Intramolecular Addition of the Riboflavin Side Chain.

The presence of higher (>0.2 M) concentrations of divalent anions A2- (hydrogenphosphate, sulfate) is found to accelerate as well as to change entirely the course of riboflavin photolysis: instead of 10-dealkylation to yield lumichrome, intramolecular addition of the 2'-hydroxyl group is found to occur at the peri-position C(9). The reaction is analogous to the "photohydration" of the flavin nucleus in the cationic state as described by Schöllnhammer and Hemmerich [Eur. J. Biochem. (1974) 44, 561-577]. The final product of the new addition reaction arises from autoxidation of dihydroflavin intermediate and exhibits the structure ... It is thus representative for the new class of flavins ("cyclo-dehydrofalvins"). Earlier reports on "anomalous" flavin photodegradation products absorbing around 410 nm [Holmström (1964) Ark. Kem. 22, 281; Massey and Atherton (1962) J. Biol. Chem. 237, 2965] are readily explained. The reaction is found to depend strictly on the presence of a nucleophilic function in the N(10)-side chain, e.g. N(10)-CH2-C(OH)RR' or even N(10)-(CH2)2-SO3-. Quenching experiments suggest that the new reaction occurs via the singlet state ¹Flox*, while the normal photolysis is mediated by the triplit ³ Flox*. The new photoaddition is thought to occur via a Flavin-A2- complex which creates sterically favorable conditions for C(9)/O(2'α)-interaction. [ABSTRACT FROM AUTHOR]