Fluorescence quantum yield determination of pyrimidine (6-4) pyrimidone photoadducts

An extensive study of the fluorescence characteristics of pyrimidine (6-4) pyrimidone photoadducts, a major class of far-UV-induced DNA lesions, was carried out on dinucleoside monophosphate (6-4) photoadducts, including thymidylyl-(3'-->5')-thymidine (TpT), 2'-deoxycytidylyl-(3'-5')-thymidine, thymidylyl-(3'-->5')-2'-deoxycytidine, 2'-deoxyuridylyl-(3'-->5')-thymidine, 5-methyl-2'-deoxycytidylyl-(3'-5')- thymidine (6-4) photoadducts and the corresponding base (6-4) photoadducts, 6-4'-(5'-methylpyrimidin-2'-one) thymine (TT), 5-hydroxy-6-4'-(5'-methylpyrimidin-2'-one)-5,6-dihydrothymine (CT), 5-amino-6-4'-(pyrimidin-2'-one)-5,6-dihydrothymine (UC) obtained by mild acidic hydrolysis of the former derivatives. The fluorescence quantum yield (phi F) of these compounds was found to depend on one hand, on the nature of the two bases involved and the base substituent and, on the other hand, on the presence of the phosphate group. The hydrolysis of the phosphodiester bond was shown to enhance phi F, the larger effect being observed in the case of the thymine-thymine photoadducts with a seven-fold increase of the phi F value in the case of TT as compared to TpT (0.21 and 0.03, respectively). These results are discussed in terms of structural considerations.