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 HisNsbd+) 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 pKa 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.