Influence of cysteine on the photochemical decomposition of thymidyl-(5′-3′)-bromodeoxyuridine

Irradiating a solution of thymidyl-(5′-3′)-bromodeoxyuridine with monochromatic, ultraviolet light yields one main photo-product, an intramolecular dimer, containing no Br and a cyclobutene ring structure. In the presence of cysteine and ultraviolet radiation the dimer is further decomposed to a photo-product characterized by an end-absorption spectrum. This compound contains one S atom per dinucleotide and cannot be reverted by irradiation at 2400 Å. The quantum yields of this sulfhydryl addition are measured as a function of wavelength. The reaction kinetics and the biological implication of this process, which is possibly radio-protective, are discussed.