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

The photolysis of cyanocobalamin (B12) and aquocobalamin (B12a)/hydroxocobalamin (B12b) in the presence of ascorbic acid (AH2) 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 (kobs) at pH 2.0-12.0 range from 0.37 to 2.63 × 10−4 s−1 and 0.21-6.35 × 10−4 s−1, for B12 and B12a/B12b, respectively. The second-order rate constants (k2) for the photochemical interaction of AH2 and B12 and AH2 and B12a/B12b range from 0.20 (pH 2.0) to 1.09 × 10−2 M−1 s−1 (pH 5.0) and 5.88 (pH 2.0) to 91.08 × 10−2 M−1 s−1 (pH 5.0), respectively. The values of the k2 for AH2-B12a/B12b interaction are 30-80 times greater than those of AH2-B12 suggesting a greater susceptibility of B12a/B12b to photodegradation compared to that of B12 in this pH range. The k2-pH profiles for both B12 and B12a/B12b are bell-shaped curves indicating the effect of AH2 ionization on the rates of interaction. The complete discoloration of B12 and B12a/B12b 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 B12 and B12a/B12b in the presence of AH2 have been presented.