Abstract: Under the irradiation of a middle pressure mercury lamp(MPML), we found that the photo lyses of nucleobases, nucleosides and nucleotides were strongly enhanced by phosphates (termed phosphate effect). In order to explain this phenomenon, the photolytic systems of cytosine in phosphate solutions were selected and studied. Experimental results indicated that the UV irradiation (190-220 nm) in the emission spectra of MPML is responsible for the phosphate effect. Phosphates absorbing the UV (190-220 nm) energy are converted to phosphate radical anions, which react with cytosine, leading to a novel photoproduct with phosphate group (C₄H₆N₃O₅P). It has been isolated, purified and characterized by means of elementary analysis, UV, IR, EI MS, 1H NMR, ¹³ C NMR and ³¹ P NMR. The phosphate effect and its formation mechanism have been discussed.

Key words: Phosphate effect, Photolysis of cytosine, Photoproduct 6-phosphocytosine