RESUMO
UV irradiation (lambda = 254 nm) of thymine and uracil in aqueous solution containing salts of phosphinic acid (hypophosphites) results in the formation of the corresponding dihydropyrimidine derivatives. The peculiarities of this new photochemical reaction consist of a specificity towards 2,4-dioxophyrimidines, 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 concentration is also found for uracil and uridine; quantum yields of their transformations 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 characteristic of the photoinduced reaction above open up the possibility of obtaining DNA molecules which contain among pyrimidine photoproducts mainly dihydropyrimidine residues. A correlation between various types of thymine modifications in UV-irradiated double-stranded DNA (dimers and dihydrothymidine residues) and the amplitude of the intense negative band in the circular dichroism (CD) spectra specific for DNA liquid crystalline dispersions has been established. A possible application of this base-specific modification to the investigation of nucleic acids is considered.
Assuntos
Dano ao DNA , DNA/efeitos dos fármacos , DNA/efeitos da radiação , Ácidos Fosfínicos/farmacologia , Radiossensibilizantes , Timina/efeitos da radiação , Raios Ultravioleta , Uracila/efeitos da radiação , Dicroísmo Circular , DNA/química , Cinética , Oxirredução , Poli T/efeitos da radiação , Pirimidinas/efeitos da radiação , Teoria Quântica , Relação Estrutura-AtividadeRESUMO
The principal UV-induced (lambda = 2546nm) reaction of N4-hydroxy- and N4methoxycytidines and N6-methoxyadenosine in neutral aqueous solutions is cleavage of the exocyclic N-O bond with the respective formation of cytidine and adenosine. Quantum yields are 2.8x10(-3) and 2.2x10(-3) M/E for the first two compounds and 9.1x10(-3) M/E for N6-methoxyadenosine.