Evaluation of Hydroxyl Radical Reactivity by Thioether Group Proximity in Model Peptide Backbone: Methionine versus S-Methyl-Cysteine.

Hydroxyl radicals (HO•) have long been regarded as a major source of cellular damage. The reaction of HO• with methionine residues (Met) in peptides and proteins is a complex multistep process. Although the reaction mechanism has been intensively studied, some essential parts remain unsolved. In the present study we examined the reaction of HO• generated by ionizing radiation in aqueous solutions under anoxic conditions with two compounds representing the simplest model peptide backbone CH3C(O)NHCHXC(O)NHCH3, where X = CH2CH2SCH3 or CH2SCH3, i.e., the Met derivative in comparison with the cysteine-methylated derivative. We performed the identification and quantification of transient species by pulse radiolysis and final products by LC-MS and high-resolution MS/MS after γ-radiolysis. The results allowed us to draw for each compound a mechanistic scheme. The fate of the initial one-electron oxidation at the sulfur atom depends on its distance from the peptide backbone and involves transient species of five-membered and/or six-membered ring formations with different heteroatoms present in the backbone as well as quite different rates of deprotonation in forming α-(alkylthio)alkyl radicals.

Early Events of Photosensitized Oxidation of Sulfur-Containing Amino Acids Studied by Laser Flash Photolysis and Mass Spectrometry.

The mechanism of photooxidation of methionine (N-Ac-Met-NH-CH3, 1) and methyl-cysteine (N-Ac-MeCys-NH-CH3, 2) analogues by 3-carboxybenzophenone triplet (3CB*) in neutral aqueous solution was studied using techniques of nanosecond laser flash photolysis and steady-state photolysis. The short-lived transients derived from 3CB and sulfur-containing amino acids were identified, and their quantum yields and kinetics of formation and decay were determined. The stable photoproducts were analyzed using liquid chromatography coupled with high-resolution mass spectrometry. Substantial differences in the mechanisms were found for methionine and S-methyl-cysteine analogues for both primary and secondary photoreactions. A new secondary reaction channel (back hydrogen atom transfer from the ketyl radical to the carbon-centered α-thioalkyl radical yielding reactants in the ground states) was suggested. The detailed mechanisms of 3CB* sensitized photooxidation of 1 and 2 are proposed and discussed.

Photoinduced CC-coupling reactions of rigid diastereomeric benzophenone-methionine dyads.

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.

Steady-state, one-, and two-color laser flash photolysis studies of alpha-bond cleavage of S-acyl-4-phenylthiophenols in solution.

Photochemical properties of alpha-cleavage of the C-S bond in excited states of p-biphenyl thioacetate and p-biphenyl thiobenzoate (Me-SBP and Ph-SBP) in solution are investigated using steady-state and laser flash photolyses in comparison with those of S-phenyl thiobenzoate, where the photo-Fries rearrangement was reported to be absent. Although Me-SBP and Ph-SBP decompose upon 254 nm photolysis in acetonitrile irrespective of the amount of the dissolved oxygen, no definite photoproducts due to the photo-Fries rearrangement were found. Laser flash photolysis (266 nm) of these molecules reveals the occurrence of the C-S bond cleavage in the excited state based on the observation of the formation of the biphenylylthiyl radical (BTR) in the transient absorption. Quantum yields (Phi(rad)) of the BTR formation were determined to be 0.20 and 0.15 for Me-SBP and Ph-SBP, respectively. Triplet sensitization of Ph-SBP using xanthone (XT) as a sensitizer shows that the lowest triplet (T(1)) state of Ph-SBP is dissociative for the C-S bond with an efficiency of >or.56. In contrast, triplet sensitization of Me-SBP using acetone as a sensitizer demonstrates the efficient formation of triplet Me-SBP, and the molar absorption coefficient of the triplet-triplet absorption was determined. No photochemical reactions are found in the T(1) state of Me-SBP. Upon 355 nm laser flash photolysis of the T(1) state of Me-SBP, the formation of BTR is confirmed in the transient absorption. This observation indicates the C-S bond cleavage in a highly excited triplet (T(n)) state of Me-SBP. The quantum yield (Phi(dec)) of the alpha-cleavage in the T(n) state of Me-SBP was determined to be 0.69. Photochemical features of Me-SBP and Ph-SBP are discussed from the viewpoint of the spin-multiplicity of the bond dissociative states.

Stereoselectivity of the hydrogen-atom transfer in benzophenone-tyrosine dyads: an intramolecular kinetic solvent effect.

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).