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.

The mechanism of 4-carboxybenzophenone (CB)-sensitized photooxidation of methionine-containing dipeptides (Met-Gly and Gly-Met) and tripeptides (Met-Gly-Gly, Gly-Met-Gly, and Gly-Gly-Met) was investigated using nanosecond flash photolysis and steady-state photolysis. The rate constants for quenching of the CB triplet by sulfur-containing peptides were determined to be in the range (1.8-2.3) x lo9 M-' s-l for neutral and alkaline solutions. The presence of the various electron-transfer intermediates accompanying the CB triplet quenching events was identified through the use of a multiple-regression procedure that was used to resolve the experimental transient spectra into components. The intermediates identified were the CB ketyl radical anion, the CB ketyl radical, intermolecularly ( S : . S)-bonded radical cations, and intramolecularly ( S : . N)-bonded radical cations derived from peptides. The spectra of appropriate ( S : . S)+ and ( S :.N)+ intermediates for the peptides were determined from complementary pulse radiolysis studies in acidified aqueous solutions of the peptides. The types of intermediates were found to depend on the pH of the solution and on the location of the methionine unit with respect to the terminal functions. The quantum yields of all the transients and the kinetics of their formation and decay were measured by flash photolysis, and quantum yields of COZ formation were measured by steady-state photolysis. These results were based on the resolution of the spectral components in the transient absorption spectra at various delays after the flash. A detailed mechanism of the CB-sensitized photooxidation of methionine-containing peptides was discussed and compared to that for methionine.

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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 acid-base reactions in excited states are some of the most elementary processes both in photochemistry and biochemistry and are, therefore, well-suited to form the basis for an undergraduate physical chemistry experiment.