Abstract. The degradation kinetics of 5×10−5 M cyanocobalamin (B 12 ) and hydroxocobalamin (B 12b ) in the presence of ascorbic acid (AH 2 ) was studied in the pH range of 1. , respectively, indicating a greater effect of AH 2 on B 12b compared to that of B 12 . The k obs -pH profiles for both B 12 and B 12b show the highest rates of degradation around pH 5. The degradation of B 12 and B 12b by AH 2 is affected by the catalytic effect of phosphate ions on the oxidation of AH 2 in the pH range 6.0-8.0.
Riboflavin (RF) is a light sensitive compound and is known to form a number of photoproducts. These photoproducts possess the same nucleus and may interfere in the analysis of RF by UV and visible spectrometry. Therefore, it is necessary to apply the methods of multicomponent spectrometric analysis to quantify the vitamin and its photoproducts accurately. Such methods are useful in the study of the kinetics of photodegradation reactions of RF to obtain accurate and reliable results. Any interference in these methods due to linear or nonlinear irrelevant absorption of the minor unknown products can be accounted for by the application of appropriate correction procedures prior to kinetic treatment. Various factors affecting the accuracy, precision and selectivity of these analytical procedures are also discussed. This review highlights the principles and applications of multicomponent spectrometric methods and their application to the simultaneous determination of RF and its major photoproducts in degraded solutions to evaluate the kinetics of degradation.
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.
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