Photochemistry of biopterin and neopterin: quantum yields and reactive oxygen species formation

MARIANA VIGNONI; MICAELA GONZALEZ; M. LAURA DÁNTOLA; GABRIELA PETROSELLI; CAROLINA LORENTE; ANDRÉS H. THOMAS

Los Cocos, Córdoba, Argentina

Congreso; IX Conferencia Latinoamericana de Físico-Química Orgánica; 2007

Biopterin (BPT) and neopterin (NPT) are heterocyclic compounds present in different types of cells. BPT and its reduced forms work as enzymatic cofactors in some redox reactions. NPT is synthesized by human monocyte-derived macrophages. Increased NPT concentrations in human serum indicate activation of cell-mediated immune response. BPT and NPT undergo photooxidation in air-equilibrated aqueous solutions under UV-A radiation1. These oxidations chemically modify the 6-subtituent but do not affect the pterins moiety or, at least, do so to a smaller extent. BPT and NPT are not photostable even in the absence of O2. Red compounds are generated when Ar-purged solutions of these pterins, are exposed to UV-A radiation. It has been proposed that these compounds with long wavelength (max 480 nm) absorption bands of low intensity are 6-acyl-5,8-dihydropterins. Therefore, the reactions observed imply the reduction of the pterin moiety and the oxidation of the substituent through an intramolecular redox reaction. The 6-acyl-5,8-dihydropterins are thermally oxidized quickly on admission of O2 to yield the corresponding oxidized pterins. In this work we studied some aspects of the mechanism of the photolysis of BPT and NPT in aqueous solutions under UV-A radiation. We have investigated the photochemical production of reactive oxygen species, in particular superoxide anion radical (O2·-) and hydrogen peroxide (H2O2) We report the consumption quantum yields (F) of BPT and NPT in aqueous solutions, in both aerobic and anaerobic conditions. The biological implications are discussed in connection with the physiopathology of skin diseases, in which protection against UV radiation fails.   1) Mengel R., Pfleiderer W. y Knappe W. R.; Tetrahedron Lett., 32, 2817 (1977)