The Eosin‐sensitized Photooxidation of Substituted Phenylalanines and Tyrosines*

Abstract— The eosin‐sensitized photooxidation of tyrosine and a number of compounds related to tyrosine (substituted phenylalanines) was studied by steady‐state kinetic and flash photolysis techniques. In particular, the role of the phenolic group and the amino and carboxyl groups of the alanyl side chain in the photooxidation mechanism was investigated in detail. Several relationships between substrate structure and susceptibility to photooxidation as well as effects of substrate structure on photooxidation mechanisms were found. For example, phenylalanine is not photooxidizable, hut substitution of electron‐donating (activating) groups such as‐OH (as in tyrosine) or‐NH 2 (as in p‐aminophenylalanine) results in rapidly photooxidized derivatives. However, substituting deactivating groups such as—C1 (as in p ‐chlorophenylalanine) or weakly activating groups such as ‐ OCH 3 (as in 4‐methoxyphenylalanine) result in non‐photooxidiz‐able derivatives. Substitution of additional activating groups to the ring of hydroxy‐substituted phenylalanines results in increased rates of photooxidation, whereas additional deactivating groups result in decreased photooxidation rates. The rate‐determining step in the photooxidation mechanism is shown to be dependent on the presence and position of an electron‐donating substituent on the benzenoid ring. Only minor involvement of the side chain amino and carboxyl groups was found. Both singlet oxygen and hydrogen abstraction mechanisms are involved in the eosin‐sensitized photooxidation of hydroxy‐substituted phenylalanines (e.g. tyrosine). The hydrogen abstraction mechanism probably predominates at both pH 8 and 11.