REDUCTION POTENTIAL OF HISTIDINE FREE-RADICALS - A PULSE-RADIOLYSIS STUDY

The technique of pulse radiolysis has been used to demonstrate that al l histidine free radicals (designated His(.+)) produced by oxidation o f histidine by Br-2(.-) radical anions can oxidise the water soluble v itamin E analogue, Trolox C (k = 1.0 +/- 0.2 x 10(9) dm(3) 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 Delta E values [E (Trp(.+)/Trp) - E(His(.+)/His)] range from - 140 to - 161 mV and indic ate an E-7(His(.+)/His) value of 1170 mV [based on E-7(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 r adical, designated His (-H+)(.). The pK(a) for this dissociation was e stimated to be in the range 5-7. The implications of the relatively hi gh reduction potential for His(.+) in its possible participation in th e mechanism of action of non-heme metalloenzymes is discussed.