Photolysis of cyanocobalamin in aqueous solution.

Cyanocobalamin is photolysed in aqueous solution to produce hydroxocobalamin. The kinetics of photolysis has been studied at pH 1-12 using a newly developed spectrophotometric method for the simultaneous determination of the two compounds at 550 and 525 nm or 361 and 351 nm. Cyanocobalamin follows zero-order kinetics at the pH values studied and the rate is catalysed by both hydrogen and hydroxyl ions. The log k-pH profile indicates that cyanocobalamin has maximum stability near pH 7. The cationic species appears to be more susceptible to photolysis than the neutral form. The rate constant for the reaction at pH 1 is 1.32 x 10(-7) mol l-1 min-1 compared with 0.050 x 10(-7) mol l-1 min-1 at pH 7.