Photolysis of folic acid and 6-formylpterin in acid aqueous solutions

ANDRÉS H. THOMAS; GUSTAVO SUÁREZ; FRANCO M. CABRERIZO; RAÚL MARTINO; ALBERTO L. CAPPARELLI

Dresden, Alemania

Congreso; XXVIII IUPAC Symposium of Photochemistry; 2000

Folates and pterins are heterocyclic compounds widespread in biological systems. Folic acid or pteroylglutamic acid, a conjugated pterin, is a vitamin of the B group and acts as a coenzyme in reactions related to the synthesis of puric and pirimidinic bases. Pterins participate in relevant biological functions too. Light sensitivity of pterins derivatives is well known and they have been found in photosensitive organs such as the eyes. Moreover, it has been suggested that pterins may play some role in photosynthesis and may act as blue antennas in superior plants and other organisms. Reports on photochemistry and photophysics of pterins can be found in the literature. However, much is still left to be known on the photochemistry of folic acid and pterins. Amide group in pterins moiety suffers deprotonation to the corresponding fenolate in alkaline media. The aim of this work is to study the photolysis of the acid forms of folic acid and 6-formylpterin by UV-A light (320 - 400 nm.). Thus, the pH conditions (4.5 - 6.0) were chosen, so that only the acid forms are present.             In the present work, products of photolysis of folic acid and 6-formylpterin were identified and quantified and the role of O2 was analysed. The photochemical reactions were followed by UV-VIS spectrophotometry, thin layer chromatography (TLC), high performance liquid chromatography (HPLC) and the oxygen consumption was measured with an O2 selective electrode. The quantum yields of substrate disappearance and of photoproducts formation were determined in experiments performed under different conditions. Scheme below summarises the results obtained. In the absence of oxygen, folic acid is photostable, whereas excitation in the presence of oxygen leads to cleavage and oxidation of the molecule, yielding 6-formylpterin and p-aminobenzoylglutamic acid as photoproducts. No other photoproducts were detected under our experimental conditions. This degradation reaction of folic acid is accelerated in the presence of 6-formylpterin. This process requires photoexcitation of 6-formylpterin but not requires the absorption of light by folic acid. Therefore, excitation of 6-formylpterin leads to the generation of some reactive specie that induces the degradation of folic acid. As the photolysis proceeds, 6-carboxypterin arises from 6-formylpterin, as observed when solutions only containing the latter compound are irradiated in the presence of oxygen. Photolysis of 6-formylpterin, under anaerobic conditions, leads to the formation of a “red intermediate” (RI) with a longwave absorbing band of low extinction at 480 nm. RI is thermally oxidised producing 6-carboxypterin on admission of oxygen. Due to its spectral features and reactivities, the "red intermediate" detected in our experiments could be 6-carboxy-5,8-dihidropterin