Photochemistry of proteins: a Review

Proteins are an important target of photochemical damage to the eye. The wavelength of irradiation is a major determinant of the initial mechanisms. The effects of ultraviolet radiation are initiated by absorption in the aromatic amino acid residues and cystine. Photoionization of tyrosine and tryptophan residues leads to aromatic free radicals. The ejected electrons are stabilized in the aqueous medium as hydrated electrons and may be temporarily trapped by cystyl bridges. N-formylkynurenine is an important tryptophan photoproduct, which can act as an endogenous photosensitizer of near-ultraviolet radiation by generating singlet oxygen and superoxide. Singlet oxygen is produced also by exogenous sensitizers absorbing in the visible and near ultraviolet regions. The mechanisms are illustrated by porphyrins and furocoumarins, in which dark binding interactions influence the photosensitization pathways.