Folic acid oxidation photoinduced by aromatic pterins

M. LAURA DÁNTOLA; BEATRIZ ZURBANO; CARLOS S. GIMENEZ; CAROLINA LORENTE; ANDRÉS H. THOMAS

La Serena

Congreso; X Encuentro Latinoamericano de Fotoquímica y Fotobiología; 2010

Folic acid (FA) is a vitamin that participates in several metabolic pathways. Its chemical structure consists of a pterin ring system, a p-aminobenzoic acid portion, and the amino acid glutamic acid. Interest in the photochemistry of FA has increased since it was demonstrated that, in human beings, UV-A exposure causes its degradation.1 Moreover, it has been proposed that human skin pigmentation is a mechanism of defense that prevents photolysis of folic acid.2 Upon UV-A irradiation in the presence of O2, FA is broken and oxidized yielding 6-formylpterin (Fop) and p-aminobenzoyl-glutamic acid (PABA-Glu) as products. It has been observed that when an air-equilibrated aqueous solution of FA is irradiated, the rate of reactant oxidation increases as a function of irradiation time.3,4 In spite of the importance of the photochemical reaction, the mechanism of this accelerated process remains unknown. In this contribution we present a study of the photooxidation of FA in the presence of aromatic pterins. The experiments were performed under steady UV-A irradiation in air-equilibrated aqueous solutions at pH in the range 5-6. The irradiated solutions were analyzed by spectrophotometry, HPLC and determination of H2O2. Additional experiments were carried out in the presence of triplet state quenchers and superoxide dismutase. Evidence obtained indicates that aromatic pterins photoinduce the oxidation of FA. Results suggest that electron transfer from FA to the photosensitizer takes place. The FA radical cation yields oxidized products, whereas photosensitizer radical anion reduces dissolved O2 to form O2•‒, which finally generates H2O2. In the light of the overall data obtained, the mechanism shown in the scheme is proposed to explain the autoacceleration of the FA photolysis.Evidence obtained indicates that aromatic pterins photoinduce the oxidation of FA. Results suggest that electron transfer from FA to the photosensitizer takes place. The FA radical cation yields oxidized products, whereas photosensitizer radical anion reduces dissolved O2 to form O2•‒, which finally generates H2O2. In the light of the overall data obtained, the mechanism shown in the scheme is proposed to explain the autoacceleration of the FA photolysis. References 1 R. F. Branda, J. W. Eaton, Science 201, 625 (1978). 2 N. G. Jablonski, G. Chaplin, J. Human Evolution 39, 57 (2000). 3 A. H. Thomas et al, J. Photochem. Photobiol. A 135, 147 (2000). 4 M. K. Off et al, J. Photochem. Photobiol. B 80, 47 (2005).