Aqueous radiation chemistry of cysteine. I. Deaerated acidic solutions

Abstract

Results of studies on the cobalt-60 y radiolysis of acidic cysteine (RSH) solutions, pH 1.0 and 0.5, are reported. Irradiation products which have been identified are H2, H2S, RSH, and H202 with radiation yields at 10 ~3 M cysteine of 3.2, 0.95, 3.0, and 0.68 molecules/100 eV absorbed. Alanine was also observed qualitatively as a radiation product. Yields for the disappearance of the thiol group were measured polarographically during the irradiation and (?(-RSH) was found to be 7.0 at 10-3 M cysteine. A mechanism is postulated for strongly acid cysteine solutions on the basis of competition kinetics, material balances, and comparison with theoretical yields. Analysis of results of competition studies provides evidence for a rate constant of approximately 3 X 10" 1. mol""1 sec-1 for the reaction of the hydrated electron with the protonated-amine form of cysteine. Results of scavenger studies at concentrations higher than 10~2 M cysteine require that additional processes be postulated in order to account for high H2S yields.

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

“…Re/~(iir 01' P(i/~(iitz f i~( i c t i~~~i / c~/ by Hj~drogen Perax-ick~ (i) Reactions 4 and 5 ~iormally proceed to complctioli at 3 0°C in thc post-inoctivatioli repair assay due to the 30 min treatment with 21 1000-fold excess of cysteine (8). This repair 111ay 21lso occur to sonie cxtcnt at 25 "C with lower cystcine concciitrations.…”

“…values. The curves were calculated from then1 using [21] and show good fits to the experiniental points for both solutions. The same procedure was applied to the percentage repairable activity losses obtained from the of this product the above integrated second order rate law niay be rearranged to the form a = ?…”

“…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.…”

“…At p H = 6 the main reactions are [8] and [9]: [8 I .OH + CysSH 4 Hz0 + CysS. 0. measured 30 min after irradiation, Glor; 0. calculated from the percentage repairable activity loss extrapolated to the midpoint of the irradiation period using [21]. The arrows at [CysSH]/[papain] = 2 and 5 are the difference between the calculated and experimental values of Gnr in Fig.…”

The radiation chemistry of 2-mercaptoethanol

“…Their reactions with the free radicals formed in irradiated aqueous solutions are therefore of considerable interest. The following reaction mechanism, with RSH as the sulphydryl molecule, has been shown to adequately describe the products observed for cysteine (3,4), penicillamine (S), and glutathione (6) in I to 10 mM aqueous solutions subjected to 60Co or fast electron radiolysis. These are also the main primary reactions in dithiothreitol, but in that case there are some additional secondary reactions (7,8).…”