Effect of phosphate buffer on photodegradation reactions of riboflavin in aqueous solution

The effect of phosphate buffer on aerobic photodegradation reactions of riboflavin (RF) at pH 7.0 has been studied. The photoproducts of the two major reactions, viz., intramolecular photoreduction and intramolecular photoaddition, have been determined by a specific multicomponent spectrophotometric method. The overall photodegradation of riboflavin in the presence of phosphate buffer involves the participation of both H 2 PO 4 - and HPO 4 2 - species. The second-order rate constants for the H 2 PO 4 - -catalysed photodegradation of riboflavin (normal photolysis) to lumichrome (LC) and HPO 4 2 - -catalysed photodegradation of riboflavin (photoaddition) to cyclodehydroriboflavin (CDRF) are 0.93 × 10−4 and 4.0 × 10−4 M−1 s−1, respectively. The addition of 0.25–2.00 M phosphate to RF solutions at pH 7.0 gives rise to RF – HPO 4 2 - complex and hence the quenching of 4–36% fluorescence, respectively. This results in the suppression of normal photolysis leading to the formation of LC in favour of photoaddition to yield CDRF. The present study shows the involvement of H 2 PO 4 - anions in the base-catalysed degradation of riboflavin by normal photolysis vis-a-vis the involvement of HPO 4 2 - anions in photoaddition reactions of riboflavin suggested earlier [M. Schuman Jorns, G. Schollnhammer, P. Hemmerich, Intramolecular addition of the riboflavin side chain. Anion-catalysed neutral photochemistry, Eur. J. Biochem. 57 (1975) 35–48].