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‐(hydroxymethyl)pteridin‐4(1 H )‐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 H 2 O 2 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 H 2 O 2 during photooxidation was monitored, and the number of mol of H 2 O 2 released per mol of HPT consumed corresponded to a 1 : 1 stoichiometry. HPT was also investigated for efficiency of singlet‐oxygen ( 1 O 2 ) production and quenching in aqueous solution. The quantum yield of 1 O 2 production ( Φ Δ =0.21±0.01) was determined by measurements of the 1 O 2 luminescence in the near‐IR (1270 nm) upon continuous excitation of the sensitizer. The rate constant of 1 O 2 total quenching by HPT was determined ( k t =3.1⋅10 6 M −1 s −1 ), indicating that this compound was able to quench 1 O 2 . However, 1 O 2 did not participate in the photooxidation of HPT to FPT.