Effect of divalent anions on photodegradation kinetics and pathways of riboflavin in aqueous solution.

The present investigation is based on a study of the effect of buffer and non-buffer divalent anions (phosphate, sulphate, tartrate, succinate, malonate) on the kinetics, product distribution and photodegradation pathways of riboflavin (RF) at pH 6.0-8.0. RF solutions (5x10(-5)M) were photodegraded in the presence of divalent anions (0.2-1.0M) using a visible light source and the photoproducts, cyclodehydroriboflavin (CDRF), formylmethylflavin (FMF), lumichrome (LC) and lumiflavin (LF) were assayed by a specific multicomponent spectrophotometric method. RF degradation in the presence of divalent anions follows parallel first-order kinetics to give CDRF and LC as the final products through photoaddition and photoreduction reactions, respectively. The divalent anion-catalysed CDRF formation is affected in the order: phosphate>sulphate>tartrate>succinate>malonate, showing maximum activity of the anions around pH 7. The divalent anions cause deviation of the photoreduction pathway in favour of the photoaddition pathway to form CDRF. The first- and second-order rate constants for the reactions involved in the photodegradation of RF have been determined and the rate-pH profiles and pathway relationships discussed. The catalytic activity of the divalent anions appears to be a function of the relative strength and chemical reactivity of the RF-divalent anion complex acting as a mediator in the photoaddition reaction.

Photolysis of riboflavin in aqueous solution: a kinetic study.

The kinetics of photolysis of aqueous riboflavin solutions on UV and visible irradiation has been studied in the pH range 1-12 using a specific multicomponent spectrophotometric method for the simultaneous determination of riboflavin and its major photoproducts (formylmethylflavin, lumichrome and lumiflavin). The apparent first-order rate constants for the photodegradation reactions in the pH range have been determined. The log k-pH profiles indicate that riboflavin has maximum photostability around pH 5-6, at which the rate of oxidation-reduction of the molecule is lowest. The cationic and anionic forms of riboflavin are non-fluorescent and less susceptible to photolysis than the non-ionised molecule as indicated by the relatively slow rates below pH 3.0 and above pH 10.0. The rate of photolysis is increased up to 80-fold at pH 10.0, compared to that at pH 5.0, due to increase in redox potentials with an increase in pH and consequently the ease with which the molecule is oxidised. The increase in rate at pH 3.0, compared to that at pH 5.0, appears to be due to the involvement of the excited singlet state as well as the triplet state in riboflavin degradation. The apparent first-order rate constants for the photolysis of riboflavin at pH 5.0-10.0 with UV and visible radiation are 0.185 x 10(-2) to 13.182 x 10(-2)min(-1) and 0.098 x 10(-2) to 7.762 x 10(-2)min(-1), respectively.

Chromatographic study of photolysis of aqueous cyanocobalamin solution in presence of vitamins B and C.

Aqueous cyanocobalamin solutions (pH 1-7) have been photolysed in the presence of individual B (thiamine HCl, riboflavin, nicotinamide and pyridoxine HCl) and C (ascorbic acid) vitamins with visible light. The degraded solutions were subjected to thin-layer chromatography using several solvent systems and the Rf values of the vitamins and their photoproducts were determined. The major photoproducts have been identified by comparison of their Rf values with those of the authentic compounds. Cyanocobalamin leads to the formation of hydroxocobalamin. Thiamine HCl gives rise to 4-methyl-5-(Beta-hydroxyethyl) thiazole and 2-methyl-4-amino-5-hydroxymethyl-pyrimidine in trace amounts whereas riboflavin degrades extensively to formylmethylflavin and lumichrome, and to a smaller extent to lumiflavin and carboxymethylflavin. Ascorbic acid is oxidized to dehydroascorbic acid. Nicotinamide and pyridoxine HCl do not undergo any degradation. The extent of degradation depends upon the pH.

Spectral study of photolysis of aqueous cyanocobalamin solutions in presence of vitamins B and C.

The UV and visible absorption characteristics of B and C vitamins have been studied in the pH range 2.0-7.0. The overlapping of the absorption bands of thiamine hydrochloride, nicotinamide, pyridoxine hydrochloride and ascorbic acid with cyanocobalamin in the UV region and those of riboflavin in the UV and visible region may influence the rate of photolysis of cyanocobalamin in aqueous solution due to mutual interaction on exposure to light. The spectral variations in cyanocobalamin solutions at pH 2.0-7.0 containing appropriate amounts of the individual B/C vitamins, during photolysis, have been monitored and the effect of pH on the rates of reaction has been discussed. The rates of photolysis, in general, decrease with an increase in pH probably due to gradual deprotonation of cyanocobalamin cation (B12 H+) as indicated by the magnitude of absorbance loss at the maxima at 361 and 550 nm.

Effect of nicotinamide on the photolysis of cyanocobalamin in aqueous solution.

The photolysis of cyanocobalamin by visible light in the presence of nicotinamide at pH 1.0-7.0 has been studied. The second-order rate constants for the bimolecular interaction of these vitamins have been determined which vary from 9.50 x 10(-3) (pH 1.0) to 1.25 x 10(-3) M(-1) min(-1) (pH 7.0). The rate-pH profile indicates a gradual slow decrease in rate in the pH range 1-3 followed by a relatively fast decrease in the pH range 3-7. The protonated form of cyanocobalamin appears to be more susceptible to photolysis than the non-ionised form. Cyanocobalamin photolysis leads to the formation of hydroxocobalamin at pH 1.0-7.0.