Effect of ascorbic acid on the photolysis of cyanocobalamin and aquocobalamin/hydroxocobalamin in aqueous solution: A kinetic study

摘要

The photolysis of cyanocobalamin (B-12) and aquocobalamin (B-12a)/hydroxocobalamin (B-12b) in the presence of ascorbic acid (AH(2)) at pH 2.0-12.0, under aerobic conditions, has been studied. It follows first-order kinetics and the values of apparent first-order rate constants (k(obs)) at pH 2.0-12.0 range from 0.37 to 2.63 x 10(-4) s(-1) and 0.21-6.35 x 10(-4) s(-1), for B-12 and B-12a/B-12s, respectively. The second-order rate constants (k(2)) for the photochemical interaction of AH(2) and B-12 and AH(2) and B-12a/B-12b range from 0.20 (pH 2.0) to 1.09 x 10(-2) M-1 s(-1) (pH 5.0) and 5.88 (pH 2.0) to 91.08 x 10(-2) M-1 s(-1) (pH 5.0), respectively. The values of the k(2) for AH(2)-B-12a/B-12b interaction are 30-80 times greater than those of AH(2)-B-12 suggesting a greater susceptibility of B-12a/B-12b to photodegradation compared to that of B-12 in this pH range. The k(2)-pH profiles for both B-12 and B-12a/B-12b are bell-shaped curves indicating the effect of AH(2) ionization on the rates of interaction. The complete discoloration of B-12 and B-12a/B-12b solutions on prolonged photolysis indicates the formation of corrin ring cleavage (oxidation) products in acid and alkaline solutions. These oxidation products do not absorb in the visible region. Reaction schemes for the mode of photodegradation of B-12 and B-12a/B-12b in the presence of AH(2) have been presented. (C) 2016 Elsevier B.V. All rights reserved.