Photochemical reactions of 4-thiouridine disulfide and 4-benzylthiouridine--the involvement of the 4-pyrimidinylthiyl radical.

The reactions of a disulfide and a benzylsulfide derived from 4-thiouridine were studied in aqueous acetonitrile using stationary and laser flash photolysis methods. Irradiation of the compounds results in specific cleavage of the S-S bond in the disulfide and the S-CH(2) bond in the sulfide. Identical pyrimidine-derived intermediates were observed in the transient absorption spectra (lambda(max) = 420 nm, epsilon(max) approximately 2500 M(-1) cm(-1)) recorded for both compounds in laser flash photolysis experiments. The intermediate was identified as the 4-pyrimidinylthiyl radical. Irradiation of the disulfide in the absence of oxygen gives 4-thiouridine while the sulfide under identical conditions produced, additionally, 3-benzyl-4-thiouridine as a stable photoproduct. The formation of the latter photoproduct provides evidence for the existence of the N-centered 4-thioxopyrimidynyl radical formed from the initially produced S-centered (thiyl) radical. The 4-thiouridine is formed from the radicals generated in the primary photochemical step by an H abstraction reaction from the solvent (acetonitrile) or from additives (alcohols) that were purposely added. Interestingly, in contrast to the benzylsulfide, the photoreaction of the disulfide is quenched by molecular oxygen with the concomitant formation of uridine. However it appears that uridine is not produced as a result of the reaction of the radicals with oxygen. A mechanism is proposed for the photochemical transformations of the disulfide and benzylsulfide derived from 4-thiouridine. The proposed mechanism is based on the structures of the identified stable photoproducts, the values of the photoreaction quantum yields determined under differing irradiation conditions, and the flash photolysis results.