MECHANISM OF THE OXIDATION OF DL-PENICILLAMINE AND GLUTATHIONE BY CHROMIUM(VI) IN AQUEOUS-SOLUTION

The kinetics of the oxidation of DL-penicillamine (3-sulfanyl-D-valine ) and glutathione (gamma-glutamylcysteinylglycine) by potassium chroma te has been studied at pH greater than or equal to 7 under pseudo-firs t-order conditions of an excess of thiol, at / = 0.50 mol dm(-3) (NaCl O4). The glutathione reaction is biphasic, while for penicillamine it is monophasic. The first stage of the oxidation of glutathione obeys t he simple expression (i). The second stage of the same reaction is slo wer than the first by a factor of 10(4) and (i) k(obs) = k(1)[RSH] + k (-1) obeys the rate expression (ii). The biphasic nature is consistent with the formation and decay of a (ii) Rate = (k(1)k(2)[RSH](2) + k(1 )k(3)[RSH])/(k(-1) + k(3) + k(2)[RSH]) [Cr-VI](T) chromate-glutathione adduct. Parameter k(1)/k(-1) at 25.3 degrees C has been determined as 184 +/- 4 dm(3) mol(-1). The second-order rate constant, k(2), for th e decomposition of the intermediate is (1.07 +/- 0.04) x 10(-2) dm(3) mol(-1) s(-1). The activation parameters were calculated as Delta H-do uble dagger = 90 +/- 7 kJ mol(-1) and Delta S-double dagger = 63 +/- 2 9 J K-1 mol(-1). The oxidation of DL-penicillamine shows simple first- order kinetics with respect to [DL-penicillamine], and may also be rep resented by expression (ii). The second-order rate constant, k(1), obt ained at 25.2 degrees C for the formation of a chromate-penicillamine intermediate is 0.23 +/- 0.01 dm(3) mol(-1) s(-1). The corresponding a ctivation parameters are Delta H-double dagger = 72 +/- 7 kJ mol(-1) a nd Delta S-double dagger = -18 +/- 28 J K-1 mol(-1). A common mechanis m which is compatible with the kinetics of both reactions is proposed.