Photolysis Mechanism of Aqueous Tyrosine and Tyrosyl Peptides*

Laser flash photolysis at 265 nm has been employed for measuring the initial hydrated electron (e ‐ aq ) and p ‐alanylphenoxyl radical (Tyr) in aqueous Tyr, small Tyr peptides and R Nase A. The results indicate that monophotonic photolysis not involving the fluorescent or triplet states is the principal initial process. Equivalent yields of e and Tyr were found in all cases except Tyr, where the Tyr yield was 60% higher than e ‐ aq attributed to splitting of the phenolic bond. Computer analysis of e ‐ aq and Tyr decays for Tyr indicates the importance of electron‐radical recombination in competition with electron scavenging and bimolecular radical‐radical reactions. Evidence for intramolecular electron migration has been obtained in cystinyl‐bis‐Tyr.