The Photoreactions of Thymine with Hypoxanthine and Imidazole*

Photocycloaddition, Thymine, Azetidine, Cyclobutane Adducts Photochemical reactions of thymine linked to hypoxanthine or imidazole by a trimethylene chain were studied in aqueous solution. Irradiation (A = 254 nm) of thymine-hypoxanthine pair yielded two internal cycloadducts with azetidine and cyclobutane part structures. Sensitization and quenching experiments suggested that the excited singlet was the reactive state in the photocycloaddition reactions. Only cyclobutanes were isolated from irradiated (A = 290 nm) solutions of thymine-imidazole pairs. Photocycloadditions were reversible upon irradiation at A = 254 nm.