Photochemistry of dihydrobiopterin in aqueous solution

Abstract

Dihydrobiopterin (H₂Bip) and its oxidized analogue, biopterin (Bip), accumulate in the skin of patients suffering from vitiligo, a chronic depigmentation disorder in which the protection against UV radiation fails. The photochemistry of H₂Bip was studied in neutral aqueous solutions upon UV-A irradiation (320–400 nm) at room temperature. The photochemical reactions were followed by UV/vis spectrophotometry, HPLC and enzymatic methods for hydrogen peroxide (H₂O₂) determination. Photoproducts were analyzed by means of electrospray ionization mass spectrometry. Under anaerobic conditions, excitation of H₂Bip leads to the formation of at least two isomeric dimers with molecular masses equal to exactly twice the molecular mass of the reactant. This reaction takes place from the singlet excited state of the reactant. To the best of our knowledge, this is the first time that the photodimerization of a dihydropterin is reported. In the presence of air, the dimers are again the main photoproducts at the beginning of the reaction, but a small proportion of the reactant is converted into Bip. As the reaction proceeds and enough Bip accumulates in the solution, a photosensitized process starts, where Bip photoinduces the oxidation of H₂Bip to Bip, and H₂O₂ is formed. As a consequence, the rates of H₂Bip consumption and Bip formation increase as a function of irradiation time, resulting in an autocatalytic photochemical process. In this process, Bip in its triplet excited state reacts with the ground state of H₂Bip. The mechanisms involved are analyzed and the biological implications of the results are discussed.