Formamidopyrimidines as major products in the low- and high-intensity UV irradiation of guanine derivatives.

Two major UV-induced transformation products of guanosine (Guo) in oxygen-free aqueous solution were isolated and characterized using reverse-phase HPLC-ESI-MS and UV absorption spectrophotometric techniques. These products were identified as two different sugar isomers of 2,6-diamino-4-hydroxy-5-formamidopyrimidine ribonucleosides, FapyGuo. A formation quantum yield of the order of 10(-3) for these products was obtained at a 75 microM concentration of Guo, while an increase in the ground state concentration of Guo from 10(-5) to 10(-4) M decreased their formation yield by a factor of ten. The formation of FapydGuo was also observed in the 254 nm photolysis of 2'-deoxyguanosine. In addition, the formation FapyGua was identified in the UV photolysis of all the guanine derivatives investigated. A formation quantum yield of the order of 10(-4) was obtained for the latter product, except in the photolysis of Gua in which a higher formation yield (10(-3)) was determined. These results suggest that hydration of the radical cation of guanine, followed by opening of the imidazole ring to form Fapy products, is one of the main low-intensity UV radiation-induced photo-transformation pathway of the guanine derivatives in oxygen-free aqueous solutions. Furthermore, the formation of the Fapy products and base release occurs through a similar photo-transformation pathways using a high-intensity UV radiation source.