Coenzyme models. Part 24. Micellar catalysis of flavin-mediated reactions. Influence of the flavin structure on the reactivity

The catalytic effect of a cationic (CTAB) micelle on the flavin-mediated oxidation of 1-benzyl-1,4-dihydronicotinamide (6), nitroethane carbanion (7), and thiophenol (8) is reported. The oxidation of (6) was subject to a small extent to micellar catalysis, whereas the oxidation of (7) and (8) which does not proceed in a simple aqueous solution was efficiently catalysed by the CTAB micelle. The rate of oxidation of (7) was profoundly dependent upon the structure of the flavin: flavins which have either a long alkyl group or a carboxy-group gave rate constants greater by 10³-10⁴ fold than unmodified flavin, and the rate constant for flavin (5) which has both groups was further enhanced (>10⁶ fold). On the other hand, the oxidation of (8) was less affected by the change in the flavin structure. The reactivity order for the oxidation of (7) was (1) (unmodified neutral flavin) ≪ (2) (neutral flavin with a hexadecyl group) < (4) (anionic flavin with a carboxy-group) < (5) (anionic flavin with carboxy-and tetradecanoyl groups), whereas that for the oxidation of (8) was (1) < (4) < (5) < (2). The results indicate that the reactivity of flavins is variable, depending not only on the type of reaction but also on the environment. The results provide useful information on the versatile reactivity of flavin coenzymes bound to apoenzymes.