Simultaneous Photoaddition, Photoreduction and Chemical Reduction of Riboflavin by Sulfur Containing Dianions: A Kinetic Study

The present investigation involves the evaluation of the effect of reducing dianions (SO32-, S2O32-, S2O52-) on the kinetics of simultaneous photoaddition, photoreduction and chemical reduction reactions of riboflavin (RF) at pH 6-8. The photoproducts of these reactions have been identified and assayed during the degradation of RF. The final products of the above reactions are cyclodehydroriboflavin (CDRF), lumichrome (LC) and dihydroriboflavin (DHRF), respectively. The first-order rate constants for the loss of RF (k(obc)) (0.61-3.07 x 10(-3) min(-1)) and formation of photoproducts (0.21-1.61 x 10(-3) min(-1)) in the presence of dianions (0.2-1.0 M), and the second-order rate constants for the photochemical interaction of RF and dianions (1.42-2.01 x 10(-3) M(-1)min(-1)) and for the chemical reduction of RF (dark) (2.31-3.25 x 10(-3) M(-1)min(-1)) have been determined. The UV and visible spectral and fluorescence characteristics of RF in the presence of dianions have been studied. The fluorescence of RF is quenched by dianions depending upon the extent of their interaction with the molecule. The photoaddition of RF is optimum under neutral pH conditions and is catalyzed by the dianions in the order: S2O52- > SO32- > S2O32-, whereas the chemical reduction is enhanced in the reverse order. The S(2)O(3)(2-)dianion is most effective as a reducing agent on the basis of its redox potential which is lower than that of the SO(3)(2-)dianion. The quantum yields of photodegradation of RF in the presence of dianions (0.018-0.046) have been determined. The mode of photodegradation reactions of RF has been discussed.