A study of simultaneous photolysis and photoaddition reactions of riboflavin in aqueous solution

The photodegradation reactions of riboflavin (RF) in the presence of 0.05–2.00 M phosphate (pH 7.0) have been studied using a specific multicomponent spectrophotometric method. The reactions involve simultaneous photolysis (intramolecular photoreduction) and photoaddition (intramolecular photoaddition) leading to lumichrome (LC) and cyclodehydroriboflavin (CDRF), respectively, as major products. The contribution of each reaction in the overall photodegradation depends upon the phosphate concentration, i.e., higher the phosphate concentration higher the extent of photoaddition. The apparent first-order rate constants for the photodegradation of RF and for the formation of LC and CDRF at 0.25–2.00 M phosphate concentration range from 0.65 to 3.03 × 10^-2, and from 0.41 to 0.99 × 10^-2 and 0.12 to 1.63 × 10^-2 min^-1, respectively. The second-order rate constants for the phosphate catalysed photodegradation of RF and for the formation of LC and CDRF are 2.12 × 10^-4 and 0.61 × 10^-4 and 1.41 × 10^-4 M^-1 s^-1, respectively. Since the formation of CDRF by photoaddition is catalysed by HPO4^2- ions [Eur. J. Biochem. 57 (1975) 35], it is suggested that H2PO4^- ions may be involved in the formation of LC by photolysis. Thus, both H2PO4^- and HPO4^2- ions may catalyse the two major reaction pathways of riboflavin photodegradation, respectively.