Divalent anion catalyzed photodegradation of riboflavin: A kinetic study.

Graphical abstract The formation of cyclodehydroriboflavin (CDRF) involves the interaction of riboflavin (RF) with divalent anions (e.g. CO 3 2−) to form a RF−CO 3 2− complex. This complex on excitation to singlet state 1[RF ox ] undergoes cyclization to give a dihydroflavin intermediate which on autoxidation yields CDRF. Highlights • Photolysis of riboflavin (RF) in the presence of divalent anions is highest at pH 7.0. • Carbonate, oxalate, phthalate from cyclodehydroriboflavin by RF photoaddition. • Rate of photoaddition depends on RF fluorescence quenching by divalent anions. • Divalent anions alter photoreduction pathway of RF to photoaddition pathway. • A multicomponent spectrometric method is used for the assay of RF and photoproducts. Abstract A study of the effect of some divalent anions (carbonate, oxalate, phthalate, phosphate) on the kinetics of photodegradation reactions of riboflavin (RF) and its pathways, product composition and fluorescence quenching in the pH range 6.0–8.0 has been conducted. The photodegradation of RF in the presence of 0.2–1.0 M divalent anions on visible irradiation leads to the formation of cyclodehydroriboflavin (CDRF) by photoaddition pathway and lumichrome (LC) by photoreduction pathway as the major end products. These products are formed by simultaneous first–order reactions. The divalent anions deviate the photoreduction pathway of RF in favor of photoaddition pathway to yield CDRF. The extent of this variation depends on the strength of RF–divalent anion complex as indicated by an increase in the loss of RF fluorescence due to complex formation. The first– and second–order rate constants for the photodegradation of RF have been determined and the effect of pH on the rates of these reactions evaluated. The rate of divalent anion catalyzed formation of CDRF is maximum at pH 7 and occurs in the order: carbonate > phosphate > oxalate > phthalate. The catalytic activity of the divalent anions increases with the strength of RF–divalent anion complex resulting in an increase in the rate of formation of CDRF and a decrease in the rate of formation of LC by different pathways. [ABSTRACT FROM AUTHOR]