THE REDUCTION OF THYMINE RESIDUES IN DNA BY THE COMBINED ACTION OF UV-LIGHT AND HYPOPHOSPHITE

UV irradiation (lambda = 254 nm) of thymine and uracil in aqueous solu tion containing salts of phosphinic acid (hypophosphites) results in t he formation of the corresponding dihydropyrimidine derivatives. The p eculiarities of this new photochemical reaction consist of a specifici ty towards 2,4-dioxopyrimidines, a high quantum yield and a neutral pH optimum. The quantum yield of photoconversion of thymine at pH 7.0 is equal to 1.9 x 10(-2) in 1 M NaH2PO2; it is diminished to 8.5 x 10(-3 ) in 0.1 M NaH2PO2. The same decrease in quantum yield with concentrat ion is also found for uracil and uridine; quantum yields of their tran sformations in 0.1 M NaH2PO2 at pH 7.0 are 6.6 x 10(-2) and 1.5 x 10(- 1) respectively. The mild conditions and high quantum yields character istic of the photoinduced reaction above open up the possibility of ob taining DNA molecules which contain among pyrimidine photoproducts mai nly dihydropyrimidine residues. A correlation between various types of thymine modifications in UV-irradiated double-stranded DNA (dimers an d dihydrothymidine residues) and the amplitude of the intense negative band in the circular dichroism (CD) spectra specific for DNA liquid c rystalline dispersions has been established. A possible application of this base-specific modification to the investigation of nucleic acids is considered.