Search for an adenine photoproduct in DNA.

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Abstract
Poly(d[14 C]A), p(dA)2, and [14C]adenosine-labeled DNA were irradiated at 254 nm with fluences up to 50 J/m2, and then following formic acid hydrolysis at 170°C were subjected to paper chromatography using a butanol:water:acetic acid (80:30:12) solvent system. For poly(dA), up to 25% of the radioactivity appeared as fluorescent material located in the Rf 0.21–0.29 region. The hydrolysate of the purified photoproduct, p(dA)2*, isolated from irradiated p(dA)2 by DEAE chromatography also had an Rf of 0.29 as well as an absorbance maximum at 310 nm. In all cases studied, however, the photoproduct yield in the Rf 0.29 region for native DNA was less than 2%. Denaturation of the DNA appeared to enhance the yield slightly, although no pronounced peak in this region of the chromatogram was discerned. Mechanistic studies indicate that the yield of the adenine photoproduct in poly(dA) is favored by base stacking, has a singlet excimer as a precursor, and is quenched by hydrogen bonding to a pyrimidine. It is concluded that the yield of the adenine photoproduct in both native and denatured DNA is considerably less than in poly(dA) and in all probability does not represent a biologically significant product.