PHOTOCHEMISTRY OF DIHYDROBIOPTERIN AND DIHYDRONEOPTERIN IN AQUEOUS SOLUTION

MARIANA VIGNONI, MARIANA PAULA SERRANO, FRANCO M. CABRERIZO, CAROLINA LORENTE, CATHERINE CLAPAROLS, ANDRÉS H. THOMAS, ESTHER OLIVEROS

Ferrara, Italy

Simposio; XXIII IUPAC Symposium on Photochemistry; 2010

IUPAC

Dihydrobiopterin (H2Bip) and its oxidized analogue, biopterin (Bip), accumulate in the skinof patients suffering vitiligo, a chronic depigmentation disorder in which the protection against UVradiation fails [1]. Dihydroneopterin (H2Nep) and neopterin (Nep) are secreted by humanmacrophages upon stimulation with interferon-g. The photochemistry of H2Bip and H2Nep was studied in neutral aqueous solutions upon UV-A irradiation (320-400 nm) at room temperature [3]. 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 and H2Nep leads to the formation of at leasttwo isomeric dimers with molecular masses equal to exactly twice the molecular mass of thereactants. In the presence of O2 the same reactions were observed, but, in the case of H2Bip, a small proportion of the reactant is converted into Bip. As the reaction proceeds and enough Bipaccumulates 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. Other oxidized pterins (Pt) can also act as photosensitizers of H2Bip oxidation, thus revealing a general mechanism. In the case of H2Nep, the oxidative pathway does not occur. However, in the presence of an oxidized pterin, photosensitized oxidation of H2Nep takes place to yield dihydroxantopterin (H2Xap).