Identification of the α and β anomers of 1-(2-deoxy-D-erythro-pentofuranosyl)-oxaluric acid at the site of riboflavin-mediated photooxidation of guanine in 2′-deoxyguanosine and thymidylyl-(3′-5′)2′-deoxyguanosine

Products of riboflavin-mediated photosensitization of 2' deoxyguanosine (dG) and thymidylyl-(3'-5')-2'-deoxyguanosine (TpdG) by 350-nm light in oxygen-saturated aqueous solution have been isolated and identified as 1-(2-deoxy-beta-D-erythro-pentofuranosyl) oxaluric acid (beta-dOx) and thymidylyl(3'-5')-1-(2-deoxy-beta-D-erythro-pentofuranosyl) oxaluric acid (Tp beta-dOx), respectively. In aqueous solution the modified beta-deoxyribonucleoside is slowly converted to the a-anomer, generating alpha-dOx and Tp alpha-dOx. These modified nucleosides and dinucleoside monophosphates have been isolated by HPLC and characterized by proton and carbon NMR spectroscopy, fast atom bombardment mass spectrometry, and enzymatic analyses. Both alpha-dOx and Tp alpha-dOx slowly convert back into the modified beta-deoxyribonucleoside, indicating that the furanosidic anomers are in dynamic equilibrium. Relative to TpdG, the rate of hydrolysis of Tp beta-dOx and Tp alpha-dOx by spleen phosphodiesterase is greatly reduced. Hot piperidine (1.0 M, 90 degrees C, 30 min) destroys Tp beta-dOx and Tp alpha-dOx. Riboflavin-mediated photosensitization of TpdG in D2O instead of H2O has no detectable effect on the yield of Tp beta-dOx, suggesting that oxaluric acid is generated through a Type-I reaction mechanism, likely through the intermediary on initially generated 8-oxo-7,8-dihydro-2'-deoxyguanosine.