Real-time monitoring of “self-oxidation” of cysteine in presence of Cu2+: novel findings in the oxidation mechanism

Reaction of N-Acetylcysteine with Cu2+: Appearance of Intermediates with High Free Radical Scavenging Activity: Implications for Anti-/Pro-Oxidant Properties of Thiols

We studied the kinetics of the reaction of N-acetyl-l-cysteine (NAC or RSH) with cupric ions at an equimolar ratio of the reactants in aqueous acid solution (pH 1.4−2) using UV/Vis absorption and circular dichroism (CD) spectroscopies. Cu2+ showed a strong catalytic effect on the 2,2′-azino-bis(3-ethylbenzothiazoline-6-sulfonate) radical (ABTSr) consumption and autoxidation of NAC. Difference spectra revealed the formation of intermediates with absorption maxima at 233 and 302 nm (ε302/Cu > 8 × 103 M−1 cm−1) and two positive Cotton effects centered at 284 and 302 nm. These intermediates accumulate during the first, O2-independent, phase of the NAC autoxidation. The autocatalytic production of another chiral intermediate, characterized by two positive Cotton effects at 280 and 333 nm and an intense negative one at 305 nm, was observed in the second reaction phase. The intermediates are rapidly oxidized by added ABTSr; otherwise, they are stable for hours in the reaction solution, undergoing a slow pH- and O2-dependent photosensitive decay. The kinetic and spectral data are consistent with proposed structures of the intermediates as disulfide-bridged dicopper(I) complexes of types cis-/trans-CuI2(RS)2(RSSR) and CuI2(RSSR)2. The electronic transitions observed in the UV/Vis and CD spectra are tentatively attributed to Cu(I) → disulfide charge transfer with an interaction of the transition dipole moments (exciton coupling). The catalytic activity of the intermediates as potential O2 activators via Cu(II) peroxo-complexes is discussed. A mechanism for autocatalytic oxidation of Cu(I)−thiolates promoted by a growing electronically coupled −[CuI2(RSSR)]n− polymer is suggested. The obtained results are in line with other reported observations regarding copper-catalyzed autoxidation of thiols and provide new insight into these complicated, not yet fully understood systems. The proposed hypotheses point to the importance of the Cu(I)−disulfide interaction, which may have a profound impact on biological systems.

Structure prediction of neutral physiological copper(II) compounds with l-cysteine and l-histidine

Bis(aminoacidato)copper(II) [Cu^II(aa)₂] coordination compounds are the physiological species of copper(II) amino acid compounds in blood plasma. Since there are no experimental data in the literature about the geometries that physiological Cu^II(aa)₂ could form with l-cysteine (Cys), that is, for bis(l-cysteinato)copper(II) [Cu(Cys)₂] and the ternary (l-histidinato)(l-cysteinato)copper(II) [Cu(His)(Cys)], this paper computationally examines the possible conformations that the two compounds could form with the Cys ligand having a protonated sulfur, as in the conventional zwitterion, which was determined to be prevailing in aqueous solution. These two amino acids can bind metals in a tridentate fashion and thus form many possible coordination patterns. Density functional calculations were performed for the conformational analyses in the gas phase and in implicitly modeled aqueous solution using a polarizable continuum model. Additionally, we examine which coordination mode, with thiol or thiolate group, is more stable. The Cys coordination via the amino N and carboxylato O atoms (a glycinato mode) is obtained as the most stable one in aqueous Cu(Cys)₂, and also in Cu(His)(Cys) when the His glycinato or histaminato mode combines with the intact thiol group. Whereas the conformers with N and thiol S as the copper(II) donor atoms are predicted to be the least stable, those with the Cu-N and Cu-S(thiolate) bonding (and protonated carboxylato group) are the most stable. The differences are explained by different covalent and ionic contributions of Cu-S(thiol) vs. Cu-S(thiolate). The study can contribute to the insight into formation and reactivity of the copper(II) cysteinato complexes in solution.

The formation mechanism and chirality evolution of chiral carbon dots prepared via radical assisted synthesis at room temperature

We report on a Cu(ii) catalyzed process for the production of cysteine based chiral carbon dots; the process does not require any thermal treatment and the carbon dot formation is driven by the production of reactive radical species that are generated in the reaction media by the catalytic role played by the multivalent transition metal. The nanomaterial presents a well-defined chirality and the enantioselectivity of the synthesis is proved by the isolation of both the carbon dot enantiomers. We focused our attention on the processes that take place during the carbon dot formation and the relationship with the structure of the organic starting material. Thanks to the comparison of reactions conducted with different organic substrates whose thiyl radical chemistry is known, we recognized a non-trivial role of the radical hydrogen abstraction reactions in the carbon dot formation process. The reported process allows access to a large variety of analyses to monitor the reaction mixtures during the reaction course. Finally, we report a detailed analysis on the evolution of optical chirality during the synthesis and related this feature with the formation mechanism of the nanomaterial revealing significant evidence on the chirality origin and structure of chiral carbon dots.

Theoretical modeling of pKa's of thiol compounds in aqueous solution

The pK_a's of different kinds of thiols (R-SH) were investigated by using the M06-2X method with a SMD_sSAS model.

Electroanalytical Determination of Cysteine Using the Electrodes Based on Ternary Silver-Copper Sulfides

The amperometric determination of cysteine, using an electrode based on ternary silver-copper sulfide, is presented. Electrochemical characterizations (using cyclic voltammetry) of three electrode materials revealed that the electrode based on the material that consists of jalpaite (Ag₃CuS₂), doped with a small amount of metallic silver, has the best electrocatalytical performance for cysteine oxidation. For the amperometric determination, 0.142 V at pH 5 and 0.04 V at pH 7 vs. Ag/AgCl, related to the electrocatalytical oxidation of thiol group, were chosen. Electrochemical impedance spectroscopy together with Fourier transform infrared spectroscopy (FTIR) revealed that oxidation takes place on the electrode surface with fouling effect, which does not affect a wide linear working range between 1 μM and 100 μM. Sensitivities, at pH 5 and pH 7, are calculated to be 0.11 μA μM-1 and 0.10 μA μM-1, respectively. The detection limits were determined to be 0.036 μM and 0.024 μM for pH 5 and pH 7, respectively. In the presence of uric acid, folic acid, ascorbic acid, and glucose no interference was noticed. This electrode showed remarkable stability and excellent reproducibility. The electrode was exploited for the determination of cysteine in a dietary supplement with the excellent recoveries.

Applying Ion Mobility-Mass Spectrometry Techniques for Explicitly Identifying the Products of Cu(II) Reactions of 2His-2Cys Motif Peptides

The Cu(II) and pH titrations of four structurally similar 2His-2Cys motif peptides were investigated by electrospray ionization-ion mobility-mass spectrometry. The results provided insight into the pH dependent redox processes that took place in solution and identified the number of inter- or intramolecular disulfide bridges, the number of Cu(I) or Cu(II) ions, the deprotonation sites, and likely Cu(I/II) coordination of the various products. Competitive Cu(II) titrations of binary peptide mixtures at pH 5 indicated which species would preferably bind Cu(I) ions over forming the intramolecular disulfide bridge. Moreover, these reactions were pH dependent and included the formation of various multimers and multiple Cu(I/II) binding. For example, for the mildly acidic solution (pH ∼ 3-6) each monomer (whether it was free or in a multimer) primarily bound up to 3 Cu(I) ions, whereas at pH ∼ 8-11 the fully oxidized monomer or multimer (where all Cys formed a disulfide bond) primarily bound up to 2 Cu(II) ions. This behavior was indicative of linear bridging of Cu(I) by Cys thiolate and His imidazole groups, whereas the coordination of Cu(II) involved His and the nitrogens of deprotonated backbone amide groups, resulting in either distorted T-shaped or square planar geometries.