Extent of formation of a dimeric adenine photoproduct in polynucleotides and DNA

Abstract

Quantum yields are reported for the formation of a dimeric adenine photoproduct, A=A, in adenine homopolymers and DNA irradiated at 254 nm. The A=A content of irradiated samples was assayed by using reversed-phase HPLC to isolate the 4,6-diamino-5-guanidinopyrimidine (DGPY) which is produced from A=A on acid hydrolysis. Acid hydrolysates derived from DNA radiolabelled with [ 14C] 2′-deoxyadenosine were spiked with unlabelled DGPY before fractionation on HPLC and the recovered material was further purified by chromatography on Sephadex G-10 followed by co-crystallization with DGPY sulphate. Although A=A is formed with a relatively high quantum yield of 1.6 × 10 −3 mol einstein −1 in single-stranded poly(dA) the photoaddition reaction is strongly quenched in base-paired poly(dA)·poly(dT) and undetectable in poly(rA)·poly(dT). Respective quantum yields of 6 × 10 −5 and 9 × 10 −6 were estimated for the formation of A=A in single- and double-stranded E. coli DNA implying that the photoproduct has very limited biological significance. From studies with d(ApG), d(GpA), ApG, GpA, d(A) 20 and d(A 4G) 4 it is concluded that adjacent guanine and adenine bases do not form a photoadduct analogous to A=A and also that guanine residues have no local or long-range quenching effect on photodimerization within A-A doublets.