Investigation 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 (H ₂ BPT), which, when excited, underwent reduction to form 5,6,7,8-tetrahydrobiopterin (H ₄ BPT). 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-H ₂ BPT) in addition to H₂BPT. UV irradiation of H₂BPT induced its oxidation to biopterin and unidentified products.