Transient Absorption Spectra and Kinetics of Folic Acid (Vitamin B11) and Some Kinetic Data of Folinic Acid and Methotrexate

Abstract

Folic acid (FA; vitamin B11) exhibits antitumor properties. Its transients, resulting from reaction with OH radicals, are of special interest and were studied by pulse radiolysis. The complex composition of FA offers the formation of numerous transients having different electronic structure stability (lifetime). Investigating their kinetic profiles of decay at various wavelengths, it was established that transients with very unstable electron structure (very short-lived radicals) tend to convert into more stable electronic structures (longer lived) by intramolecular electron transfer process. The biological importance of a FA radical depends on its concentration and on its specific reaction rate constant (k value) for a given process. The rate constant for the OH attack is k(OH + FA) = 1.1 x 10(10) L mol(-1) s(-1). Superimposed absorption spectra of FA radicals as well as formation and decay rate constants of total processes are presented. The primary spectrum is characterized by lambda = 425 nm, epsilon = 1.64 x 10(4) L mol(-1) s(-1). The chemical structure of FA is similar to that of folinic acid (FNA) and of methotrexate (MTX), but their biological properties are different. Therefore, their rate constants for the reactions with e-aq and OH were also determined for comparison: k(FNA + e-aq) = 3.1 x 10(10) L mol(-1) s(-1), k(FNA + OH) = 0.8 x 10(10) L mol(-1) s(-1), k(MTX + e-aq) = 1 x 10(10) L mol(-1) s(-1); k(MTX + OH) = 2.3 x 10(10) L mol(-1) s(-1), and k(FA + e-aq) = 1.9 x 10(10) L mol(-1) s(-1).