Dixon, Donovan A.; Dasgupta, Tara P.; Sadler-McKnight N.
Author Affiliation, Ana.
Department of Chemistry
Oxidation of thiol-containing substrates by the pentaamminechromatocobalt(III) complex ion
Inorganic Reaction Mechanisms
Date of Publication
The reactions of the pentaamminechromatocobalt(III) complex ion with a series of thiols, RSH (where RSH = glutathione, L-cysteine, and DL-penicillamine) have been studied by spectrophotometry at pH > 7, over the range 20°C £ T £ 34.4°C, with the thiols in excess at ionic strength 0.50 M (NaC1O4). The reactions occur in two stages; the first is a rapid reaction between reactants leading to formation of outer-sphere adducts, which are facilitated by electrostatic interaction between the chromato complex and each thiol. For L-cysteine, the equation representing the first stage is rate = k2K1[complex][RSH]T / (1 + K1[RSH]T). (i) At 25°C, the formation of constant for the adduct is K1 = (800 ± 110) M-1 and the rate constant for its decomposition to pentaamminehydroxocobalt(III) and chromate is k2 = (4.1 ± 0.2) x 10-3 s-1 (pH = 7.40). Rate parameters for the reactions of DL-penicillamine and glutathione with the complex are presented. The second lower stage of all three reactions is postulated to be oxidation of the thiols by the free chromium(VI) ion formed in the first stage. From schemes derived for the oxidation of the thiols, a general equation obtained may be expressed as follows: kobs = (k5k6[RSH]2 + k5k7[RSH]) / (k-5 + k7 + k6[RSH]), (ii) where k5 represents the formation of a chromium(VI) ester intermediate, k-5 its reversible decomposition, k6 is the reduction to chromium(IV) by excess of the reductant and finally k7 represents intramolecular transfer of an electron resultung in the reduction of the intermediate to chromium(V). For L-cysteine and DL-penicillamine, k5 = (0.47 ± 0.01) M-1 s-1 and (2.47 ± 0.01) x 10-1 M-1 s-1, respectively. The pentaamminecobalt(III) moiety is not reduced, but reacts slowly with the thiols to form complexes. These latter complexation reactions are significantly slower than the electron transfer reactions.....