Photooxidation of Pterin in Aqueous Solutions: Biological and Biomedical Implications

Studies of the photochemical reactivity of pterin (=2‐aminopteridin‐4(3 H )‐one; PT) in acidic (pH 5.0–6.0) and alkaline (pH 10.2–10.8) aqueous solutions have been 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. PT is not light‐sensitive in the absence of molecular oxygen, but it undergoes photooxidation in the presence of O 2 , yielding several nonpteridinic products. The quantum yields for PT disappearance were found to be 8.2 (±0.6)×10 −4 and 1.2 (±0.2)×10 −3 in acidic and alkaline media, respectively. H 2 O 2 was detected and quantified in irradiated solutions of PT; and its importance from a biomedical point of view is discussed. The rate constant of the chemical reaction between singlet oxygen ( 1 O 2 ) and PT was determined to be 2.5 (±0.2)×10 5  l mol −1 s −1 in alkaline medium, and the role of 1 O 2 in the photooxidation of pterin was evaluated.