Study of the photochemical properties of biopterin and its reduced forms

UV-irradiation (270-390 nm, 20 min, I = 3.2 W m(-2)) of deaerated biopterin solution containing electron donor (Na2-EDTA) led to the formation of 7,8-dihydrobiopterin (H2BPT), which, when excited, underwent reduction to form 5,6,7,8-tetrahydrobiopterin (H4BPT). Protonated molecules of H4BPT were resistant to oxidation by O2 both in the "dark" incubated and UV-irradiated solutions at pH below 3.0. The rate of H4BPT oxidation dramatically increased at pH above 3.0, and, then, up to pH 10.0, it did not change, showing no dependence on UV-irradiation. At the initial stage (5 min) of H4BPT oxidation in neutral solution, UV-irradiation stimulated the accumulation of quinonoid 6,7-dihydrobiopterin (q-H2BPT) in addition to H2BPT. UV-irradiation of H2BPT induced its oxidation to biopterin and unidentified products.