Abstract
AbstractCyanocobalamin (B12) is a photosensitive vitamin, and its photodegradation to hydroxocobalamin (B12b) in liposomes has been investigated. The values of apparent first‐order rate constants (kobs) for the photodegradation of B12 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 B12 in aqueous solution (pH 5.0). The entrapment efficiency of B12 in liposomes is 26.4‐38.8%. The values of kobs show a linear relation with phosphatidylcholine (PC) content in liposomes, indicating the influence of PC in inhibiting the rate of photolysis of B12. The value of the bimolecular rate constant for photochemical interaction of B12 and PC is 0.32 M–1 min–1, indicating the stabilizing effect of PC on the photolysis of B12. The ratio of B12 stabilization in liposomal preparations is in the range 2‐6 compared to that of the unentrapped vitamin The stabilization of B12 is mediated by a photoinduced charge‐transfer B12‐PC complex that leads to the reduction of B12 to B12r, which is then oxidized to B12b that has low susceptibility to photolysis. The extent of stabilization of B12 probably depends on the degree of interaction between the two compounds under the reaction conditions, indicated by the loss of B12 fluorescence.
Published Version
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