Activation and transfer of oxygen—IX : Nonenzymic hydroxylation of phenylalanine by model systems of dihydroalloxazine/O2 dihydroalloxazine/H2O2 and alloxazinium cation/H2O

Efficient hydroxylations were effected without addition of metal compounds. In the dihydroalloxazine system HO· radicals were the hydroxylating species according to the stoichiometry and the distribution of the hydroxyphenylalanine isomers. The OH radicals were generated in one-electron reductions of A^R-OOH or H₂O₂, in which a dihydroalloxazine or a semiquinone acted as the reducing agent. The yield of hydroxylation varied in dependence on the further oxidation of the hydroxycyclohexadienyl radicals. Quantitative disproportionation occurred in 6N H₂SO₄, while an attack by O₂, H₂O₂ or A^R-OOH predominated in the pH region 0–7. The influence of a HO- consuming aliphatic compound e.g. EDTA was studied.Hydroxylating species are also formed in the attack of an alloxazinium cation by H₂O₂, depending on the acidity of the medium.