Oxidation of DL-penicillamine by chromium(VI). Kinetics of formation of the thioester intermediate

Abstract

, 1637 (1994). The kinetics of formation of the thioester involved as an intermediate in the reaction between chromium(V1) and or.-penicillamine in aqueous media (pH = 1-8) containing different buffers (acetate, citrate, and phosphate) has been studied by monitoring the disappearance of chromium(V1) at 370 nm and application of the initial-rates method. The initial rate is directly proportional to the initial concentrations of both oxidant and reductant, and the rate vs. pH plots show bell-shaped profiles. The reaction is catalyzed by the buffer present in the medium, the catalytic power of each buffer increasing in the order acetate < citrate < phosphate. This is explained in terms of a mechanism involving the formation of a complex between the acidic form of the buffer and HCr04-previous to the formation of the thioester. Potassium chloride and sodium sulfate do not seem to have important specific effects on the reaction rate, their effect being that of an acceleration of the reaction as the ionic strength increases. On the contrary, the sulfates of magnesium, manganese(II), and zinc (the latter only in the presence of acetate buffer) have specific effects, indicating the probable formation of several complexes. The spectrophotometric detection of the thioester at 430 nm has allowed to confirm some of the conclusions extracted from the measurement of initial rates, and suggests that this intermediate might approach a steady-state behavior in the three buffers at pH > 6.25, and also that a bimolecular reaction with DL-penicillamine might be involved in its destruction. JOAQUIN F. PEREZ-BENITO, DRISS LAMRHARI et CONCHITA ARIAS. Can. J. Chem. 72, 1637 (1994).En examinant la vitesse de disparition du chrome(V1) B 370 nm et en appliquant la mCthode des vitesses initiales, on a CtudiC la cinitique de formation du thioester impliquC comme intermkdiaire dans la rCaction du chrome(V1) et la DLpinicillamine, en milieu aqueux (pH = 1 B 8) contenant divers tampons (acCtate, citrate et phosphate). La vitesse initiale est directement proportionnelle aux concentrations B la fois de I'oxydant et du rCducteur et les courbes des vitesses en fonction du pH prksentent des profils en forme de cloche. La rCaction est catalysCe par le tampon prCsent dans le milieu, le pouvoir catalytique de chacun des tampons augmentant dans l'ordre acCtate < citrate < phosphate. On peut expliquer ces rCsultats en fonction d'un mCcanisme impliquant la formation, avant la formation du thioester, d'un complexe entre la forme acide du tampon et le HCr04-. Le chlorure de potassium et le sulfate de sodium ne semblent pas avoir d'effets spCcifiques importants sur la vitesse de la reaction; leur effet se limite B une accClCration de la rCaction lorsque la force ionique augmente. Au contraire, les sulfates de magnCsium, de manganbse(I1) et de zinc (dans ce dernier cas, uniquement en prCsence du tampon acCtate) posskdent des effets spCcifiques, suggCrant la formation probable de plusieurs complexes. La dCtection spectrophotomCtrique du thioester 2 430 nm a p...

Formation of Superoxochromium(III) by a Novel Mechanism

“…The mechanism proposed for the formation of CrOO 2+ , Equations (6)-(9), can explain the finding that the yield of the intermediate CrOO 2+ in the reactions of chromate with three different thiols follows the sequence: DL-penicillamine > L-cysteine > glutathione [8]. This sequence is the opposite of that corresponding to the yield of the intermediates HRSCrO À 3 in the same reactions, and it is coincident with the sequence DL-penicillamine (k 2 ¼ 9:62Â 10 À3 s À1 Þ > L-cysteine ðk 2 ¼ 1:66 Â 10 À3 s À1 Þ > glutathione ðk 2 ¼ 0:80 Â 10 À3 s À1 Þ, corresponding to the rate constants for the redox decay of those intermediates (k 2 in Scheme 1) measured under the same experimental conditions (acetate buffer, ionic strength 0.20 M, pH 5.60, 25.0°C) [22].…”

“…spectra obtained by difference spectroscopy for the intermediate CrOO 2+ in the reactions of chromate ion with two sulfhydryl-containing amino acids, L-cysteine (present work) and DL-penicillamine [8], are compared with those corresponding to the u.v. spectrum of authenticated CrOO 2+ obtained by ordinary spectroscopy in Table 2.…”

“…Although it is known to be formed as a long-lived intermediate in the reactions of chromium(VI) and chromium(IV) with many organic reductants in aerated acid solutions [6,7], its formation in neutral solutions has rarely been observed. The only report on this topic is my previous communication on the detection of CrOO 2+ as a longlived intermediate in the reactions of chromate ion with biological thiols in near-neutral solutions [8], but the mechanism of its formation is still largely in the dark. Particularly interesting is the observation that this intermediate can be observed in those reactions only when the concentration of dissolved O 2 is extremely low.…”

Combined Kinetic Response of Two Substrates with Similar Reactive Patterns

“…Much of our effort was directed to the oxidations by Cr(VI) of thiolic type substrates such as thiols, thioacids, thioethers [5][6][7][8][9]. Same area studies by other groups are also found in the literature [5,[10][11][12] and some general patterns are now well established. Among them there is a first and often fast step in which one Cr(VI) and one or more thio compound molecules form a condensation intermediate (adduct).…”

The interaction of cysteine with chromium(VI) ions under UV irradiation

“…Although investigations on the interactions of Cr(VI) with thiol compounds have been extensively reported [9,10,[18][19][20][21], no electrochemical and spectroscopic studies dealing with the fotocatalytic effect of UV light on the interaction of Cr(VI) with cysteine have appeared in the literature. Therefore, in the present work, the role of UV irradiation in the reduction of Cr(VI) with cysteine has been investigated with cyclic voltammetry and electronic spectroscopy techniques.…”