Quantum yields of adenine photodimerization in poly(deoxyadenylic acid) and DNA

The photodimerization of adjacent adenine bases on the same polydeoxyribonucleotide strand gives rise to two distinct dimeric photoadducts, designated A=A and AA*, which are converted by acid hydrolysis into 4,6-diamino-5-guanidinopyrimidine (DGPY) and 8-(5-aminoimidazol-4-yl) adenine (8-AIA) respectively. A radiochemical assay utilizing reversed-phase high performance liquid chromatography (HPLC) has been used to isolate and quantify DGPY and 8-AIA present in acid hydrolysates of tritium-labelled poly(deoxyadenylic acid) (poly(dA)) and E. coli DNA after UV irradiation at 254 nm. Individual quantum yields of formation of A=A and AA* have thus been calculated, and summed to provide the following overall quantum yields (mu mol einstein(-1)) of adenine photodimerization: poly(dA), 3000; poly(dA).poly(dT), 380; denatured DNA, 60-120; native DNA, 10-40. The photoaddition reaction is strongly inhibited by base pairing, and the very low quantum yield in native DNA indicates that dimeric adenine photoproducts constitute no more than 1% of the total photodamage induced by direct excitation. (C) 1997 Elsevier Science S.A.