Quantification of Thiols and Disulfides

The native chemical ligation reaction (NCL) involves reacting a C-terminal peptide thioester with an N-terminal cysteinyl peptide to produce a native peptide bond between the two fragments. This reaction has considerably extended the size of polypeptides and proteins that can be produced by total synthesis and has also numerous applications in bioconjugation, polymer synthesis, material science, and micro- and nanotechnology research. The aim of the present review is to provide a thorough mechanistic overview of NCL and extended methods. The most relevant properties of peptide thioesters, Cys peptides, and common solvents, reagents, additives, and catalysts used for these ligations are presented. Mechanisms, selectivity and reactivity are, whenever possible, discussed through the insights of computational and physical chemistry studies. The inherent limitations of NCL are discussed with insights from the mechanistic standpoint. This review also presents a palette of O, S-, N, S-, or N, Se-acyl shift systems as thioester or selenoester surrogates and discusses the special molecular features that govern reactivity in each case. Finally, the various thiol-based auxiliaries and thiol or selenol amino acid surrogates that have been developed so far are discussed with a special focus on the mechanism of long-range N, S-acyl migrations and selective dechalcogenation reactions.

Native Chemical Ligation and Extended Methods: Mechanisms, Catalysis, Scope, and Limitations

An introduction to methods for analyzing thiols and disulfides: Reactions, reagents, and practical considerations

Particulate matter (PM)-mediated reactive oxygen species (ROS) generation has been implicated in health effects posed by PM. Humic-like substances (HULIS) are an unresolved mixture of water-extracted organic compounds from atmospheric aerosol particles or isolated from fog/cloudwater samples. In this study, we use a cell-free dithiothreitol (DTT) assay to measure ROS production mediated by HULIS. The HULIS samples are isolated from aerosols collected at a rural location and a suburban location in the Pearl River Delta, China. In our experiments, ROS activities by residue metal ions in the HULIS fraction are suppressed by including a strong chelating agent in the DTT assay. Under conditions of DTT consumption not exceeding 90%, the HULIS-catalyzed oxidation of DTT follows the zero-order kinetics with respect to DTT concentration, and the rate of DTT oxidation is proportional to the dose of HULIS. The ROS activity of the aerosol HULIS, on a per unit mass basis is 2% of the ROS activity by a reference quinon...

Generation of reactive oxygen species mediated by humic-like substances in atmospheric aerosols.

The effects of wine composition and postbottling oxygen exposure on 3-mercaptohexanol (3-MH), hydrogen sulfide (H2S), and methyl mercaptan (MeSH) were investigated. A Sauvignon blanc wine with initial copper concentration of 0.1 mg/L was treated with copper sulfate and/or glutathione (GSH) prior to bottling to give final concentrations of 0.3 and 20 mg/L, respectively. The wines were bottled with a synthetic closure previously stored in either ambient air or nitrogen to study the effect of the oxygen normally present in the closure. Bottled wines were stored for 6 months in either air or nitrogen to study the effect of oxygen ingress through the closure. Copper addition resulted in a rapid initial decrease in 3-MH. During storage, a further decrease of 3-MH was observed, which was lower with GSH addition and lowered oxygen exposure. H2S accumulated largely during the second 3 months of bottle storage, with the highest concentrations attained in the wines treated with GSH and copper. Lower oxygen from and ...

Evolution of 3-mercaptohexanol, hydrogen sulfide, and methyl mercaptan during bottle storage of Sauvignon blanc wines. Effect of glutathione, copper, oxygen exposure, and closure-derived oxygen.

Side self-oxidation of thiols was studied. It was found that these reactions in neutral and alkaline solutions are induced by impurities of variable-valence metals. The ability of transition metals to catalyze oxidation of thiols changes in the order Cu > Mn > Fe > Ni ≫ Co. The plot of the self-oxidation rate vs. pH passes through a maximum whose position on the pH scale depends on both the nature of metal and the structure of the thiol oxidized. For thiols having different structures, the kinetic orders in reactions catalyzed by copper ions differently vary with pH, which is apparently associated with the formation of complexes possessing different catalytic activity.

Oxidation of thiol compounds by molecular oxygen in aqueous solutions

The kinetics of the catalytic oxidation reactions of thiol compounds with molecular oxygen in aqueous solutions in the presence of copper ions was studied in relation to the structures of oxidized thiols and the pH of the solution. A modified procedure used for the determination of [O2] allowed us to obtain the kinetic characteristics of more than 30 thiols over a wide pH range. We found that weakly chelating thiols exhibited a first order of reaction with respect to [Cu+] and [O2] under conditions when the [(Cu+)(RS–)2] complex occurred. In the oxidation of strongly chelating thiols in an alkaline medium, the order of reaction with respect to [Cu+] was equal to 2, and the rate of reaction was independent of [O2]. We found that the introduction of small amounts of strongly chelating thiols into Cu+ solutions containing difficult-to-oxidize mercaptans resulted in a dramatic acceleration of mercaptan oxidation. We hypothesized that O2 was effectively bound to the [(Cu+)(RS–)2] complexes in an alkaline medium in the case of strongly chelating thiols, and this was not the case with the complexes of weakly chelating thiols.

Kinetics of the catalytic oxidation reactions of thiol compounds in aqueous solutions in the presence of copper ions

Catalysis of oxidation of aminothiols by copper ions was studied depending on the structure of aminothiols and pH of the medium. The catalytic reaction proceeds in the inner coordination sphere of Cu+. At pH 7—9, oxidation of bidentate aminothiols involves reduction of O2 to H2O2. At pH 9—13, oxidation of chelating aminothiols is accompanied by reduction of O2 to H2O, whereas oxidation of weak-chelating aminothiols still proceeds by the former mechanism. In this process, the thiolate anions coordinated to the Cu+ ions lose one electron each and are oxidized to amino disulfides, which go from the inner sphere of the Cu+ complex into a solution. Procedures developed for the determination of amino disulfides, the chemiluminescence determination of H2O2 in the presence of aminothiols as luminescence quenchers, and a modified polarographic procedure for the determination of O2 allowed us to establish that oxidation of aminothiols is not accompanied by catalytic decomposition of H2O2 that formed.

Oxidation of aminothiols by molecular oxygen catalyzed by copper ions. Stoichiometry of the reaction

1.

A study was made of the interaction of aminothiols (AT) with H2O2, as affected by the acidity of the medium, the temperature, the ionic strength of the solution, and the structure of the AT; the reacting particles were the thiolate anion and the molecular form of hydrogen peroxide.




2.

The reaction was promoted by hydrogen bonding between the OH− group splitting off and the amino group of the AT, and by the attraction between the OH− group and the positively charged end of the zwitterion form of the AT.




3.

Intermolecular hydrogen bonding and the electrostatic effect introduced equal contributions to the activation of the reaction.

The kinetics and mechanism of the reaction of the oxidation of aminothiols by hydrogen peroxide in aqueous solutions

Methods of chemical kinetics have been used in a study of the mechanism of hydrogen sulfide oxidation by iodine. It has been shown that the stage of electron transfer from HS− to the I3/− complex proceeds through a tunneling mechanism. A proposed “twinkling” model of the reaction mechanism provides an explanation for the observed experimental facts: the dependence of the rate constant on the acidity, viscosity, and ionic strength of the solution; the inverse temperature dependence of the reaction rate constant; the dependence of the reaction rate constant on the concentrations of iodide ion and maleic acid, which are not involved directly in the reaction.

G. A. Bagiyan

Papers

Oxidation of thiol compounds by molecular oxygen in aqueous solutions

Kinetics of the catalytic oxidation reactions of thiol compounds in aqueous solutions in the presence of copper ions

Oxidation of aminothiols by molecular oxygen catalyzed by copper ions. Stoichiometry of the reaction

The kinetics and mechanism of the reaction of the oxidation of aminothiols by hydrogen peroxide in aqueous solutions

Tunneling transfer of an electron in oxidation of the HS− ion by an I3− complex in aqueous solution