Salicylic Acid As a Photosensitizer for Thymidine Dimerization Induced by UV

When a neutral solution of a nucleoside mixture was irradiated with UV light having wavelength longer than 300 nm, addition of salicylic acid to the solution greatly accelerated the reaction of thymidine. The UV light irradiation of thymidine solution in the presence of salicylic acid resulted in four major product peaks in HPLC. All the products were identified as isomers of cyclobutane thymidine dimers by MS and NMR. The cyclobutane thymidine dimers were generated from thymidine almost exclusively. UV irradiation with the longer wavelength of 350 nm induced almost no reaction. The results indicate that salicylic acid is a photosensitizer for thymidine dimerization excited by UV light of wavelength 300 to 350 nm.