INFLUENCE OF THE PEPTIDE BOND ON THE SINGLET MOLECULAR OXYGEN-MEDIATED (O2[g]) PHOTOOXIDATION OF HISTIDINE and METHIONINE DIPEPTIDES. A KINETIC STUDY

The dye-sensitized photooxidation of l -histidine (His) and l -methionine (Met) and their simplest dipeptides with glycine (Gly) (His-Gly, Gly-His, Gly-Met) and Met-methyl ester (Met-ME) mediated by singlet molecular oxygen (O_2g]) was studied. The overall rate constants in acetonitrile-H₂O (K_t) for O₂(¹_g) quenching were measured by time-resolved phosphorescence detection. In H₂O a competitive kinetic method was employed. In both solvents the reactive rate constants (K_t,) were determined to discriminate between the overall and physical contributions to the quenching. The kinetic and mechanistic aspects of the interaction are discussed. For His-Gly, the peptide bond has practically no effect on the kinetics of photooxidation. For Gly-His the overall rate constant is much higher than that for His and His-Gly, in both H₂O and acetonitrile-H₂O. The main contribution to k₁ (for Gly-His) is the physical quenching of O₂(¹_g)- In water the k_t/k_r ratio for free His and His-Gly is 1.0, reaching a value of 2.0 in the organic solvent-H₂O mixture. The rates of-NH₂ loss upon sensitized photooxidation in all cases parallel the trend of k_r values. The main results for the His series indicate that: (1) a polar environment favors autoprotection (i.e. an increase in the contribution of physical quenching) against photodynamic effects; (2) only the rate constant for reactive interaction with O_2g] does not depend on the location of the peptide bond involving His. For Met derivatives the k_t, values are higher in both solvents than that for free Met. Only for the free amino acid in H₂O is the interaction with O₂(¹_g) totally reactive. For Gly-Met and Met-ME the physical quenching prevails: k_t, is, in both solvents, about one order of magnitude higher than k_r. According to our results on -NH₂ loss and on the basis of previous investigations by others, the photooxidative products distribution in the Met series indicates that Gly-Met yields only dehydroMet, whereas Met and Met-ME produce a mixture of Met-sulfoxide and the Met-dehydro compound.