effects in the ground, first and second excited electronic states of the trifluoromethane radical cation are theoretically examined here. Extensive ab initio computation of electronic potential energy surfaces and their coupling surfaces are performed. Full quantum dynamics are obtained by both time-independent and time-dependent quantum mechanical methods. This system belongs to (E+A)⊗e JT-PJT family. Our results compare well with the experimental data. JT interactions are fairly strong in the second excited B˜2 E electronic state and the PJT interaction between A˜2A2 - B˜2 E electronic states is stronger which cause an increase of the spectral line density of the vibronic spectrum.

  16. The mechanism of the retro-Diels-Alder reaction in 4-vinylcyclohexene cation radical

    NASA Astrophysics Data System (ADS)

    Pancíř, J.; Tureček, F.

    1984-06-01

    Butadiene cation radicals are produced symmetrically from the ring and side-chain of the vinylcyclohexene cation radical near the onset of the fragmentation. The appearance energies of C 4H 6+- and C 4H 2D 4+- from (3,3,6,6-D 4)vinylcyclohex ene were measured as 11.07 ± 0.05 and 11.06 ± 0.06 eV, respectively. This sets the barrier to retro-Diels-Alder decomposition at 1140 kJ mol -1 above the energy of 1 and 44 kJ mol -1 above the thermochemical threshold corresponding to C 4H 6+- + C 4H 6. Topological molecular orbital calculations indicate that this lowest-energy path involves a sequential rupture of the C 3C 4 and C 5C 6 bonds, with a calculated barrier of 211 kJ mol -1. The second, two-step reaction channel proceeds by subsequent fission of the C 5C 6 and C 3C 4 bonds with a barrier of 299 kJ mol -1. This channel is found experimentally as a break on the ionization efficiency curve at 12.1 eV. Both the supra-supra and the supra-antara pericyclic reactions go through energy maxima and are therefore forbidden. The supra-supra process is the most favorable route for decomposition from the first excited state, the activation energy being 333 kJ mol -1. The preference for the two-step mechanism is due to hyperconjugative stabilization of intermediate molecular configurations.

  17. Synthesis, Physical Properties and Reactivity of Stable Antiaromatic 1,4-DIHYDROPYRAZINES and Their Associated Radical Cations

    NASA Astrophysics Data System (ADS)

    Brook, David James Rawsthorne

    oxidation resulted in cleavage of the pyrazine ring and in trifluoroacetic acid only the DDTTA radical cation was observed as an oxidation product. With iron carbonyls, HTTA produced an unstable pi coordinated intermediate, detected by NMR spectroscopy. This decomposed to give an insoluble iron carbonyl with stoichiometry Fe_{5 }(CO)_{8}. With cobalt and manganese carbonyls, reaction resulted in the formation of 4.9, probably through metal-hydrido intermediates. Reaction of DDTTA with tetracyanoquinodimathane (TCNQ) results in three phases of composition DDTTA(TCNQ)_ {2}, two of which were characterised by single crystal X-ray diffraction, the third by X-ray powder diffraction. The materials are semiconductors with infinite TCNQ stacks. HTTA also gave a TCNQ salt of composition HTTAcdotTCNQcdotCH _{3}CN with the TCNQ molecules arranged in discrete dimers. Magnetic properties of the TCNQ salts along with the trifluoracetate and tetraphenyl borate salts of DDTTA^{+} were investigated. These suggest that the hydrogen bonded arrangement of DDTTA cations within the P1 form of DDTTA(TCNQ) _{2} give rise to short range ferromagnetic interactions between cations.

  18. Simultaneous electrochemical and electron paramagnetic resonance studies of carotenoid cation radicals and dications

    SciTech Connect

    Khaled, M.; Hadjipetrou, A.; Kispert, L.D. ); Allendoerfer, R.D. )

    1991-03-21

    Comproportionation equilibrium constants have been determined from simultaneous electrochemical and EPR measurements for the carotenoid cation radicals (CAR{sup {sm bullet}+}) and dications (CAR {sup 2+}) of {beta}-carotene (1), {beta}-apo-8{prime}-carotenal (2), and canthaxanthin (3). K(1){sub com} = 2.4 {times} 10{sup {minus}2}, K{sub com}(2) = 1.8 {times} 10{sup {minus}2}, K{sub com}(3) = 2.1 {times} 10{sup 3}. These indicated that, upon oxidation of 3, 96% CAR{sup {sm bullet}+} would be formed while 99.7% CAR{sup 2+} would be formed for 1 and 2 if the oxidation potential was 100 mV anodic of the first observed voltammetric wave. This explains the reason for the strong EPR spectrum observed for 3 and the weak EPR spectra observed for 1 and 2. Rotating disk experiments confirm that oxidation of carotenoids occurs by an EE rather than by an ECE mechanism and are highly quasireversible systems. The second oxidation peak in the CV spectrum of 2 has been shown not to be due to a dication analogous to the CV of 3 but to a radical apparently from the oxidation of a decay product of the dication from 2.

  19. One-electron oxidation of alcohols by the 1,3,5-trimethoxybenzene radical cation in the excited state during two-color two-laser flash photolysis.

    PubMed

    Cai, Xichen; Sakamoto, Masanori; Fujitsuka, Mamoru; Majima, Tetsuro

    2007-03-15

    One-electron oxidation of alcohols such as methanol, ethanol, and 2-propanol by 1,3,5-trimethoxybenzene radical cation (TMB*+) in the excited state (TMB*+*) was observed during the two-color two-laser flash photolysis. TMB*+ was formed by the photoinduced bimolecular electron-transfer reaction from TMB to 2,3,5,6-tetrachlorobenzoquinone (TCQ) in the triplet excited-state during the first 355-nm laser flash photolysis. Then, TMB*+* was generated from the selective excitation of TMB*+ during the second 532 nm laser flash photolysis. Hole transfer rate constants from TMB*+* to methanol, ethanol, and 2-propanol were calculated to be (5.2 +/- 0.5) x 10(10), (1.4 +/- 0.3) x 10(11), and (3.2 +/- 0.6) x 10(11) M-1 s-1, respectively. The order of the hole transfer rate constants is consistent with oxidation potentials of alcohol. Formation of TCQH radical (TCQH*) with a characteristic absorption peak at 435 nm was observed in the microsecond time scale, suggesting that deprotonation of the alcohol radical cation occurs after the hole transfer and that TCQ radical anion (TCQ*-), generated together with TMB*+ by the photoinduced electron-transfer reaction, reacts with H+ to give TCQH*. PMID:17295459

  20. On the dissociation of the naphthalene radical cation: new iPEPICO and tandem mass spectrometry results.

    PubMed

    West, Brandi; Joblin, Christine; Blanchet, Valerie; Bodi, Andras; Sztáray, Bálint; Mayer, Paul M

    2012-11-15

    The dissociation of the naphthalene radical cation has been reinvestigated here by a combination of tandem mass spectrometry and imaging photoelectron photoion coincidence spectroscopy (iPEPICO). Six reactions were explored: (R1) C(10)H(8)(•+) → C(10)H(7)(+) + H (m/z = 127); (R2) C(10)H(8)(•+) → C(8)H(6)(•+) + C(2)H(2) (m/z = 102); (R3) C(10)H(8)(•+) → C(6)H(6)(•+) + C(4)H(2) (m/z = 78); (R4) C(10)H(8)(•+) → C(10)H(6)(•+) + H(2) (m/z = 126); (R5) C(10)H(7)(+) → C(6)H(5)(+) + C(4)H(2) (m/z = 77); (R6) C(10)H(7)(+) → C(10)H(6)(•+) + H (m/z = 126). The E(0) activation energies for the reactions deduced from the present measurements are (in eV) 4.20 ± 0.04 (R1), 4.12 ± 0.05 (R2), 4.27 ± 0.07 (R3), 4.72 ± 0.06 (R4), 3.69 ± 0.26 (R5), and 3.20 ± 0.13 (R6). The corresponding entropies of activation, ΔS(‡)(1000K), derived in the present study are (in J K(-1) mol(-1)) 2 ± 2 (R1), 0 ± 2 (R2), 4 ± 4 (R3), 11 ± 4 (R4), 5 ± 15 (R5), and -19 ± 11 (R6). The derived E(0) value, combined with the previously reported IE of naphthalene (8.1442 eV) results in an enthalpy of formation for the naphthyl cation, Δ(f)H°(0K) = 1148 ± 14 kJ mol(-1)/Δ(f)H°(298K) = 1123 ± 14 kJ mol(-1) (site of dehydrogenation unspecified), slightly lower than the previous estimate by Gotkis and co-workers. The derived E(0) for the second H-loss leads to a Δ(f)H° for ion 7, the cycloprop[a]indene radical cation, of Δ(f)H°(0K) =1457 ± 27 kJ mol(-1)/Δ(f)H°(298K)(C(10)H(6)(+)) = 1432 ± 27 kJ mol(-1). Detailed comparisons are provided with values (experimental and theoretical) available in the literature. PMID:23088182

  1. Radical formation in the coma from photodissociation of ice grains

    NASA Technical Reports Server (NTRS)

    Jackson, William M.; Gerth, Christopher

    1990-01-01

    Long ago visual observations of comets suggested that there are jets in comets but it has only been recently that A'Hearn et al. have proven that some of these jets are due to emission from the CN radical. Recent studies in the lab have shown that CN radicals can be ejected directly into the gas phase from the photolysis of frozen vapors if the parent molecular has been excited to repulsive excited state. This later observation suggests that the jets that have been observed may be due to photodissociation of icy grains in the coma. A theory of radical formation from icy grains is presented. It is shown that direct formation of free radicals in the coma is an effective way to produce radicals from icy grains in the coma. The model predicts that icy grains could produce from 6 to 800,000 OH radicals/s per grain depending upon whether the radius of the grain is 0.3 to 100 micron.

  2. Formation of ions and radicals from icy grains in comets

    NASA Technical Reports Server (NTRS)

    Jackson, William M.; Gerth, Christopher; Hendricks, Charles

    1991-01-01

    Ion and radical formation in comets are thought to occur primarily by photodissociation of gas phase molecules. Experimental evidence and theoretical calculations are presented that show that some of the radical and ions can come directly from ice grains. The experimental evidence suggest that if the frozen molecules on the surface of grains undergo direct dissociation then they may be able to release radicals directly in the gas phase. If the molecules undergo predissociation it is unlikely that they will release radicals in the gas phase since they should be quenched. Calculations of this direct photodissociation mechanism further indicate that even if the parent molecule undergoes direct dissociation, the yield will not be high enough to explain the rays structure in comets unless the radicals are stored in the grains and then released when the grain evaporates. Calculations were also performed to determine the maximum number of ions that can be stored in an icy grain's radius. This number is compared with the ratio of the ion to neutral molecular density. The comparison suggests that some of the ions observed near the nucleus of the comet could have originally been present in the cometary nucleus. It is also pointed out that the presence of these ions in icy grains could lead to radical formation via electron recombination. Finally, an avalanche process was evaluated as another means of producing ions in comets.

  3. Synthesis of the iron phthalocyaninate radical cation μ-nitrido dimer and its interaction with hydrogen peroxide

    NASA Astrophysics Data System (ADS)

    Grishina, E. S.; Makarova, A. S.; Kudrik, E. V.; Makarov, S. V.; Koifman, O. I.

    2016-03-01

    The iron phthalocyaninate μ-nitrido dimer radical cation, as well as the μ-nitrido dimer complexes of iron phthalocyaninate, was found to have high catalytic activity in the oxidation of organic compounds. It was concluded that this compound is of interest as a model of active intermediates—catalase and oxidase enzymes.

  4. Review: Formation of Peptide Radical Ions Through Dissociative Electron Transfer in Ternary Metal-Ligand-Peptide Complexes

    SciTech Connect

    Chu, Ivan K.; Laskin, Julia

    2011-12-31

    The formation and fragmentation of odd-electron ions of peptides and proteins is of interest to applications in biological mass spectrometry. Gas-phase redox chemistry occurring during collision-induced dissociation of ternary metal-ligand-peptide complexes enables the formation of a variety of peptide radicals including the canonical radical cations, M{sup +{sm_bullet}}, radical dications, [M{sup +}H]{sup 2+{sm_bullet}}, radical anions, [M-2H]{sup -{sm_bullet}}. In addition, odd-electron peptide ions with well-defined initial location of the radical site are produced through side chain losses from the radical ions. Subsequent fragmentation of these species provides information on the role of charge and the location of the radical site on the competition between radical-induced and proton-driven fragmentation of odd-electron peptide ions. This account summarizes current understanding of the factors that control the efficiency of the intramolecular electron transfer (ET) in ternary metal-ligand-peptide complexes resulting in formation of odd-electron peptide ions. Specifically, we discuss the effect of the metal center, the ligand and the peptide structure on the competition between the ET, proton transfer (PT), and loss of neutral peptide and neutral peptide fragments from the complex. Fundamental studies of the structures, stabilities, and the energetics and dynamics of fragmentation of such complexes are also important for detailed molecular-level understanding of photosynthesis and respiration in biological systems.

  5. Reactions of 5-methylcytosine cation radicals in DNA and model systems: thermal deprotonation from the 5-methyl group vs. excited state deprotonation from sugar

    PubMed Central

    Adhikary, Amitava; Kumar, Anil; Palmer, Brian J.; Todd, Andrew D.; Heizer, Alicia N.; Sevilla, Michael D.

    2014-01-01

    Purpose To study the formation and subsequent reactions of the 5-methyl-2′-deoxycytidine cation radical (5-Me-2′-dC•+) in nucleosides and DNA-oligomers and compare to one electron oxidized thymidine. Materials and methods Employing electron spin resonance (ESR), cation radical formation and its reactions were investigated in 5-Me-2′-dC, thymidine (Thd) and their derivatives, in fully double stranded (ds) d[GC*GC*GC*GC*]2 and in the 5-Me-C/A mismatched, d[GGAC*AAGC:CCTAATCG], where C* = 5-Me-C. Results We report 5-Me-2′-dC•+ production by one-electron oxidation of 5-Me-2′-dC by Cl2•− via annealing in the dark at 155 K. Progressive annealing of 5-Me-2′-dC•+ at 155 K produces the allylic radical (C-CH2•). However, photoexcitation of 5-Me-2′-dC•+ by 405 nm laser or by photoflood lamp leads to only C3′• formation. Photoexcitation of N3-deprotonated thyminyl radical in Thd and its 5′-nucleotides leads to C3′• formation but not in 3′-TMP which resulted in the allylic radical (U-CH2•) and C5′• production. For excited 5-Me-2′,3′-ddC•+, absence of the 3′-OH group does not prevent C3′• formation. For d[GC*GC*GC*GC*]2 and d[GGAC*AAGC:CCTAATCG], intra-base paired proton transferred form of G cation radical (G(N1-H)•:C(+H+)) is found with no observable 5-Me-2′-dC•+ formation. Photoexcitation of (G(N1-H)•:C(+H+)) in d[GC*GC*GC*GC*]2 produced only C1′• and not the expected photoproducts from 5-Me-2′-dC•+. However, photoexcitation of (G(N1-H)•:C(+H+)) in d[GGAC*AAGC:CCTAATCG] led to C5′• and C1′• formation. Conclusions C-CH2• formation from 5-Me-2′-dC•+ occurs via ground state deprotonation from C5-methyl group on the base. In the excited 5-Me-2′-dC•+ and 5-Me-2′,3′-ddC•+, spin and charge localization at C3′ followed by deprotonation leads to C3′• formation. Thus, deprotonation from C3′ in the excited cation radical is kinetically controlled and sugar C-H bond energies are

  6. Cationic PAMAM Dendrimers Aggressively Initiate Blood Clot Formation

    PubMed Central

    Jones, Clinton F.; Campbell, Robert A.; Brooks, Amanda E.; Assemi, Shoeleh; Tadjiki, Soheyl; Thiagarajan, Giridhar; Mulcock, Cheyanne; Weyrich, Andrew S.; Brooks, Benjamin D.; Ghandehari, Hamidreza; Grainger, David W.

    2012-01-01

    Poly(amidoamine) (PAMAM) dendrimers are increasingly studied as model nanoparticles for a variety of biomedical applications, notably in systemic administrations. However, with respect to blood contacting applications, amine-terminated dendrimers have recently been shown to activate platelets and cause a fatal, disseminated intravascular coagulation (DIC)-like condition in mice and rats. We here demonstrate that, upon addition to blood, cationic G7 PAMAM dendrimers induce fibrinogen aggregation, which may contribute to the in vivo DIC-like phenomenon. We demonstrate that amine-terminated dendrimers act directly on fibrinogen in a thrombin-independent manner to generate dense, high-molecular-weight fibrinogen aggregates with minimal fibrin fibril formation. In addition, we hypothesize this clot-like behavior is likely mediated through electrostatic interactions between the densely charged cationic dendrimer surface and negatively charged fibrinogen domains. Interestingly, cationic dendrimers also induced aggregation of albumin, suggesting that many negatively charged blood proteins may be affected by cationic dendrimers. To investigate this further, zebrafish embryos (ZFE) were employed to more specifically determine the speed of this phenomenon and the pathway- and dose-dependency of the resulting vascular occlusion phenotype. These novel findings show that G7 PAMAM dendrimers significantly and adversely impact many blood components to produce rapid coagulation and strongly suggest that these effects are independent of classic coagulation mechanisms. These results also strongly suggest the need to fully characterize amine-terminated PAMAM dendrimers in regards to their adverse effects on both coagulation and platelets, which may contribute to blood toxicity. PMID:23062017

  7. Formation and Stabilization of Environmentally Persistent Free Radicals Induced by the Interaction of Anthracene with Fe(III)-Modified Clays.

    PubMed

    Jia, Hanzhong; Nulaji, Gulimire; Gao, Hongwei; Wang, Fu; Zhu, Yunqing; Wang, Chuanyi

    2016-06-21

    Environmentally persistent free radicals (EPFRs) are occasionally detected in Superfund sites but the formation of EPFRs induced by polycyclic aromatic hydrocarbons (PAHs) is not well understood. In the present work, the formation of EPFRs on anthracene-contaminated clay minerals was quantitatively monitored via electron paramagnetic resonance (EPR) spectroscopy, and surface/interface-related environmental influential factors were systematically explored. The obtained results suggest that EPFRs are more readily formed on anthracene-contaminated Fe(III)-montmorillonite than in other tested systems. Depending on the reaction condition, more than one type of organic radicals including anthracene-based radical cations with g-factors of 2.0028-2.0030 and oxygenic carbon-centered radicals featured by g-factors of 2.0032-2.0038 were identified. The formed EPFRs are stabilized by their interaction with interlayer surfaces, and such surface-bound EPFRs exhibit slow decay with 1/e-lifetime of 38.46 days. Transformation pathway and possible mechanism are proposed on the basis of experimental results and quantum mechanical simulations. Overall, the formation of EPFRs involves single-electron-transfer from anthracene to Fe(III) initially, followed by H2O addition on formed aromatic radical cation. Because of their potential exposure in soil and atmosphere, such clay surface-associated EPFRs might induce more serious toxicity than PAHs and exerts significant impacts on human health. PMID:27224055

  8. Formation of Chlorotriophenoxy Radicals from Complete Series Reactions of Chlorotriophenols with H and OH Radicals

    PubMed Central

    Xu, Fei; Shi, Xiangli; Zhang, Qingzhu; Wang, Wenxing

    2015-01-01

    The chlorothiophenoxy radicals (CTPRs) are key intermediate species in the formation of polychlorinated dibenzothiophenes/thianthrenes (PCDT/TAs). In this work, the formation of CTPRs from the complete series reactions of 19 chlorothiophenol (CTP) congeners with H and OH radicals were investigated theoretically by using the density functional theory (DFT) method. The profiles of the potential energy surface were constructed at the MPWB1K/6-311+G(3df,2p)//MPWB1K/6-31+G(d,p) level. The rate constants were evaluated by the canonical variational transition-state (CVT) theory with the small curvature tunneling (SCT) contribution at 600–1200 K. The present study indicates that the structural parameters, thermal data, and rate constants as well as the formation potential of CTPRs from CTPs are strongly dominated by the chlorine substitution at the ortho-position of CTPs. Comparison with the study of formation of chlorophenoxy radicals (CPRs) from chlorophenols (CPs) clearly shows that the thiophenoxyl-hydrogen abstraction from CTPs by H is more efficient than the phenoxyl-hydrogen abstraction from CPs by H, whereas the thiophenoxyl-hydrogen abstraction from CTPs by OH is less impactful than the phenoxyl-hydrogen abstraction from CPs by OH. Reactions of CTPs with H can occur more readily than that of CTPs with OH, which is opposite to the reactivity comparison of CPs with H and OH. PMID:26270566

  9. Formation of Chlorotriophenoxy Radicals from Complete Series Reactions of Chlorotriophenols with H and OH Radicals.

    PubMed

    Xu, Fei; Shi, Xiangli; Zhang, Qingzhu; Wang, Wenxing

    2015-01-01

    The chlorothiophenoxy radicals (CTPRs) are key intermediate species in the formation of polychlorinated dibenzothiophenes/thianthrenes (PCDT/TAs). In this work, the formation of CTPRs from the complete series reactions of 19 chlorothiophenol (CTP) congeners with H and OH radicals were investigated theoretically by using the density functional theory (DFT) method. The profiles of the potential energy surface were constructed at the MPWB1K/6-311+G(3df,2p)//MPWB1K/6-31+G(d,p) level. The rate constants were evaluated by the canonical variational transition-state (CVT) theory with the small curvature tunneling (SCT) contribution at 600-1200 K. The present study indicates that the structural parameters, thermal data, and rate constants as well as the formation potential of CTPRs from CTPs are strongly dominated by the chlorine substitution at the ortho-position of CTPs. Comparison with the study of formation of chlorophenoxy radicals (CPRs) from chlorophenols (CPs) clearly shows that the thiophenoxyl-hydrogen abstraction from CTPs by H is more efficient than the phenoxyl-hydrogen abstraction from CPs by H, whereas the thiophenoxyl-hydrogen abstraction from CTPs by OH is less impactful than the phenoxyl-hydrogen abstraction from CPs by OH. Reactions of CTPs with H can occur more readily than that of CTPs with OH, which is opposite to the reactivity comparison of CPs with H and OH. PMID:26270566

  10. A quantum chemical topological analysis of the C-C bond formation in organic reactions involving cationic species.

    PubMed

    Domingo, Luis R; Pérez, Patricia

    2014-07-21

    ELF topological analysis of the ionic Diels-Alder (I-DA) reaction between the N,N-dimethyliminium cation and cyclopentadiene (Cp) has been performed in order to characterise the C-C single bond formation. The C-C bond formation begins in the short range of 2.00-1.96 Åvia a C-to-C pseudoradical coupling between the most electrophilic center of the iminium cation and one of the two most nucleophilic centers of Cp. The electron density of the pseudoradical center generated at the most electrophilic carbon of the iminium cation comes mainly from the global charge transfer which takes place along the reaction. Analysis of the global reactivity indices indicates that the very high electrophilic character of the iminium cation is responsible for the negative activation energy found in the gas phase. On the other hand, the analysis of the radical P(k)(o) Parr functions of the iminium cation, and the nucleophilic P(k)(-) Parr functions of Cp makes the characterisation of the most favourable two-center interaction along the formation of the C-C single bond possible. PMID:24901220

  11. Reactive Pathways in the Chlorobenzene-Ammonia Dimer Cation Radical: New Insights from Experiment and Theory

    NASA Astrophysics Data System (ADS)

    Nyambo, Silver; Uhler, Brandon; Kalume, Aimable; Muzangwa, Lloyd; Reid, Scott

    2014-06-01

    Previously, we have studied non-covalent interactions in mono-halogenated benzene clusters using mass selected resonant 2-photon ionization methods. We have extended our studies by investigating the interaction between these mono-halobenzenes with a prototypical N atom donor (NH_3). Thus, we have obtained electronic spectra of PhX…(NH_3)n ( X=F, Cl, Br and n=1,2….) complexes in the region of the PhX monomer S_0-S_1 (ππ*) transition. Here we are mainly focusing on PhCl…NH_3 dimer. We found that upon ionization of the dimer, three reactive pathways of the [PhCl…NH_3]+. have been evidenced. The primary pathway is the Cl atom elimination, previously evidenced. The second and third pathways, HCl elimination and H atom elimination are identified for the first time in the R2PI studies of the dimer. Electronic spectra obtained for the three pathways shows that they originate from a common precursor. The reactive pathways in this system were extensively characterized computationally. We used DFT and post-Hartree Fock electronic structure calculations, Frank-Condon analysis to support our experimental findings. The results were consistent with previous direct ab initio molecular dynamics calculations, we found two nearly iso-energetic Wheland intermediates which lie significantly lower in energy than the initially formed dimer cation radical [PhCl…NH_3]+..

  12. Solvent effects on the resonance Raman and electronic absorption spectra of bacteriochlorophyll a cation radical

    SciTech Connect

    Misono, Yasuhito; Itoh, Koichi; Limanatara, Leenawaty; Koyama, Yasushi

    1996-02-08

    Resonance Raman and electronic absorption spectra of bacteriocholrophyll a cation radical (BChl a{sup .+}) were recorded in 14 different kinds of solvents. The frequency of the ring-breathing Raman band of BChl a{sup .+} was in the region of 1596-1599 cm{sup -1} in solvents forming the pentacoordinated state in neutral bacteriochlorophyll a (BChl a), while it was in the region of 1584-1588 cm{sup -1} in solvents forming the hexacoordinated state. BChl a{sup .+} exhibited a key absorption band in the regions 546-554 and 557-563 nm in the above penta- and hexa-coordinating solvents. Therefore, it has been concluded that the penta- and hexa-coordinated states are retained even after conversion of BChl a into BChl a{sup .+} (one-electron oxidization). Application of this rule to the case of 2-propanol solution showed transformation from the penta- to the hexa-coordinated state upon one-electron oxidation in this particular solution. The coordination states of BChl a{sup .+} could be correlated with the donor number(DN) and the Taft parameters, {Beta} and {pi}{sup *}, of the solvent: The hexacoordinated state was formed in solvents with DN >= 18 or {Beta} > 0.5 showing higher electron donating power, while the pentacoordinated state was formed in solvents with {pi}{sup *} > 0.65 showing higher dielectric stabilization. 27 refs., 8 figs., 3 tabs.

  13. Unconventional hydrogen bonding to organic ions in the gas phase: Stepwise association of hydrogen cyanide with the pyridine and pyrimidine radical cations and protonated pyridine

    NASA Astrophysics Data System (ADS)

    Hamid, Ahmed M.; El-Shall, M. Samy; Hilal, Rifaat; Elroby, Shaaban; Aziz, Saadullah G.

    2014-08-01

    Equilibrium thermochemical measurements using the ion mobility drift cell technique have been utilized to investigate the binding energies and entropy changes for the stepwise association of HCN molecules with the pyridine and pyrimidine radical cations forming the C5H5N+.(HCN)n and C4H4N2+.(HCN)n clusters, respectively, with n = 1-4. For comparison, the binding of 1-4 HCN molecules to the protonated pyridine C5H5NH+(HCN)n has also been investigated. The binding energies of HCN to the pyridine and pyrimidine radical cations are nearly equal (11.4 and 12.0 kcal/mol, respectively) but weaker than the HCN binding to the protonated pyridine (14.0 kcal/mol). The pyridine and pyrimidine radical cations form unconventional carbon-based ionic hydrogen bonds with HCN (CHδ+⋯NCH). Protonated pyridine forms a stronger ionic hydrogen bond with HCN (NH+⋯NCH) which can be extended to a linear chain with the clustering of additional HCN molecules (NH+⋯NCH..NCH⋯NCH) leading to a rapid decrease in the bond strength as the length of the chain increases. The lowest energy structures of the pyridine and pyrimidine radical cation clusters containing 3-4 HCN molecules show a strong tendency for the internal solvation of the radical cation by the HCN molecules where bifurcated structures involving multiple hydrogen bonding sites with the ring hydrogen atoms are formed. The unconventional H-bonds (CHδ+⋯NCH) formed between the pyridine or the pyrimidine radical cations and HCN molecules (11-12 kcal/mol) are stronger than the similar (CHδ+⋯NCH) bonds formed between the benzene radical cation and HCN molecules (9 kcal/mol) indicating that the CHδ+ centers in the pyridine and pyrimidine radical cations have more effective charges than in the benzene radical cation.

  14. GLYCOLALDEHYDE FORMATION VIA THE DIMERIZATION OF THE FORMYL RADICAL

    SciTech Connect

    Woods, Paul M.; Viti, Serena; Slater, Ben; Raza, Zamaan; Brown, Wendy A.; Burke, Daren J.

    2013-11-10

    Glycolaldehyde, the simplest monosaccharide sugar, has recently been detected in low- and high-mass star-forming cores. Following our previous investigation into glycolaldehyde formation, we now consider a further mechanism for the formation of glycolaldehyde that involves the dimerization of the formyl radical, HCO. Quantum mechanical investigation of the HCO dimerization process upon an ice surface is predicted to be barrierless and therefore fast. In an astrophysical context, we show that this mechanism can be very efficient in star-forming cores. It is limited by the availability of the formyl radical, but models suggest that only very small amounts of CO are required to be converted to HCO to meet the observational constraints.

  15. Investigation of the molecular structure of radical cation of s-trioxane: quantum chemical calculations and low-temperature EPR results

    NASA Astrophysics Data System (ADS)

    Janovský, I.; Naumov, S.; Knolle, W.; Mehnert, R.

    2003-06-01

    s-Trioxane radical cation was radiolytically generated in freon matrix and the changes of the EPR spectra with temperature, arising from conformational interconversion involving ring, were observed. The equilibration, leading to six equivalent protons (hfs splitting constant 5.9 mT) characteristic of the average planar geometry of the radical cation, occurs at ˜120 K in CF 3CCl 3. Supplementary experiments with 1,3-dioxane, which forms a radical cation with a similar electronic structure, were also performed. DFT quantum chemical calculations were used to support the experimental results.

  16. The Guanine Cation Radical: Investigation of Deprotonation States by ESR and DFT

    PubMed Central

    Adhikary, Amitava; Kumar, Anil; Becker, David; Sevilla, Michael D.

    2008-01-01

    This work reports ESR studies that identify the favored site of deprotonation of the guanine cation radical (G•+) in an aqueous medium at 77 K. Using ESR and UV-visible spectroscopy, one-electron oxidized guanine is investigated in frozen aqueous D2O solutions of 2′-deoxyguanosine (dGuo) at low temperatures at various pHs at which the guanine cation, G•+ (pH 3–5), singly deprotonated species, G(-H)• (pH 7–9) and doubly deprotonated species, G(-2H)•− (pH>11) are found. C-8-deuteration of dGuo to give 8-D-dGuo removes the major proton hyperfine coupling at C-8. This isolates the anisotropic nitrogen couplings for each of the three species and aids our analyses. These anisotropic nitrogen couplings were assigned to specific nitrogen sites by use of 15N substituted derivatives at N1, N2 N3 atoms in dGuo. Both ESR and UV-visible spectra are reported for each of the species: G•+, G(-H)•, and G(-2H)•−. The experimental anisotropic ESR hyperfine couplings are compared to those obtained from DFT calculations for the various tautomers of G(-H)•. Using the B3LYP/6–31G(d) method, the geometries and energies of G•+ and its singly deprotonated state in its two tautomeric forms, G(N1-H)• and G(N2-H)•, were investigated. In a non-hydrated state G(N2-H)• is found to be more stable than G(N1-H)• but on hydration with 7 water molecules G(N1-H)• is found to be more stable than G(N2-H)•. The theoretically calculated hyperfine coupling constants (HFCC) of G•+, G(N1-H)• and G(-2H)•− match the experimentally observed HFCCs best on hydration with 7 or more waters. For G(-2H)•−, the hyperfine coupling constant (HFCC) at the exocyclic nitrogen atom (N2) is especially sensitive to the number of hydrating water molecules; good agreement with experiment is not obtained until 9 or 10 waters of hydration are included. PMID:17125389

  17. The dynamical behavior of the s-trioxane radical cation-A low-temperature EPR and theoretical study.

    PubMed

    Naumov, Sergej S; Knolle, Wolfgang; Naumov, Sergej P; Pöppl, Andreas; Janovský, Igor

    2014-01-01

    The radical cation of s-trioxane, radiolytically generated in a freon (CF3CCl3) matrix, was studied in the 10-140 K temperature region. Reversible changes of the EPR spectra were observed, arising from both ring puckering and ring inversion through the molecular plane. The ESREXN program based on the Liouville density matrix equation, allowing the treatment of dynamical exchange, has been used to analyze the experimental results. Two limiting conformer structures of the s-trioxane radical cation were taken into account, namely "rigid" half-boat and averaged planar ones, differing strongly in their electron distribution. The spectrum due to the "rigid" half-boat conformer can be observed only at very low (<60 K) temperatures, when the exchange of conformers is very slow. Two transition states for interconversion by puckering and ring-inversion were identified, close in activation energy (2.3 and 3.0 kJ/mol calculated). Since the energy difference is very small, both processes set on at a comparable temperature. In the case of nearly complete equilibration (fast exchange) between six energetically equivalent structures at T > 120 K in CF3CCl3, a septet due to six equivalent protons (hfs splitting constant 5.9 mT) is observed, characteristic of the dynamically averaged planar geometry of the radical cation. DFT quantum chemical calculations and spectral simulation including intramolecular dynamical exchange support the interpretation. PMID:25353382

  18. To jump or not to jump? Cα hydrogen atom transfer in post-cleavage radical-cation complexes.

    PubMed

    Bythell, Benjamin J

    2013-02-14

    Conventionally, electron capture or transfer to a polyprotonated peptide ion produces an initial radical-cation intermediate which dissociates "directly" to generate complementary c(n)' and z(m)(•) sequence ions (or ions and neutrals). Alternatively, or in addition, the initial radical-cation intermediate can undergo H(•) migration to produce c(n)(•) (or c(n) - H(•)) and z(m)' (or z(m)(•) + H(•)) species prior to complex separation ("nondirect"). This reaction significantly complicates spectral interpretation, creates ambiguity in peak assignment, impairs effective algorithmic processing (reduction of the spectrum to solely (12)C m/z values), and reduces sequence ion signal-to-noise. Experimental evidence indicates that the products of hydrogen atom transfer reactions are substantially less prevalent for higher charge state precursors. This effect is generally rationalized on the basis of decreased complex lifetime. Here, we present a theoretical study of these reactions in post N-C(α) bond cleavage radical-cation complexes as a function of size and precursor charge state. This approach provides a computational estimate of the barriers associated with these processes for highly charged peptides with little charge solvation. The data indicate that the H(•) migration is an exothermic process and that the barrier governing this reaction rises steeply with precursor ion charge state. There is also some evidence for immediate product separation following N-C(α) bond cleavage at higher charge state. PMID:22809411

  19. A search for blues brothers: X-ray crystallographic/spectroscopic characterization of the tetraarylbenzidine cation radical as a product of aging of solid magic blue.

    PubMed

    Talipov, Marat R; Hossain, Mohammad M; Boddeda, Anitha; Thakur, Khushabu; Rathore, Rajendra

    2016-03-14

    Magic blue (MB+˙ SbCl6− salt), i.e. tris-4-bromophenylamminium cation radical, is a routinely employed one-electron oxidant that slowly decomposes in the solid state upon storage to form so called ‘blues brothers’, which often complicate the quantitative analyses of the oxidation processes. Herein, we disclose the identity of the main ‘blues brother’ as the cation radical and dication of tetrakis-(4-bromophenyl)benzidine (TAB) by a combined DFT and experimental approach, including isolation of TAB+˙ SbCl6− and its X-ray crystallography characterization. The formation of TAB in aged magic blue samples occurs by a Scholl-type coupling of a pair of MB followed by a loss of molecular bromine. The recognition of this fact led us to the rational design and synthesis of tris(2-bromo-4-tert-butylphenyl)amine, referred to as ‘blues cousin’ (BC: Eox1 = 0.78 V vs. Fc/Fc+, λmax(BC+˙) = 805 nm, εmax = 9930 cm−1 M−1), whose oxidative dimerization is significantly hampered by positioning the sterically demanding tert-butyl groups at the para-positions of the aryl rings. A ready two-step synthesis of BC from triphenylamine and the high stability of its cation radical (BC+˙) promise that BC will serve as a ready replacement for MB and an oxidant of choice for mechanistic investigations of one-electron transfer processes in organic, inorganic, and organometallic transformations. PMID:26878458

  20. The Influence of Zeolites on Radical Formation During Lignin Pyrolysis.

    PubMed

    Bährle, Christian; Custodis, Victoria; Jeschke, Gunnar; van Bokhoven, Jeroen A; Vogel, Frédéric

    2016-09-01

    Lignin from lignocellulosic biomass is a promising source of energy, fuels, and chemicals. The conversion of the polymeric lignin to fuels and chemicals can be achieved by catalytic and noncatalytic pyrolysis. The influence of nonporous silica and zeolite catalysts, such as silicalite, HZSM5, and HUSY, on the radical and volatile product formation during lignin pyrolysis was studied by in situ high-temperature electron paramagnetic resonance spectroscopy (HTEPR) as well as GC-MS. Higher radical concentrations were observed in the samples containing zeolite compared to the sample containing only lignin, which suggests that there is a stabilizing effect by the inorganic surfaces on the formed radical fragments. This effect was observed for nonporous silica as well as for HUSY, HZSM5, and silicalite zeolite catalysts. However, the effect is far larger for the zeolites owing to their higher specific surface area. The zeolites also showed an effect on the volatile product yield and the product distribution within the volatile phase. Although silicalite showed no effect on the product selectivity, the acidic zeolites such as HZSM5 or HUSY increased the formation of deoxygenated products such as benzene, toluene, xylene (BTX), and naphthalene. PMID:27486717

  1. Effects of Hofmeister salt series on gluten network formation: Part I. Cation series.

    PubMed

    Tuhumury, H C D; Small, D M; Day, L

    2016-12-01

    Different cationic salts were used to investigate the effects of the Hofmeister salt series on gluten network formation. The effects of cationic salts on wheat flour dough mixing properties, the rheological and the chemical properties of the gluten extracted from the dough with different respective salts, were investigated. The specific influence of different cationic salts on the gluten structure formation during dough mixing, compared to the sodium ion, were determined. The effects of different cations on dough and gluten of different flours mostly followed the Hofmeister series (NH4(+), K(+), Na(+), Mg(2+) and Ca(2+)). The impacts of cations on gluten structure and dough rheology at levels tested were relatively small. Therefore, the replacement of sodium from a technological standpoint is possible, particularly by monovalent cations such as NH4(+), or K(+). However the levels of replacement need to take into account sensory attributes of the cationic salts. PMID:27374596

  2. Procainamide, but not N-Acetylprocainamide, Induces Protein Free Radical Formation on Myeloperoxidase: A Potential Mechanism of Agranulocytosis

    PubMed Central

    Siraki, Arno G.; Deterding, Leesa J.; Bonini, Marcelo G.; Jiang, JinJie; Ehrenshaft, Marilyn; Tomer, Kenneth B.; Mason, Ronald P.

    2009-01-01

    Procainamide (PA) is a drug that is used to treat tachycardia in post-operative patients or for long term maintenance of cardiac arrythmias. Unfortunately, its use has also been associated with agranulocytosis. Here we have investigated the metabolism of PA by myeloperoxidase (MPO) and the formation of an MPO protein free radical. We hypothesized that PA oxidation by MPO/H2O2 would produce a PA cation radical that, in the absence of a biochemical reductant, would lead to the free-radical oxidation of MPO. We utilized a novel anti-DMPO antibody to detect DMPO (5,5-dimethyl-1-pyrroline N-oxide) covalently bound to protein, which forms only by the reaction of DMPO with a protein free radical. We found that PA metabolism by MPO/H2O2 induced the formation of DMPO-MPO, which was inhibited by MPO inhibitors and ascorbate. N-acetyl-PA did not cause DMPO-MPO formation, indicating that the unsubstituted aromatic amine was more oxidizable. PA had a lower calculated ionization potential than N-acetyl-PA. The DMPO adducts of MPO metabolism, as analyzed by electron spin resonance spectroscopy, included a nitrogen-centered radical and a phenyl radical derived from PA, either of which may be involved in the free radical formation on MPO. Furthermore, we also found protein-DMPO adducts in MPO-containing, intact human promyelocytic leukemia cells (HL-60). MPO was affinity-purified from HL-60 cells treated with PA/H2O2 and was found to contain DMPO using the anti-DMPO antibody. Mass spectrometry analysis confirmed the identity of the protein as human MPO. These findings were also supported by the detection of protein free radicals with electron spin resonance in the cellular cytosolic lysate. The formation of an MPO protein free radical is believed to be mediated by free radical metabolites of PA, which we characterized by spin trapping. We propose that drug-induced free radical formation on MPO may play a role in the origin of agranulocytosis. PMID:18489081

  3. Non-photochemical Fluorescence Quenching in Photosystem II Antenna Complexes by the Reaction Center Cation Radical.

    PubMed

    Paschenko, V Z; Gorokhov, V V; Grishanova, N P; Korvatovskii, B N; Ivanov, M V; Maksimov, E G; Mamedov, M D

    2016-06-01

    efficiency for non-photochemical antenna fluorescence quenching by RC cation radical in comparison to that of photochemical quenching are discussed. PMID:27301286

  4. Hydroxyl radical formation in phagocytic cells of the rat.

    PubMed

    Drath, D B; Karnovsky, M L; Huber, G L

    1979-01-01

    Polymorphonuclear leukocytes (PMN) and macrophages, harvested from the peritoneum and lung, release superoxide (O-.2) and hydrogen peroxide (H2O2) during phagocytosis. These two agents are thought to react with each other to produce a highly active oxidative substance known as hydroxyl radical (OH.). We present evidence suggesting that these radicals are generated by phagocytic cells of the rat. Our findings are based upon an assay where ethylene gas is generated from methional by the action of this radical. Ethylene generation was shown to be inhibited by superoxide dismutase, catalase, and scavengers of OH.. Of the cells examined, PMN generated the most ethylene from methional, exhibiting a fourfold increase during phagocytosis. Pulmonary and peritoneal macrophages caused smaller amounts of this gas to be formed. Regardless of cell type, an intact cell was required for ethylene generation. Zymosan appeared to be the most effective particle for all cells in ethylene formation from methional, although opsonization was critical only for PMN. Ethylene generation was dependent on cell concentration to an extent and increased with time. PMID:222719

  5. The loss of NH2O from the N-hydroxyacetamide radical cation CH3C(O)NHOH+

    NASA Astrophysics Data System (ADS)

    Jobst, Karl J.; Burgers, Peter C.; Ruttink, Paul J. A.; Terlouw, Johan K.

    2006-08-01

    A previous study [Ch. Lifshitz, P.J.A. Ruttink, G. Schaftenaar, J.K. Terlouw, Rapid Commun. Mass Spectrom. 1 (1987) 61] shows that metastable N-hydroxyacetamide ions CH3C(O)NHOH+ (HA-1) do not dissociate into CH3CO+ + NHOH by direct bond cleavage but rather yield CH3CO+ + NH2OE The tandem mass spectrometry based experiments of the present study on the isotopologue CH3C(O)NDOD+ reveal that the majority of the metastable ions lose the NH2O radical as NHDO rather than ND2O. A mechanistic analysis using the CBS-QB3 model chemistry shows that the molecular ions HA-1 rearrange into hydrogen-bridged radical cations [OCC(H2)H...N(H)OH]+ whose acetyl cation component then catalyses the transformation NHOH --> NH2O prior to dissociation. The high barrier for the unassisted 1,2-H shift in the free radical, 43 kcal mol-1, is reduced to a mere 7 kcal mol-1 for the catalysed transformation which can be viewed as a quid-pro-quo reaction involving two proton transfers.

  6. Free Radical Formation after Intensive Exercise in Thoroughbred Skeletal Muscles

    PubMed Central

    MINAMI, Yoshio; KAWAI, Minako; MIGITA, Taiko C.; HIRAGA, Atsushi; MIYATA, Hirofumi

    2011-01-01

    Although high oxygen consumption in skeletal muscle may result in severe oxidative stress, there are no direct studies that have documented free radical production in horse muscles after intensive exercise. To find a new parameter indicating the muscle adaptation state for the training of Thoroughbred horses, we examined free radical formation in the muscle by using electron paramagnetic resonance (EPR). Ten male Thoroughbred horses received conventional training for 18 weeks. Before and after the training period, all horses performed an exhaustive incremental load exercise on a 6% incline treadmill. Muscle samples of the middle gluteal muscle were taken pre-exercise and 1 min, 1 hr, and 1 day after exercise. Muscle fiber type composition was also determined in the pre-exercise samples by immunohistochemical staining with monoclonal antibody to myosin heavy chain. We measured the free radical in the muscle homogenate using EPR at room temperature, and the amount was expressed as relative EPR signal intensity. There was a significant increase in Type IIA muscle fiber composition and a decrease in Type IIX fiber composition after the training period. Before the training period, the mean value of the relative EPR signal intensity showed a significant increase over the pre-exercise value at 1 min after the exercise and an incomplete recovery at 24 hr after the exercise. While no significant changes were found in the relative EPR signal intensity after the training period. There was a significant relationship between percentages of Type IIA fiber and change rates in EPR signal intensity at 1 min after exercise. The measurement of free radicals may be useful for determining the muscle adaptation state in the training of Thoroughbred horses. PMID:24833984

  7. Hydroxyl radical substitution in halogenated carbonyls: oxalic acid formation.

    PubMed

    Christiansen, Carrie J; Dalal, Shakeel S; Francisco, Joseph S; Mebel, Alexander M; Gaffney, Jeffrey S

    2010-03-01

    An ab initio study of OH radical substitution reactions in halogenated carbonyls is conducted. Hydroxyl radical substitution into oxalyl dichloride [ClC(O)C(O)Cl] and oxalyl dibromide [BrC(O)C(O)Br], resulting in the formation of oxalic acid, is presented. Analogous substitution reactions in formyl chloride [ClCH(O)], acetyl chloride [ClC(O)CH(3)], formyl bromide [BrCH(O)], and acetyl bromide [BrC(O)CH(3)] are considered. Energetics of competing hydrogen abstraction reactions for all applicable species are computed for comparison. Geometry optimizations and frequency computations are performed using the second-order Møller-Plesset perturbation theory (MP2) and the 6-31G(d) basis set for all minimum species and transition states. Single point energy computations are performed using fourth-order Møller-Plesset perturbation theory (MP4) and coupled cluster theory [CCSD(T)]. Potential energy surfaces, including activation energies and enthalpies, are determined from the computations. These potential energy surfaces show that OH substitution into ClC(O)C(O)Cl and BrC(O)C(O)Br, resulting in the formation of oxalic acid and other minor products, is energetically favorable. Energetics of analogous reactions with ClCH(O), BrCH(O), ClC(O)CH(3), and BrC(O)CH(3) are also computed. PMID:20131850

  8. Phthalocyanine. pi. -cation-radical species: photochemical and electrochemical preparation of (ZnPc(-1))/sup. +/ in solution

    SciTech Connect

    Nyokong, T.; Gasyna, Z.; Stillman, M.J.

    1987-02-25

    The ..pi..-cation-radical species of ZnPc, (ZnPc(-1))/sup .+/ (Pc = phthalocyanine), has been formed quantitatively as a stable product in solution following photochemical reactions with visible-region light in the presence of electron acceptors. The photolyses were carried out by excitation into the phthalocyanine's Q band (lambda > 580 nm), with carbon tetrabromide as an irreversible electron acceptor. The neutral parent species could be regenerated following photooxidation by the addition of sodium dithionite. Cyclic voltammetry of neutral ZnPcL species identified the ring oxidation potentials; for ZnPc(py) and ZnPc(im) (py = pyridine; im = imidazole). In dimethylacetamide, there is one oxidation couple at 0.70 and 0.71 V vs. SCE, respectively, and there are two reduction couples, at -0.96 and -1.28 V vs SCE, for the pyridine complex, and at -0.98 and -1.53 V vs SCE, for the imidazole complex. Each of these reactions was reversible on the cyclic voltammetry time scale. The electrochemical and photochemical oxidation products were characterized by absorption, magnetic circular dichroism (MCD), and electron paramagnetic resonance (EPR) spectroscopies. Four clearly resolvable, optical transitions, centered at 440, 500, 720, and 825 nm, are observed in the absorption spectrum of the cation radical species. EPR spectra obtained from frozen solutions of the ..pi.. cation radicals gave isotropic g values that are characteristic of the oxidation at the phthalocyanine ligand (the g values were between 2.0055 and 2.0068). 52 references, 4 figures, 2 tables.

  9. A Radical-Mediated Pathway for the Formation of [M + H](+) in Dielectric Barrier Discharge Ionization.

    PubMed

    Wolf, Jan-Christoph; Gyr, Luzia; Mirabelli, Mario F; Schaer, Martin; Siegenthaler, Peter; Zenobi, Renato

    2016-09-01

    Active capillary plasma ionization is a highly efficient ambient ionization method. Its general principle of ion formation is closely related to atmospheric pressure chemical ionization (APCI). The method is based on dielectric barrier discharge ionization (DBDI), and can be constructed in the form of a direct flow-through interface to a mass spectrometer. Protonated species ([M + H](+)) are predominantly formed, although in some cases radical cations are also observed. We investigated the underlying ionization mechanisms and reaction pathways for the formation of protonated analyte ([M + H](+)). We found that ionization occurs in the presence and in the absence of water vapor. Therefore, the mechanism cannot exclusively rely on hydronium clusters, as generally accepted for APCI. Based on isotope labeling experiments, protons were shown to originate from various solvents (other than water) and, to a minor extent, from gaseous impurities and/or self-protonation. By using CO2 instead of air or N2 as plasma gas, additional species like [M + OH](+) and [M - H](+) were observed. These gas-phase reaction products of CO2 with the analyte (tertiary amines) indicate the presence of a radical-mediated ionization pathway, which proceeds by direct reaction of the ionized plasma gas with the analyte. The proposed reaction pathway is supported with density functional theory (DFT) calculations. These findings add a new ionization pathway leading to the protonated species to those currently known for APCI. Graphical Abstract ᅟ. PMID:27380388

  10. A Radical-Mediated Pathway for the Formation of [M + H]+ in Dielectric Barrier Discharge Ionization

    NASA Astrophysics Data System (ADS)

    Wolf, Jan-Christoph; Gyr, Luzia; Mirabelli, Mario F.; Schaer, Martin; Siegenthaler, Peter; Zenobi, Renato

    2016-07-01

    Active capillary plasma ionization is a highly efficient ambient ionization method. Its general principle of ion formation is closely related to atmospheric pressure chemical ionization (APCI). The method is based on dielectric barrier discharge ionization (DBDI), and can be constructed in the form of a direct flow-through interface to a mass spectrometer. Protonated species ([M + H]+) are predominantly formed, although in some cases radical cations are also observed. We investigated the underlying ionization mechanisms and reaction pathways for the formation of protonated analyte ([M + H]+). We found that ionization occurs in the presence and in the absence of water vapor. Therefore, the mechanism cannot exclusively rely on hydronium clusters, as generally accepted for APCI. Based on isotope labeling experiments, protons were shown to originate from various solvents (other than water) and, to a minor extent, from gaseous impurities and/or self-protonation. By using CO2 instead of air or N2 as plasma gas, additional species like [M + OH]+ and [M - H]+ were observed. These gas-phase reaction products of CO2 with the analyte (tertiary amines) indicate the presence of a radical-mediated ionization pathway, which proceeds by direct reaction of the ionized plasma gas with the analyte. The proposed reaction pathway is supported with density functional theory (DFT) calculations. These findings add a new ionization pathway leading to the protonated species to those currently known for APCI.

  11. A Radical-Mediated Pathway for the Formation of [M + H]+ in Dielectric Barrier Discharge Ionization

    NASA Astrophysics Data System (ADS)

    Wolf, Jan-Christoph; Gyr, Luzia; Mirabelli, Mario F.; Schaer, Martin; Siegenthaler, Peter; Zenobi, Renato

    2016-09-01

    Active capillary plasma ionization is a highly efficient ambient ionization method. Its general principle of ion formation is closely related to atmospheric pressure chemical ionization (APCI). The method is based on dielectric barrier discharge ionization (DBDI), and can be constructed in the form of a direct flow-through interface to a mass spectrometer. Protonated species ([M + H]+) are predominantly formed, although in some cases radical cations are also observed. We investigated the underlying ionization mechanisms and reaction pathways for the formation of protonated analyte ([M + H]+). We found that ionization occurs in the presence and in the absence of water vapor. Therefore, the mechanism cannot exclusively rely on hydronium clusters, as generally accepted for APCI. Based on isotope labeling experiments, protons were shown to originate from various solvents (other than water) and, to a minor extent, from gaseous impurities and/or self-protonation. By using CO2 instead of air or N2 as plasma gas, additional species like [M + OH]+ and [M - H]+ were observed. These gas-phase reaction products of CO2 with the analyte (tertiary amines) indicate the presence of a radical-mediated ionization pathway, which proceeds by direct reaction of the ionized plasma gas with the analyte. The proposed reaction pathway is supported with density functional theory (DFT) calculations. These findings add a new ionization pathway leading to the protonated species to those currently known for APCI.

  12. Detection of the short-lived radical cation intermediate in the electrooxidation of N,N-dimethylaniline by mass spectrometry.

    PubMed

    Brown, Timothy A; Chen, Hao; Zare, Richard N

    2015-09-14

    The N,N-dimethylaniline (DMA) radical cation DMA(.+) , a long-sought transient intermediate, was detected by mass spectrometry (MS) during the electrochemical oxidation of DMA. This was accomplished by coupling desorption electrospray ionization (DESI) MS with a waterwheel working electrode setup to sample the surface of the working electrode during electrochemical analysis. This study clearly shows that DESI-based electrochemical MS is capable of capturing electrochemically generated intermediates with half-lives on the order of microseconds, which is 4-5 orders of magnitude faster than previously reported electrochemical mass spectrometry techniques. PMID:26352029

  13. Formation and Stabilization of Combustion-Generated, Environmentally Persistent Radicals on Ni(II)O Supported on a Silica Surface

    PubMed Central

    Vejerano, Eric; Lomnicki, Slawomir M.; Dellinger, Barry

    2013-01-01

    Previous studies have indicated Environmentally Persistent Free Radicals (EPFRs) are formed when hydroxyl- and chlorine-substituted aromatics chemisorbed on Cu(II)O and Fe(III)2O3 surfaces and were stabilized through their interactions with the surface metal cation. The current study reports our laboratory investigation on the formation and stabilization of EPFRs on an Ni(II)O surface. The EPFRs were produced by the chemisorption of adsorbates on the supported metal oxide surface and transfer of an electron from the adsorbate to the metal center, resulting in reduction of the metal cation. Depending on the temperature and the nature of the adsorbate, more than one type of organic radical was formed. A phenoxyl-type radical, with g-value between 2.0029 and 2.0044, and a semiquinone-type radical, with g-value from 2.0050 to as high as 2.0081, were observed. The half-lives on Ni(II)O were long and ranged from 1.5 to 5.2 days, which were similar to what were observed on Fe(III)2O3,. The yields of the EPFRs formed on Ni(II)O was ~ 8x higher than on Cu(II)O and ~50x higher than on Fe(III)2O3. PMID:22831558

  14. Radical formation, chemical processing, and explosion of interstellar grains

    NASA Technical Reports Server (NTRS)

    Greenberg, J. M.

    1976-01-01

    The ultraviolet radiation in interstellar space is shown to create a sufficient steady-state density of free radicals in the grain mantle material consisting of oxygen, carbon, nitrogen, and hydrogen to satisfy the critical condition for initiation of chain reactions. The criterion for minimum critical particle size for maintaining the chain reaction is of the order of the larger grain sizes in a distribution satisfying the average extinction and polarization measures. The triggering of the explosion of interstellar grains leading to the ejection of complex interstellar molecules is shown to be most probable where the grains are largest and where radiation is suddenly introduced; i.e., in regions of new star formation. Similar conditions prevail at the boundaries between very dark clouds and H II regions. When the energy released by the chemical activity of the free radicals is inadequate to explode the grain, the resulting mantle material must consist of extremely large organic molecules which are much more resistant to the hostile environment of H II regions than the classical dirty-ice mantles made up of water, methane, and ammonia.

  15. Positive exchange interaction in the radical ion pair of benzophenone anion and 1,4-diazabicyclo[2,2,2]octane cation radicals studied by FT-EPR spectroscopy

    NASA Astrophysics Data System (ADS)

    Sekiguchi, Shinji; Akiyama, Kimio; Tero-Kubota, Shozo

    1996-12-01

    Electron spin polarization generated from the photoreduction of benzophenone (BP) and its derivatives in the presence of 1,4-diazabicyclo[2,2,2]octane (DABCO) was studied in various solvents. The DABCO cation radical obtained showed a CIDEP spectrum with A/E (absorption/emission) polarization by RPM, while other neutral radicals gaee an E/A pattern. Using triplet quenchers, it was confirmed that these RPM signals were generated through the triplet reaction process. The present results revealed that the radical ion pair including the BP anion and DABCO +. cation radicals has a positive J, while the neutral radical pairs generated under the same condition have a negative J. The sign of J is independent of the polarity of the organic solvents used.

  16. Electrochemical Behavior of meso-Substituted Porphyrins: The Role of Cation Radicals to the Half-Wave Oxidation Potential Splitting.

    PubMed

    Tran, Thai T H; Chang, Yan-Ru; Hoang, Tuan K A; Kuo, Ming-Yu; Su, Yuhlong O

    2016-07-21

    In this study, the electrochemical behavior of free base and zinc meso-substituted porphyrins is examined by cyclic voltammetry (CV) and density functional theory (DFT). The results show that the half-wave oxidation potential splitting of the two oxidation states (ΔE= second E1/2 - first E1/2) of tetraphenylporphyrin (H2TPP) and its zinc complex (ZnTPP) are higher than those of porphyrins and their zinc complexes with meso-substituted five-membered heterocylic rings. The ΔE values follow the trend of TPP > T(3'-thienyl)P > T(3'-furyl)P > T(2'-thienyl)P for both meso-porphyrins and their respective zinc complexes. By employing DFT calculations, we have found that the trend of ΔE values is consistent with that of highest spin density (HSD) distribution and HOMO-LUMO energy gaps of cationic radicals as well as the π-conjugation between central porphyrin and meso-substituted rings. Also, they exhibit the better resonance between the porphyrin ring with meso-substituted rings as moving from porphyrins and their zinc complexes with phenyl rings to five-membered heterocyclic rings. A good agreement between calculated and experimental results indicates that cationic radicals, especially their spin density distribution, do play an important role in half-wave oxidation potential splitting of meso-porphyrins and their zinc complexes. PMID:27379447

  17. Key Role of End-Capping Groups in Optoelectronic Properties of Poly-p-phenylene Cation Radicals

    PubMed Central

    2015-01-01

    Poly-p-phenylenes (PPs) are prototype systems for understanding the charge transport in π-conjugated polymers. In a combined computational and experimental study, we demonstrate that the smooth evolution of redox and optoelectronic properties of PP cation radicals toward the polymeric limit can be significantly altered by electron-donating iso-alkyl and iso-alkoxy end-capping groups. A multiparabolic model (MPM) developed and validated here rationalizes this unexpected effect by interplay of the two modes of hole stabilization: due to the framework of equivalent p-phenylene units and due to the electron-donating end-capping groups. A symmetric, bell-shaped hole in unsubstituted PPs becomes either slightly skewed and shifted toward an end of the molecule in iso-alkyl-capped PPs or highly deformed and concentrated on a terminal unit in PPs with strongly electron-donating iso-alkoxy capping groups. The MPM shows that the observed linear 1/n evolution of the PP cation radical properties toward the polymer limit originates from the hole stabilization due to the growing chain of p-phenylene units, while shifting of the hole toward electron-donating end-capping groups leads to early breakdown of these 1/n dependencies. These insights, along with the readily applicable and flexible multistate parabolic model, can guide studies of complex donor–spacer–acceptor systems and doped molecular wires to aid the design of the next generation materials for long-range charge transport and photovoltaic applications. PMID:25264475

  18. Unconventional hydrogen bonding to organic ions in the gas phase: Stepwise association of hydrogen cyanide with the pyridine and pyrimidine radical cations and protonated pyridine

    SciTech Connect

    Hamid, Ahmed M.; El-Shall, M. Samy; Hilal, Rifaat; Elroby, Shaaban; Aziz, Saadullah G.

    2014-08-07

    Equilibrium thermochemical measurements using the ion mobility drift cell technique have been utilized to investigate the binding energies and entropy changes for the stepwise association of HCN molecules with the pyridine and pyrimidine radical cations forming the C{sub 5}H{sub 5}N{sup +·}(HCN){sub n} and C{sub 4}H{sub 4}N{sub 2}{sup +·}(HCN){sub n} clusters, respectively, with n = 1–4. For comparison, the binding of 1–4 HCN molecules to the protonated pyridine C{sub 5}H{sub 5}NH{sup +}(HCN){sub n} has also been investigated. The binding energies of HCN to the pyridine and pyrimidine radical cations are nearly equal (11.4 and 12.0 kcal/mol, respectively) but weaker than the HCN binding to the protonated pyridine (14.0 kcal/mol). The pyridine and pyrimidine radical cations form unconventional carbon-based ionic hydrogen bonds with HCN (CH{sup δ+}⋯NCH). Protonated pyridine forms a stronger ionic hydrogen bond with HCN (NH{sup +}⋯NCH) which can be extended to a linear chain with the clustering of additional HCN molecules (NH{sup +}⋯NCH··NCH⋯NCH) leading to a rapid decrease in the bond strength as the length of the chain increases. The lowest energy structures of the pyridine and pyrimidine radical cation clusters containing 3-4 HCN molecules show a strong tendency for the internal solvation of the radical cation by the HCN molecules where bifurcated structures involving multiple hydrogen bonding sites with the ring hydrogen atoms are formed. The unconventional H-bonds (CH{sup δ+}⋯NCH) formed between the pyridine or the pyrimidine radical cations and HCN molecules (11–12 kcal/mol) are stronger than the similar (CH{sup δ+}⋯NCH) bonds formed between the benzene radical cation and HCN molecules (9 kcal/mol) indicating that the CH{sup δ+} centers in the pyridine and pyrimidine radical cations have more effective charges than in the benzene radical cation.

  19. Structural Basis for Glycyl Radical Formation By Pyruvate Formate-Lyase Activating Enzyme

    SciTech Connect

    Vey, J.L.; Yang, J.; Li, M.; Broderick, W.E.; Broderick, J.B.; Drennan, C.L.

    2009-05-26

    Pyruvate formate-lyase activating enzyme generates a stable and catalytically essential glycyl radical on G{sup 734} of pyruvate formate-lyase via the direct, stereospecific abstraction of a hydrogen atom from pyruvate formate-lyase. The activase performs this remarkable feat by using an iron-sulfur cluster and S-adenosylmethionine (AdoMet), thus placing it among the AdoMet radical superfamily of enzymes. We report here structures of the substrate-free and substrate-bound forms of pyruvate formate-lyase-activating enzyme, the first structures of an AdoMet radical activase. To obtain the substrate-bound structure, we have used a peptide substrate, the 7-mer RVSGYAV, which contains the sequence surrounding G{sup 734}. Our structures provide fundamental insights into the interactions between the activase and the G{sup 734} loop of pyruvate formate-lyase and provide a structural basis for direct and stereospecific H atom abstraction from the buried G{sup 734}4 of pyruvate formate-lyase.

  20. Electronic and vibrational spectra of matrix isolated anthracene radical cations - Experimental and theoretical aspects

    NASA Technical Reports Server (NTRS)

    Szczepanski, Jan; Vala, Martin; Talbi, Dahbia; Parisel, Olivier; Ellinger, Yves

    1993-01-01

    The IR vibrational and visible/UV electronic absorption spectra of the anthracene cation, An(+), were studied experimentally, in argon matrices at 12 K, as well as theoretically, using ab initio calculations for the vibrational modes and enhanced semiempirical methods with configuration interaction for the electronic spectra. It was found that both approaches predicted well the observed photoelectron spectrum. The theoretical IR intensities showed some remarkable differences between neutral and ionized species (for example, the CH in-plane bending modes and CC in-plane stretching vibrations were predicted to increase by several orders of magnitude upon ionization). Likewise, estimated experimental IR intensities showed a significant increase in the cation band intensities over the neutrals. The implication of these findings for the hypothesis that polycyclic aromatic hydrocarbon cations are responsible for the unidentified IR emission bands from interstellar space is discussed.

  1. Theoretical study of electronically excited radical cations of naphthalene and anthracene as archetypal models for astrophysical observations. Part I. Static aspects.

    PubMed

    Ghanta, S; Reddy, V Sivaranjana; Mahapatra, S

    2011-08-28

    Motivated by the recent discovery of new diffuse interstellar bands and results from laboratory experiments, ab initio quantum chemistry calculations are carried out for the lowest six electronic states of naphthalene and anthracene radical cations. The calculated adiabatic electronic energies are utilized to construct suitable diabatic electronic Hamiltonians in order to perform nuclear dynamics studies in Part II. Complex entanglement of the electronic states is established for both the radical cations and the coupling surfaces among them are also derived in accordance with the symmetry selection rules. Critical examination of the coupling parameters of the Hamiltonian suggests that 29 (out of 48) and 31 (out of 66) vibrational modes are relevant in the nuclear dynamics in the six lowest electronic states of naphthalene and anthracene radical cations, respectively. PMID:21750790

  2. N-substituted phenothiazine derivatives: how the stability of the neutral and radical cation forms affects overcharge performance in lithium-ion batteries.

    PubMed

    Narayana, Kishore Anand; Casselman, Matthew D; Elliott, Corrine F; Ergun, Selin; Parkin, Sean R; Risko, Chad; Odom, Susan A

    2015-04-27

    Phenothiazine and five N-substituted derivatives were evaluated as electrolyte additives for overcharge protection in LiFePO4 /synthetic graphite lithium-ion batteries. We report on the stability and reactivity of both the neutral and radical-cation forms of these six compounds. While three of the compounds show extensive overcharge protection, the remaining three last for only one to a few cycles. UV/Vis studies of redox shuttle stability in the radical cation form are consistent with the overcharge performance: redox shuttles with spectra that show little change over time exhibit extensive overcharge performance, whereas those with changing spectra have limited overcharge protection. In one case, we determined that a C-N bond cleaves upon oxidation, forming the phenothiazine radical cation and leading to premature overcharge protection failure; in another case, poor solubility appears to limit protection. PMID:25504135

  3. Xe-bearing hydrocarbon ions: Observation of Xe.acetylene+rad and Xe.benzene+rad radical cations and calculations of their ground state structures

    NASA Astrophysics Data System (ADS)

    Cui, Zhong-hua; Attah, Isaac K.; Platt, Sean P.; Aziz, Saadullah G.; Kertesz, Miklos; El-Shall, M. S.

    2016-04-01

    This work reports evidence for novel types of Xe-bearing hydrocarbon radical cations. The Xe.acetylene+rad radical cation adduct is observed at nearly room temperature using the mass-selected drift cell technique. The irreversible addition of the Xe atom and the lack of back dissociation to HCCH+rad + Xe is consistent with the calculated binding energy of 0.85 eV to be contrasted with the metastable nature of the neutral Xe.acetylene adduct. The observed Xe.benzene+rad radical cation appears to be a weakly bound complex stabilized mainly by ion-induced dipole interaction consistent with a calculated binding energy in the range of 0.14-0.17 eV.

  4. Ne matrix spectra of the sym-C6Br3F3+ radical cation

    USGS Publications Warehouse

    Bondybey, V.E.; Sears, T.J.; Miller, T.A.; Vaughn, C.; English, J.H.; Shiley, R.S.

    1981-01-01

    The electronic absorption and laser excited, wavelength resolved fluorescence spectra of the title cation have been observed in solid Ne matrix and vibrationally analysed. The vibrational structure of the excited B2A2??? state shows close similarity to the parent compound. The X2E??? ground state structure is strongly perturbed and irregular owing to a large Jahn-Teller distortion. The data are analysed in terms of a recently developed, sophisticated multimode Jahn-Teller theoretical model. We have generated the sym-C6Br3F3+ cations in solid Ne matrix and obtained their wavelength resolved emission and absorption spectra. T ground electronic X2E??? state exhibits an irregular and strongly perturbed vibrational structure, which can be successfully modeled using sophisticated multimode Jahn-Teller theory. ?? 1981.

  5. Interfacial/free radical polymerization microencapsulation: kinetics of particle formation.

    PubMed

    Mahabadi, H K; Ng, T H; Tan, H S

    1996-01-01

    Microcapsules containing pigment and polymer were prepared by dispersing a viscous mixture of pigment, core monomers, initiators and oil-soluble shell monomer in an aqueous solution of surfactants, forming oil-in-water droplets. Subsequently, a water-soluble shell monomer was added to these droplets, encapsulating them via interfacial (IF) polycondensation. These microcapsules were then heated for free radical (FR) polymerization of the core monomers. Effects of primary variables, such as the shearing time during particle formation, surfactant concentration, organic phase concentration, and mode of water-soluble shell monomer addition, were studied. The results indicated that polyvinylalcohol (PVOH), used as the surfactant/stabilizer, reacted with the oil-soluble shell monomers. The depletion of PVOH, especially when PVOH concentration was low, resulted in rapid growth of particle size and, eventually, suspension failure. The kinetic data revealed a particle formation mechanism which consists of two processes. The first process is the formation of an equilibrium particle size by the equilibrium process of particle breakage due to the mechanical shearing force and coalescence due to collisions among particles and surface tension forces. The second process is the reaction between PVOH and oil-soluble shell monomer which leads to the depletion of PVOH and consequently causes more coalescence of particles and a significant increase in the equilibrium particle size. The net effect of these two processes shows an optimum shearing time where the smallest particle size can be attained, and this optimum time is a function of several primary variables. Methods to prevent the reaction and therefore the depletion of PVOH are proposed. PMID:8864993

  6. Theoretical study of the electronically excited radical cations of naphthalene and anthracene as archetypal models for astrophysical observations. Part II. Dynamics consequences.

    PubMed

    Ghanta, S; Reddy, V Sivaranjana; Mahapatra, S

    2011-08-28

    Nuclear dynamics is investigated theoretically from first principles by employing the ab initio vibronic models of the prototypical naphthalene and anthracene radical cations developed in Part I. This Part is primarily aimed at corroborating a large amount of available experimental data with a specific final goal to establish an unambiguous link with the current observations in astrophysics and astronomy. The detailed analyses presented here perhaps establish that these two prototypical polycyclic aromatic hydrocarbon radical cations are indeed potential carriers of the observed diffuse interstellar bands. PMID:21750791

  7. Synthesis of a calix[4]arene derivative for isolation of a stable cation radical salt for use as a colorimetric sensor of nitric oxide.

    PubMed

    Rathore, Rajendra; Abdelwahed, Sameh H; Guzei, Ilia A

    2004-10-27

    We have designed and synthesized a modified calixarene derivative (1) that allows, for the first time, the isolation of a stable cation radical salt that binds a single molecule of nitric oxide deep within its cavity with remarkable efficiency (KNO >108 M-1), as demonstrated by isolation of a crystalline complex [1, NO]+ and its characterization by X-ray crystallography as well as by optical spectroscopy. Furthermore, the ready accessibility of the calixarene cation radical will allow the exploration of its use for developing efficient sensing devices for nitric oxide based on the accompanied color changes. PMID:15493892

  8. Mechano-chemical radical formation and polymerization initiation during wet grinding of alumina.

    PubMed

    Damm, C; Peukert, W

    2011-11-01

    The formation of free radicals during wet grinding of alumina in a stirred media mill was studied by using the test radical 2,2-diphenyl-1-picrylhydracyl (DPPH). The kinetics of mechano-chemical radical formation follows a zeroth-order rate law. Particle breakage as well as mechanical activation of the surface of the alumina particles contributes to the radical formation. The rate constants of the radical formation due to mechanical activation of the particle surface k(A) and due to particle breakage k(B) depend on the milling process parameters. The radical formation during wet grinding of alumina was exploited to initiate mechano-chemical polymerization reactions of acrylic acid and acryl amide, respectively. In this way nanoparticles functionalized with polyacrylic acid and polyacryl amide, respectively, are obtained. The influence of the milling process parameters on the kinetics of mechano-chemical radical formation and on the grafted amount of polymer is discussed on the basis of stress energy and number of stress events in the mill. A correlation between the grafted amount of polyacryl amide on the alumina particles and the total radical formation rate was found showing that the concentration of mechano-chemically formed free radicals governs the efficacy of a chemical reaction at activated particle surfaces. PMID:21820123

  9. Formation of bromate in sulfate radical based oxidation: mechanistic aspects and suppression by dissolved organic matter.

    PubMed

    Lutze, Holger V; Bakkour, Rani; Kerlin, Nils; von Sonntag, Clemens; Schmidt, Torsten C

    2014-04-15

    Sulfate radical based oxidation is discussed being a potential alternative to hydroxyl radical based oxidation for pollutant control in water treatment. However, formation of undesired by-products, has hardly been addressed in the current literature, which is an issue in other oxidative processes such as bromate formation in ozonation of bromide containing water (US-EPA and EU drinking water standard of bromate: 10 μg L(-1)). Sulfate radicals react fast with bromide (k = 3.5 × 10(9) M(-1) s(-1)) which could also yield bromate as final product. The mechanism of bromate formation in aqueous solution in presence of sulfate radicals has been investigated in the present paper. Further experiments were performed in presence of humic acids and in surface water for investigating the relevance of bromate formation in context of pollutant control. The formation of bromate by sulfate radicals resembles the well described mechanism of the hydroxyl radical based bromate formation. In both cases hypobromous acid is a requisite intermediate. In presence of organic matter formation of bromate is effectively suppressed. That can be explained by formation of superoxide formed in the reaction of sulfate radicals plus aromatic moieties of organic matter, since superoxide reduces hypobromous acid yielding bromine atoms and bromide. Hence formation of bromate can be neglected in sulfate radical based oxidation at typical conditions of water treatment. PMID:24565691

  10. Vibrational studies of reactive intermediates of aromatic amines. IV. Radical cation time-resolved resonance Raman investigation of N, N-dimethylaniline and N, N-diethylaniline derivatives

    NASA Astrophysics Data System (ADS)

    Poizat, O.; Guichard, V.; Buntinx, G.

    1989-05-01

    The radical cation time-resolved resonance Raman spectra of various isotopic derivatives of N, N-dimethylaniline (DMA), N, N-diethylaniline (DEA), N, N-dimethyl-p-toluidine (4MDMA) and 3, 5, N, N-tetramethylaniline (3,5DMDMA) are reported in the 300-1800 cm-1 range. Excitation was in the weak radical cation absorption around 480 nm. Complete vibrational assignments are proposed. The band activity and the changes in frequency with respect to the neutral molecules are consistent with a quinoidal-type conformation of the framework close to planarity. Stabilization of this conformation is observed when the phenyl ring contains methyl substituents. The analysis of the Raman enhancements suggests that the quinoidal character of the radical structure is significantly lowered in the resonant excited state. An obvious analogy is found between the spectra of DMA+ ṡ and of the biphenyl radical cation, which clearly indicates that (i) a nearly common chromophore structure characterizes these two radical cations and (ii) the distortion of this chromophore structure in the resonant excited state is comparable in both compounds, i.e., the biphenyl+ ṡ* ←biphenyl+ ṡ and DMA+ ṡ* ←DMA+ ṡ transitions are of similar nature. These results are consistent with structural previsions from simple molecular orbital considerations and a comprehensive interpretation of the Raman spectra is given in terms of HOMO population.

  11. Comparison of the Reactivity of the Three Distonic Isomers of the Pyridine Radical Cation Toward Tetrahydrofuran in Solution and in the Gas Phase

    NASA Astrophysics Data System (ADS)

    Widjaja, Fanny; Jin, Zhicheng; Nash, John J.; Kenttämaa, Hilkka I.

    2013-04-01

    The reactivity of the three distonic isomers of the pyridine radical cation toward tetrahydrofuran is compared in solution and in the gas phase. In solution, the distonic ions were generated by UV photolysis at 300 nm from iodo-precursors in acidic 50:50 tetrahydrofuran/water solutions. In the gas phase, the ions were generated by collisionally activated dissociation (CAD) of protonated iodo-precursors in an FT-ICR mass spectrometer, as described in the literature. The same major reaction, hydrogen atom abstraction, was observed in solution and in the gas phase. Attempts to cleave the iodine atom from the 2-iodopyridinium cation in the gas phase and in solution yielded the 2-pyridyl cation in addition to the desired 2-dehydropyridinium cation. In the gas phase, this ion was ejected prior to the examination of the desired ion's chemical properties. This was not possible in solution. This study suggests that solvation effects are not significant for radical reactions of charged radicals. On the other hand, the even-electron ion studied, the 2-pyridyl cation, shows substantial solvation effects. For example, in solution, the 2-pyridyl cation forms a stable adduct with tetrahydrofuran, whereas in the gas phase, only addition/elimination reactions were observed.

  12. Umbrella motion of the methyl cation, radical, and anion molecules. I. Potentials, energy levels and partition functions

    NASA Astrophysics Data System (ADS)

    Ragni, Mirco; Bitencourt, Ana Carla P.; Prudente, Frederico V.; Barreto, Patricia R. P.; Posati, Tamara

    2016-03-01

    A study of the umbrella motion of the methyl cation, radical, and anion molecules is presented. This is the floppiest mode of vibration of all three species and its characterization is of fundamental importance for understanding their reactivity. Minimum Energy Paths of the umbrella motions according to the hyperspherical treatment were obtained, by single point calculations, at the CCSD(T)/aug-cc-pVQT level of theory in the Born-Oppenheimer approximation. These energy profiles permit us to calculate the vibrational levels through the Hyperquantization algorithm, which is shown appropriated for the description of the umbrella motion of these three molecules. The adiabatic electron affinity and ionization potentials were estimated to good accuracy. Partition functions are also calculated in order to obtain information on the reaction rates involving these groups.

  13. Formaldehyde mediated proton-transport catalysis in the ketene-water radical cation CH2C(O)OH2+

    NASA Astrophysics Data System (ADS)

    Lee, Richard; Ruttink, Paul J. A.; Burgers, Peter C.; Terlouw, Johan K.

    2006-09-01

    Previous studies have shown that the solitary ketene-water ion CH2C(O)OH2+ (1) does not isomerize into CH2C(OH)2+ (2), its more stable hydrogen shift isomer. Tandem mass spectrometry based collision experiments reveal that this isomerization does take place in the CH2O loss from low-energy 1,3-dihydroxyacetone ions (HOCH2)2CO+. A mechanistic analysis using the CBS-QB3 model chemistry shows that such molecular ions rearrange into hydrogen-bridged radical cations [CH2C(O)O(H)-H...OCH2]+ in which the CH2O molecule catalyzes the transformation 1 --> 2 prior to dissociation. The barrier for the unassisted reaction, 29 kcal mol-1, is reduced to a mere 0.6 kcal mol-1 for the catalysed transformation. Formaldehyde is an efficient catalyst because its proton affinity meets the criterion for facile proton-transport catalysis.

  14. (Bi)sulfite Oxidation by Copper,Zinc-Superoxide Dismutase: Sulfite-Derived, Radical-Initiated Protein Radical Formation

    PubMed Central

    Ranguelova, Kalina; Bonini, Marcelo G.; Mason, Ronald P.

    2010-01-01

    Background Sulfur dioxide, formed during the combustion of fossil fuels, is a major air pollutant near large cities. Its two ionized forms in aqueous solution, sulfite and (bi)sulfite, are widely used as preservatives and antioxidants to prevent food and beverage spoilage. (Bi)sulfite can be oxidized by peroxidases to form the very reactive sulfur trioxide anion radical (•SO3−). This free radical further reacts with oxygen to form the peroxymonosulfate anion radical (−O3SOO•) and sulfate anion radical (SO4• −). Objective To explore the critical role of these radical intermediates in further oxidizing biomolecules, we examined the ability of copper,zinc-superoxide dismutase (Cu,Zn-SOD) to initiate this radical chain reaction, using human serum albumin (HSA) as a model target. Methods We used electron paramagnetic resonance, optical spectroscopy, oxygen uptake, and immuno-spin trapping to study the protein oxidations driven by sulfite-derived radicals. Results We found that when Cu,Zn-SOD reacted with (bi)sulfite, •SO3− was produced, with the concomitant reduction of SOD-Cu(II) to SOD-Cu(I). Further, we demonstrated that sulfite oxidation mediated by Cu,Zn-SOD induced the formation of radical-derived 5,5-dimethyl-1-pyrroline N-oxide (DMPO) spin-trapped HSA radicals. Conclusions The present study suggests that protein oxidative damage resulting from (bi)sulfite oxidation promoted by Cu,Zn-SOD could be involved in oxidative damage and tissue injury in (bi)sulfite-exacerbated allergic reactions. PMID:20348042

  15. Fluorescence of the perylene radical cation and an inaccessible D0/D1 conical intersection: An MMVB, RASSCF, and TD-DFT computational study

    NASA Astrophysics Data System (ADS)

    Tokmachev, Andrei M.; Boggio-Pasqua, Martial; Mendive-Tapia, David; Bearpark, Michael J.; Robb, Michael A.

    2010-01-01

    The photophysics of the perylene radical cation (Pe•+) was studied using the molecular mechanics-valence bond (MMVB) hybrid force field. Potential energy surfaces of the first three electronic states were investigated. Geometry optimizations of critical points—including conical intersections between the relevant electronic states—were performed using the MMVB analytical energy gradient for cations. No accessible planar conical intersection between the D0 and D1 states of Pe•+ was found; this is consistent with the experimentally observed D1 lifetimes and the observation of D1 emission from this cation in the condensed phase. Benchmark RASSCF and TD-DFT calculations support the reliability of the MMVB results.

  16. Cationic cluster formation versus disproportionation of low-valent indium and gallium complexes of 2,2'-bipyridine

    PubMed Central

    Lichtenthaler, Martin R.; Stahl, Florian; Kratzert, Daniel; Heidinger, Lorenz; Schleicher, Erik; Hamann, Julian; Himmel, Daniel; Weber, Stefan; Krossing, Ingo

    2015-01-01

    Group 13 MI compounds often disproportionate into M0 and MIII. Here, however, we show that the reaction of the MI salt of the weakly coordinating alkoxyaluminate [GaI(C6H5F)2]+[Al(ORF)4]− (RF=C(CF3)3) with 2,2'-bipyridine (bipy) yields the paramagnetic and distorted octahedral [Ga(bipy)3]2+•{[Al(ORF)4]−}2 complex salt. While the latter appears to be a GaII compound, both, EPR and DFT investigations assign a ligand-centred [GaIII{(bipy)3}•]2+ radical dication. Surprisingly, the application of the heavier homologue [InI(C6H5F)2]+[Al(ORF)4]− leads to aggregation and formation of the homonuclear cationic triangular and rhombic [In3(bipy)6]3+, [In3(bipy)5]3+ and [In4(bipy)6]4+ metal atom clusters. Typically, such clusters are formed under strongly reductive conditions. Analysing the unexpected redox-neutral cationic cluster formation, DFT studies suggest a stepwise formation of the clusters, possibly via their triplet state and further investigations attribute the overall driving force of the reactions to the strong In−In bonds and the high lattice enthalpies of the resultant ligand stabilized [M3]3+{[Al(ORF)4]−}3 and [M4]4+{[Al(ORF)4]−}4 salts. PMID:26478464

  17. A 1,2,3-dithiazolyl-o-naphthoquinone: a neutral radical with isolable cation and anion oxidation states.

    PubMed

    Smithson, Chad S; MacDonald, Daniel J; Matt Letvenuk, T; Carello, Christian E; Jennings, Michael; Lough, Alan J; Britten, James; Decken, Andreas; Preuss, Kathryn E

    2016-06-21

    Under aprotic conditions, the reaction of 4-amino-1,2-naphthoquinone with excess S2Cl2 generates 4,5-dioxo-naphtho[1,2-d][1,2,3]dithiazol-2-ium chloride in a typical Herz condensation. By contrast, prior literature reports an imine (NH) product, 4,5-dioxo-1H-naphtho[1,2-d][1,2,3]dithiazole, for the same reaction performed in acetic acid. Herein, the cation product is isolated with four different counter-anions (Cl(-), GaCl4(-), FeCl4(-) and OTf(-)). Reduction of the cation generates a neutral radical 1,2,3-dithiazolyl-o-naphthoquinone, with potential ligand properties. Further reduction generates a closed shell anion, isolated as a water-stable Li(+) complex and exhibiting O,O-bidentate chelation. The hydroxy (OH) isomer of the original imine (NH) product is reported, and this can be readily deprotonated and acylated (OAc). All species are structurally characterized. Solution redox behaviour and EPR are discussed where appropriate. PMID:27216412

  18. Vinylogous tetrathiafulvalene (TTF) {pi}-electron donors and derived radical cations: ESR spectroscopic, magnetic, and X-ray structural studies

    SciTech Connect

    Bryce, M.R.; Moore, A.J.; Tanner, B.K.

    1996-06-01

    The properties of new 2,2`-ethanediylidene(1,3-diethile) derivatives 5, 6 and 8-11 are reported. Cyclic voltammetric studies establish that they are efficient donor molecules, with the extended conjugation resulting in stabilization of dications, relative to tetrathiafulvalene TTF (1). Radical cations are generated by oxidation of the neutral compounds with trifluoroacetic acid or anhydrous silver perchlorate in dichloromethane, and their ESR and proton ENDOR spectra are reported. The bulk of the spin population resides in the central S{sub 2} {double_bond}C-C{double_bond}CS{sub 2} part of the {pi}-system. The X-ray crystal structure of donor 6 reveals that the 2,2`-ethanediylidene(1,3-dithiole) framework is planar. Donor 6 forms a crystalline 1:1 charge-transfer complex with TCNQ, the X-ray crystal structure of which shows a mixed stack structure. A solution of this complex in acetonitrile exhibits ESR spectra of both radical ions, 6{sup {lg_bullet}}{sup +} and TCNQ{sup {lg_bullet}}{sup +}. Static susceptibility data are reported for TCNQ complexes of some of these donors. 20 refs., 9 figs., 7 tabs.

  19. 15N electron nuclear double resonance of the primary donor cation radical P+.865 in reaction centers of Rhodopseudomonas sphaeroides: additional evidence for the dimer model.

    PubMed Central

    Lubitz, W; Isaacson, R A; Abresch, E C; Feher, G

    1984-01-01

    Four 15N hyperfine coupling constants, including signs, have been measured by electron nuclear double resonance (ENDOR) and electron nuclear nuclear triple resonance (TRIPLE) for the bacteriochlorophyll a radical cation, BChla+., in vitro and for the light-induced primary donor radical cation, P+.865, in reaction centers of Rhodopseudomonas sphaeroides R-26. A comparison of the data shows that the hyperfine coupling constants have the same sign in both radicals and are, on the average, smaller by a factor of 2 in P+.865. These results provide additional evidence that P+.865 is a bacteriochlorophyll dimer and are in contradiction with the monomer structure of P+.865 recently proposed by O'Malley and Babcock. The reduction factors of the individual 15N couplings, together with the evidence from proton ENDOR data and molecular orbital calculations, indicate a dimer structure in which only two rings (either I and I or III and III) of the bacteriochlorophyll macrocycles overlap. PMID:6096857

  20. Cationic Pillararenes Potently Inhibit Biofilm Formation without Affecting Bacterial Growth and Viability.

    PubMed

    Joseph, Roymon; Naugolny, Alissa; Feldman, Mark; Herzog, Ido M; Fridman, Micha; Cohen, Yoram

    2016-01-27

    It is estimated that up to 80% of bacterial infections are accompanied by biofilm formation. Since bacteria in biofilms are less susceptible to antibiotics than are bacteria in the planktonic state, biofilm-associated infections pose a major health threat, and there is a pressing need for antibiofilm agents. Here we report that water-soluble cationic pillararenes differing in the quaternary ammonium groups efficiently inhibited the formation of biofilms by clinically important Gram-positive pathogens. Biofilm inhibition did not result from antimicrobial activity; thus, the compounds should not inhibit growth of natural bacterial flora. Moreover, none of the cationic pillararenes caused detectable membrane damage to red blood cells or toxicity to human cells in culture. The results indicate that cationic pillararenes have potential for use in medical applications in which biofilm formation is a problem. PMID:26745311

  1. Complex formation of alkaline-earth cations with crown ethers and cryptands in methanol solutions

    SciTech Connect

    Buschman, H.J.

    1986-06-01

    The complexation of alkaline-earth cations by different crown ethers, azacrown ethers, and cryptands has been studied in methanol solutions by means of calorimetric and potentiometric titrations. The smallest monocyclic ligands examined from 2:1 complexes (ratio of ligand to cation) with cations which are too large to fit into the ligand cavity. With the smallest cryptand, only Sr/sup 2 +/ and Ba/sup 2 +/ ions are able to form exclusive complexes. In the case of the reaction of cryptand (211) with Ca/sup 2 +/, a separate estimation of stability constants for the formation of exclusive and inclusive complexes was possible for the first time. Higher values for stability constants are found for the reaction of alkaline-earth cations with cryptands compared to the reaction with alkali ions. This increase is only caused by favorable entropic contributions.

  2. Time-resolved fluorescence spectroscopic investigation of cationic polymer/DNA complex formation

    NASA Astrophysics Data System (ADS)

    D'Andrea, Cosimo; Bassi, Andrea; Taroni, Paola; Pezzoli, Daniele; Volonterio, Alessandro; Candiani, Gabriele

    2011-07-01

    Since DNA is not internalized efficiently by cells, the success of gene therapy depends on the availability of carriers to efficiently deliver genetic material into target cells. Gene delivery vectors can be broadly categorized into viral and non-viral ones. Non-viral gene delivery systems are represented by cationic lipids and polymers rely on the basics of supramolecular chemistry termed "self-assembling": at physiological pH, they are cations and spontaneously form lipoplexes (for lipids) and polyplexes (for polymers) complexing nucleic acids. In this scenario, cationic polymers are commonly used as non-viral vehicles. Their effectiveness is strongly related to key parameters including DNA binding ability and stability in different environments. Time-resolved fluorescence spectroscopy of SYBR Green I (DNA dye) was carried out to characterize cationic polymer/DNA complex (polyplex) formation dispersed in aqueous solution. Both fluorescence amplitude and lifetime proved to be very sensitive to the polymer/DNA ratio (N/P ratio, +/-).

  3. C---lH...O and O...H...O bonded intermediates in the dissociation of low energy methyl glycolate radical cations

    NASA Astrophysics Data System (ADS)

    Suh, Dennis; Kingsmill, Carol A.; Ruttink, Paul J. A.; Burgers, Peter C.; Terlouw, Johan K.

    1995-08-01

    Low energy methyl glycolate radical cations HOCH2C(=O)OCH3+, 1, abundantly lose HCO, yielding protonated methyl formate H---C(OH)OCH3+. Tandem mass spectrometry based experiments on 2H, 13C and 18O labelled isotopologues show that this loss is largely (about 75%) atom specific. Analysis of the atom connectivity in the product ions indicates that the reaction proceeds analogously to the loss of HCO and CH3CO from ionized acetol HOCH2C(=O)CH3+ and acetoin HOCH(CH3)C(=O)CH3+, respectively. The mechanism, it is proposed, involves isomerization of 1 to the key intermediate CH2=O... H---C(=O)OCH3+, an H-bridged ion-dipole complex of neutral formaldehyde and ionized methyl formate. Next, charge transfer takes place to produce CH3OC(H)=O...HC(H)=O+, an H-bridged ion-dipole complex of ionized formaldehyde and neutral methyl formate, followed by proton transfer to generate the products. Preliminary ab initio calculations executed at the UMP3/6-31G*//6-31G*+ZPVE level of theory are presented in support of this proposal. The non-specific loss of HCO from 1 (about 25%) is rationalized to occur via the same mechanism, but after communication with isomeric dimethyl carbonate ions CH3OC(=O)OCH3+, 2, via the O...H...O bonded intermediate [CH2=O...H...O=C---OCH3]+. The latter pathway is even more important in the formation of CH2OH+ ions from 1 which, it is shown, is not a simple bond cleavage reaction, but may involve consecutive or concerted losses of CH3 and CO2 from the above O...H...O bonded species. Ionized methyl lactate HOCH(CH3)C(=O)OCH3+, the higher homologue of 1, shows a unimolecular chemistry which is akin to that of 1.

  4. Compound ES of cytochrome c peroxidase contains a Trp {pi}-cation radical. Characterization by CW and pulsed Q-band ENDOR spectroscopy

    SciTech Connect

    Huyett, J.E.; Doan, P.E.; Gurbiel, R.; Houseman, A.L.P.; Sivaraja, M.; Hoffman, B.M.; Goodin, D.B.

    1995-09-06

    The fully oxidized state of cytochrome c peroxidase (CcP), called ES, contains two oxidizing equivalents, one as an oxyferryl heme and the other as an organic radical on an amino acid residue. The unusual electron paramagnetic resonance spectrum of ES has been shown to be due to a weak distributed exchange coupling between the two paramagnetic redox centers. Various residues have been proposed as the radical site over the years. In this paper continuous wave and pulsed Q-band electron nuclear double resonance (ENDOR) spectroscopy confirms that the radical is located on Trp-191, as previously proposed. The paper completes the characterization of the active site of compound ES as being comprized of an oxyferryl heme coupled to the Trp-191 {pi}-cation radical by a weak spin exchange. 47 refs., 11 figs., 2 tabs.

  5. Theoretical study of reactivity of methane, methyl fluoride, and methyl chloride: Interaction with their radical cations and proton donors

    SciTech Connect

    Hess, B.A. Jr. ); Zahradnik, R. )

    1990-07-18

    This work deals with interactions between CH{sub 4} and CH{sub 4}{sup {sm bullet}+}, CH{sub 3}F and CH{sub 3}F{sup {sm bullet}+}, and CH{sub 3}Cl and CH{sub 3}Cl{sup {sm bullet}+}. The calculated {Delta}H{sub 0} values (MP4/6-31G**//MP2/6-31G**, ZPE included) for processes leading to CH{sub 5}{sup +} and CH{sub 3}{sup {sm bullet}}, and to CFH{sub 4} and CH{sub 2}F{sup {sm bullet}} amount to {minus}1.8 and {minus}21.0 kcal/mol, respectively. The {Delta}H{sub 0} leading to CClH{sub 4}{sup +} and CH{sub 2}Cl{sup {sm bullet}} (MP4/6-31G**//SCF/6-31G**, ZPE included) is {minus}2.3 kcal/mol. The calculated reaction heat for the first interaction is significantly closer to experimental values ({minus}4.16 and {minus}6 kcal/mol, respectively) than their previous theoretical estimates. The structures of the radical cations (CH{sub 4}{sup {sm bullet}+}, CH{sub 3}F{sup {sm bullet}+}) possess features of van der Waals associates.

  6. Dimerization of the octaethylporphyrin {pi} cation radical complex of cobalt(II): Thermodynamic, kinetic, and spectroscopic studies

    SciTech Connect

    Ni, Y.; Lee, S.; Wayland, B.B.

    1999-08-23

    One electron oxidation of cobalt(II) can occur from either the cobalt d or porphyrin {pi} orbitals depending on the choice of porphyrin and reaction media. Oxidation of (octaethylporphyrinato)cobalt(II), (OEP)Co{sup II} (1), in the presence of ligands such as H{sub 2}O and CO produces diamagnetic five and six coordinate complexes of cobalt(III). In the absence of additional ligands to coordinate with Co(III) the first oxidation of [(OEP)Co{sup II}]{sup +} (2). Metalloporphyrin {pi} cation radical complexes and dimers of the OEP derivatives have been extensively investigated. This article reports on the interconversion of the paramagnetic (S = 1) monomer, [(OEP)Co{sup II}]{sup +} (2), with a diamagnetic dimer, [(OEP)-Co{sup II}]{sub 2}{sup 2+} (3), in dichloromethane solvant. {sup 1}H NMR shift and line width studies in CD{sub 2}Cl{sub 2} are applied in evaluating the thermodynamic and activation parameters for homolytic dissociation of the diamagnetic dimer (3).

  7. Chlorophyll-quinone photochemistry in liposomes: mechanisms of radical formation and decay

    SciTech Connect

    Hurley, J.K.; Tollin, G.

    1980-01-01

    Laser flash photolysis has been used to investigate the mechanism of formation and decay of the radical species generated by light induced electron transfer from chlorophyll a triplet to quinone in egg phosphatidyl choline bilayer vesicles. Chlorophyll triplet quenching by quinone is controlled by diffusion occurring within the bilayer membrane and reflects bilayer viscosity. Radical formation via separation of the intermediate ion pair is also inhibited by increased bilayer viscosity. Cooperativity is observed in this process due to an enhancement of radical separation by electron transfer from semiquinone anion radical to a neighboring quinone molecule. Two modes of radical decay are observed, a rapid recombination occurring within the bilayer and a much slower recombination occurring across the bilayer. The slow decay is only observed with quinones which are not tightly anchored into the bilayer, and is probably the result of electron transfer from semiquinone anion radical formed within the bilayer to a quinone molecule residing at the bilayer-water interface. With benzoquinone, approximately 60% of the radical decay occurs via the slow mode. Triplet to radical conversion efficiencies in the bilayer systems are comparable to those obtained in fluid solution (approx. 60%). However, radical recombination, at least for the slow decay mechanism, is considerably retarded.

  8. Formation of environmentally persistent free radicals from the heterogeneous reaction of ozone and polycyclic aromatic compounds.

    PubMed

    Borrowman, Cuyler K; Zhou, Shouming; Burrow, Timothy E; Abbatt, Jonathan P D

    2016-01-01

    In the 1980s long-lived radical species were identified in cigarette tar. Since then, environmentally persistent free radicals (EPFRs) have been observed in ambient particulate matter, and have been generated in particulate matter generated from internal combustion engines. For the first time, we measure in situ the formation and decay of EPFRs through the heterogeneous reaction of ozone and several polycyclic aromatic compounds (PAC). Solid anthracene (ANT), pyrene (PY), benzo[a]pyrene (BAP), benzo[ghi]perylene (BGHIP), 1,4-naphthoquinone (1,4NQ), and 9,10-anthraquinone (AQ) were reacted with gas-phase ozone in a flow system placed in the active cavity of an electron paramagnetic resonance (EPR) spectrometer, and the formation of radicals was measured on the timescale of tens of minutes at ambient levels of ozone down to 30 ppb. For most substrates the net radical production is initially rapid, slows at intermediate times, and is followed by a slow decay. For oxidized solid BAP, radical signal persists for many days in the absence of ozone. To evaluate the effect of substrate phase, the solid PAHs were also dissolved in squalane, an organic oil inert to ozone, which yielded a much higher maximum radical concentration and faster radical decay when exposed to ozone. With higher mobility, reactants were apparently able to more easily diffuse and react with each other, yielding the higher radical concentrations. The EPR spectra exhibit three radicals types, two of which have been assigned to semiquinone species and one to a PAH-derived, carbon-centered radical. Although our system uses levels of PAC not typically found in the environment it is worth noting that the amounts of radical formed, on the order of 10(18) radicals per g, are comparable to those observed in ambient particulate matter. PMID:26603953

  9. Radical Formation Initiates Solvent-Dependent Unfolding and β-sheet Formation in a Model Helical Peptide.

    PubMed

    Owen, Michael C; Strodel, Birgit; Csizmadia, Imre G; Viskolcz, Béla

    2016-06-01

    We examined the effects of Cα-centered radical formation on the stability of a model helical peptide, N-Ac-KK(AL)10KK-NH2. Three, 100 ns molecular dynamics simulations using the OPLS-AA force field were carried out on each α-helical peptide in six distinct binary TIP4P water/2,2,2-trifluoroethanol (TFE) mixtures. The α-helicity was at a maximum in 20% TFE, which was inversely proportional to the number of H-bonds between water molecules and the peptide backbone. The radial distribution of TFE around the peptide backbone was highest in 20% TFE, which enhanced helix stability. The Cα-centered radical initiated the formation of a turn within 5 ns, which was a smaller kink at high TFE concentrations, and a loop at lower TFE concentrations. The highest helicity of the peptide radical was measured in 100% TFE. The formation of hydrogen bonds between the peptide backbone and water destabilized the helix, whereas the clustering of TFE molecules around the radical center stabilized the helix. Following radical termination, the once helical structure converted to a β-sheet rich state in 100% water only, and this transition did not occur in the nonradical control peptide. This study gives evidence on how the formation of peptide radicals can initiate α-helical to β-sheet transitions under oxidative stress conditions. PMID:27169334

  10. Multiphoton processes in cyclohexane and trans-decalin and the formation of high-mobility cations

    SciTech Connect

    Liu, A.; Sauer, M.C. Jr.; Trifunac, A.D. )

    1993-10-28

    The dependence of the absolute efficiencies of production of free electrons and HM[sub +] (positive ions with anomalously high mobility) on the intensity of 248- and 308-nm laser pulses has been measured for neat cyclohexane and trans-decalin and for solutions containing aromatic compounds. In the neat solvents, the yields of electrons and HM[sub +] have the same intensity dependence; for ionization of these two alkanes, two photons are required at 248 nm and three photons at 308 nm. In solutions containing aromatic solutes, where the major fraction of the light absorption is by the solute, yields of both free electron and HM[sub +] are markedly higher, and at both 248 and 308 nm the intensity dependences indicate two photons are required for ionization but that three photons are required to create HM[sub +]. This is consistent with the explanation, based on previously reported product analysis studies from this laboratory, that the aromatic solute is ionized when its excited state, created by the first photon, absorbs a second photon and the radical cation absorbs a third photon, which enables it to react with the solvent, creating HM[sub +]. Examination of previously reported results on anthracene in 2-propanol supports a similar explanation for the observed decrease in the quantum yield of the anthracene radical cation with increasing intensity. 39 refs., 12 figs., 4 tabs.

  11. Communication: Ion mobility of the radical cation dimers: (Naphthalene)2+• and naphthalene+•-benzene: Evidence for stacked sandwich and T-shape structures

    NASA Astrophysics Data System (ADS)

    Platt, Sean P.; Attah, Isaac K.; Aziz, Saadullah; El-Shall, M. Samy

    2015-05-01

    Dimer radical cations of aromatic and polycyclic aromatic molecules are good model systems for a fundamental understanding of photoconductivity and ferromagnetism in organic materials which depend on the degree of charge delocalization. The structures of the dimer radical cations are difficult to determine theoretically since the potential energy surface is often very flat with multiple shallow minima representing two major classes of isomers adopting the stacked parallel or the T-shape structure. We present experimental results, based on mass-selected ion mobility measurements, on the gas phase structures of the naphthalene+ṡ ṡ naphthalene homodimer and the naphthalene+ṡ ṡ benzene heterodimer radical cations at different temperatures. Ion mobility studies reveal a persistence of the stacked parallel structure of the naphthalene+ṡ ṡ naphthalene homodimer in the temperature range 230-300 K. On the other hand, the results reveal that the naphthalene+ṡ ṡ benzene heterodimer is able to exhibit both the stacked parallel and T-shape structural isomers depending on the experimental conditions. Exploitation of the unique structural motifs among charged homo- and heteroaromatic-aromatic interactions may lead to new opportunities for molecular design and recognition involving charged aromatic systems.

  12. Spectroscopic and Kinetic Characterization of Peroxidase-Like π-Cation Radical Pinch-Porphyrin-Iron(III) Reaction Intermediate Models of Peroxidase Enzymes.

    PubMed

    Hernández Anzaldo, Samuel; Arroyo Abad, Uriel; León García, Armando; Ramírez Rosales, Daniel; Zamorano Ulloa, Rafael; Reyes Ortega, Yasmi

    2016-01-01

    The spectroscopic and kinetic characterization of two intermediates from the H₂O₂ oxidation of three dimethyl ester [(proto), (meso), (deuteroporphyrinato) (picdien)]Fe(III) complexes ([FePPPic], [FeMPPic] and [FeDPPic], respectively) pinch-porphyrin peroxidase enzyme models, with s = 5/2 and 3/2 Fe(III) quantum mixed spin (qms) ground states is described herein. The kinetic study by UV/Vis at λmax = 465 nm showed two different types of kinetics during the oxidation process in the guaiacol test for peroxidases (1-3 + guaiacol + H₂O₂ → oxidation guaiacol products). The first intermediate was observed during the first 24 s of the reaction. When the reaction conditions were changed to higher concentration of pinch-porphyrins and hydrogen peroxide only one type of kinetics was observed. Next, the reaction was performed only between pinch-porphyrins-Fe(III) and H₂O₂, resulting in only two types of kinetics that were developed during the first 0-4 s. After this time a self-oxidation process was observed. Our hypotheses state that the formation of the π-cation radicals, reaction intermediates of the pinch-porphyrin-Fe(III) family with the ligand picdien [N,N'-bis-pyridin-2-ylmethyl-propane-1,3-diamine], occurred with unique kinetics that are different from the overall process and was involved in the oxidation pathway. UV-Vis, ¹H-NMR and ESR spectra confirmed the formation of such intermediates. The results in this paper highlight the link between different spectroscopic techniques that positively depict the kinetic traits of artificial compounds with enzyme-like activity. PMID:27355940

  13. Formation of free radicals during mechanical degradation of elastomers.

    NASA Technical Reports Server (NTRS)

    Devries, K. L.; Williams, M. L.; Roylance, D. K.

    1971-01-01

    Solithane 113 (an amorphous polyurethane elastomer) was prepared by curing equal proportions of castor oil and trifunctional isocyanate for 6 hr 45 min at 170 F. The sample material was mechanically degraded by grinding below and above its glass transition point at liquid nitrogen and room temperatures. The EPR spectra of ground samples were recorded and the number of free radicals were determined by a computer double-integration of the recorded spectra and by a comparison of the values with those of a standard material. Curves of EPR spectra suggest that different molecular mechanisms may be active in degradation of this material below and above its glass transition temperature.

  14. C{sub 8}H{sub 8} radical cations of cyclooctatetraene, semibullvalene, and their common bisallylic rearrangement product. Electronic structure and potential energy surfaces

    SciTech Connect

    Bally, T.; Truttmann, L.; Dai, S.; Williams, F.

    1995-08-02

    The recently discovered access paths to the radical cation of bicyclo[3,3,0]octa-2,6-diene-4,8-diyl (BOD{sup +}) are explored by electronic absorption (EA) spectroscopy whereby previous ESR results are confirmed. The electronic and molecular structure of BOD{sup +} and of its photoprecursor, the radical cation of cyclooctatetraene (COT{sup +}), are discussed on the basis of their EA spectra and ab initio calculations. The ground and excited state potential surfaces common to the title cations are explored, and it is shown that the COT{sup +} {yields} BOD{sup +} photorearrangement proceeds mainly by virtue of a pronounced Jahn-Teller distortion of the second excited state of COT{sup +} ({sup 2}E). This distortion competes effectively with internal conversion to the first excited state, leads to an inversion of the ground state symmetry, and covers a substantial part of the reaction path leading to the bisallylic cation. 37 refs., 8 figs., 4 tabs.

  15. Electronic Characterization of Reaction Intermediates: The Fluorenylium, Phenalenylium, and Benz[f]indenylium Cations and Their Radicals.

    PubMed

    Fulara, Jan; Chakraborty, Arghya; Maier, John P

    2016-03-01

    Three vibrationally resolved absorption systems commencing at 538, 518, and 392 nm were detected in a 6 K neon matrix after mass-selected deposition of C13 H9 (+) ions (m/z=165) produced from fluorene in a hot-cathode discharge ion source. The benz[f]indenylium (BfI(+) : 538 nm), fluorenylium (FL9(+) : 518 nm), and phenalenylium (PHL(+) : 392 nm) cations are the absorbing molecules. Two electronic systems corresponding to neutral species are apparent at 490 and 546 nm after irradiation of the matrix with λ<260 nm photons and were assigned to the FL9 and BfI radicals, respectively. The strongest peak at 518 nm is the origin of the 2 (1) B2 ←X̃ (1) A1 absorption of FL9(+) , and the 490 nm band is the 2 (2) A2 ←X̃ (2) B1 origin of FL9. The electronic systems commencing at 538 nm and 546 nm were assigned to the 1 (1) A1 ←X̃ (1) A1 and 1 (2) A2 ←X̃ (2) A2 transitions of BfI(+) and BfI. The 392 nm band is the 1 (1) E'←X̃ (1) A1 ' transition of PHL(+). The electronic spectra of C13 H9 (+) /C13 H9 were assigned on the basis of the vertical excitation energies calculated with SAC-CI and MS-CASPT2 methods. PMID:26845059

  16. Free radical formation in vivo and hepatotoxicity due to anesthesia with halothane

    SciTech Connect

    Plummer, J.L.; Beckwith, A.L.; Bastin, F.N.; Adams, J.F.; Cousins, M.J.; Hall, P.

    1982-09-01

    In vivo studies were undertaken to determine whether free radical formation in the liver during administration of various halogenated anesthetics is associated with hepatotoxicity of these agents in an animal model. In addition to the anesthetics halothane, enflurane, and isoflurane, carbon tetrachloride was studied as an example of a hepatotoxic halogenated compound acting by a free radical mechanism. Free radicals were trapped in vivo during anesthesia as stable adducts using the spin trap, alpha-phenyl-t-butyl nitrone. These adducts were extracted from the liver and studied by electron spin resonance spectrometry. Free radicals were detected after administration of halothane or carbon tetrachloride, compounds which were hepatotoxic under the conditions of the experiment, but were not found after anesthesia induced with enflurane or isoflurane, anesthetics which were not hepatotoxic under identical conditions. The free radical trapped after alpha-phenyl-t-butyl nitrone treatment of halothane-anesthetized rats appeared to be a metabolic intermediate of halothane.

  17. Formation of Stable Cationic Lipid/DNA Complexes for Gene Transfer

    NASA Astrophysics Data System (ADS)

    Hofland, Hans E. J.; Shephard, Lee; Sullivan, Sean M.

    1996-07-01

    Stable cationic lipid/DNA complexes were formed by solubilizing cationic liposomes with 1% octylglucoside and complexing a DNA plasmid with the lipid in the presence of detergent. Removal of the detergent by dialysis yielded a lipid/DNA suspension that was able to transfect tissue culture cells up to 90 days after formation with no loss in activity. Similar levels of gene transfer were obtained by mixing the cationic lipid in a liposome form with DNA just prior to cell addition. However, expression was completely lost 24 hr after mixing. The transfection efficiency of the stable complex in 15% fetal calf serum was 30% of that obtained in the absence of serum, whereas the transient complex was completely inactivated with 2% fetal calf serum. A 90-day stability study comparing various storage conditions showed that the stable complex could be stored frozen or as a suspension at 4 degrees C with no loss in transfection efficiency. Centrifugation of the stable complex produced a pellet that contained approximately 90% of the DNA and 10% of the lipid. Transfection of cells with the resuspended pellet and the supernatant showed that the majority of the transfection activity was in the pellet and all the toxicity was in the supernatant. Formation of a stable cationic lipid/DNA complex has produced a transfection vehicle that can be stored indefinitely, can be concentrated with no loss in transfection efficiency, and the toxicity levels can be greatly reduced when the active complex is isolated from the uncomplexed lipid.

  18. Peroxy radical concentration and ozone formation rate at a rural site in the southeastern United States

    NASA Astrophysics Data System (ADS)

    Kleinman, Lawrence; Lee, Yin-Nan; Springston, Stephen R.; Lee, Jai H.; Nunnermacker, Linda; Weinstein-Lloyd, Judith; Zhou, Xianliang; Newman, Leonard

    1995-04-01

    As part of the Southern Oxidants Study, Brookhaven National Laboratory operated an intensive measurement site near Metter, Georgia, during parts of the summers of 1991 and 1992. Measurements were made of photochemically active trace gases and meteorological parameters relevant to determining causes for elevated ambient ozone concentration. The 1992 data set was used to calculate peroxy radical concentration and ozone formation rate based on determining the departure from the photostationary state (PSS) and based on a radical budget equation, such as applied previously to the 1991 data set. Averaged over the 28-day experimental period, we find maximum radical production occurring near noon at 2.5 ppb h-1, maximum peroxy radical concentration also occurring near noon at 80 ppt, and maximum ozone production of 8 ppb h-1 occurring near 1000 EST. Ozone photolysis accounts for 55% of radical production, HCHO and other carbonyl compounds about 40%. The radical budget and PSS methods depend in different ways on atmospheric photochemistry and a comparison between them affords a test of our understanding of the photochemical production of O3. We find that these methods agree to the extent expected based on uncertainty estimates. For the data set as a whole, the median estimate for fractional error in hourly average peroxy radical concentration determined from the radical budget method is approximately 30% and from the PSS method, 50%. Error estimates for the PSS method are highly variable, becoming infinite as peroxy radical concentration approaches zero. This behavior can be traced back to the difference form of the PSS equations. To conduct a meaningful comparison between the methods, the data set was segregated into subsets based on PSS uncertainty estimates. For the low-uncertainty subset, consisting of a third of the whole data set, we find that the ratio of peroxy radical concentration predicted from the PSS method to that predicted from the radical budget method to be

  19. Velocity Map Imaging Study of Ion-Radical Chemistry: Charge Transfer and Carbon-Carbon Bond Formation in the Reactions of Allyl Radicals with C(.).

    PubMed

    Pei, Linsen; Farrar, James M

    2016-08-11

    We present an experimental and computational study of the dynamics of collisions of ground state carbon cations with allyl radicals, C3H5, at a collision energy of 2.2 eV. Charge transfer to produce the allyl cation, C3H5(+), is exoergic by 3.08 eV and proceeds via energy resonance such that the electron transfer occurs without a significant change in nuclear velocities. The products have sufficient energy to undergo the dissociation process C3H5(+) → C3H4(+) + H. Approximately 80% of the reaction products are ascribed to charge transfer, with ∼40% of those products decaying via loss of a hydrogen atom. We also observe products arising from the formation of new carbon-carbon bonds. The experimental velocity space flux distributions for the four-carbon products are symmetric about the centroid of the reactants, providing direct evidence that the products are mediated by formation of a C4H5(+) complex living at least a few rotational periods. The primary four-carbon reaction products are formed by elimination of molecular hydrogen from the C4H5(+) complex. More than 75% of the nascent C4H3(+) products decay by C-H bond cleavage to yield a C4H2(+) species. Quantum chemical calculations at the MP2/6-311+g(d,p) level of theory support the formation of a nonplanar cyclic C4H5(+) adduct that is produced when the p-orbital containing the unpaired electron on C(+) overlaps with the unpaired spin density on the terminal carbon atoms in allyl. Product formation then occurs by 1,2-elimination of molecular hydrogen from the cyclic intermediate to form a planar cyclic C4H3(+) product. The large rearrangement in geometry as the C4H3(+) products are formed is consistent with high vibrational excitation in that product and supports the observation that the majority of those products decay to form the C4H2(+) species. PMID:27434380

  20. Formation of radical-anions and radicals in the reaction of sodium sulfide with aromatic halogen compounds

    SciTech Connect

    Annenkova, V.Z.; Antonik, L.M.; Vakul'skaya, T.I.; Voronkov, M.G.

    1986-03-20

    The ESR and UV spectroscopic methods were used to establish the mechanism of the substitution of a chlorine atom by sulfide sulfur in the reactions of 2,5-dichloro-nitrobenzene (i) and p-dichlorobenzene (II) with sodium sulfide in N-methyl-2-pyrrolidone. The S/sub 2//sup -./ and S/sub 3//sup -./ radical-anions were detected and identified. The former corresponds to a narrow singlet with a g factor of 2.005 (lambdamax 440 nm), while the latter corresponds to a broad ESR signal with a g value of 2.028 (lambdamax 618 nm) (1-3). The formation of the radical-anion of the reagent gave grounds for supposing that the reaction of (I) and (II) with sodium sulfide takes place through a one-electron transfer stage. Thus, a stable radical-anion characterized by hyperfine structure (hfs) (3/sub N/ x 2/sub H/ x 3/sub H/ x 3/sub H/ with constants of 11.6, 3.7, 3.5, and 0.7 Oe respectively) is formed in the nitrobenzene-sodium sulfide system.

  1. Sulfate radical-based water treatment in presence of chloride: formation of chlorate, inter-conversion of sulfate radicals into hydroxyl radicals and influence of bicarbonate.

    PubMed

    Lutze, Holger V; Kerlin, Nils; Schmidt, Torsten C

    2015-04-01

    Sulfate radical (SO4(-)) based oxidation is discussed as a potential water treatment option and is already used in ground water remediation. However, the complex SO4(-) chemistry in various matrices is poorly understood. In that regard, the fast reaction of SO4(-) with Cl(-) is of high importance since Cl(-) belongs to the main constituents in aqueous environments. This reaction yields chlorine atoms (Cl) as primary products. Cl initiate a cascade of subsequent reactions with a pH dependent product pattern. At low pH (<5) formation of chlorine derived oxidation products such as chlorate (ClO3(-)) is favoured. This is undesired because ClO3(-) may reveal adverse effects on the environment and human health. At pH > 5 Cl mainly react with water yielding hydroxyl radicals. Thus, at moderate Cl(-) concentrations (mM range) the SO4(-)-based process may be converted into a conventional (hydroxyl radical -based) advanced oxidation process. The conversion of SO4(-) into OH, however, is interrupted in presence of bicarbonate by scavenging of Cl. PMID:25455043

  2. DNA strand exchange stimulated by spontaneous complex formation with cationic comb-type copolymer.

    PubMed

    Kim, Won Jong; Akaike, Toshihiro; Maruyama, Atsushi

    2002-10-30

    Cationic comb-type copolymers (CCCs) composed of a polycation backbone and water-soluble side chains accelerate by 4-5 orders the DNA strand exchange reaction (SER) between double helical DNA and its homologous single-strand DNA. The accelerating effect is considered due to alleviation of counterion association during transitional intermediate formation in sequential displacement pathway. CCCs stabilize not only matured hybrids but also the nucleation complex to accelerate hybridization. PMID:12392411

  3. Free-radical Destruction of Sphingolipids Resulting in 2-hexadecenal Formation

    PubMed Central

    Shadyro, Oleg; Lisovskaya, Alexandra; Semenkova, Galina; Edimecheva, Irina; Amaegberi, Nadezda

    2015-01-01

    The action of hypochlorous acid (HOCl) and γ-radiation on aqueous lysosphingolipid dispersions was found to produce 2-hexadecenal (Hex). This process includes the stages of formation of nitrogen-centered radicals from the starting molecules and the subsequent fragmentation of these radicals via the rupture of C–C and O–H bonds. These findings prove the existence of a nonenzymatic pathway of sphingolipid destruction leading to the formation of Hex, which possesses a wide spectrum of biological activity. Analysis of the effect of HOCl on transplantable rat glioma C6 cells and human embryonic kidney 293 cells points to the formation of Hex. This suggests that the described mechanism of free-radical destruction of sphingolipids may be replicated on cell culture under the stress of active chlorine forms. PMID:25861222

  4. Möbius-Hückel topology switching in an expanded porphyrin cation radical as studied by EPR and ENDOR spectroscopy.

    PubMed

    Möbius, Klaus; Plato, Martin; Klihm, Gudrun; Laurich, Christoph; Savitsky, Anton; Lubitz, Wolfgang; Szyszko, Bartosz; Stępień, Marcin; Latos-Grażyński, Lechosław

    2015-03-01

    The symmetry of the arrangement of objects has fascinated philosophers, artists and scientists for a long time, and still does. Symmetries often exist in nature, but are also created artificially, for instance by chemical synthesis of novel molecules and materials. The one-sided, non-orientable Möbius band topology is a paradigm of such a symmetry-based fascination. In the early 1960s, in synthetic organic chemistry the interest in molecules with Möbius symmetry was greatly stimulated by a short paper by Edgar Heilbronner. He predicted that sufficiently large [n]annulenes with a closed-shell electron configuration of 4n π-electrons should allow for sufficient π-overlap stabilization to be synthesizable by twisting them with a 180° phase change into the Möbius symmetry of their hydrocarbon skeleton. In 2007, the group of Lechosław Latos-Grażyński succeeded in synthesizing the compound di-p-benzi[28]hexa-phyrin(1.1.1.1.1.1), compound 1, which can dynamically switch between Hückel and Möbius conjugation depending, in a complex manner, on the polarity and temperature of the surrounding solvent. This discovery of "topology switching" between the two-sided (Hückel) and one-sided (Möbius) molecular state with closed-shell electronic configuration was based primarily on the results of NMR spectroscopy and DFT calculations. The present EPR and ENDOR work on the radical cation state of compound 1 is the first study of a ground-state open-shell system which exhibits a Hückel-Möbius topology switch that is controlled by temperature, like in the case of the closed-shell precursor. The unpaired electron interacting with magnetic nuclei in the molecule is used as a sensitive probe for the electronic structure and its symmetry properties. For a Hückel conformer with its higher symmetry, we expect - and observe - fewer ENDOR lines than for a Möbius conformer. The ENDOR results are supplemented by and in accordance with theoretical calculations based on density

  5. Radical Pathways for the Prebiotic Formation of Pyrimidine Bases from Formamide.

    PubMed

    Nguyen, Huyen Thi; Jeilani, Yassin A; Hung, Huynh Minh; Nguyen, Minh Tho

    2015-08-20

    The prebiotic formation of nucleobases, the building blocks of RNA/DNA, is of current interest. Highly reactive radical species present in the atmosphere under irradiation have been suggested to be involved in the prebiotic synthesis of nucleobases from formamide (FM). We studied several free radical reaction pathways for the synthesis of pyrimidine bases (cytosine, uracil, and thymine) from FM under cold conditions. These pathways are theoretically determined using density functional theory (DFT) computations to examine their kinetic and thermodynamic feasibilities. These free radical reaction pathways share some common reaction types such as H-rearrangement, (•)H/(•)OH/(•)NH2 radical loss, and intramolecular radical cyclization. The rate-determining steps in these pathways are characterized with low energy barriers. The energy barriers of the ring formation steps are in the range of 3-7 kcal/mol. Although DFT methods are known to significantly underestimate the barriers for addition of (•)H radical to neutral species, many of these reactions are highly exergonic with energy release of -15 to -52 kcal/mol and are thus favorable. Among the suggested pathways for formation of cytosine (main route, routes 7a and 1a), uracil (main route, routes 7b and 1b), and thymine (main route and route 26a), the main routes are in general thermodynamically more exergonic and more kinetically favored than other alternative routes with lower overall energy barriers. The reaction energies released following formation of cytosine, uracil, and thymine from FM via the main radical routes amount to -59, -81, and -104 kcal/mol, respectively. Increasing temperature induces unfavorable changes in both kinetic and thermodynamic aspects of the suggested routes. However, the main routes are still more favored than the alternative pathways at the temperature up to the boiling point of FM. PMID:26196536

  6. Radical S-adenosylmethionine enzyme catalyzed thioether bond formation in sactipeptide biosynthesis.

    PubMed

    Flühe, Leif; Marahiel, Mohamed A

    2013-08-01

    Sactipeptides represent a new emerging class of ribosomally assembled and posttranslationally modified peptides that show diverse bioactivities. Their common hallmark is an intramolecular thioether bond that crosslink the sulfur atom of a cysteine residue with the α-carbon of an acceptor amino acid. This review summarizes recent achievements concerning the biosynthesis of sactipeptides in general and with special focus on the common enzymatic radical SAM mechanism leading to the thioether linkage formation. In addition this mechanism is compared to the mechanism of thioether bond formation during lanthipeptide biosynthesis and to other radical based thioether bond forming reactions. PMID:23891473

  7. Nighttime formation of peroxy and hydroxyl radicals during the BERLIOZ campaign: Observations and modeling studies

    NASA Astrophysics Data System (ADS)

    Geyer, Andreas; BäChmann, Kurt; Hofzumahaus, Andreas; Holland, Frank; Konrad, Stefan; Klüpfel, Thomas; PäTz, Hans-Werner; Perner, Dieter; Mihelcic, Djuro; SchäFer, Hans-Jürgen; Volz-Thomas, Andreas; Platt, Ulrich

    2003-02-01

    Traditionally, tropospheric radical chemistry is discussed in terms of the daytime photochemically produced hydroxyl radical (OH). Radicals, however, are also important during nighttime: this is especially true for ozone and the nitrate radical (NO3), which both act as key initiators of the degradation of alkenes such as biogenic monoterpenes. These reactions lead to the formation of peroxy radicals (HO2 and RO2) and hydroxyl radicals at night. We present recent observations of nighttime concentrations of NO3, RO2, HO2, and OH by differential optical absorption spectroscopy (DOAS), matrix isolation electron spin resonance (MIESR), laser-induced fluorescence (LIF), and a chemical amplifier (CA) in the framework of the Berliner Ozonexperiment (BERLIOZ) campaign at Pabstthum, Germany, together with modeling studies of nocturnal radical chemistry. Modeled RO2 mixing ratios reached 40 ppt while the measured ROx level went up to 22 ppt at the same time. Modeled and measured HO2 mixing ratios were up to 6 and 4 ppt, respectively. In the case of OH, a nocturnal concentration of (1.85 ± 0.82) × 105 cm-3 was measured during one night. At this time, the model yielded an OH level of (4.1 ± 0.7) × 105 cm-3. This overestimation by the model could point to a missing nocturnal sink of OH. Nitrate radical reactions with terpenes were found responsible for producing 77% of the RO2 radicals, 53% of the HO2, and 36% of the OH radicals during night. Nighttime ozonolysis formed 12% of the RO2, 47% of the HO2, and 64% of the OH radicals. Another 11% of the RO2 radicals were formed by OH-volatile organic compound (VOC) reactions. A positive linear correlation of RO2 and NO3 was observed and could be reproduced in model calculations originating from the loss of both radicals by reaction with NO and the NO3-initiated RO2 production. The contribution of nighttime OH to the atmosphere's oxidation capacity (oxidation rate of VOCs, CO, and CH4) was found negligible (<0.5%).

  8. Domino Fragmentations in Traceless Directing Groups of Radical Cascades: Evidence for the Formation of Alkoxy Radicals via C-O Scission.

    PubMed

    Harris, Trevor; Gomes, Gabriel Dos Passos; Clark, Ronald J; Alabugin, Igor V

    2016-07-15

    Direct evidence for the formation of alkoxy radicals is reported in radical cascades using traceless directing groups. Despite the possibility of hydrogen abstraction in the fragmenting step, followed by loss of R-OH, β-scission is preferred for the formation of alkoxy radicals. For the first time, the C-O radical was intermolecularly trapped using a silyl enol ether. Various C-X fragmenting groups were explored as possible traceless directing groups for the preparation of extended polyaromatics. Computational evidence shows that a combination of aromatization, steric and stereoelectronic effects assists the fragmentation to alkoxy radicals. Additionally, a new through-space interaction was discovered between O and Sn in the fragmentation as a specific transition state stabilizing effect. PMID:27304982

  9. Effect of gas-containing microspheres and echo contrast agents on free radical formation by ultrasound.

    PubMed

    Kondo, T; Misík, V; Riesz, P

    1998-09-01

    Stabilized microbubbles (microspheres) are widely used to enhance the contrast of ultrasound imaging. Our data provide direct evidence that the contrast agents, Levovist, PVC-AN (polyvinylidene chloride-acrylonitryl copolymer), and Albunex (compared to 5% human albumin), at concentrations comparable to those used for ultrasound imaging, enhance H2O2 production (through the superoxide-dependent pathway) in air-saturated aqueous solutions exposed to 47 kHz ultrasound above the cavitation threshold. These agents also act as scavengers of .H atoms and .OH radicals, thus lowering H2O2 formation (by recombination of .OH radicals) in argon-saturated solutions. EPR spin trapping also reveals that secondary radicals derived from the contrast agents are produced by reactions with .H and .OH which are formed by pyrolysis of water inside cavitation bubbles. In addition, the contrast agents themselves undergo pyrolysis reactions in the cavitation bubbles as demonstrated by formation of methyl radicals. Possible deleterious consequences of the formation of sonochemical intermediates may have to be assessed, particularly since some of the echo contrast agents have been shown to lower the cavitation threshold of diagnostic ultrasound. Unlike the microspheres formed from organic molecules, inorganic microspheres, Eccospheres, because of their stability and inert nature with respect to participation in free radical processes, appear to be suitable tools for enhancing the yields of aqueous sonochemical reactions. PMID:9741598

  10. Formation of stable radicals in catechin/nitrous acid systems: participation of dinitrosocatechin.

    PubMed

    Morina, Filis; Takahama, Umeo; Mojović, Miloš; Popović-Bijelić, Ana; Veljović-Jovanović, Sonja

    2016-03-01

    Catechins are transformed into dinitrosocatechins (diNOcats) and then oxidized to the quinones by salivary nitrite under conditions simulating the stomach. This manuscript deals with formation of stable radicals in the NO group of diNOcat during nitrite-induced oxidation of (+)-catechin and diNOcat at pH 2. We postulated two mechanisms for the stable radical formation; one is nitrous acid-induced oxidation of diNOcat in the A-ring, and the other intermolecular charge transfer from the A-ring of diNOcat and/or diNOcat quinone to the quinone moiety of the B-ring of diNOcat quinone. In addition, an unstable phenoxyl radical, which might be transformed into quinone, was also produced, accompanying the formation of the stable radical on the NO group. Taking the above results into account, we mainly focus on the adverse effects of the radicals and quinone, which may be produced from (+)-catechin in the stomach under the conditions of high salivary nitrite concentrations. PMID:26471661

  11. Spin trapping evidence for myeloperoxidase-dependent hydroxyl radical formation by human neutrophils and monocytes

    SciTech Connect

    Ramos, C.L.; Pou, S.; Britigan, B.E.; Cohen, M.S.; Rosen, G.M. )

    1992-04-25

    Using the electron spin resonance/spin trapping system, 4-pyridyl 1-oxide N-tert-butylnitrone (4-POBN)/ethanol, hydroxyl radical was detected as the alpha-hydroxyethyl spin trapped adduct of 4-POBN, 4-POBN-CH(CH3)OH, from phorbol 12-myristate 13-acetate-stimulated human neutrophils and monocytes without the addition of supplemental iron. 4-POBN-CH(CH3)OH was stable in the presence of a neutrophil-derived superoxide flux. Hydroxyl radical formation was inhibited by treatment with superoxide dismutase, catalase, and azide. Treatment with a series of transition metal chelators did not appreciably alter 4-POBN-CH(CH3)OH, which suggested that hydroxyl radical generation was mediated by a mechanism independent of the transition metal-catalyzed Haber-Weiss reaction. Kinetic differences between transition metal-dependent and -independent mechanisms of hydroxyl radical generation by stimulated neutrophils were demonstrated by a greater rate of 4-POBN-CH(CH3)-OH accumulation in the presence of supplemental iron. Detection of hydroxyl radical from stimulated monocyte-derived macrophages, which lack myeloperoxidase, required the addition of supplemental iron. The addition of purified myeloperoxidase to an enzymatic superoxide generating system resulted in the detection of hydroxyl radical that was dependent upon the presence of chloride and was inhibited by superoxide dismutase, catalase, and azide. These findings implicated the reaction of hypochlorous acid and superoxide to produce hydroxyl radical. 4-POBN-CH(CH3)OH was not observed upon stimulation of myeloperoxidase-deficient neutrophils, whereas addition of myeloperoxidase to the reaction mixture resulted in the detection of hydroxyl radical. These results support the ability of human neutrophils and monocytes to generate hydroxyl radical through a myeloperoxidase-dependent mechanism.

  12. Formation of highly oxidized multifunctional compounds: Autoxidation of peroxy radicals formed in the oxidation of alkenes

    NASA Astrophysics Data System (ADS)

    Mentel, Thomas; Ehn, Mikael; Thornton, Joel; Kleist, Einhard; Pullinen, Iida; Springer, Monika; Wahner, Andreas; Wildt, Jürgen

    2015-04-01

    Recent studies show that peroxy radicals are key intermediates in particle formation. Permutation reactions involving highly oxidized peroxy radicals form stable products with extremely low volatility (ELVOC). We suggest that ELVOC are the postulated organic compounds that explain growth of small particles (Ehn et al., Nature, 2014). To elucidate the pathways of ELVOC formation, experiments were performed in the Juelich Plant Atmosphere Chamber. We applied High Resolution Nitrate-Chemical Ionization Mass Spectrometry for detection of ELVOC including highly oxidized peroxy radicals. ELVOC were produced by ozonolysis of a-pinene and other cyclic alkenes (Rissanen et al., JACS, 2014, Mentel et al., ACPD, 2015), as well as by reactions of the target compounds with OH. ELVOC with C10 skeletons carry a large number of oxygens, still containing 14 or 16 H-atoms. ELVOC-dimers with twice the number of C-atoms of the reactant were also observed. The formation of ELVOC can be explained by fast intramolecular H-shifts in combination with classical peroxy radical termination reactions, leading to ketones, alcohols, and hydroperoxides (including peroxy acids). The subsequent H-shifts enable the formation of an increasing number of hydroperoxide groups under reproduction of a peroxy radical (containing now two more oxygens). Addition of NOX to the system increases the concentrations of nitrates at the expense of the corresponding peroxy radicals, confirming their identification as peroxy radicals. Furthermore, the concentrations of ELVOC dimers decrease strongly with increasing NOX suggesting that they are indeed formed by peroxy-peroxy permutation reactions. ELVOC are involved in new particle formation, and can explain the major fraction of the early growth observed in field studies. ELVOC dimers are very likely key in new particle formation as their formation is strongly suppressed with increasing NOX in accordance with the observed NOX dependence of new particle formation (Ehn

  13. Pseudomonas and neutrophil products modify transferrin and lactoferrin to create conditions that favor hydroxyl radical formation.

    PubMed Central

    Britigan, B E; Edeker, B L

    1991-01-01

    In vivo most extracellular iron is bound to transferrin or lactoferrin in such a way as to be unable to catalyze the formation of hydroxyl radical from superoxide (.O2-) and hydrogen peroxide (H2O2). At sites of Pseudomonas aeruginosa infection bacterial and neutrophil products could possibly modify transferrin and/or lactoferrin forming catalytic iron complexes. To examine this possibility, diferrictransferrin and diferriclactoferrin which had been incubated with pseudomonas elastase, pseudomonas alkaline protease, human neutrophil elastase, trypsin, or the myeloperoxidase product HOCl were added to a hypoxanthine/xanthine oxidase .O2-/H2O2 generating system. Hydroxyl radical formation was only detected with pseudomonas elastase treated diferrictransferrin and, to a much lesser extent, diferriclactoferrin. This effect was enhanced by the combination of pseudomonas elastase with other proteases, most prominently neutrophil elastase. Addition of pseudomonas elastase-treated diferrictransferrin to stimulated neutrophils also resulted in hydroxyl radical generation. Incubation of pseudomonas elastase with transferrin which had been selectively iron loaded at either the NH2- or COOH-terminal binding site yielded iron chelates with similar efficacy for hydroxyl radical catalysis. Pseudomonas elastase and HOCl treatment also decreased the ability of apotransferrin to inhibit hydroxyl radical formation by a Fe-NTA supplemented hypoxanthine/xanthine oxidase system. However, apotransferrin could be protected from the effects of HOCl if bicarbonate anion was present during the incubation. Apolactoferrin inhibition of hydroxyl radical generation was unaffected by any of the four proteases or HOCl. Alteration of transferrin by enzymes and oxidants present at sites of pseudomonas and other bacterial infections may increase the potential for local hydroxyl radical generation thereby contributing to tissue injury. Images PMID:1655825

  14. On the Electronic Spectroscopy of Closed Shell Cations Derived from Resonance Stabilized Radicals: Insights from Theory and Franck-Condon Analysis

    NASA Astrophysics Data System (ADS)

    Troy, Tyler P.; Kable, Scott H.; Schmidt, Timothy W.; Reid, Scott A.

    2012-06-01

    Recent attention has been directed on closed shell aromatic cations as potential carriers of the diffuse interstellar bands. The spectra of mass-selected, matrix-isolated benzylium and tropylium cations were recently reported [Nagy, A., Fulara, J., Garkusha, I. and Maier, J. P. (2011), Angew. Chem. Int. Ed., 50: 3022-3025]. The benzylium spectrum shows an extended progression in a low frequency (510 cm-1) ring distortion mode. Modeling of the benzylium spectrum using (TD)DFT and MCSCF-PT2 methods in concert with multidimensional Franck-Condon (FC) analysis is found to yield excellent agreement with the experimental spectrum. We extended this analysis to larger (2 and 3 ring) PAH cations derived from resonance stabilized radicals, which are predicted to show strong S0 → Sn transitions in the visible region. The FC progression is significantly quenched in the larger species, and our results for 1-napthylmethylium are in excellent agreement with very recent experiments [Nagy, A., Fulara, J., and Maier, J. P. (2011), J. Am. Chem. Soc., 133, 19796]. Since carriers of the DIBs should exhibit spectra dominated by a single vibronic transition, our results demonstrate that closed-shell cations may present spectra with the required properties. Furthermore, the calculated ionization energies of a range of CSCs were found to be in the 13-14 eV range, consistent with variations in behaviour of the DIB carriers with respect to various astrophysical environments.

  15. Ion/molecule reactions of 2-chloro- and 2-bromopropene radical cations with methanol and ethanol--FT-ICR spectrometry and DFT calculations

    NASA Astrophysics Data System (ADS)

    Grützmacher, Hans-Friedrich; Büchner, Michael; Zipse, Hendrik

    2005-02-01

    Continuing the studies of ion/molecule reactions of haloalkene radical cations with nucleophiles, the reactions of the radical cations of 2-chloropropene, 1+, and 2-bromopropene. 2+, with methanol and ethanol, respectively, have been investigated by FT-ICR spectrometry and by computational analysis using DFT calculation (BHLYP/6-311 + G(2d,p)//BHLYP/6-31 + G(d) level). Only slow reactions (reaction efficiency <1%) are observed for 1+/methanol and 2+/methanol. Slow proton transfer is the main process for 1+/methanol besides minor addition of methanol to 1+ followed by loss of HCl or Cl. Addition of methanol accompanied by loss of Br is the exclusive process observed for 2+/methanol. In contrast, both 1+ and 2+ react efficiently with ethanol yielding protonated acetaldehyde as the exclusive (1+) or by far dominant (2+) primary reaction product. The computational analysis of these ion/molecule reactions shows that in the case of 1+/methanol and 2+/methanol all processes are either endothermic or blocked by large activation energies. Nonetheless, addition of methanol to the ionized CC double bond of 1+ or 2+ is exothermic, yielding in each case a pair of isomeric [beta]-distonic methoxonium ions. A new reaction mechanism has been found for the HX (X = Cl, Br) elimination from the less stable isomer of the distonic intermediates. Further, an energetically favorable transition state has been detected for hydrogen atom transfer from the [alpha]-CH2 group of alcohol to the halogenoalkene radical cations. These findings lead to a revised mechanism of the oxidation process and provide a plausible explanation for the excessive H/D exchange between 1+ and CD3OH during their slow reaction.

  16. Cu(II) and Cu(I) coordination complexes involving two tetrathiafulvalene-1,3-benzothiazole hybrid ligands and their radical cation salts.

    PubMed

    Yokota, Sayo; Tsujimoto, Keijiro; Hayashi, Sadayoshi; Pointillart, Fabrice; Ouahab, Lahcène; Fujiwara, Hideki

    2013-06-01

    Preparations, crystal structure analyses, and magnetic property investigations on a new Cu(II)(hfac)2 complex coordinated with two TTF-CH═CH-BTA ligands, where hfac is hexafluoroacetylacetonate, TTF is tetrathiafulvalene, and BTA is 1,3-benzothiazole, are reported together with those of its dicationic AsF6(-) salt, [Cu(hfac)2(TTF-CH═CH-BTA)2](AsF6)2, in which each TTF part is in a radical cation state. In these Cu(II)(hfac)2 complexes, two ligands are bonded to the central Cu atom of the Cu(hfac)2 part through the nitrogen atom of the 1,3-benzothiazole ring and occupy the two apical positions of the Cu(hfac)2 complex with an elongated octahedral geometry. These two ligands are located parallelly in a transverse head-to-tail manner, and the Cu(hfac)2 moiety is closely sandwiched by these two ligands. In the AsF6(-) salt of the Cu(hfac)2 complex, each TTF dimer is separated by the AsF6(-) anions and has no overlap with each other within the one-dimensional arrays, resulting in an insulating behavior. Both Cu(hfac)2 complexes showed the simple Curie-like temperature dependence of paramagnetic susceptibilities (χM), indicating that no interaction exists between the paramagnetic Cu(II) d spins. Furthermore, crystal structure analysis and magnetic/conducting properties of a radical cation ReO4(-) salt of the Cu(I) complex with two TTF-CH═CH-BTA ligands, [Cu(TTF-CH═CH-BTA)2](ReO4)2, are also described. Two nitrogen atoms of the ligands are connected to the central Cu(I) in a linear dicoordination with a Cu-N bond length of 1.879(9) Å. Two TTF parts of the neighboring complexes form a dimerized structure, and such a TTF dimer forms a one-dimensional uniform array along the a direction with a short S-S contact of 3.88 Å. Magnetic property measurement suggested the existence of a strongly antiferromagnetic one-dimensional uniform chain of S = 1/2 spins that originate from the radical cation states of the TTF dimers. Due to the construction of the one

  17. Discovery and Mechanistic Studies of Facile N-Terminal Cα–C Bond Cleavages in the Dissociation of Tyrosine-Containing Peptide Radical Cations

    SciTech Connect

    Mu, Xiaoyan; Song, Tao; Xu, Minjie; Lai, Cheuk-Kuen; Siu, Chi-Kit; Laskin, Julia; Chu, Ivan K.

    2014-03-28

    Gas phase fragmentations of protein and peptide (M) ions in a mass spectrometer—induced by, for example, electron-capture dissociation1-2 and electron-transfer dissociation3-422 —form the foundation for top-down amino acid sequencing approaches for the rapid identification of protein components in complex biological samples. During these processes, protonated protein and peptide radicals ([M + nH]•(n – 1)+)5–8 are generated; their fragmentations are governed largely by the properties of the unpaired electron. Because of their importance in modern bioanalytical chemistry, considerable attention has been drawn recently toward understanding the radical cation chemistry behind the fragmentations of these odd-electron biomolecular ions in the gas phase.

  18. A complete map of the ion chemistry of the naphthalene radical cation? DFT and RRKM modeling of a complex potential energy surface

    NASA Astrophysics Data System (ADS)

    Solano, Eduardo A.; Mayer, Paul M.

    2015-09-01

    The fragmentation mechanisms of the naphthalene molecular ion to [M-C4H2]+•, [M-C2H2]+•, [M-H2]+•, and [M-H•]+ were obtained at the UB3LYP/6-311+G(3df,2p)//UB3LYP/6-31G(d) level of theory and were subsequently used to calculate the microcanonical rate constants, k(E)'s, for all the steps by the Rice-Ramsperger-Kassel-Marcus formalism. The pre-equilibrium and steady state approximations were applied on different regions of the potential energy profiles to obtain the fragmentation k(E)'s and calculate the relative abundances of the ions as a function of energy. These results reproduce acceptably well the imaging photoelectron-photoion coincidence spectra of naphthalene, in the photon-energy range 14.0-18.8 eV that was previously reported by our group. Prior to dissociation, the molecular ion rapidly equilibrates with a set of isomers that includes the Z- and E-phenylvinylacetylene (PVA) radical cations. The naphthalene ion is the predominant isomer below 10 eV internal energy, with the other isomers remaining at steady state concentrations. Later on, new steady-state intermediates are formed, such as the azulene and 1-phenyl-butatriene radical cations. The naphthalene ion does not eject an H atom directly but eliminates an H2 molecule in a two-step fragmentation. H• loss occurs instead from the 1-phenyl-butatriene ion. The PVA ions initiate the ejection of diacetylene (C4H2) to yield the benzene radical cation. Acetylene elimination yields the pentalene cation at low energies (where it can account for 45.9%-100.0% of the rate constant of this channel), in a three-step mechanism starting from the azulene ion. However, above 7.6 eV, the major [M-C2H2]+• structure is the phenylacetylene cation.

  19. A complete map of the ion chemistry of the naphthalene radical cation? DFT and RRKM modeling of a complex potential energy surface.

    PubMed

    Solano, Eduardo A; Mayer, Paul M

    2015-09-14

    The fragmentation mechanisms of the naphthalene molecular ion to [M-C4H2](+•), [M-C2H2](+•), [M-H2](+•), and [M-H(•)](+) were obtained at the UB3LYP/6-311+G(3df,2p)//UB3LYP/6-31G(d) level of theory and were subsequently used to calculate the microcanonical rate constants, k(E)'s, for all the steps by the Rice-Ramsperger-Kassel-Marcus formalism. The pre-equilibrium and steady state approximations were applied on different regions of the potential energy profiles to obtain the fragmentation k(E)'s and calculate the relative abundances of the ions as a function of energy. These results reproduce acceptably well the imaging photoelectron-photoion coincidence spectra of naphthalene, in the photon-energy range 14.0-18.8 eV that was previously reported by our group. Prior to dissociation, the molecular ion rapidly equilibrates with a set of isomers that includes the Z- and E-phenylvinylacetylene (PVA) radical cations. The naphthalene ion is the predominant isomer below 10 eV internal energy, with the other isomers remaining at steady state concentrations. Later on, new steady-state intermediates are formed, such as the azulene and 1-phenyl-butatriene radical cations. The naphthalene ion does not eject an H atom directly but eliminates an H2 molecule in a two-step fragmentation. H(•) loss occurs instead from the 1-phenyl-butatriene ion. The PVA ions initiate the ejection of diacetylene (C4H2) to yield the benzene radical cation. Acetylene elimination yields the pentalene cation at low energies (where it can account for 45.9%-100.0% of the rate constant of this channel), in a three-step mechanism starting from the azulene ion. However, above 7.6 eV, the major [M-C2H2](+•) structure is the phenylacetylene cation. PMID:26374033

  20. A complete map of the ion chemistry of the naphthalene radical cation? DFT and RRKM modeling of a complex potential energy surface

    SciTech Connect

    Solano, Eduardo A.; Mayer, Paul M.

    2015-09-14

    The fragmentation mechanisms of the naphthalene molecular ion to [M–C{sub 4}H{sub 2}]{sup +•}, [M–C{sub 2}H{sub 2}]{sup +•}, [M–H{sub 2}]{sup +•}, and [M–H{sup •}]{sup +} were obtained at the UB3LYP/6-311+G(3df,2p)//UB3LYP/6-31G(d) level of theory and were subsequently used to calculate the microcanonical rate constants, k(E)’s, for all the steps by the Rice-Ramsperger-Kassel-Marcus formalism. The pre-equilibrium and steady state approximations were applied on different regions of the potential energy profiles to obtain the fragmentation k(E)’s and calculate the relative abundances of the ions as a function of energy. These results reproduce acceptably well the imaging photoelectron-photoion coincidence spectra of naphthalene, in the photon-energy range 14.0–18.8 eV that was previously reported by our group. Prior to dissociation, the molecular ion rapidly equilibrates with a set of isomers that includes the Z- and E-phenylvinylacetylene (PVA) radical cations. The naphthalene ion is the predominant isomer below 10 eV internal energy, with the other isomers remaining at steady state concentrations. Later on, new steady-state intermediates are formed, such as the azulene and 1-phenyl-butatriene radical cations. The naphthalene ion does not eject an H atom directly but eliminates an H{sub 2} molecule in a two-step fragmentation. H{sup •} loss occurs instead from the 1-phenyl-butatriene ion. The PVA ions initiate the ejection of diacetylene (C{sub 4}H{sub 2}) to yield the benzene radical cation. Acetylene elimination yields the pentalene cation at low energies (where it can account for 45.9%–100.0% of the rate constant of this channel), in a three-step mechanism starting from the azulene ion. However, above 7.6 eV, the major [M–C{sub 2}H{sub 2}]{sup +•} structure is the phenylacetylene cation.

  1. Post-translational activation introduces a free radical into pyruvate formate-lyase.

    PubMed Central

    Knappe, J; Neugebauer, F A; Blaschkowski, H P; Gänzler, M

    1984-01-01

    Pyruvate formate-lyase (formate acetyltransferase; EC 2.3.1.54) of Escherichia coli cells is post-translationally interconverted between inactive and active forms. Conversion of the inactive to the active form is catalyzed by an Fe2+-dependent activating enzyme and requires adenosylmethionine and dihydroflavodoxin. This process is shown here to introduce a paramagnetic moiety into the structure of pyruvate formate-lyase. It displays an EPR signal at g = 2 with a doublet splitting of 1.5 mT and could comprise an organic free radical located on an amino acid residue of the polypeptide chain. Hypophosphite was discovered as a specific reagent that destroys both the enzyme radical and the enzyme activity; it becomes covalently bound to the protein. The enzymatic generation of the radical, which is linked to adenosylmethionine cleavage into 5'-deoxyadenosine and methionine, possibly occurs through an Fe-adenosyl complex. These results suggest a radical mechanism for the catalytic cycle of pyruvate formate-lyase. PMID:6369325

  2. HYDROXYL FREE RADICAL MEDIATED FORMATION OF 8-HYDROXYGUANINE IN ISOLATED DNA (JOURNAL VERSION)

    EPA Science Inventory

    Formation of 8-hydroxyguanine within calf thymus DNA has been studied after exposure to uv-H2O2 as a hydroxyl free radical generating system. Using high-pressure liquid chromatography with electrochemical detection, the amount of 8-hydroxy-2-deoxyguanosine (8-OHdG) was measured. ...

  3. Modeling the radical chemistry in an oxidation flow reactor: radical formation and recycling, sensitivities, and the OH exposure estimation equation.

    PubMed

    Li, Rui; Palm, Brett B; Ortega, Amber M; Hlywiak, James; Hu, Weiwei; Peng, Zhe; Day, Douglas A; Knote, Christoph; Brune, William H; de Gouw, Joost A; Jimenez, Jose L

    2015-05-14

    Oxidation flow reactors (OFRs) containing low-pressure mercury (Hg) lamps that emit UV light at both 185 and 254 nm ("OFR185") to generate OH radicals and O3 are used in many areas of atmospheric science and in pollution control devices. The widely used potential aerosol mass (PAM) OFR was designed for studies on the formation and oxidation of secondary organic aerosols (SOA), allowing for a wide range of oxidant exposures and short experiment duration with reduced wall loss effects. Although fundamental photochemical and kinetic data applicable to these reactors are available, the radical chemistry and its sensitivities have not been modeled in detail before; thus, experimental verification of our understanding of this chemistry has been very limited. To better understand the chemistry in the OFR185, a model has been developed to simulate the formation, recycling, and destruction of radicals and to allow the quantification of OH exposure (OHexp) in the reactor and its sensitivities. The model outputs of OHexp were evaluated against laboratory calibration experiments by estimating OHexp from trace gas removal and were shown to agree within a factor of 2. A sensitivity study was performed to characterize the dependence of the OHexp, HO2/OH ratio, and O3 and H2O2 output concentrations on reactor parameters. OHexp is strongly affected by the UV photon flux, absolute humidity, reactor residence time, and the OH reactivity (OHR) of the sampled air, and more weakly by pressure and temperature. OHexp can be strongly suppressed by high OHR, especially under low UV light conditions. A OHexp estimation equation as a function of easily measurable quantities was shown to reproduce model results within 10% (average absolute value of the relative errors) over the whole operating range of the reactor. OHexp from the estimation equation was compared with measurements in several field campaigns and shows agreement within a factor of 3. The improved understanding of the OFR185 and

  4. Asymmetric catalytic formation of quaternary carbons by iminium ion trapping of radicals.

    PubMed

    Murphy, John J; Bastida, David; Paria, Suva; Fagnoni, Maurizio; Melchiorre, Paolo

    2016-04-14

    An important goal of modern organic chemistry is to develop new catalytic strategies for enantioselective carbon-carbon bond formation that can be used to generate quaternary stereogenic centres. Whereas considerable advances have been achieved by exploiting polar reactivity, radical transformations have been far less successful. This is despite the fact that open-shell intermediates are intrinsically primed for connecting structurally congested carbons, as their reactivity is only marginally affected by steric factors. Here we show how the combination of photoredox and asymmetric organic catalysis enables enantioselective radical conjugate additions to β,β-disubstituted cyclic enones to obtain quaternary carbon stereocentres with high fidelity. Critical to our success was the design of a chiral organic catalyst, containing a redox-active carbazole moiety, that drives the formation of iminium ions and the stereoselective trapping of photochemically generated carbon-centred radicals by means of an electron-relay mechanism. We demonstrate the generality of this organocatalytic radical-trapping strategy with two sets of open-shell intermediates, formed through unrelated light-triggered pathways from readily available substrates and photoredox catalysts--this method represents the application of iminium ion activation (a successful catalytic strategy for enantioselective polar chemistry) within the realm of radical reactivity. PMID:27075098

  5. Asymmetric catalytic formation of quaternary carbons by iminium ion trapping of radicals

    NASA Astrophysics Data System (ADS)

    Murphy, John J.; Bastida, David; Paria, Suva; Fagnoni, Maurizio; Melchiorre, Paolo

    2016-04-01

    An important goal of modern organic chemistry is to develop new catalytic strategies for enantioselective carbon–carbon bond formation that can be used to generate quaternary stereogenic centres. Whereas considerable advances have been achieved by exploiting polar reactivity, radical transformations have been far less successful. This is despite the fact that open-shell intermediates are intrinsically primed for connecting structurally congested carbons, as their reactivity is only marginally affected by steric factors. Here we show how the combination of photoredox and asymmetric organic catalysis enables enantioselective radical conjugate additions to β,β-disubstituted cyclic enones to obtain quaternary carbon stereocentres with high fidelity. Critical to our success was the design of a chiral organic catalyst, containing a redox-active carbazole moiety, that drives the formation of iminium ions and the stereoselective trapping of photochemically generated carbon-centred radicals by means of an electron-relay mechanism. We demonstrate the generality of this organocatalytic radical-trapping strategy with two sets of open-shell intermediates, formed through unrelated light-triggered pathways from readily available substrates and photoredox catalysts—this method represents the application of iminium ion activation (a successful catalytic strategy for enantioselective polar chemistry) within the realm of radical reactivity.

  6. Hydrogen exchange of the glycyl radical of pyruvate formate-lyase is catalyzed by cysteine 419.

    PubMed

    Parast, C V; Wong, K K; Lewisch, S A; Kozarich, J W; Peisach, J; Magliozzo, R S

    1995-02-28

    Pyruvate formate-lyase (PFL) catalyzes the reversible conversion of CoA and pyruvate into acetyl-CoA and formate. Active enzyme contains a glycyl radical whose alpha-hydrogen undergoes rapid exchange with solvent (t1/2 approximately 5 min at 0 degree C). We have investigated this exchange using site-directed mutagenesis and mechanism-based inactivation. Mutation of the active-site cysteine 419 into a serine, which renders the enzyme catalytically inactive, abolishes alpha-hydrogen exchange in the radical. This suggests that the exchange process is not an intrinsic property of the glycyl radical but is a consequence of its interaction with cysteine 419. This residue is also demonstrated to be involved in the transfer of the radical to acetylphosphinate, a mechanism-based inactivator of the enzyme. In contrast, mutation of the other essential cysteine 418 to a serine has no effect on the hydrogen exchange or the transfer of the radical to acetylphosphinate. A mechanism for the hydrogen exchange catalyzed by cysteine 419 consistent with a redox role for this residue in the normal catalytic reaction is proposed. PMID:7873518

  7. Photochemical Formation of Hydroxyl Radical in Red-Soil-Polluted Seawater in Okinawa, Japan -Potential Impacts on Marine Organisms

    NASA Astrophysics Data System (ADS)

    Arakaki, T.; Hamdun, A. M.; Okada, K.; Kuroki, Y.; Ikota, H.; Fujimura, H.; Oomori, T.

    2004-12-01

    Development of pineapple farmlands and construction of recreational facilities caused runoff of red soil into coastal ocean (locally termed as red-soil-pollution) in the north of Okinawa Island, Japan. In an attempt to understand the impacts of red soil on oxidizing power of the seawater, we studied formation of hydroxyl radical (OH radical), the most potent oxidant in the environment, in red-soil-polluted seawaters, using 313 nm monochromatic light. Photo-formation rates of OH radical showed a good correlation with dissolved iron concentrations (R = 0.98). The major source of OH radical was found to be the Fenton reaction (a reaction between Fe(II) and HOOH). The un-filtered red-soil-polluted seawater samples exhibited faster OH radical formation rates than the filtered samples, suggesting that iron-bearing red soil particles enhanced formation of OH radical.

  8. Formation and Implications of Alpha-Synuclein Radical in Maneb- and Paraquat-Induced Models of Parkinson's Disease.

    PubMed

    Kumar, Ashutosh; Leinisch, Fabian; Kadiiska, Maria B; Corbett, Jean; Mason, Ronald P

    2016-07-01

    Parkinson's disease (PD) is a debilitating, progressive, neurodegenerative disorder characterized by progressive loss of dopaminergic neurons and motor deficits. Alpha-synuclein-containing aggregates represent a feature of a variety of neurodegenerative disorders, including PD; however, the mechanism that initiates and promotes intraneuronal alpha-synuclein aggregation remains unknown. We hypothesized protein radical formation as an initiating mechanism for alpha-synuclein aggregation. Therefore, we used the highly sensitive immuno-spin trapping technique to investigate protein radical formation as a possible mechanism of alpha-synuclein aggregation as well as to investigate the source of protein radical formation in the midbrains of Maneb- and paraquat-coexposed mice. Coexposure to Maneb and paraquat for 6 weeks resulted in active microgliosis, NADPH oxidase activation, and inducible nitric oxide synthase (iNOS) induction, which culminated in protein radical formation in the midbrains of mice. Results obtained with immuno-spin trapping and immunoprecipitation experiments confirmed formation of alpha-synuclein radicals in dopaminergic neurons of exposed mice. Free radical formation requires NADPH oxidase and iNOS, as indicated by decreased protein radical formation in knockout mice (P47phox(-/-) and iNOS(-/-)) and in mice treated with inhibitors such as FeTPPS (a peroxynitrite decomposition catalyst), 1400 W (an iNOS inhibitor), or apocynin (a NADPH oxidase inhibitor). Concurrence of protein radical formation with dopaminergic neuronal death indicated a link between protein radicals and disease progression. Taken together, these results show for the first time the formation and detection of the alpha-synuclein radical and suggest that NADPH oxidase and iNOS play roles in peroxynitrite-mediated protein radical formation and subsequent neuronal death in the midbrains of Maneb- and paraquat-coexposed mice. PMID:25952542

  9. Free radical mediated formation of 3-monochloropropanediol (3-MCPD) fatty acid diesters.

    PubMed

    Zhang, Xiaowei; Gao, Boyan; Qin, Fang; Shi, Haiming; Jiang, Yuangrong; Xu, Xuebing; Yu, Liangli Lucy

    2013-03-13

    The present study was conducted to test the hypothesis that a free radical was formed and mediated the formation of 3-monochloropropanediol (3-MCPD) fatty acid diesters, a group of food contaminants, from diacylglycerols at high temperature under a low-moisture condition for the first time. The presence of free radicals in a vegetable oil kept at 120 °C for 20 min was demonstrated using an electron spin resonance (ESR) spectroscopy examination with 5,5-dimethylpyrroline-N-oxide (DMPO) as the spin trap agent. ESR investigation also showed an association between thermal treatment degree and the concentration of free radicals. A Fourier transform infrared spectroscopy (FT-IR) analysis of sn-1,2-stearoylglycerol (DSG) at 25 and 120 °C suggested the possible involvement of an ester carbonyl group in forming 3-MCPD diesters. On the basis of these results, a novel free radical mediated chemical mechanism was proposed for 3-MCPD diester formation. Furthermore, a quadrupole-time of flight (Q-TOF) MS/MS investigation was performed and detected the DMPO adducts with the cyclic acyloxonium free radical (CAFR) and its product MS ions, proving the presence of CAFR. Furthermore, the free radical mechanism was validated by the formation of 3-MCPD diesters through reacting DSG with a number of organic and inorganic chlorine sources including chlorine gas at 120 and 240 °C. The findings of this study might lead to the improvement of oil and food processing conditions to reduce the level of 3-MCPD diesters in foods and enhance food safety. PMID:23425600

  10. Spontaneous formation of biocompatible vesicles in aqueous mixtures of amino acid-based cationic surfactants and SDS/SDBS.

    PubMed

    Shome, Anshupriya; Kar, Tanmoy; Das, Prasanta K

    2011-02-01

    The spontaneous formation of vesicles by six amino acid-based cationic surfactants and two anionic surfactants (sodium dodecylbenzene sulfonate (SDBS) and sodium dodecyl sulfate (SDS)) is reported. The head-group structure of the cationic surfactants is minutely altered to understand their effect on vesicle formation. To establish the regulatory role of the aromatic group in self-aggregation, both aliphatic and aromatic side-chain-substituted amino acid-based cationic surfactants are used. The presence of aromaticity in any one of the constituents favors the formation of vesicles by cationic/anionic surfactant mixtures. The formation of vesicles is primarily dependent on the balance between the hydrophobicity and hydrophilicity of both cationic and anionic surfactants. Vesicle formation is characterized by surface tension, fluorescence anisotropy, transmission electron microscopy, dynamic light scattering, and phase diagrams. These vesicles are thermally stable up to 65 °C, determined by temperature-dependent fluorescence anisotropy. According to the MTT assay, these catanionic vesicles are nontoxic to NIH3T3 cells, thus indicating their wider applicability as delivery vehicles to cells. Among the six cationic surfactants examined, tryptophan- and tyrosine-based surfactants have the ability to reduce HAuCl(4) to gold nanoparticles (GNPs), which is utilized to obtain in-situ-synthesized GNPs entrapped in vesicles without the need for any external reducing agent. PMID:21275029

  11. Photochemistry at interfaces: a source of radicals impacting on aerosol formation and properties

    NASA Astrophysics Data System (ADS)

    George, C.; D'anna, B.; Monge, M.; Dupart, Y.

    2011-12-01

    Recent findings confirm the presence of light absorbing organic material (HULIS, biomass burning particles, PAHs) in atmospheric aerosols... but also at the air/ocean interface. The presence of such light absorbing material allows photosensitised processes to occur. The latter may be sources of radicals in the troposphere that may alter its oxidation capacity (as for instance through the formation of HONO) or change the pathways leading to particle formation and ageing. We will exemplify such processes by presenting new data on the generation of radicals from dust particles or at the air/sea interface leading to particle formation in presence of SO2. These are recent findings that are questioning our current understanding of tropospheric photochemistry.

  12. Synthesis and Characterization of [n]CPP (n = 5, 6, 8, 10, and 12) Radical Cation and Dications: Size-Dependent Absorption, Spin, and Charge Delocalization.

    PubMed

    Kayahara, Eiichi; Kouyama, Takahiko; Kato, Tatsuhisa; Yamago, Shigeru

    2016-01-13

    Radical cations and dications of [n]cyclo-p-phenylenes ([n]CPPs, n = 5, 6, 10, and 12), which are the models of those of linear oligo-p-phenylenes without a terminus, were synthesized as hexafluoroantimonate salts by the one- and two-electron chemical oxidation of CPP by NOSbF6 or SbF5. The radical cations, [n]CPP(•+), and dications, [n]CPP(2+), exhibited remarkable bathochromic shifts in their UV-vis-NIR absorption bands, suggesting that [n]CPP(•+) and larger [n]CPP(2+) exhibit longer polyene character than the shorter analogues. The larger bathochromic shift was consistent with the narrower HOMO-SOMO and HOMO-LUMO gaps in larger [n]CPP(•+) and [n]CPP(2+), respectively. In [n]CPP(•+), the spins and charges were equally and fully delocalized over the p-phenylene rings of the CPPs, as noted by ESR. (1)H NMR revealed that the hydrogen of [n]CPP(2+) shifted to a high magnetic field from the neutral compounds due to the diamagnetic ring current derived from the in-plane aromaticity of [n]CPP(2+). The single resonances observed in all [n]CPP(2+) strongly suggest the complete delocalization of the charges over the CPPs. Furthermore, the contribution of biradical character was clarified for [10]- and [12]CPP by VT-NMR experiment and theoretical calculation. PMID:26675620

  13. [Riboflavin-radical formation by mechanochemical solid-state reaction using stainless steel vessel].

    PubMed

    Kondo, Shin-ichi; Furuta, Youji; Okita, Shintarou; Sasai, Yasushi; Aramaki, Hideki; Kuzuya, Masayuki

    2004-03-01

    The mechanochemical reaction of free riboflavin (FR) due to vibratory ball milling was carried out in a stainless steel vessel at room temperature under anaerobic conditions. The ESR of the fractured sample showed a broad single-line spectrum. It is suggested that the solid-state single-electron transfer (SSET) reaction from the surface of the stainless steel vessel to FR proceeded during the vibratory milling, resulting in the formation of the corresponding anion radicals. When the mechanochemical reaction of FR in the presence of calcium pantothenate (PC) was carried out, the radical concentration increased with the increasing PC content. It was shown that the anion radical in the metal complex was stable for a lengthy period of time even in highly humid air. PMID:15049132

  14. Photoinduced formation of peroxyl radicals in aqueous solutions of nucleobase derivatives at 77 K

    NASA Astrophysics Data System (ADS)

    Lozinova, T. A.; Lander, A. V.

    2015-05-01

    It is shown that the formation of free radicals photoinduced by near-UV irradiation at 77 K in aqueous solutions of guanosine-5'-monophosphate (GMP), adenosine (Ado), adenine (A), and thymine (T) containing NaCl (0.1 M) is intensified in the presence of O2. Signals of peroxyl radicals O{2/-·} and HO{2/·} are indicated in analyzing EPR spectra, and their overall yield is 20-40% of the total amount of the formed paramagnetic products. It is found that the concentration ratio of O{2/-·} and HO{2/·} radicals depends not only on pH of the solutions before freezing but also on the duration of irradiation and method of freezing the samples. Plausible mechanisms of the processes are discussed.

  15. Acid-base equilibria involved in secondary reactions following the 4-carboxybenzophenone sensitized photooxidation of methionylglycine in aqueous solution. Spectral and time resolution of the decaying (S...N){sup +} radical cation

    SciTech Connect

    Hug, G.L.; Marciniak, B. |; Bobrowski, K. ||

    1996-09-05

    A radical cation with an intramolecular sulfur-nitrogen bond was formed in the photoinitiated transfer of an electron from the sulfur atom of the dipeptide Met-Gly to 4-carboxybenzophenone in its triplet state. The sulfur-nitrogen coupling involved two-center, three-electron bonds. The kinetics of the reactions of these radical cations, which were initiated by a laser flash, were followed over time. The principal method of implementing the spectral resolutions was accomplished through a multiple linear regression technique. This spectral analysis was repeated for numerous time windows during the lifetime of the transients` decays. The resulting concentrations of the transients were consistent with an independent factor analysis. It was found that the decay of the radical cations was multiexponential and that the decay varied with pH. A simplified reaction scheme was proposed whereby the absorbing radical cations can alternatively decay by an irreversible channel or react reversibly with OH{sup -}. Rate constants for the three elementary reactions of this scheme were determined from an analysis of the decay of the concentration of the radical cations. In addition, the equilibrium constant for the reversible reaction was determined by two separate procedures. 35 refs., 7 figs., 2 tabs.

  16. Absence of an effect of vitamin E on protein and lipid radical formation during lipoperoxidation of LDL by lipoxygenase

    PubMed Central

    Ganini, Douglas; Mason, Ronald P.

    2014-01-01

    LDL oxidation is the primary event in atherosclerosis, where LDL lipoperoxidation leads to modifications in the apolipoprotein B-100 (apo B-100) and lipids. Intermediate species of lipoperoxidation are known to be able to generate amino acid-centered radicals. Thus, we hypothesized that lipoperoxidation intermediates induce protein-derived free radical formation during LDL oxidation. Using DMPO and immuno spin-trapping, we detected the formation of protein free radicals on LDL incubated with Cu2+ or the soybean lipoxidase (LPOx)/phospholipase A2 (PLA2). With low concentrations of DMPO (1 mM), Cu2+ dose-dependently induced oxidation of LDL and easily detected apo B-100 radicals. Protein radical formation in LDL incubated with Cu2+ showed maximum yields after 30 minutes. In contrast, the yields of apo B-100-radicals formed by LPOx/PLA2 followed a typical enzyme-catalyzed kinetics that was unaffected by DMPO concentrations of up to 50 mM. Furthermore, when we analyzed the effect of antioxidants on protein radical formation during LDL oxidation, we found that ascorbate, urate and Trolox dose-dependently reduced apo B-100-free radical formation in LDL exposed to Cu2+. In contrast, Trolox was the only antioxidant that even partially protected LDL from LPOx/PLA2. We also examined the kinetics of lipid radical formation and protein radical formation induced by Cu2+ or LPOx/PLA2 for LDL supplemented with α-tocopherol. In contrast to the potent antioxidant effect of α-tocopherol on the delay of LDL oxidation induced by Cu2+, when we used the oxidizing system LPOx/PLA2, no significant protection was detected. The lack of protection of α-tocopherol on the apo B-100 and lipid free radical formation by LPOx may explain the failure of vitamin E as a cardiovascular protective agent for humans. PMID:25091900

  17. Role of the Filters in the Formation and Stabilization of Semiquinone Radicals Collected from Cigarette Smoke

    PubMed Central

    Maskos, Zofia; Dellinger, Barry

    2013-01-01

    The fractional pyrolysis of Bright tobacco was performed in nitrogen atmosphere over the temperature range of 240 – 510 °C in a specially constructed, high temperature flow reactor system. Electron paramagnetic resonance (EPR) spectroscopy was used to analyze the free radicals in the initially produced total particular matter (TPM) and in TPM after exposure to ambient air (aging). Different filters have been used to collect TPM from tobacco smoke: cellulosic, cellulose nitrate, cellulose acetate, nylon, Teflon and Cambridge. The collection of the primary radicals (measured immediately after collection of TPM on filters), the formation and stabilization of the secondary radicals (defined as radicals formed during aging of TPM samples on the filters) depend significantly on the material of the filter. A mechanistic explanation about different binding capability of the filters decreasing in the order: cellulosic < cellulose nitrate < cellulose acetate < nylon ~ teflon is presented. Different properties were observed for the Cambridge filter. Specific care must be taken using the filters for identification of radicals from tobacco smoke to avoid artifacts in each case. PMID:24265513

  18. The inhibitory effects of γ-glutamylcysteine derivatives from fresh garlic on glycation radical formation.

    PubMed

    Shi, Fei; Bai, Bing; Ma, Shufeng; Ji, Shujuan; Liu, Ling

    2016-03-01

    The effects of three reactive peptides, γ-glutamylmethylcysteine (γ-GMC), γ-glutamylpropylcysteine (γ-GPC), and γ-glutamylbutylcysteine (γ-GBC) on the suppression of reactive radicals during the heating of l-lysine in the presence or absence of glucose was studied by electron spin resonance spectroscopy. γ-GMC and γ-GPC were extracted from fresh garlic, and γ-GBC was a synthetic peptide. The results showed that γ-GMC and γ-GPC effectively suppress formation of l-lysine radicals, but that γ-GBC exhibits low radical inhibition. The origin of the short peptides, and the length of their side chain, influenced their surface hydrophobicity and subsequent radical inhibition. In addition, the oxidation of l-lysine was inhibited by the peptides in a similar manner to their inhibition of the Maillard reaction (MR), and their radical inhibition was consistent with similar activity towards N(ε)-(carboxymethyl)lysine (CML). PMID:26471590

  19. Properties of compositional volume grating formation with photoinitiated cationic-ring-opening polymerization

    NASA Astrophysics Data System (ADS)

    Paraschis, Loukas; Sugiyama, Yasuyuki; Akella, Annapoorna; Honda, Tokuyuki; Hesselink, Lambertus

    1998-11-01

    We investigated compositional volume grating formation in the Polaroid medium that utilizes the cationic-ring-opening photoinitiated polymerization process, and compared our conclusions with the current physical model describing polymer holographic recording. We identified the effects of diffusion and polymerization during illumination, as well as significant postexposure grating development. Holographic recording in this medium allows for final strong gratings with high recording sensitivity (S approximately 2 cm/mJ), that were not limited at the higher recording intensities (I less than or equal to 250 mW/cm2) corresponding to photon (exposure) limited recording. The results of the present analysis allow for more comprehensive physical description of grating formation in the photoinitiated CROP process, and evaluation of the polymer recording process in a nonvolatile holographic storage system.

  20. Effect of electrolytes and temperature on dications and radical cations of carotenoids: Electrochemical, optical absorption, and high-performance liquid chromatography studies

    SciTech Connect

    He, Z.; Kispert, L.D.

    1999-11-25

    The effect of supporting electrolytes and temperature on the behavior of dications and radical cations of carotenoids is studied. Cyclic voltammograms (CVs) of canthaxanthin (I) at 23 and {minus}25 C show that Car{sup sm{underscore}bullet+} of I has similar stability during the time of the CV scan, when using tetrabutylammonium perchlorate (TBAPC), tetrabutylammonium tetrafluoroborate (TBATFB), or tetrabutylammonium hexafluorophosphate (TBAHFP) as supporting electrolyte. However, the stability of Car{sup 2+} decreases when using TBAPC or TBATFB; {beta}-carotene (II) shows similar behavior. The CV of I at {minus}25 C shows a strong cathodic wave (wave 6) near {minus}0.15 V (vs Ag) with an intensity about half that of the neutral oxidation wave when TBAPC or TBATFB is the supporting electrolyte. When TBAHFP is used, wave 6 (ca. {minus}0.05 V vs Ag) is ca. 8 times weaker than when TBAPC or TBATFB is used. This wave results from the reduction of a species that may be a decay product of Car{sup 2+} of I. Results show that these electrolytes commonly used in electrochemical studies may affect the studied systems to different extents. In simultaneous bulk electrolysis (BE) and optical absorption spectroscopic measurements, the absorption band of Car{sup 2+} of I in the presence of 0.1 M TBAHFP can be observed by lowering the BE temperature to {minus}20 C. In the presence of 0.1 M TBAPC or TBATFB, this band is not observed, even at {minus}50 C. Isomerization of neutral I (as shown by HPLC and its blue absorption band shift) is observed only when the Car{sup 2+} absorption band is absent during BE. This observation, along with an increase of the neutral absorption band after stopping BE, suggests that the equilibrium Car + Car{sup 2+} {r{underscore}equilibrium} 2Car{sup {sm{underscore}bullet}+} is shifted to the left because Car{sup 2+} decays more quickly than Car{sup {sm{underscore}bullet}+} in the presence of electrolyte and this is a major path for formation of cis

  1. Chemistry of polycyclic aromatic hydrocarbons formation from phenyl radical pyrolysis and reaction of phenyl and acetylene.

    PubMed

    Comandini, A; Malewicki, T; Brezinsky, K

    2012-03-15

    An experimental investigation of phenyl radical pyrolysis and the phenyl radical + acetylene reaction has been performed to clarify the role of different reaction mechanisms involved in the formation and growth of polycyclic aromatic hydrocarbons (PAHs) serving as precursors for soot formation. Experiments were conducted using GC/GC-MS diagnostics coupled to the high-pressure single-pulse shock tube present at the University of Illinois at Chicago. For the first time, comprehensive speciation of the major stable products, including small hydrocarbons and large PAH intermediates, was obtained over a wide range of pressures (25-60 atm) and temperatures (900-1800 K) which encompass the typical conditions in modern combustion devices. The experimental results were used to validate a comprehensive chemical kinetic model which provides relevant information on the chemistry associated with the formation of PAH compounds. In particular, the modeling results indicate that the o-benzyne chemistry is a key factor in the formation of multi-ring intermediates in phenyl radical pyrolysis. On the other hand, the PAHs from the phenyl + acetylene reaction are formed mainly through recombination between single-ring aromatics and through the hydrogen abstraction/acetylene addition mechanism. Polymerization is the common dominant process at high temperature conditions. PMID:22339468

  2. Polycyclic aromatic hydrocarbon (PAH) formation from benzyl radicals: a reaction kinetics study.

    PubMed

    Sinha, Sourab; Raj, Abhijeet

    2016-03-01

    The role of resonantly stabilized radicals such as propargyl, cyclopentadienyl and benzyl in the formation of aromatic hydrocarbons such as benzene and naphthalene in the high temperature environments has been long known. In this work, the possibility of benzyl recombination to form three-ring aromatics, phenanthrene and anthracene, is explored. A reaction mechanism for it is developed, where reaction energetics are calculated using density functional theory (B3LYP functional with 6-311++G(d,p) basis set) and CBS-QB3, while temperature-dependent reaction kinetics are evaluated using transition state theory. The mechanism begins with barrierless formation of bibenzyl from two benzyl radicals with the release of 283.2 kJ mol(-1) of reaction energy. The further reactions involve H-abstraction by a H atom, H-desorption, H-migration, and ring closure to gain aromaticity. Through mechanism and rate of production analyses, the important reactions leading to phenanthrene and anthracene formation are determined. Phenanthrene is found to be the major product at high temperatures. Premixed laminar flame simulations are carried out by including the proposed reactions for phenanthrene formation from benzyl radicals and compared to experimentally observed species profiles to understand their effects on species concentrations. PMID:26923612

  3. Aqueous SOA formation from radical oligomerization of methyl vinyl ketone (MVK) and methacrolein (MACR)

    NASA Astrophysics Data System (ADS)

    Renard, P.; Siekmann, F.; Ravier, S.; Temime-Roussel, B.; Clément, J.; Ervens, B.; Monod, A.

    2013-12-01

    It is now accepted that one of the important pathways of secondary organic aerosol (SOA) formation occurs through aqueous phase chemistry in the atmosphere. However, the chemical mechanisms leading to macromolecules are still not well understood. It was recently shown that oligomer production by OH radical oxidation in the aerosol aqueous phase from α-dicarbonyl precursors, such as methylglyoxal and glyoxal, is irreversible and fast. We have investigated the aqueous phase photooxidation of MACR and MVK, which are biogenic organic compounds derived from isoprene. Aqueous phase photooxidation of MVK and MACR was investigated in a photoreactor using photolysis of H2O2 as OH radical source. Electrospray high resolution mass spectrometry analysis of the solutions brought clear evidence for the formation of oligomer systems having a mass range of up to 1800 Da within less than 15 minutes of reaction. Highest oligomer formation rates were obtained under conditions of low dissolved oxygen, highest temperature (T = 298 K) and highest precursor initial concentrations ([MVK]0 = 20 mM). A radical mechanism of oligomerization is proposed to explain the formation of the high molecular weight products. Furthermore, we quantified the total amount of carbon present in oligomers. Kinetic parameters of the proposed oligomerization mechanism are constrained by means of a box model that is able to reproduce the temporal evolution of intermediates and products as observed in the laboratory experiments. Additional model simulations for atmospherically-relevant conditions will be presented that show the extent to which these radical processes contribute to SOA formation in the atmospheric multiphase system as compared to other aqueous phase as well as traditional SOA sources. MVK time profile (as measured by UV Spectroscopy) and mass spectra (obtained using UPLC-ESI-MS for the retention time range 0-5 min in the positive mode) at 5, 10 and 50 min of reaction (MVK 20 mM, 25° C, under

  4. Formation of radical species in photolyzed CH 4:N 2 ices

    NASA Astrophysics Data System (ADS)

    Hodyss, Robert; Howard, Heather R.; Johnson, Paul V.; Goguen, Jay D.; Kanik, Isik

    2011-08-01

    We report photochemical studies of thin cryogenic ice films composed of N 2, CH 4 and CO in ratios analogous to those on the surfaces of Neptune's largest satellite, Triton, and on Pluto. Experiments were performed using a hydrogen discharge lamp, which provides an intense source of ultraviolet light to simulate the sunlight-induced photochemistry on these icy bodies. Characterization via infrared spectroscopy showed that C 2H 6 and C 2H 2, and HCO are formed by the dissociation of CH 4 into H, CH 2 and CH 3 and the subsequent reaction of these radicals within the ice. Other radical species, such as C 2, C2-, CN, and CNN, are observed in the visible and ultraviolet regions of the spectrum. These species imply a rich chemistry based on formation of radicals from methane and their subsequent reaction with the N 2 matrix. We discuss the implications of the formation of these radicals for the chemical evolution of Triton and Pluto. Ultimately, this work suggests that C2-, CN, HCO, and CNN may be found in significant quantities on the surfaces of Triton and Pluto and that new observations of these objects in the appropriate wavelength regions are warranted.

  5. Rearrangement as a probe for radical formation: bromomethylcyclopropane on oxygen-covered Mo(1 1 0)

    NASA Astrophysics Data System (ADS)

    Levinson, J. A.; Kretzschmar, I.; Sheehy, M. A.; Deiner, L. J.; Friend, C. M.

    2001-05-01

    The reactions of bromomethylcyclopropane on oxygen-covered Mo(1 1 0) were studied in order to investigate the lifetimes of radical intermediates, which are important in heterogeneous oxidation catalysis. The methylcyclopropyl radical is known to rearrange on the nanosecond time scale, providing us with a means of probing for radical formation. Surprisingly, no rearrangement occurs subsequent to C-Br bond dissociation, which commences at ˜220 K. Instead, displacement of bromine by oxygen occurs to yield adsorbed methylcyclopropoxide, which is identified using infrared spectroscopy. The C-O bond of methylcyclopropoxide is cleaved at ˜400 K to yield a transient methylcyclopropyl radical. As shown previously, the methylcyclopropyl radical rearranges and the ring-opened butenyl species is trapped on the surface. Addition to oxygen yields 3-buten-1-oxy and addition to the metal affords the butenyl-Mo moiety. Infrared spectroscopy is used to identify these intermediates. The same linear species are formed from the reaction of 4-bromo-1-butene. The 3-buten-1-oxy species is also formed from reactions of 3-buten-1-ol on O-covered Mo(1 1 0). Upon further heating, the 3-buten-1-oxy reacts to form 1,3-butadiene, 1-butene, water, and dihydrogen between 450 and 600 K. Ethene is also evolved at ˜560 K. The primary mechanism for ethene evolution is elimination from metal-bound butenyl. Carbon monoxide is also formed above 900 K, due to reaction of surface carbon and oxygen. The implications of our results for studies where alkyl halides are used as models for radical reactions on surfaces are discussed.

  6. Charge transfer complexes of fullerenes containing C60˙(-) and C70˙(-) radical anions with paramagnetic Co(II)(dppe)2Cl(+) cations (dppe: 1,2-bis(diphenylphosphino)ethane).

    PubMed

    Konarev, Dmitri V; Troyanov, Sergey I; Otsuka, Akihiro; Yamochi, Hideki; Saito, Gunzi; Lyubovskaya, Rimma N

    2016-04-12

    The reduction of Co(II)(dppe)Cl2 with sodium fluorenone ketyl produces a red solution containing the Co(I) species. The dissolution of C60 in the obtained solution followed by the precipitation of crystals with hexane yields a salt {Co(I)(dppe)2(+)}(C60˙(-))·2C6H4Cl2 and a novel complex {Co(dppe)2Cl}(C60) (). With C70, only the crystals of {Co(dppe)2Cl}(C70)·0.5C6H4Cl2 () are formed. Complex contains zig-zag fullerene chains whereas closely packed double chains are formed from fullerenes in . According to the optical spectra and magnetic data charge transfer occurs in both and with the formation of the Co(II)(dppe)2Cl(+) cations and the C60˙(-) or C70˙(-) radical anions. In spite of the close packing in crystals, C60˙(-) or C70˙(-) retain their monomeric form at least down to 100 K. The effective magnetic moments of and of 1.98 and 2.27μB at 300 K, respectively, do not attain the value of 2.45μB expected for the system with two non-interacting S = 1/2 spins at full charge transfer to fullerenes. Most probably diamagnetic {Co(I)(dppe)2Cl}(0) and neutral fullerenes are partially preserved in the samples which can explain the weak magnetic coupling of spins and the absence of fullerene dimerization in both complexes. The EPR spectra of and show asymmetric signals approximated by several lines with g-factors ranging from 2.0009 to 2.3325. These signals originate from the exchange interaction between the paramagnetic Co(II)(dppe)2Cl(+) cations and the fullerene˙(-) radical anions. PMID:26956368

  7. Comparison between sonodynamic effect and photodynamic effect with photosensitizers on free radical formation and cell killing.

    PubMed

    Hiraoka, Wakako; Honda, Hidemi; Feril, Loreto B; Kudo, Nobuki; Kondo, Takashi

    2006-09-01

    Although enhancement of ultrasound-induced cell killing by photodynamic reagents has been shown, the sonochemical mechanism in detail is still not clear. Here, comparison between sonodynamic effect and photodynamic effect with photosensitizers at a concentration of 10 microM on free radical formation and cell killing was made. When electron paramagnetic-resonance spectroscopy (EPR) was used to detect 2,2,6,6-tetramethyl-4-piperidone-N-oxyl (TAN) after photo-irradiation or sonication with 2,2,6,6-tetramethyl-4-piperidone (TMPD), the order of TAN formation in the photo-irradiated samples was as follows: rhodamine 6G (R6) > sulforhodamine B (SR) > hematoporphyrin (Hp) > rhodamine 123 (R123) > rose bengal (RB)>erythrosine B (Er) = 0; although there was time-dependent TAN formation when the samples were sonicated, no significant difference among these agents were observed. All these agents suppressed ultrasound-induced OH radical formation detected by EPR-spin trapping. Sensitizer-derived free radicals were markedly observed in SR, RB and Er, while trace level of radicals derived from R6 and R123 were observed. Enhancement of ultrasound-induced decrease of survival in human lymphoma U937 cells was observed at 1.5 W/cm(2) (less than inertial cavitation threshold) for R6, R123, SR and Er, and at 2.3 W/cm(2) for R6, R123, Er, RB and SR. On the other hand, photo-induced decrease of survival was observed for R6, Hp and RB at the same concentration (10 microM). These comparative results suggest that (1) (1)O(2) is not involved in the enhancement of ultrasound-induced loss of cell survival, (2) OH radicals and sensitizer-derived free radicals do not take part in the enhancement, and (3) the mechanism is mainly due to certain mechanical stress such as augmentation of physical disruption of cellular membrane by sensitizers in the close vicinity of cells and/or cavitation bubbles. PMID:16325451

  8. Effect of Base Stacking on the Acid-Base Properties of the Adenine Cation Radical [A•+] in Solution: ESR and DFT Studies

    PubMed Central

    Adhikary, Amitava; Kumar, Anil; Khanduri, Deepti

    2015-01-01

    In this study, the acid–base properties of the adenine cation radical are investigated by means of experiment and theory. Adenine cation radical (A•+) is produced by one-electron oxidation of dAdo and of the stacked DNA-oligomer (dA)6 by Cl2•− in aqueous glass (7.5 M LiCl in H2O and in D2O) and investigated by ESR spectroscopy. Theoretical calculations and deuterium substitution at C8–H and N6–H in dAdo aid in our assignments of structure. We find the pKa value of A•+ in this system to be ca. 8 at 150 K in seeming contradiction to the accepted value of ≤ 1 at ambient temperature. However, upon thermal annealing to ≥160 K, complete deprotonation of A•+ occurs in dAdo in these glassy systems even at pH ca. 3. A•+ found in (dA)6 at 150 K also deprotonates on thermal annealing. The stability of A•+ at 150 K in these systems is attributed to charge delocalization between stacked bases. Theoretical calculations at various levels (DFT B3LYP/6-31G*, MPWB95, and HF-MP2) predict binding energies for the adenine stacked dimer cation radical of 12 to 16 kcal/mol. Further DFT B3LYP/6-31G* calculations predict that, in aqueous solution, monomeric A•+ should deprotonate spontaneously (a predicted pKa of ca. −0.3 for A•+). However, the charge resonance stabilized dimer AA•+ is predicted to result in a significant barrier to deprotonation and a calculated pKa of ca. 7 for the AA•+ dimer which is 7 pH units higher than the monomer. These theoretical and experimental results suggest that A•+ isolated in solution and A•+ in adenine stacks have highly differing acid–base properties resulting from the stabilization induced by hole delocalization within adenine stacks. PMID:18611019

  9. Complexes of DNA with cationic peptides: conditions of formation and factors effecting internalization by mammalian cells.

    PubMed

    Dizhe, E B; Ignatovich, I A; Burov, S V; Pohvoscheva, A V; Akifiev, B N; Efremov, A M; Perevozchikov, A P; Orlov, S V

    2006-12-01

    This work was devoted to the study of conditions of the formation of DNA/K8 complex and analysis of factors effecting the entry of DNA/K8 complex into mammalian cells in comparison with DNA complexes with arginine-rich fragment (47-57) of human immunodeficiency virus (type 1) transcription factor Tat (Tat peptide). The stoichiometry of positively charged DNA/K8 complexes has been studied for the first time. Non-cooperative character of DNA-K8 interaction was revealed. It has been shown that along with the positive charge of such complexes, the presence of an excess of free K8 peptide in the culture medium is a necessary condition for maximal efficiency of cell transfection with DNA/K8 complexes. A stimulatory effect of free K8 peptide on the efficiency of mammalian cell transfection by DNA/K8 complexes is likely to be mediated by the interactions of cationic peptide K8 with negatively charged proteoglycans on the cell surface, which leads to protection of DNA/K8 complexes from disruption by cellular heparan sulfates. However, the protective role of free cationic peptides depends not only on their positive charge, but also on the primary structure of the peptide. In contrast with the results obtained for DNA complexes with molecular conjugates based on poly-L-lysine, the aggregation of DNA/K8 complexes leads to a significant increase in the expression of transferred gene. PMID:17223788

  10. Visible-light-promoted iminyl radical formation from vinyl azides: synthesis of 6-(fluoro)alkylated phenanthridines.

    PubMed

    Sun, Xiaoyang; Yu, Shouyun

    2016-09-18

    An efficient strategy assisted by visible-light-promoted iminyl radical formation has been developed for the synthesis of 6-(fluoro)alkylated phenanthridine derivatives. In the reactions, addition of alkyl and trifluoromethyl radicals onto vinyl azides gives iminyl radicals, which then undergo intramolecular homolytic aromatic substitution leading to phenanthridines. These reactions can be carried out under mild conditions with high chemical yields and broad substrate scope. PMID:27530901