Generation of reactive oxygen species during the photolysis of 6-(hydroxymethyl)pterin in alkaline aqueous solutions

Photochemical studies of the reactivity of 6-(hydroxymethyl)pterin (=2-amino-6-(hydromethyl)pteridin-4(1H)-one; HPT) in alkaline aqueous solutions (pH 10.2-10.8) at 350 nm and room temperature were performed. The photochemical reactions were followed by UV/VIS spectrophotometry, thin-layer chromatography (TLC), high-performance liquid chromatography (HPLC), and an enzymatic method for H2O2 determination. In the presence of O-2, 6-formylpterin (= 2-amino-3,4-dihydro-4-oxopteridine-6-carboxaldehyde; FPT) was the only photoproduct detected. In the absence of O-2, we observed a compound with an absorbance maximum at 480 nm, which was oxidized very rapidly by O-2 in a dark reaction to yield FPT. The quantum yields of substrates disappearance and of photoproducts formation were determined. The formation of H2O2 during photooxidation was monitored, and the number of mol of H2O2 released per mol of HPT consumed corresponded to a 1:1 stoichiometry. HPT was also investigated for efficiency of singlet-oxgen (O-1(2)) production and quenching in aqueous solution. The quantum yield of O-1(2) production (phi Delta = 0.21+/-0.01) was determined by measurements of the O-1(2) luminescence in the near-IR (1270 nm) upon continuous excitation of the sensitizer. The rate constant of O-1(2) total quenching by HPT was determined (k(1) = 3.1 . 10(6) M-1 S-1), indicating that this compound was able to quench O-1(2). However, O-1(2) did not participate in the photooxidation of HPT to FPT.