E FFECT OF C OMPLEXING A GENTS ON THE P HOTODEGRADATION OF R IBOFLAVIN IN A QUEOUS S OLUTION

Abstract

The present investigation involves a study of the effect of complexing agents, i.e., borate, caffeine and divalent ions (phosphate, sulphate, tartrate, succinate, and malonate) on the photodegradation reactions of riboflavin (RF) in aqueous solutions. RF and its major products of photoreduction, formylmethylflavin (FMF), lumichrome (LC), lumiflavin (LF), and photoaddition, cyclodehydroriboflavin (CDRF), have been determined by a multicomponent spectrophotometric method. In borate buffer (0.1–0.5 M) at pH 8.0–10.5, RF (5 × 10 –5 M) undergoes photolysis by consecutive first-order reactions to yield FMF, LC, LF (major products) and carboxymethylflavin, CMF (minor product). The overall first-order rate constants (k obs ) for the photolysis of RF (1.55–4.36 × 10 –2 min –1 ) and the rate constants for the formation of FMF (1.16–3.52 × 10 –2 min –1 ) and LC (0.24–0.84 × 10 –2 min –1 ) have been determined. The values of all these rate constants decrease with an increase in buffer concentration suggesting the inhibition of the photolysis reaction by borate ions. The kinetic data support the formation of a RF–borate complex involving the ribityl side chain to cause the inhibition of the reaction. The second-order rate constants for the borate inhibited reactions (k ́) range from 1.17–3.94 × 10 –2 M –1 min –1 ). The log k–pH profiles for the reactions at various buffer concentrations indicate a gradual increase in rate, with pH, up to 10 followed by a decrease in rate at pH 10.5 probably due to the ionization of RF and quenching of fluorescence by borate ions (32%). A graph of k ́ against pH is a sigmoid curve showing that the rate of photolysis increases with an increase in pH. The results suggest the involvement of excited singlet state, in addition to excited triplet state, in the formation of LC. ivThe photolysis of RF in the presence of caffeine (0.5–2.5 × 10 –4 M) yields the same products as in the case of borate buffer. The apparent first-order rate constants (k obs ) for the photolysis reactions at pH 2.0–10.5 range from 2.71 × 10 –4 to 4.26 × 10 –2 min –1 . Similar to the effect of borate ions, the values of the rate constants decrease with increasing concentrations of caffeine indicating its inhibitory effect on the reactions. The second-order rate constants (k') for the photolysis reactions in the presence of caffeine are in the range of 0.13–5.10 × 10 –3 M –1 min –1 . The log k–pH profiles for the photolysis reactions at various caffeine concentrations involve multiple steps indicating a gradual increase in the rate up to pH 10. The lower rates at pH 2.0 and above 10.0 are due to the ionization of RF. The k'–pH profile for the interaction of RF and caffeine represents a bell-shaped curve in the pH range 3–6 followed by a sigmoid curve in the pH range 7–10. The inhibition of RF photolysis in the presence of caffeine appears to be a result of the monomeric interaction and complex formation of RF with caffeine. The photochemical interaction of RF with caffeine suggests that a pH around 6 is most appropriate for the stabilization of the vitamin. At this pH the complex shows the highest stability constant. The photodegradation of RF in the presence of divalent ions (0.2–1.0 M) at pH 6.0–8.0 involves simultaneous photolysis and photoaddition yielding FMF, LC, LF, CMF, and CDRF, respectively, by parallel first-order reactions. The rate–pH curves represent a composite profile for the overall photodegradation of RF by two simultaneous reactions involving changes in the rates of formation of CDRF and LC. The catalytic effect of divalent ions influences the reaction in the order of phosphate > sulphate > tartrate > succinate > malonate to give rise to CDRF. The mode of photodegradation of RF has been explained on the basis of the kinetic data obtained for these reactions.