Photochemical behavior of folic acid in alkaline aqueous solutions and evolution of its photoproducts

A.H. THOMAS; G. SUÁREZ; F.M. CABRERIZO; F.S. GARCÍA EINSCHLAG; R. MARTINO; C. BAIOCCHI; E. PRAMAURO; A.L. CAPPARELLI

HELVETICA CHIMICA ACTA

Año: 2002 vol. 85 p. 2300 - 2315

0018-019X

Abstract Photochemical and kinetic studies of the reactivity of folic acid in alkaline aqueous solutions were performed. The photolysis of the substrate was carried out at 350 nm at room temperature in the pH range 10.0-11.0. UV-VIS spectrophotometry, thin layer chromatography (TLC), both for analytical and preparative purposes, and high performance liquid chromatography (HPLC, HPLC/MS) were applied to follow the phototransformation of folate. This species undergoes, at least, two independent photooxidation pathways, which are not observed when the acid form of this substance is photolyzed at pH <7. The presence of dioxygen is essential in these pathways. Evidences on the role of singlet oxygen are reported. In one of these pathways folic acid undergoes cleavage, yielding 6-formylpterin and p-aminobenzoylglutamic acid as photoproducts. The other pathway yields a new photoproduct (here named as substance A) having a molecular weight of 455. This new compound A is photostable, can be isolated and stored in acidic or neutral solutions aqueous solutions. However, this compound is rather unstable in alkaline media, where it undergoes a thermal reaction, yielding a product of lower molecular weight (427). The kinetic of this thermal reaction was analyzed using a stopped flow equipment. A linear dependence of the first order rate constant with the OH- concentration is observed. The corresponding bimolecular rate constant was determined (1.1 M-1s-1). The quantum yields of substrates consumption and of photoproducts formation are reported. The photochemical behavior of folate solutions reported in the present communication departs from results in acid media, where phototransformation proceeds through the cleavage of the acid form into 6-formylpterin and p-aminobenzoylglutamic acid as the first major photoproducts and no thermal reactions are observed