Intramolecular Addition of the Riboflavin Side Chain. Anion-Catalyzed Neutral Photochemistry

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Abstract
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 a dihydroflavin intermediate and exhibits the structure It is thus representative for a new class of flavins (“cyclo-dehydroflavins”). 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-SO−3. Quenching experiments suggest that the new reaction occurs via the singlet state 1Fl*ox, while the normal photolysis is mediated by the triplet 3Fl*ox. The new photoaddition is thought to occur via a Flavin-A2- complex which creates sterically favorable conditions for C(9)/O(2′α)-interaction.

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