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 (O2[1 delta g]) was studied. The overall rate constants in acetonitrile-H2O (kt) for O2(1 delta g) quenching were measured by time-resolved phosphorescence detection. In H2O a competitive kinetic method was employed. In both solvents the reactive rate constants (kr) 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 kt (for Gly-His) is the physical quenching of O2(1 delta g). In water the kt/kr 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 kr 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 O2(1 delta g) does not depend on the location of the peptide bond involving His. For Met derivatives the kt values are higher in both solvents than that for free Met.(ABSTRACT TRUNCATED AT 250 WORDS)