Kinetic and mechanistic analysis of oxidation of 2-furoic hydrazide by hexachloroirradate(IV) in a wide pH range

Antioxidative Reactivity of L-Ascorbic Acid and D-Isoascorbic Acid Species towards Reduction of Hexachloroiridate (IV)

The pair [IrCl6]2–/[IrCl6]3– has been demonstrated to be a good redox probe in biological systems while L-ascorbic acid (AA) is one of the most important antioxidants. D-isoascorbic acid (IAA) is an epimer of AA and is widely used as an antioxidant in various foods, beverages, meat, and fisher products. Reductions of [IrCl6]2– by AA and IAA have been analyzed kinetically and mechanistically in this work. The reductions strictly follow overall second-order kinetics and the observed second-order rate constants were collected in the pH region of 0 ≤ pH ≤ 2.33 at 25.0°C. Spectrophotometric titration experiments revealed a well-defined 1 : 2 stoichiometry, namely Δ[AA] : Δ[Ir(IV)] or Δ[IAA] : Δ[Ir(IV)] = 1 : 2, indicating that L-dehydroascorbic acid (DHA) and D-dehydroisoascorbic acid (DHIA) were the oxidation products of AA and IAA, respectively. A reaction mechanism is suggested involving parallel reactions of [IrCl6]2– with three protolysis species of AA/IAA (fully protonated, monoanionic, and dianionic forms) as the rate-determining steps and formation of ascorbic/isoascorbic and ascorbate/isoascorbate radicals; in each of the steps, [IrCl6]2– acquires an electron via an outer-sphere electron transfer mode. Rate constants of the rate-determining steps have been derived or estimated. The fully protonated forms of AA and IAA display virtually identical reactivity whereas ascorbate and isoascorbate monoanions have a significant reactivity difference. The ascorbate and isoascorbate dianions are extremely reactive and their reactions with [IrCl6]2– proceed with the diffusion-controlled rate. The species versus pH and the species reactivity versus pH distribution diagrams were constructed endowing that the ascorbate/isoascorbate monoanionic form dominated the total reactivity at physiological pH. In addition, the value of pKa1 = 3.74 ± 0.05 for IAA at 25.0°C and 1.0 M ionic strength was determined in this work.

Kinetic Analysis of the Reduction Processes of a Cisplatin Pt(IV) Prodrug by Mesna, Thioglycolic Acid, and Thiolactic Acid

Although Mesna is an FDA-approved chemotherapeutic adjuvant and an antioxidant based largely on its antioxidative properties, kinetic and mechanistic studies of its redox reactions are limited. A kinetic analysis of the reduction processes of cis-diamminetetrachloroplatinum(IV) (cis-[Pt(NH3)2Cl4], a cisplatin Pt(IV) prodrug) by thiol-containing compounds Mesna, thioglycolic acid (TGA), and DL-thiolactic acid (TLA) was carried out in this work at 25.0°C and 1.0 M ionic strength. The reduction processes were followed under pseudo-first-order conditions and were found to strictly obey overall second-order kinetics; the observed second-order rate constant k′ versus pH profiles were established in a wide pH range. A general reaction stoichiometry of Δ[Pt(IV)] : Δ[Thiol]tot = 1 : 2 was revealed for all the thiols; the thiols were oxidized to their corresponding disulfides which were identified by mass spectrometry. Reaction mechanisms are proposed which involves all the prololytic species of the thiols attacking the Pt(IV) prodrug in parallel, designating as the rate-determining steps. Transient species chlorothiol and/or chlorothiolate are formed in these steps; for each particular thiol, these transient species can be trapped rapidly by another thiol molecule which is in excess in the reaction mixture, giving rise to a disulfide as the oxidation product. The rate constants of the rate-determining steps were elucidated, revealing reactivity enhancements of (1.4–8.9) × 105 times when the thiols become thiolates. The species versus pH and reactivity of species versus pH distribution diagrams were constructed, demonstrating that the species ‒SCH2CH2SO3‒ of Mesna largely governs the total reactivity when pH > 5; in contrast, the form of Mesna per se (mainly as HSCH2CH2SO3‒) makes a negligible contribution. In addition, a well-determined dissociation constant for the Mesna thiol group (pKa2 = 8.85 ± 0.05 at 25.0°C and μ = 1.0 M) is offered in this work, which was determined by both kinetic approach and spectrophotometic titration method.

Oxidations of Benzhydrazide and Phenylacetic Hydrazide by Hexachloroiridate(IV): Reaction Mechanism and Structure-Reactivity Relationship

Benz(o)hydrazide (BH) is the basic aryl hydrazide; aryl hydrazides have been pursued in the course of drug discovery. Oxidations of BH and phenylacetic hydrazide (PAH) by hexachloroiridate(IV) ([IrCl₆]²⁻) were investigated by use of stopped-flow spectral, rapid spectral scan, RP-HPLC and NMR spectroscopic techniques. The oxidation reactions followed well-defined second-order kinetics and the observed second-order rate constant k′ versus pH profiles were established over a wide pH range. Product analysis revealed that BH and PAH were cleanly oxidized to benzoic acid and phenylacetic acid, respectively. A reaction mechanism was proposed, resembling those suggested previously for the oxidations of isoniazid (INH) and nicotinic hydrazide (NH) by [IrCl₆]²⁻. Rate constants of the rate-determining steps were evaluated, confirming a huge reactivity span of the protolysis species observed previously. The enolate species of BH is extremely reactive towards reduction of [IrCl₆]²⁻. The determined middle-ranged negative values of activation entropies together with rapid scan spectra manifest that an outer-sphere electron transfer is probably taking place in the rate-determining steps. The reactivity of neutral species of hydrazides is clearly not correlated to the corresponding pK_a values of the hydrazides. On the other hand, a linear correlation, logk_enolate = (0.16 ± 0.07)pK_enol + (6.1 ± 0.8), is found for the aryl hydrazides studied so far. The big intercept and the small slope of this correlation may pave a way for a rational design of new antioxidants based on aryl hydrazides. The present work also provides the pK_a values for BH and PAH at 25.0 °C and 1.0 M ionic strength which were not reported before.