Ranendu Sekhar Das

Kinetics and mechanism of oxidation of thiourea by a bridging superoxide in the presence of Ellman’s reagent

Abstract In aqueous acetate buffer, reaction between the superoxo complex [(dien)(en)CoIII(O2)CoIII(en)(dien)]5+ (1) and thiourea (Tu, SC(NH2)2) proceeds through a complex kinetic pathway and the change of the absorbance of 1 with time follows a sigmoidal trace. Only in the presence of Ellman’s reagent (5,5′-dithio-bis-(2-nitrobenzoic acid), DTNB), the reaction becomes quantitative and Tu reduces the bound superoxo group (––) of 1 to the corresponding peroxo group (––) in [(dien)(en)CoIII(O2)CoIII(en)(dien)]4+ (2). In the presence of DTNB, with large excess of Tu over [1], the reaction follows first-order kinetics and the observed rate constant, ko, increases linearly with the increase in [1], [TTu] ([TTu] is the analytical concentration of Tu), [H+], and the media ionic strength (I). The presence of an intercept in the plot of ko versus [H+] suggests that both tautomeric forms of thiourea, i.e. TuZ and its protonated form TuZH+, are kinetically reactive reductants. The calculated rate constant values for k1 (~102) and k2 (~104) indicate that TuZH+ plays the leading role in the redox reaction. The present work also reveals the prospective involvement of Ellman’s reagent as a radical scavenger.

Penicillamine and captopril: mechanistic exploration of defensive actions of thiol drugs against a metal bound-superoxo complex

Abstract In acid-media ([H+] = 0.01–0.06 M), each of the thiol compounds, D-penicillamine (PEN, LPH2) and captopril (CAP, LCH2) exist in several proton-dependent forms which can reduce the superoxo complex [(en)(dien)CoIII(O2)CoIII(en)(dien)]5+ (1) to the corresponding peroxo [(en)(dien)CoIII(O2)CoIII(en)(dien)]4+ (2) or the hydroperoxo complex [(en)(dien)CoIII(OOH)CoIII(en)(dien)]5+ (3). The observed first-order rate constants, ko,P and ko,C for PEN and CAP increase with the increase in [TPEN] and [TCAP] (which are the analytical concentrations of the respective thiols) but decrease with the increase in the media-acidity ([H+]) and the media ionic strength (I). The protolytic equilibria in aqueous solution allow several potentially reducing forms to coexist for both PEN (LPH3+, LPH2, LPH−, and LP2−) and CAP (LCH2, LCH−, LC2−) but the kinetic analyses reveal that the order of reactivity for the species are LPH3+ ~ LPH2 <<< LPH− and LCH2 < LCH− <<< LC2−, respectively. The predominance and higher reactivities of the anionic species, LPH− and LC2− are supported by the negative slopes of the plots of ko,P or ko,C versus I. Moreover, a large value of kH/kD for PEN suggests an inner-sphere electroprotic reaction pathway while the absence of such effect for CAP strongly supports an outer-sphere electron transfer reaction. These propositions are supported by the structural features of LPH− and LC2−.