Effect of Riboflavin on the Photolysis of Cyanocobolamin in AqueousSolution

The kinetics of photolysis of cyanocobalamin (CC) by visible radiation in the presence of riboflavin (RF) at pH 2.0-12.0 has been studied. A specific two-component spectrophotometric method has been used for the simultaneous assay of CC and its photoproduct, hydroxocobalamin (HC), at 525 and 550 nm, without any interference from RF. The apparent first-order rate constants for the photolysis of CC in the presence of 1-5 � 10 -5 M concentrations of RF at pH 2.0- 12.0 range from 0.46 (pH 7.0) to 4.22 � 10 -3 min -1 (pH 3.5). The second-order rate constants for the bimolecular interaction of these vitamins are in the range of 8.66 (pH 7.5) to 45.20 M -1 min -1 (pH 2.0). The rate-pH profile indicates a gradual decrease in rate in the acid pH range followed by the slowest rate and maximum stability around pH 7-8. Thus the cationic form of CC is more susceptible to photolysis compared to the neutral form. The increase in rate in the alkaline region is probably due to the hydrolysis of the amide groups. The kinetic results indicate that RF acts as a sensitizer in the photolysis of CC and in this way promotes the degradation of the molecule. Thus the presence of RF in CC solutions adversely affects the stability of vitamin B12.