Intra and intermolecular charge effects on the reaction of the superoxide radical anion with semi-oxidized tryptophan in peptides and N-acetyl tryptophan

The reaction of the superoxide radical anion (O-2(-.)), with the semi-oxidi zed tryptophan neutral radical (Trp(.)) generated from tryptophan (Trp) by pulse radiolysis has been observed in a variety of functionalized Trp deriv atives including peptides. It is found that the reaction proceeds 4-5 times faster in positively charged peptides, such as Lys-Trp-Lys, Lys-Gly-Trp-Ly s and Lys-Gly-Trp-Lys-O-tert-butyl, than in solutions of the negatively cha rged N-acetyl tryptophan (NAT). However, the reactivity of O-2(-.) with the Trp(.) radical is totally inhibited upon binding of these peptides to mice lles of negatively charged SDS and is reduced upon binding to native DNA. B y contrast, no change in reactivity is observed in a medium containing CTAB , where the peptides cannot bind to the positively charged micelles. On the other hand, the reactivity of the Trp(.) radical formed from NAT with O-2( -.) is reduced to half that of the free Trp(.) in buffer but is markedly in creased in CTAB micelles. The models studied here incorporate elements of t he complex environment in which Trp(.) and O-2(-.) may be concomitantly for med in biological system and demonstrate the magnitude of the influence suc h elements may have on the kinetics of reactions involving these two specie s.