Reduction potential of histidine free radicals: a pulse radiolysis study

Abstract

The technique of pulse radiolysis has been used to demonstrate that all histidine free radicals (designated His^Nsbd+) produced by oxidation of histidine by Br₂˙⁻ radical anions can oxidise the water soluble vitamin E analogue, Trolox C (k = 1.0 ± 0.2 × 10⁹ d mol⁻¹ s⁻¹ at pH 6.95). It has also been shown that His˙⁺ radicals can react with tryptophan in electron transfer equilibria involving both His˙⁺ and Trp˙⁺ species over the pH range 6.4–9.0. The ΔE values [E(Trp˙⁺/Trp) − E(His˙⁺/His)] range from −140 to −161 mV and indicate an E₇(His˙⁺/His) value of 1170 mV [based on E₇(Trp˙⁺/Trp) = 1015 mV at pH 7]. The effect of pH on E(His˙⁺/His) was accounted for by assuming that His˙⁺ can deprotonate to yield a bi-allylic free radical, designated His (−H⁺)˙. The pK_a for this dissociation was estimated to be in the range 5–7. The implications of the relatively high reduction potential for His˙⁺ in its possible participation in the mechanism of action of non-heme metalloenzymes is discussed.