Three Rate Constants from a Single Kinetic Experiment: Formation, Decomposition, and Reactivity of the Chromium(VI)-Glutathione Thioester Intermediate

Article abstract of DOI:10.1021/j100099a027

The use of an exact integrated rate law hass allowed the determiantion of the three rate constants associated with the formation (k₁), decomposition (k_-1), and reactivity (k₂) of the thioester involved as a relatively-stable intermediate in the reduction of chromium(VI) by glutathione in neutral or slightly acidic aqueous solution (pH = 4.0 - 7.2).The rate constants k₁ and k₂ increase linearly as the concentration of glutathione increases, whereas k_-1 does not change.Both k₁ and k_-1 increase with rising concentration of buffer.The effect of three different buffering agents has been studied, the catalytic power of each buffer increasing in the order acetate < citrate < phosphate.At constant buffer concentration, the k₁ vs pH plots are bell-shaped, with a maximum at pH = 5-6.The three rate constants increase with rising ionic strength, and in the presence of acetate buffer, Zn(2+) exerts a catalytic effect on both k₁ and k_-1.The apparent activation energies associated with the rate constants k₁, k_-1, and k₂ are 42.3 +/- 0.8, 61.6 +/- 1.4, and 35.7 +/- 3.3 kJ mol_-1, respectively.A mechanism explaining the catalytic effects of both the buffer and Zn(2+) and involving Cr(II) as an unstable intermediate is proposed for the reaction.It is also suggested that either Cr(II) or CrO2(2+) (formed from the reaction of the latter intermediate with oxygen) might be one of the mutagenc agents implicated in the carcinogeneicity of chromium(VI) activated by glutathione.