Absorption spectra of radicals of substrates for p-hydroxybenzoate hydroxylase following electrophilic attack of the .OH radical in the 3 position

The spectra of radicals formed upon the addition of .OH radicals to the 3 position of substrates for p-hydroxybenzoate hydroxylase (4-hydroxybenzoic acid, 2,4-dihydroxybenzoic acid, and 4-aminobenzoic acid) have been determined using pulse radiolysis combined with the results from high performance liquid chromatography measurements. The 3-hydroxy radical forms of the substrates absorb maximally in the 365-410 nm region with extinction coefficients in the range 3700-5250 M-1 cm-1. Upon combining these radical spectra with the known spectrum of enzyme-bound reduced flavin that is substituted in the C(4a) position of the isoalloxazine ring, spectra are found which closely resemble the species long thought to be formed concomitantly with the introduction of an oxygen atom into substrates. On the basis of these spectral results a new radical mechanism is proposed for the functioning of this class of flavoprotein hydroxylases.