Aromatic hydroxylations by flavins: Evidence on direct attack of phenylalamine by flavin radical species

Article abstract of DOI:10.1016/S0040-4020(01)80687-6

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.