Stephen P. Mezyk

Disulfide anion radical equilibria: effects of -NH3+, -CO2–, -NHC(O)- and -CH3 groups

Equilibria between disulfide anion radicals (RS⇌SR) and thiolate anions and thiyl radicals, namely:have been studied as a function of pH with alkyl R substituents of different structure and net charge, for the purpose of obtaining thermochemical data. The thermodynamic stability of the RS⇌SR species was examined in terms of: (a) the reduction potential for its formation from RSSR, and: (b) the magnitude of the equilibrium constant for its formation from RS˙ and RS–. It was found that the RS⇌SR stability increased when protonated amino groups were present and rose with their proximity to the S⇌S group. For each protonated amino group beta to the S atoms E n o(RSSR/RS⇌SR) typically rose by about 0.1 V. In parallel with this the equilibrium constant for formation of RS⇌SR from RS˙ and RS– increased in the doubly protonated systems by a typical factor of ten. This equilibrium constant is strongly depressed by methyl groups on the C atoms adjacent to the sulfur, and is reduced in structures with ionised carboxylate groups beta to the S atom. The magnitude of the latter effect is diminished when the distance between the sulfur centers and the carboxylate groups increased. The changes in RS⇌SR stability can be understood in terms of inductive effects and Coulombic interactions. The value of E n o(RSSR/RS⇌SR)xa0=xa0–1.41 V found for glutathione disulfide is an indication of the reduction potential for the cystine residue in proteins. Estimates of the effects of nearby -NH3+ groups on the stability of disulfide anions have been made.

Ion recombination processes in the formation of krypton fluoride

The formtion of KrF∗ and XeF∗ has been studied in electron-beam irradiated Kr/Xe/Sf6 gas mixtures. An electron-beam-energy dependent process has been observed, and confirmed to be due to ion recombination processes between rare gas cations and halide anions. The anions are formed by thermal electron capture by SF6 and the cations either being directly formed in the electron pulse (Kr+), or by subsequent charge transfer (Xe+).