Cytosine Derived Heteroadduct Formation in Ultraviolet-irradiated DNA

Abstract

PYRIMIDINE photoproducts other than cyclobutyl dimers have recently been isolated from irradiated pyrimidines as well as from DNA. For example, from irradiated (254 nm) frozen solutions of thymine, two products in addition to the cis-syn cyclobutyl thymine homodimer have been isolated^1,2. One, which cochromatographs with the T–T cyclobutyl dimer in neutral or acidic solvent systems, was found to be a thymine–thymine adduct. Similarly, a uracil–thymine adduct has been found recently after irradiation of a mixture of uracil and thymine in frozen solution (A. J. V. and S. Y. Wang, unpublished results), and has also been isolated from acid hydrolysates of ultraviolet-irradiated calf thymus DNA³. These adducts have an absorption maximum at 315 nm and may be responsible for the absorbance increase at this wavelength observed in irradiated solutions of DNA^4,5. Here we report that the U–T adduct is derived from an initially formed photoproduct involving thymine and cytosine. We have, as yet, found no evidence for formation of a T–T adduct in DNA.