THE MECHANISM OF PHOTOCHEMICAL ADDITION OF CYSTEINE TO URACIL AND FORMATION OF DIHYDROURACIL

Abstract— A proposed mechanism for the photochemical addition of L -cysteine to uracil with the concurrent formation of dihydrouracil is shown to proceed through the triplet excited state of uracil which can abstract hydrogen atoms from cysteine to form dihydrouracil. This triplet state is the same one as that leading to photodimerization. The thiyl radicals generated add to ground state uracil molecules. The data permit a re-evaluation of the quantum yield for intersystem crossing of uracil in water which shows dimerization in aqueous solution to have a maximum efficiency of 56 per cent. The formation of the cross-adduct and dihydrouracil may be sensitized but the efficiency of the reaction is related to the ability of the sensitizer to be photoreduced and not to its triplet energy.