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ABSTRACT: Nanosecond laser flash photolysis was used to generate sulfur radical cations of the thioether, S-methylglutathione (S-Me-Glu), via the one-electron oxidation of this thioether by triplet 4-carboxybenzophenone. The purpose of this investigation was to follow the neighboring group effects resulting from the interactions between the sulfur radical cationic sites and nearby lone-pair electrons on heteroatoms within the radical cation, especially the electron lone-pairs on heteroatoms in the peptide bonds. The tripeptide, S-Me-Glu, offers several possible competing neighboring group effects that are characterized in this work. Quantum yields of the various radicals and three-electron bonded (both intramolecular and intermolecular) species were determined. The pH dependence of photoinduced decarboxylation yields was used as evidence for the identification of a nine-membered ring, sulfur-nitrogen, three-electron bonded species. The mechanisms of the secondary reactions of the radicals and radical cations were characterized by resolving their overlapping transient-absorption spectra and following their kinetic behavior. In particular, sulfur - oxygen and sulfur - nitrogen three electron bonded species were identified where the oxygen and nitrogen atoms were in the peptide bonds.
The Journal of Physical Chemistry B 01/2013; · 3.70 Impact Factor
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ABSTRACT: The reactions of ketone/methionine systems are widely used as efficient and selective sources of biorelevant radical species. In this study, we address intramolecular variants of this couple with respect to its photosynthetic utility and as a mechanistic model of underlying elementary reaction steps of biological importance, especially with respect to the study of photoinitiated electron transport in complex peptides. The outcomes of this study are two-fold: (1) steady-state irradiation of sterically constrained benzophenone/methionine dyads afforded stable photocyclization products with high yield and product selectivity. (2) Mechanistic insights into the triplet-triggered product formation were obtained from an analysis of the flash photolysis results and the molecular structure of the stable product formed upon irradiation. Time-resolved experiments identified (net) hydrogen-atom transfer from the methionine as the mechanism of the triplet quenching and the resulting biradicals as the major precursor of the isolated stable product. Both the analyses of triplet quenching and stable-product formation in the diastereomeric pairs point to effects of chiral center configuration, i.e., significant stereoselectivity is observed for all elementary steps. The underlying stereochemical restraints were quantitatively addressed by means of molecular dynamics simulations.
Photochemistry and Photobiology 01/2013; 89(1):14-23. · 2.41 Impact Factor
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ABSTRACT: Pulse radiolysis of acetonitrile solutions of tetra-n-butyl ammonium salts of 2- and 4-carboxybenzophenones [BP-COO−···N+(C4H9)4] were performed in order to generate directly the reduced forms of the benzophenone moieties within pre-formed ion pairs.
In earlier studies on photochemical electron transfer reactions, ion pairs containing a tetraalkyl ammonium cation and a benzophenone
radical anion were formed in an electron transfer to the triplet BP from a quencher consisting of a tetraalkyl ammonium salt
of (phenylthio)acetic acid. In the current work, the [BP•−COO−···N+(C4H9)4] ion pairs were formed by direct reduction of the salts without the complication of a third moiety, i.e., the (phenylthio)acetic
anion. The spectra and kinetic parameters of the radiolytically-reduced salts were compared to the behavior of reduced forms
of the 2- and 4-COOH substituted benzophenones. The results from the pulse radiolysis and photochemistry were compared and
explained in terms of the different structures of the ion pairs.
Research on Chemical Intermediates 04/2012; 35(4):389-399. · 0.70 Impact Factor
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ABSTRACT: The mechanism of the photoinduced reaction of the lowest excited singlet state of the 10-methylacridinium (AcrMe+) cation with benzyltrimethylsilane (BTMSi) in acetonitrile has been investigated by means of steady-state and time-resolved The mechanism of the photoinduced reaction of the lowest excited singlet state of the 10-methylacridinium (AcrMe+) cation with benzyltrimethylsilane (BTMSi) in acetonitrile has been investigated by means of steady-state and time-resolved
methods. A variety of stable products was found after irradiation (365nm) of the reaction mixture under aerobic and oxygen-free methods. A variety of stable products was found after irradiation (365nm) of the reaction mixture under aerobic and oxygen-free
conditions. The stable products were identified and analyzed using UV–Vis spectrophotometry, high performance liquid chromatography conditions. The stable products were identified and analyzed using UV–Vis spectrophotometry, high performance liquid chromatography
(HPLC), and mass spectrometry (MS). Based on Stern–Volmer plots of the AcrMe+ fluorescence quenching by BTMSi (using fluorescence intensity and lifetime measurements), the rate constants were determined (HPLC), and mass spectrometry (MS). Based on Stern–Volmer plots of the AcrMe+ fluorescence quenching by BTMSi (using fluorescence intensity and lifetime measurements), the rate constants were determined
to be k to be k
q=1.24 (±0.02)×1010M−1s−1 and k q=1.24 (±0.02)×1010M−1s−1 and k
q=1.23 (±0.02)×1010M−1s−1, i.e., close to the diffusion-controlled limit in acetonitrile, indicating the dynamic quenching mechanism. The quenching q=1.23 (±0.02)×1010M−1s−1, i.e., close to the diffusion-controlled limit in acetonitrile, indicating the dynamic quenching mechanism. The quenching
process was shown to occur via an electron-transfer reaction leading to the formation of acridinyl radicals (AcrMe•) and C6H5CH2Si(CH3)3 process was shown to occur via an electron-transfer reaction leading to the formation of acridinyl radicals (AcrMe•) and C6H5CH2Si(CH3)3
•+ radical cations. Based on stationary and flash photolysis experiments, a detailed mechanism of the secondary reactions is •+ radical cations. Based on stationary and flash photolysis experiments, a detailed mechanism of the secondary reactions is
proposed and discussed. The AcrMe• radical was shown to decay by two processes. The fast decay, observed on the nanosecond timescale, was attributed to the proposed and discussed. The AcrMe• radical was shown to decay by two processes. The fast decay, observed on the nanosecond timescale, was attributed to the
back-electron transfer occurring within the initial radical ion pair. The slow decay on the microsecond timescale was explained back-electron transfer occurring within the initial radical ion pair. The slow decay on the microsecond timescale was explained
by recombination reactions of radicals which escaped from the radical pair, including benzyl radicals formed via C–Si bond by recombination reactions of radicals which escaped from the radical pair, including benzyl radicals formed via C–Si bond
cleavage in the C6H5CH2Si(CH3)3 cleavage in the C6H5CH2Si(CH3)3
•+ radical cation. •+ radical cation.
Research on Chemical Intermediates 04/2012; 35(4):351-361. · 0.70 Impact Factor
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ABSTRACT: Radicals formed in γ-irradiated 1,3,5-trithiane (TT) and its three derivatives, α- and β-2,4,6-trimethyl-1,3,5-trithiane (α-TMT
and β-TMT), and 2,4,6-trimethyl-2,4,6-triphenyl-1,3,5-trithiane (TMTPT), were studied by the electron paramagnetic resonance
(EPR) method in the solid state. The sulfur radical cations (>S+•) were identified in all compounds at 77K. In TT and its two derivatives, α-TMT and β-TMT, the >S+• decay via deprotonation-forming C-centered radicals. Further increase of temperature up to 293K results in the appearance
of thiyl-type radicals (RS•). In TMTPT, the >S+• are stable up to 250K. They formed the intermolecularly three-electron-bonded dimeric radical cations (S∴S)+ while RS• radicals were not observed. Some of the radical assignments and their EPR parameters (g and a hyperfine splittings) obtained support from the DFT calculations.
Research on Chemical Intermediates 04/2012; 35(4):507-517. · 0.70 Impact Factor
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ABSTRACT: The early events in the triplet 4-carboxybenzophenone (CB)-induced oxidation of N-acetyl-methionine methyl ester (N-Ac-Met-OCH3) are investigated in aqueous solution. Upon electron transfer from the methionine residue of N-Ac-Met-OCH3 to 3CB*, the resulting sulfur radical cation undergoes further reactions: (1) back electron transfer, (2) escape of the radical
ions from the solvent cage, or (3) proton transfer and escape of the radicals. The yields and paths of these reactions are
shown to depend strongly on the pH of the solution, and, similar to the previously reported results for dipeptides (Met-Gly
and Gly-Met), on the structural nature of the methionine substituents. In the experiments performed in this work, low quencher
concentrations were used to avoid formation of intermolecular transients (e.g., dimeric sulfur-centered radical cation (S∴S)+). Under these experimental conditions, the one-electron oxidized sulfur does not seem to become stabilized in an (S∴N)+ three-electron bonded intramolecular complex. The proposed mechanism is further supported by the stable products analysis.
A detailed mechanism involving characterization of the transients is discussed and compared to that of methionine and methionine-containing
dipeptides (Met-Gly and Gly-Met). Moreover, a newly installed transient absorption laser system is described in details.
Research on Chemical Intermediates 04/2012; 35(4):497-506. · 0.70 Impact Factor
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ABSTRACT: The rates and the mechanism of intramolecular hydrogen-atom transfer across the rigid diketopiperazine spacer in two epimeric benzophenone-tyrosine dyads were studied by nanosecond laser flash photolysis in 12 non-protic and protic solvents. Effects of the stereochemistry on the ground-state structures and conformational equilibria and dynamics have been addressed by means of NMR-based population analysis and long-term molecular-dynamics simulations. The excited triplet state of the benzophenone is found to be quenched intramolecularly by the remote phenol moiety with rates that are highly dependent on the solvent and on the relative benzophenone-phenol orientation. In agreement with previous studies, the kinetic diversity, with rates ranging from<105 s−1 to 2× 107 s−1, can be attributed to retarding effects of the bulk viscosity and to effects of specific solvation, which can be both rate retarding and rate accelerating. As an extension of the previous knowledge, however, the phenomenology of the kinetic solvent effects, that is, the nature of the predominating solvent parameter and its absolute impact on the reactivity, is found to be highly correlated with structural effects.Utilisant la photolyse laser éclair, au niveau de la nanoseconde, on a étudié les vitesses et le mécanisme de réaction du transfert d'atome d'hydrogène intramoléculaire à travers l'espaceur dicétopipérazine rigide dans deux dyades de benzophénone-tyrosine dans douze solvants protiques et aprotiques. Faisant appel à l'analyse de populations à l'aide de la RMN et de simulations de dynamique moléculaire à long terme, on a étudié les effets de la stéréochimie sur les structures de l'état fondamental et sur la dynamique et les équilibres conformationnels. On a trouvé que l'état triplet excité de la benzophénone est piégé d'une façon intramoléculaire par la portion phénolique éloignée avec des vitesses qui dépendent beaucoup du solvant et des orientations relatives de la benzophénone et du phénol. En accord avec les études antérieures, la diversité cinétique des vitesses qui varient de moins de 105 s-1 à 2 × 107 s-1 peut être attribuée aux effets de viscosité globale qui les diminuent et aux effets de solvatation spécifique qui peuvent soit les accélérer ou les retarder. Toutefois, comme extension aux connaissances acquises antérieurement, on a trouvé que la phénoménologie des effets cinétiques du solvant, c'est-à-dire la nature du paramètre prédominant du solvant et son impact absolu sur la réactivité, est intimement reliée aux effets structuraux.
Canadian Journal of Chemistry 02/2011; 89(3):266-279. · 1.24 Impact Factor
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ABSTRACT: The dynamics of the bimolecular quenching of triplet excited benzophenone by anisole was studied by nanosecond flash photolysis. We carried out a detailed study of the solvent dependence of the reaction rates and efficiencies in a number of protic and non-protic solvents. These studies were augmented by theoretical modelling and experimental investigation of solute/solvent interactions in the triplet excited and the ground state, respectively. The triplet quenching that follows Stern-Volmer kinetics in all cases is profoundly dependent on the nature of the solvent, with the highest reactivity being consistently found in protic solvents. The results in non-protic solvents are compatible with unproductive quenching via a charge-transfer state, whereas the generally fast quenching in protic solvents is accompanied by efficient formation of free-radical products. Analysis of the solvent dependence in terms of Marcus theory reveals the impact of specific solvation of benzophenone by protic solvents on the ET driving force and kinetics. Specific solvation is found to support efficient free radical ion formation in media of moderate and low polarity as well.
ChemPhysChem 07/2010; 11(10):2108-17. · 3.41 Impact Factor
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ABSTRACT: The kinetics scheme for directly excited, photoreversible reactions is solved exactly under the assumptions of no irreversible side reactions and constant excitation intensity for the duration of the reaction. The advantages of the methodology over the extrapolation-to-zero-time and the back-reaction correction methods are (i) that the quantum yields of both the forward and reverse photoreactions can be obtained starting from either pure reactant or pure product and (ii) the conversion percentage is not limited to a narrow domain in the neighborhood of small conversions. Examples of E-Z photoisomerizations are given to illustrate the fitting procedures required. The results from these examples are compared to the photoisomerization method of extrapolating the empirical quantum yields to zero time and the back-reaction correction. The exact equations are used to justify the extrapolation-to-zero-time method and to establish criteria on extrapolation ranges for the conversion percentage of starting material.
Photochemical and Photobiological Sciences 12/2009; 8(12):1667-75. · 2.58 Impact Factor
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ABSTRACT: The intramolecular hydrogen-atom transfer between tyrosine and triplet-excited benzophenone was studied by nanosecond laser-flash photolysis in a number of nonprotic and protic solvents. The reaction rates were found to be strongly solvent dependent with a range from <105 s−1 to 5 × 107 s−1. Eventual contributions from solvent-dependent conformational equilibria on the reaction rates were found to be of minor importance. In contrast, the rate-enhancing and rate-retarding effects of the solvents could be attributed to specific solvent−solute interactions with the hydrogen donor and the hydrogen acceptor. Opposite effects were exerted by nonprotic and protic solvents. The sharp decrease of the reaction rates in nonprotic solvents with the hydrogen-bond acceptor ability of the solvent is referred to hydrogen bonding of the tyrosine to the solvent, TyrOH → S. It could be quantified in terms of an intramolecular kinetic solvent effect model. In turn, a sharp increase of the reaction rates with the hydrogen-bond donor ability of the solvents is observed in protic solvents, which is referred to solvation effects on both chromophores, SH → O(H)Tyr and SH → bp. Implications of these diverting solvent dependencies on the reaction mechanism in protic and nonprotic solvents are discussed.
The Journal of Physical Chemistry C 07/2009; 113(25):11695–11703. · 4.80 Impact Factor
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ABSTRACT: Factor analysis is introduced and applied to resolving puzzling behavior in the free-radical chemistry of the cyclic dipeptide
cyclo-(D-Met-L-Met). Previously, spectral analysis of the transient absorption spectra, following the hydroxyl radical-induced
oxidation of cyclo-(D-Met-L-Met), failed to match expectations seen when transient conductivity was used to monitor the same
reaction sequence. In the current work, factor analysis is used to show that a radical cation is formed via the stabilization
of the oxidized sulfur through the formation of two-centered three-electron bonds with the lone pairs on oxygen. This previously
undetected radical resolves the discrepancy between transient absorption and transient conductivity observations.
Research on Chemical Intermediates 06/2009; 35(4):431-442. · 0.70 Impact Factor
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ABSTRACT: Photochemical properties of 4-coumarinylmethyl derivatives (CM-X, X = Br, Cl, OH, OPh, SH, and SPh) in solution were studied by laser flash photolysis and DFT calculations. It was found that CM-Br and CM-SPh undergo photoinduced bond dissociation in the lowest excited singlet state, yielding the 4-coumarinylmethyl radical (CMR) and the corresponding radical with quantum yields of approximately 0.25. Laser flash photolysis of CM-Cl, OH, OPh, and SH provided no or very little transient absorption of the corresponding triplet state. Upon triplet sensitization of CM-X using benzophenone (BP) as a triplet sensitizer, efficient formation of triplet CM-OH and -OPh was seen, whereas CM-SH and -SPh underwent the C-S bond cleavage in the lowest triplet (T(1)) state, resulting in production of CMR and the corresponding radicals with efficiencies (alpha(rad)) of >or=0.66. CM-Br and -Cl efficiently quenched triplet BP without formation of appreciable intermediates. On the basis of the results of laser flash photolysis and DFT calculations, photochemical features of triplet CM-X were discussed in detail.
The Journal of Physical Chemistry A 05/2009; 113(20):5815-22. · 2.95 Impact Factor
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Chemical Physics Letters 04/2009; 473:348-353. · 2.34 Impact Factor
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ABSTRACT: To be or not to be solvated is the decisive parameter that controls the photoinduced hydrogen-atom transfer in diastereomeric ketone/phenol dyads. A kinetic solvent effect that refers to hydrogen bonding between the phenol and the solvent is suggested to be the main source of the stereoselective discrimination in the hydrogen transfer (see figure).
Chemistry 03/2009; 15(13):3061-4. · 5.93 Impact Factor
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Journal of Polymer Science Part A Polymer Chemistry 11/2008; 46(24):8013 - 8022. · 3.92 Impact Factor
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ABSTRACT: Methionine is a key amino acid that has numerous roles in essential vital processes. Moreover, methionine oxidation is biologically important during conditions of oxidative stress and represents an important step in the development of some severe pathologies. Considerable work has been performed to understand the mechanisms of one-electron oxidation of the Met-residue as a function of its proteic environment. The most important recent results obtained by means of time-resolved techniques (laser flash photolysis and pulse radiolysis) on model peptides containing single or multiple Met-residues and in selected naturally occurring peptides (Met-enkephalin and β-amyloid peptide) and proteins (thioredoxin and calmodulin) have been reviewed.
CHIMIA International Journal for Chemistry 08/2008; 62(9):728-734. · 1.21 Impact Factor
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ABSTRACT: The one-electron reduction of bis[1-(2',3',5'-tri-O-acetylribosyl)uracil-4-yl] disulfide, initiated by hydrated electrons in a radiation chemical study, has been shown to yield 1-(2',3',5'-tri-O-acetylribosyl)-4-thiouracil as a stable molecular product. The reduction reaction leads, in the first instance, to a transient, albeit remarkably stable disulfide radical anion. This is characterized by a 2-center-3-electron bond with two bonding sigma-electrons and an antibonding sigma*-electron in the sulfur-sulfur bridge, (-S therefore S-)(-). It receives its stability from a sandwich-structure with the two uracilyl moieties facing each other (possibly further assisted by the 2',3',5'-tri-O-acetylribosyl substituents). A considerable lengthening of the original disulfide bridge from 2.02 to 2.73 A in the radical anion seems to facilitate the interaction of the heterocycles and leads to a gain in stabilization energy of 24 and 33 kcal/mol (100 and 140 kJ/mol) as evaluated by UMP2/cc-pVTZ and UMP2/cc-pVDZ calculations, respectively. The (-S therefore S-)(-) bonded radical anion shows a broad optical absorption band with lambdamax=450 nm, epsilonmax=6000 M(-1) cm(-1), and a half-width of 1.0 eV. It exists in equilibrium with the conjugated 1-(2',3',5'-tri- O-acetylribosyl)uracil-4-yl thiyl radical -S(*), and the corresponding thiolate, -S(-). The rate determining step for the disappearance of the disulfide radical anion appears to be protonation of both the radical anion and the free thiolate by reaction with H(+)aq. Absolute rate constants have been measured for these protonation processes, for the formation of the stable thiouridine product, and for the electron transfer from the disulfide radical anion to molecular oxygen. With the (-S therefore S-)(-) <--> -S(*) + -S(-) equilibrium lying very much on the left-hand side, the reduced disulfide system exhibits predominantly reducing properties whereas any oxidizing property of the conjugated thiyl radical has only little if any chance to materialize. Besides attaching directly to the disulfide bridge, the hydrated electrons react also, with about equal efficiency, with the uracil moiety of the investigated compound. This leads to a structurally totally different and electronically distinguishable species than that with the reduced disulfide bridge. In particular, there is no face-to-face interaction between the two heterocyclic moieties and no increased electron density in the S-S bond. The C-centered radicals resulting from the reduction of the uracil and possibly also generated from the ribosyl moieties initiate further cleavage of the S-S bond and thus contribute to the formation of thiouridine. The overall yield of the latter, as determined from steady-state gamma-radiolysis, indicates a small chain process (G=1.54 micromol/J). Possible mechanisms are discussed.
The Journal of Physical Chemistry B 08/2008; 112(32):10045-53. · 3.70 Impact Factor
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ABSTRACT: The formation of head-to-tail contacts in de novo synthesized benzophenone/tyrosine dyads, bp logical sum Tyr, was probed in the ground and excited triplet state by NMR techniques and laser flash photolysis, respectively. The high affinity of triplet-excited ketones towards phenols was used to trace the geometric demands for high reactivity in the excited state. A retardation effect on the rates with increasing hydrogen-bond-acceptor ability of the solvent is correlated with ground-state masking of the phenol. In a given solvent the efficiencies of the intramolecular hydrogen-atom-transfer reaction depend strongly on the properties of the linker: rate constants for the intramolecular quenching of the triplet state cover the range of 10(5) to 10(8) s(-1). The observed order of reactivity correlates to a) the probability of close contacts (from molecular-dynamics simulations) and b) the extent of the electronic overlap between the pi systems of the donor and acceptor moieties (from NMR). A broad survey of the NMR spectra in nine different solvents showed that head-to-tail interactions between the aromatic moieties of the bp logical sum Tyr dyads already exist in the ground state. Favourable aromatic-aromatic interactions in the ground state appear to correspond to high excited-state reactivity.
Chemistry 08/2008; 14(26):7913-29. · 5.93 Impact Factor
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ABSTRACT: The reactions of a disulfide and a benzylsulfide derived from 4-thiouridine were studied in aqueous acetonitrile using stationary and laser flash photolysis methods. Irradiation of the compounds results in specific cleavage of the S-S bond in the disulfide and the S-CH(2) bond in the sulfide. Identical pyrimidine-derived intermediates were observed in the transient absorption spectra (lambda(max) = 420 nm, epsilon(max) approximately 2500 M(-1) cm(-1)) recorded for both compounds in laser flash photolysis experiments. The intermediate was identified as the 4-pyrimidinylthiyl radical. Irradiation of the disulfide in the absence of oxygen gives 4-thiouridine while the sulfide under identical conditions produced, additionally, 3-benzyl-4-thiouridine as a stable photoproduct. The formation of the latter photoproduct provides evidence for the existence of the N-centered 4-thioxopyrimidynyl radical formed from the initially produced S-centered (thiyl) radical. The 4-thiouridine is formed from the radicals generated in the primary photochemical step by an H abstraction reaction from the solvent (acetonitrile) or from additives (alcohols) that were purposely added. Interestingly, in contrast to the benzylsulfide, the photoreaction of the disulfide is quenched by molecular oxygen with the concomitant formation of uridine. However it appears that uridine is not produced as a result of the reaction of the radicals with oxygen. A mechanism is proposed for the photochemical transformations of the disulfide and benzylsulfide derived from 4-thiouridine. The proposed mechanism is based on the structures of the identified stable photoproducts, the values of the photoreaction quantum yields determined under differing irradiation conditions, and the flash photolysis results.
Photochemical and Photobiological Sciences 03/2008; 7(2):250-6. · 2.58 Impact Factor
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ABSTRACT: Abstract— Oxidation of the triplet state of 4-carboxybenzophenone (CB) by a series of five substituted methionines and three methionine-containing dipeptides was monitored under laser flash photolysis conditions in aqueous solution. Spectral resolution techniques were employed to follow the concentration profiles of the intermediates formed from the quenching events. From these concentration profiles, quantum yields for the intermediates were determined. Branching ratios were evaluated for the decay of the charge-transfer complex by the competing processes of back electron transfer, proton transfer and escape of radical ions. The relative prominence of these processes was discussed in terms of the proton-transfer tendencies of the nominal sulfur-radical-cationic species. A systematic decrease was observed in the quantum yields for the escape of radical ions along with a correlated increase in the proton-transfer yields. The enhanced propensity of the sulfur radical cations to depro-tonate is due to deprotonation at the carbons adjacent to the sulfur-cationic site and at the unsubstituted amino groups when present. This scheme was supported by an observed decrease in the yields of dimeric sulfur radical cations with an increase in the electron-withdrawing abilities of the substituents, making the radical-cationic species stronger acids. The involvement of protons on the amino groups was implicated by the correlation of the quantum yields of ketyl radical formation in the photochemistry experiments with the rate constants for the reaction of the CB radical anion with the sulfur-containing substrates in pulse radiolysis experiments.
Photochemistry and Photobiology 01/2008; 68(6):785 - 796. · 2.41 Impact Factor
