The kinetics of photostabilization of cyanocobalamin in liposomal preparations

Abstract

Abstract Cyanocobalamin (B 12 ) is a photosensitive vitamin, and its photodegradation to hydroxocobalamin (B 12b ) in liposomes has been investigated. The values of apparent first‐order rate constants ( k obs ) for the photodegradation of B 12 in liposomes (nine preparations) are in the range of (0.52‐2.24) × 10 –3 min –1 , compared to 3.21 × 10 –3 min –1 for B 12 in aqueous solution (pH 5.0). The entrapment efficiency of B 12 in liposomes is 26.4‐38.8%. The values of k obs show a linear relation with phosphatidylcholine (PC) content in liposomes, indicating the influence of PC in inhibiting the rate of photolysis of B 12 . The value of the bimolecular rate constant for photochemical interaction of B 12 and PC is 0.32 M –1 min –1 , indicating the stabilizing effect of PC on the photolysis of B 12 . The ratio of B 12 stabilization in liposomal preparations is in the range 2‐6 compared to that of the unentrapped vitamin The stabilization of B 12 is mediated by a photoinduced charge‐transfer B 12 ‐PC complex that leads to the reduction of B 12 to B 12r , which is then oxidized to B 12b that has low susceptibility to photolysis. The extent of stabilization of B 12 probably depends on the degree of interaction between the two compounds under the reaction conditions, indicated by the loss of B 12 fluorescence.