MECHANISM OF 4-CARBOXYBENZOPHENONE-SENSITIZED PHOTOOXIDATION OF METHIONINE-CONTAINING DIPEPTIDES AND TRIPEPTIDES IN AQUEOUS-SOLUTION

The mechanism of 4-carboxybenzophenone (CB)-sensitized photooxidation of methionine-containing dipeptides (Met-Gly and Gly-Met) and tripepti des (Met-Gly-Gly, Gly-Met-Gly, and Gly-Gly-Met) was investigated using nanosecond flash photolysis and steady-state photolysis. The rate con stants for quenching of the CB triplet by sulfur-containing peptides w ere determined to be in the range (1.8-2.3) x 10(9) M(-1) s-(1) for ne utral and alkaline solutions. The presence of the various electron-tra nsfer intermediates accompanying the CB triplet quenching events was i dentified through the use of a multiple-regression procedure that was used to resolve the experimental transient spectra into components. Th e intermediates identified were the CB ketyl radical anion, the CB ket yl radical, intermolecularly (S therefore S)-bonded radical cations, a nd intramolecularly (S therefore N)-bonded radical cations derived fro m peptides. The spectra of appropriate (S therefore S)(+) and (S there fore N)(+) intermediates for the peptides were determined from complem entary 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 an d the kinetics of their formation and decay were measured by flash pho tolysis, and quantum yields of CO2 formation were measured by steady-s tate photolysis. These results were based on the resolution of the spe ctral components in the transient absorption spectra at various delays after the flash. A detailed mechanism of the CB-sensitized photooxida tion of methionine-containing peptides was discussed and compared to t hat for methionine.