Effect of phosphate buffer on the complexation and photochemical interaction of riboflavin and caffeine in aqueous solution: A kinetic study

A study of the photodegradation of 5 × 10−5 M riboflavin (RF) in 0.2–1.0 M phosphate buffer in the presence and absence of 2.50 × 10−4 M caffeine at pH 6.0–8.0 has been carried out. RF in phosphate buffer is photodegraded simultaneously by normal photolysis (photoreduction) and photoaddition reactions giving rise to lumichrome (LC) and cyclodehydroriboflavin (CDRF) as the main final products, respectively. RF and its photoproducts, formylmethylflavin (FMF), lumiflavin (LF), LC and CDRF in degraded solution have been determined by a specific multicomponent spectrophotometric method with an accuracy of ±5%. The apparent first-order rate constants for the photodegradation of RF and for the formation of LC and CDRF are 5.47–15.05 × 10−3 min−1, 1.06–8.30 × 10−3 min−1 and 4.31–8.05 × 10−3 min−1, respectively. An increase in phosphate concentration leads to an increase in the rate of formation of CDRF and alters the photodegradation of RF in favor of the photoaddition reaction. This photoaddition reaction is further enhanced in the presence of caffeine which results in a further decrease of the fluorescence of RF in phosphate buffer. Caffeine may facilitate the photoaddition reaction by suppression of the photoreduction pathway of RF.