Photochemical Behavior of Folic Acid in Alkaline Aqueous Solution and Evolution of Its Photoproducts

ANDRES H. THOMAS; GUSTAVO SUÁREZ; FRANCO M. CABRERIZO; FERNANDO S. GARCÍA EINSCHLAG; RAUL MARTINO; CLAUDIO BAIOCCHI; EDMONDO PRAMAURO; ALBERTO CAPARELLI

HELVETICA CHIMICA ACTA

wiley-vhc

Año: 2002 p. 2300 - 2315

0018-019X

The photolysis of folic acid (_N-(4-{[(2-amino-1,4-dihydro-4-oxopteridin-6-yl)methyl]amino}benzoyl)-_-glutamic acid) in alkaline aqueous solution (pH 10.0 ± 11.0) was carried out at 350 nm at room temperature and monitored by UV/VIS spectrophotometry, anal. and prep. thin-layer chromatography (TLC), and highperformance liquid chromatography (HPLC, HPLC/MS). The folate species underwent at least two independent photo-oxidation pathways, which were not observed when the acid form was photolyzed at pH_7. The presence of O2 was essential in these oxidation pathways. Evidence for the role of singlet oxygen was established. In one of the pathways, the folate underwent cleavage, yielding 6-formylpterin (_2-amino-1,4-dihydro-4-oxopteridine-6-carboxaldehyde) and (4-aminobenzoyl)glutamic acid as photoproducts. The other pathway yielded a new photostable product A of molecular mass 455, which could be isolated and stored in acidic or neutral aqueous solution. However, A was rather unstable in alkaline media undergoing a thermal reaction to a product B of lower molecular mass (427). The kinetics of this thermal reaction was analyzed with a stopped-flow spectrophotometer. A linear dependence of the first-order rate constant with the OH_ concentration was observed. The corresponding bimolecular rate constant was 1.1 M-1 s-1. The quantum yields of substrate consumption and of photoproduct formation were determined. The here-reported photochemical behavior of folate solutions departs from results in acid media, where phototransformation proceeded via the cleavage of the acid form into 6-formylpterin and (4-aminobenzoyl)glutamic acid as the first major photoproducts, and where no thermal reactions were observed.