Aromatic hydroxylations by flavins: evidence on direct attack of phenylalanine by flavin radical species.

Abstract

In 0.05–12.0 N acidic solutions, the 5-ethyl-3-methyllumiflavosemiquinone 5 (and/or 5H +) spontaneously arose from the corresponding flavinium cation 4. Raising the temperature from 20 to 50°C, considerably increased the reaction rates with no significant changes in the yields of 5 ( 5H +). The spontaneous one-electron reduction of 4 requires a coupling with a one-electron oxidation of another flavin such as 5-ethyl-4 a-hydroxy-3-methyl-4 a,5-dihydroflavin pseudobase 1. The latter, being in equilibrium with 4, can be oxidized to give the transient 5-ethyl-4 a-hydroxy-3-methyllumiflavin radical 2. This is the protonated form of a flavinoxy radical 3 a,b , a product of a homolysis of the OO bond in a dihydroflavin hydroperoxide. As an alternative to the homolysis mentioned, the one-electron oxidation of 1 provides the principle to develop a new hydroxylating model system that does not require a dihydroflavin hydroperoxide as a starting compound. Using phenylalanine as a test substrate, the anaerobic formation of tyrosine and its o- and m-hydroxyphenylalanine isomers was established. This achievement is a strong experimental support for the hypothesis that flavin radical species like 2 may directly attack an aromatic. Evidence was obtained on some accumulation of an intermediate that is not a hydroxycyclohexadienyl radical. It was shown to react in a secondary, oxidative chain reaction, remarkably increasing the yields of aromatic hydroxylation without any further supply of flavin.