Determination of the Rate Constant for the Reaction of Hydroxyl and Oxide Radicals with Cysteine in Aqueous Solution

The techniques of pulse radiolysis and absorption spectroscopy has been used to determine absolute rate constants for the reaction of hydroxyl and oxide radicals with cysteine (CysS) species in aqueous solution. Using measured equilibrium constants for radical disulfide anion formation, over the pH range 6.0-13.0, observed reaction rate constants were resolved into their constituent reactions <tex-math>${}^{\bullet}{\rm OH}+{\rm CysSH}\rightarrow {\rm H}_{2}{\rm O}+{\rm CysS}^{\bullet}$</tex-math> and <tex-math>${}^{\bullet}{\rm O}{}^{-}+{\rm CysS}^{-}(+{\rm H}_{2}{\rm O})\rightarrow {\rm CysS}^{\bullet}+20{\rm H}^{-}$</tex-math> with calculated limiting rat constants of <tex-math>$(5.35\pm 0.82)\times 10^{9}$</tex-math> and <tex-math>$(3.28\pm 0.49)\times 10^{8}\ {\rm dm}^{3}\ {\rm mol}^{-1}\ {\rm s}^{-1}$</tex-math>, respectively. The rate constant for hydroxyl radical reaction with <tex-math>${\rm CysS}^{-}$</tex-math> was found to be the same as that for CysSH.