Photodegradation of formylmethylflavin by side–chain and isoalloxazine ring cleavage in alkaline solution: A kinetic study

Abstract

The photodegradation of fromylmethylflavin (FMF), a major intermediate product in the photolysis of riboflavin, in the pH range 8.0–12.0 has been studied. FMF undergoes photodegradation by side–chain cleavage to give lumichrome (LC) and lumiflavin (LF) in alkaline solution. It is also degraded by the hydrolysis of isoalloxazine ring to form 1,2–dihydro–1–methyl–2–keto–3–quinoxaline carboxylic acid (KA) and 1,2,3,4–tetrahydro–1–methyl–2,3–dioxoquinoxaline (DQ) and is oxidized to carboxymethylflavin (CMF) in alkaline solution. The photodegradation of FMF follows simultaneous first–order kinetics to form the side–chain, ring cleavage and oxidation products. The apparent first–order rate constants (kobs) for these reactions have been determined and are in the range of 0.15 (pH 8.0) to 4.98 × 102 min–1 (pH 12.0). The rate constants for the formation of the photoproducts (LC, LF, CMF, KA, DQ) have also been determined. The formation of the photoproducts is enhanced with pH due to the sensitivity of flavin triplet to alkaline hydrolysis. FMF and photodegradation products have been determined by a multicomponent spectrofluorimetric method in degraded solutions. It involves the separation of LC and LF by chloroform extraction at pH 2 followed by the choloroform extraction of undegraded FMF at pH 6.5 and their assay at 478 (LC) and 530 nm (LF and FMF), respectively. The aqueous phase is used for the assay of CMF, KA and DQ at 530, 443 and 420 nm, respectively. The mode of photodegradation of FMF has been discussed.