THE PHOTOREACTIVITY OF PYRIMIDINE‐PURINE SEQUENCES IN SOME DEOXYDINUCLEOSIDE MONOPHOSPHATES AND ALTERNATING DNA COPOLYMERS

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Abstract
Abstract—A reversed‐phase HPLC system has been developed which separates the common nucleo‐bases from the 6‐methylimidazo[4,5‐b]pyridin‐5‐one (6‐MIP) produced on acid hydrolysis of a thymine‐adenine photoadduct (TA*) that is formed between adjacent thymine and adenine bases in UV‐irradiated polydeoxyribonucleotides. By measuring the relative amounts of adenine and 6‐MIP in acid hydrolysates, this system has been used to investigate how polynucleotide conformation affects the yield of TA* in poly(dA‐dT) irradiated at 254 nm. The photoreactivity of other pyrimidine‐purine sequences has been examined with the deoxydinucleoside monophosphates d(TpI) and d(m5CpA) and with the alternating DNA copolymers poly(dA‐dU), poly(dI‐dC), poly(dG‐dC) and poly(dA‐dC).poly(dG‐dT). Samples were irradiated at 254 nm in aqueous solution and in ice, and at wavelengths >290 nm with acetone as photosensitizer. A photoproduct resembling TA*, and giving 6‐MIP on acid hydrolysis, was isolated from d(TpI) irradiated at 254 nm in solution or in ice; d(m5CpA) was comparatively unreactive. Acid hydrolysates of the irradiated DNA copolymers were screened by HPLC and by TLC and paper electrophoresis, for the presence of imidazo[4,5‐b]pyridin‐5‐one, 6‐MIP, or other species attributable to specific photoproduct formation. By this criterion, however, none of the copolymers showed evidence of significant photoreactivity in either their single‐ or double‐stranded conformational states. The formation of mixed pyrimidine‐purine photoadducts in DNA is therefore probably restricted to T‐A doublets.