Density functional theory study of possible mechanisms of folic acid photodecomposition

Density functional theory (DFT) calculations were performed at the B3LYP/6-31+G*//B3LYP/6−31G* level of theory on a series of folic acid, pterin, and p-aminobenzoic acid (PABA) models to further understand the anaerobic mechanism of folic acid photodecomposition. After initial excitation of the pterin ring at 350 nm, an energetically favored intramolecular electron-transfer reaction occurs from the PABA ring to the pterin (−11.9 kcal/mol; PCM using water as the dielectric constant). The resulting zwitterionic species can then undergo an exothermic heterolytic bond cleavage which results in bond fragmentation between the PABA and pterin rings (−15.9 kcal/mol). The pterin radical is then hypothesized to undergo oxidation to produce the reported 6-carboxypterin. Natural population analyses (NPA) were performed on all radical and radical ion structures to establish reasonable resonance structures used in the proposed photodecomposition mechanism of folic acid by determining the most reasonable location of the radicals and ions on the aromatic rings. A proposed mechanism of the photolysis of folic acid in the absence of oxygen is then proposed based upon the DFT calculations which agree well with the currently existing data of this process.