Reactions of OH, CH₂OH, Br₂⁻, and (CNS)₂⁻ Free Radicals with Cysteine at pH 5.3

Abstract

GREESH CHAND GOYAL and DAVID ANTHONY AKMSTRONG. Can. J. Chem. 53,1475Chem. 53, (1975. The products of radiolysis of cysteine in nitrous oxide saturated solutions at p H 5.3 have been investigated both in the absence of other solutes and in the presence of 1 M methanol, 0.5 M KBr, and 0.2 MNaCNS. The results can be explained by a mechanism in which the free radicals *OH, .CHZOH, .BrZ-, and .(CNS)2-all react with the cysteine (CysSH) sulfhydryl group to form CysS. as a major intermediate product. Minor yields (0.2 molecules per 100 eV) of ammonia observed in the case of attack by .Brz-may result from attack on the NH3+ group or a-hydrogen by this species. Cysteine trisulfide was observed in all systems with a yield 0.12 f 0.03 molecules per 100 eV and is probably formed from minor secondary reactions of CysS* radicals. In agreement with previous conclusions based on the analysis of products from deaerated solutions the majority of CysS* radicals form CysSSCys.GREESH CHAND GOYAL et DAVID ANTHONY ARMSTRONG. Can. J. Chem. 53,1475Chem. 53, (1975. On a examine les produits de radiolyse de la cysteine dans des soluti~ns saturees en oxyde nitreux a un p H de 5.3 en I'absence d'autres solutes de m&me qu'en presence de methanol 1 M, de KBr 0.5 M e t de NaCNS 0.2 M. Les resultats peuvent &re expliquts par un mecanisme dans lequel les radicaux libres .OH, .CH20H, .Br2-et .(CNS)2-rtagissent tous avec le groupe sulfhydryle de la cysteine pour former le radical CysS* comme un produit intermtdiaire majeur. Les rendements mineurs (0.2 molecules par 100 eV) d'ammoniac observes dans le cas de I'attaque par -Brz-peuvent provenir d'une attaque, par cette espece, soit sur le groupe NH3+ soit sur I'hydrogene en a. Dans tous les cas, on a observe la formation de trisulfure de cysteine et le rendement est de 0.12 f 0.03 molecules par 100 eV; ce compose se forme probablement a partir de reactions secondaires mineures des radicaux CysS. La majorite des radicaux CysS* forment CysSSCys; cette conclusion est en accord avec d'autres Bnonces preckdemment et qui ttaient bases sur I'analyse des produits provenant de solutions desquelles I'air avait kt6 enlevee.[Traduit par le journal] Introduction Previous studies of the y (1, 2) and pulse radiolysis (3,4) of aqueous oxygen free solutions of cysteine (HSCH,CH(NH,+)CO,-) and the related molecule cysteamine (HSCH,CH,NH,+) (5) have shown that the major products can be explained by the following mechanism, in which the abbreviation CysSH is used for the cysteine structure :

Cysteine-sensitised formation and repair of mixed disulfide in the oxidation of papain by H2O2 and OH radicals

“…20 also shows that, with increasing [CysSH]/[papain], G(-SH) rises well above 0.5 and then tends to level off. To explain this one must first corisider the proccsscs which occur in nitrous oxide-saturated cysteine solutions without papain (3,(21)(22)(23)(24)(25). At p H = 6 the main reactions are [8] and [9]: [8 I .OH + CysSH 4 Hz0 + CysS.…”

“…Taking Go11 = 5.6 (16) and G I~ = 0.5 the total radical yield is 6.1 per lOOeV and when all radicals are scavenged there should be an equivalent net loss in cysteine SH (25) [papaiti] = 10 are already within 12y0 of the total radical yield of 6.1 per 100 eV. The differences are much greater than the small SH loss ( 5 0 .…”