Autoxidation Of Thiols Research Articles

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.

Rocket fuel for the quantification of S-nitrosothiols. Highly specific reduction of S-nitrosothiols to thiols by methylhydrazine

Reduction of S-nitrosothiols to the corresponding thiol function is the key step in analyzing S-nitrosocysteinyl residues in proteins. Though it has been shown to give low yields, ascorbate-dependent reduction is commonly performed in the frequently used biotin-switch technique. We demonstrate that the compound methylhydrazine can act as a specific and efficient reducing agent for S-nitrosothiols. The corresponding thiol function is exclusively generated from low molecular weight and proteinaceous S-nitrosothiols while methylhydrazine failed to reduce disulfides. It was possible to optimize the experimental conditions so that thiol autoxidation is excluded, and high reaction yields (> 90%) are obtained for the thiol function. The biotin-switch technique performed with methylhydrazine-dependent reduction shows remarkably improved sensitivity compared to the ascorbate-dependent procedure.

Aerobic oxygenation of phenylboronic acid promoted by thiol derivatives under gold-free conditions: a warning against gold nanoparticle catalysis

Oxygenation of phenylboronic acid to phenol is promoted by thiol derivatives such as 2-aminothiophenol under aerobic conditions in water without metal catalyst. A plausible mechanism involves autoxidation of thiol to generate hydrogen peroxide in situ, which converts phenylboronic acid into phenol under basic conditions. Since thiols are often utilized as protective ligands for gold nanoparticles (AuNPs), this result is a warning that excess thiols and free thiols liberated from the Au surface could participate in aerobic oxidations catalyzed by thiol-protected AuNPs.

The dynamics of cysteine, glutathione and their disulphides in astrocyte culture medium

Glutathione (GSH) plays an important neuroprotective role, and its synthesis depends on the amount of available cysteine (CSH) in the cells. Various kinds of evidence suggest that astrocytes can provide CSH or GSH to neurons, but the delivery mechanism of the thiol-compounds has not been elucidated. In this study, the dynamics of CSH, GSH and their disulphides in astrocyte culture medium were investigated by following the time-course of concentration changes and by computer simulation and curve fitting to experimental data using a mathematical model. The model consists of seven reactions and three transports, which are grouped into four categories: autoxidation of thiols into disulphides, thiol-disulphide exchange and reactions of thiols with medium components, as well as the cellular influx and efflux of thiols and disulphides. The obtained results are interpreted that cystine (CSSC) after entering astrocyte is reduced to CSH, most of which is released to medium and autoxidized to CSSC. The efflux of GSH was estimated to be considerably slower than that of CSH, and most of the excreted GSH is converted to cysteine-glutathione disulphide principally through the thiol-disulphide exchange. The results seem to indicate that astrocytes provide neurons mainly with CSH, rather than GSH, as the antioxidant material for neuroprotection.

Specific toxins destabilize virus inhibitors (e.g. AIDS viruses)

There are two approaches to the study of viral infections – the A–Z hypothesis of Sprietsma involving thiol-zinc inhibition of protease activation in the virus coat, and the activity of toxins in depleting the cell of thiols, or the activity of toxins activating (not detoxifying) the mixed function oxidase (MFO) system. Both of these systems generate free radicals and utilize thiols in the process.Zinc forms stable mercaptides with thiols, inhibits cyclic reduction–autoxidation of thiols and superoxide generation. When the MFO system acts as an activator and not as a detoxifier in that intermediate products are more toxic than the original compound, zinc inhibits the oxidation. An example of increased toxicity with increased MFO activity is the toxin 3-methylindole (3-MI), a toxic product of intestinal bacterial putrifaction which reactivates the infectious bovine rhinotracheitis virus (IBR).Zinc reduces MFO activity and in this regard it functions synergistically with antioxidants in protecting cell membranes.It is hypothesized that stable zinc complexes inhibit activity of proteases in the virus nucleocapside (NC) proteins in the virus coat, both directly and indirectly because zinc also inactivates some toxins that are thiol depleters or virus reactivators.

Mutagenicity of thiol compounds in Escherichia coli WP2 tester strain IC203, deficient in OxyR: effects of S9 fractions from rat liver and kidney

Low doses of l-cysteine (CYS), cysteinyl-glycine (CYSGLY) and reduced glutathione (GSH) activated by γ-glutamyl transpeptidase (GGT) were mutagenic in strain IC203 ( oxyR), whereas higher doses were required to observe a weak mutagenicity in the oxyR + strain WP2 uvrA/pKM101 (denoted IC188). This indicates that thiol mutagenesis is suppressed by OxyR-regulated antioxidant defenses and confirms its oxidative character. The mutagenesis by low doses of CYS, CYSGLY and GSH+GGT detected in IC203 was abolished by rat liver S9, through the activity of catalase, as well as by the metal chelator diethyldithiocarbamate (DETC), supporting the dependence of this mutagenesis on H 2O 2 production, probably in thiol autoxidation reactions in which transition metals are involved. Surprisingly, low DETC concentrations greatly potentiate the mutagenicity of low CYS doses. Mutagenesis by high doses of CYS and CYSGLY occurred in both IC203 and IC188 in the presence of liver S9, and was resistant to inhibition by catalase, although it was prevented by DETC. Mutagenesis by GSH activated by rat kidney S9, rich in GGT, was detected in IC203 and IC188 only at high doses since catalase and glutathione peroxidase, both present in kidney S9, might inhibit its induction by low GSH doses. In the presence of liver S9, almost deficient in GGT, GSH was not mutagenic. The mutagenicity of a high GSH dose occurring in the presence either of GGT plus liver S9 or of kidney S9 was weakly prevented by DETC.

UV-vis and Binding Studies of Cobalt Tetrasulfophthalocyanine–Thiolate Complexes as Intermediates of the Merox Process

Intermediates of the cobalt tetrasulfophthalocyanine ( CoTSPc )-catalyzed thiol autoxidation were studied by UV-vis spectroscopy. All thiolates react with CoTSPc resulting in the formation of 1:1 complexes. Three major factors control both the stability and aggregation of the complexes: thiolate basicity, metal-to-ligand charge transfer (MLCT) and π stacking. Basic thiolates partially reduce C oII TSPc , whereas CoTSPc complexes with low-basicity aliphatic thiolates ( p K a < 4) do not exhibit Co (II) reduction, based on the absence of the characteristic Co (I) charge transfer band at 450 nm. CoTSPc complexes with aliphatic and bulky aromatic thiolates appear to be aggregated in aqueous solutions and are characterized by a broad band at 650 nm. Non-bulky aromatic thiolates of low basicity ( p K a < 6) form unique stable monomeric Co II TSPc complexes. This unique spectral feature can be attributed to π stacking between the phthalocyanine ring and thiolate. Comparison of binding constants shows that the partial reduction of Co (II) significantly contributes to the thiolate binding. A combination of aromatic π stacking and MLCT appears to be responsible for the observed 1000-fold stronger binding of non-basic aromatic thiolates as compared with aliphatic ligands of similar basicity. Kinetic studies confirm the importance of the thiolate binding type for catalysis.

Heterogeneous catalysis of the air oxidation of thiols by the cobalt porphyrin intercalated into a phosphatoantimonic acid host

Cobalt porphyrin (TPPCo) has been prepared in situ as a guest molecule in the layered host material phosphatoantimonic acid, HSb(PO 4) 2, here denoted SbP. The powder X-ray diffraction of the purified sample, the SbP-intercalated cobalt porphyrin (SbPTPPCo), showed the expanded interlamellar distance of SbP upon the intercalation. The electronic absorption and electron paramagnetic resonance spectra revealed the retention of cobalt porphyrin in the layered host material. The catalytic behaviour of the novel catalyst SbPTPPCo for the thiol autoxidation was investigated in quantitative detail. In our experimental conditions, the catalytic autoxidation proceeded without induction period for oxygen consumption. The intercalated cobalt porphyrin appears to be a promising catalyst owing to its good catalytic capability. No loss in activity was observed for SbPTPPCo after eight reaction cycles and a total of more than 5390 turnovers. The proposed reaction scheme for the autoxidation catalyzed by SbPTPPCo involves the formation of catalytic center of mononuclear complex by which the electron of RS − is transferred to bound and activated dioxygen.

Role of copper and ceruloplasmin in oxidative mutagenesis induced by the glutathione-γ-glutamyl transpeptidase system and by other thiols

Glutathione is activated to a mutagen by gamma-glutamyl transpeptidase. Other thiols, such as cysteine, penicillamine, cysteine ethylester, and cysteinylglycine, are direct mutagens in the Ames Salmonella mutagenicity test. Thiol mutagenesis is oxidative in nature and involves H2O2 and possibly hydroxyl radicals. Transition metals are crucial for thiol autoxidation. The role of copper and ceruloplasmin (CP) in thiol-dependent mutagenesis was studied in Salmonella typhimurium strain TA102. Cu and CP at low concentrations enhanced thiol-dependent mutagenesis in the presence, but not in the absence, of added Fe. The degree of enhancement depended on the type of thiol. At high Cu or CP concentrations, thiol mutagenesis was inhibited. Cu also decreased the mutagenicity of H2O2. Cu- and CP-enhanced mutagenesis were inhibited by radical scavengers, catalase, and peroxidase but not by superoxide dismutase. The effects of Cu and CP on thiol-dependent mutagenesis were similar to their effects on thiol-driven lipid peroxidation. The results indicate that the role of Cu and CP in the enhancement of thiol mutagenesis is the facilitation of the transfer of electrons from a thiol to iron, rather than in catalysis of the Fenton reaction.

Thiol-oxidation with zeolite encapsulated phthalocyanines: influence of the solid support

The catalytic activity of zeolite encapsulated cobalt-pathalocyanine for thiol autoxidation varies with the composition of the zeolite support. Different zeolites were compared with carbon supports. The Si Al ratio in the zeolite appears to be the key factor: it determines the hydrophobicity of the support, and it influences the loading of active complex in the zeolite. Two different types of reaction were tested: in an aqueous alkaline medium, the activity increases with hydrophobicity of the support (thus giving carbon a substantial advantage), while in an organic medium, were the commonly used systems are less efficient, sodium Y zeolite is more active than the usual carbon support.

Functional Metallomacrocycles and Their Polymers XXXIII. Autoxidation of Thiol Catalyzed by Thermoelastic Polyurethane Resin Containing a Cobalt(II)phthalocyanine

Functional Metallomacrocycles and Their Polymers XXXIII. Autoxidation of Thiol Catalyzed by Thermoelastic Polyurethane Resin Containing a Cobalt(II)phthalocyanine

Spin Trap Studies on the Decomposition of Peroxynitrite

The observation of the hydroxyl radical spin adduct of 5,5-dimethyl-1-pyrroline- N-oxide (DMPO) is evaluated as a probe for the production of hydroxyl radicals from the decomposition of peroxynitrite. Although a weak signal corresponding to the DMPO-hydroxyl radical spin adduct ( . DMPO-OH) is observed when peroxynitrite is allowed to decompose in the presence of DMPO, it is concluded that this does not constitute proof of the presence of free hydroxyl radicals. The observed rate constant for the decay of peroxynitrite increases from 0.35 to 0.51 s −1 (46% increase) when the concentration of DMPO is increased from 0 to 75 mM. This strongly suggests there is a reaction between DMPO and HOONO, or between DMPO and an activated intermediate of HOONO, to produce the hydroxyl radical spin adduct. The addition of glutathione or cysteine produces a large increase in the intensity of the . DMPO-OH spin adduct signal; experiments employing superoxide dismutase suggest that the increases in the amounts of . DMPO-OH adduct are produced from the decomposition of the spin adduct of the superoxide radical ( . DMPO-OOH). The superoxide adduct arises as a result of the autoxidation of thiols, a process known to produce superoxide. The results presented here are incompatible with the formation of free hydroxyl radicals but can be explained in terms of an intermediate of HOONO that is less reactive and more selective than the free hydroxyl radical.

Functional metallomacrocycles and their polymers. Part 31. Autoxidation of thiol by temperature-sensitive polymer catalyst containing cobalt(II) phthalocyanine complex

Copolymers (I-IV) of 2-acryloylamino-9,16,23-tri( t-butyl)phthalocyanine (APc( t-Bu) 3) and acrylamide derivatives were prepared and their catalytic activity for oxidation of 2-mercaptoethanol was examined. Copolymers (II and IV) of APc( t-Bu) 3 and N-isopropylacrylamide revealed a lower critical solution temperature phenomenon at 29°C in carbonate buffer, that is to say, the copolymers dissolving in buffer solution below 29°C separated out as precipitate above 29°C. The catalytic activity of polymer IV, which was prepared by metalation of copolymer II, markedly changed around 29°C. Copolymer IV served as a homogeneous catalyst of autoxidation of 2-mercaptoethanol below 29°C. In contrast, the catalytic activity drastically decreased above 29°C because of separation as precipitate.

Functional metallomacrocycles and their polymers, 26. Synthesis of 2‐acryloylamino‐9,16,23‐tri‐tert‐butylphthalocyanine and autoxidation of thiol with water‐soluble polymers containing cobalt(II) phthalocyaninate complex

AbstractThe catalytic autoxidation of 2‐mercaptoethanol was studied by using new water‐soluble polymers containing a cobalt(II) phthalocyaninate complex as a catalyst. The phthalocyanine vinyl‐monomer, 2‐acryloylamino‐9,16,23‐tri‐tert‐butylphthalocyanine (APc(t‐Bu)3), was synthesized and copolymerized with three kinds of water‐soluble monomers to result in a favorite electrostatic environment as a catalyst. The resulting cationic polymer containing a cobalt(II) phthalocyaninate complex has high activity as a catalyst for autoxidation of 2‐mercaptoethanol independent of pH. The catalystic system obeys Michaelis‐Menten kinetics with respect to substrate concentration. The rate constant k5 of the rate‐controlling step and the apparent Michaelis constant K were obtained from a Lineweaver‐Burk plot. The cationic environment on the polymeric catalyst enhances enzyme‐like activity in the neutral pH region for autoxidation of 2‐mercaptoethanol, because of the promotion of substrate dissociation and the accumulation of thiolate anions.

Influence of the molecular weight of ionenes on the cobaltphthalocyanine‐catalysed autoxidation of mercaptoethanol

AbstractIn order to study the promoting effect of polycations on the cobalt(II)phthalocyanine‐tetrasodiumsulphonate (CoPc(NaSO3)4)‐catalysed autoxidation of thiols, it is imperative to know the molecular weight dependence of the polymer. Monodisperse oligomers can often supply a lot of information. To elecidate the mechanism of the promoting effect of 2,4‐ionene, a poly(quaternary ammonium)salt, on the CoPc(NaSO3)4‐catalysed autoxidation of 2‐mercaptoethanol, a series of monodisperse 2,4‐ionene oligomers were prepared. Trimeric 2,4‐ionenes, containing two quaternary ammonium groups separated by four methylene groups, showed a high co‐catalytic activity, and active Co‐aggregates were detected with visible light spectroscopy. The spectroscopic behaviour was in close agreement with that of high molecular 2,4‐ionene/CoPc(NaSO3)4 complexes. In order to achieve this aggregation several oligomeric ionenes have to act concertedly. If the distance between the ionic sites is short the trimer acts as a simple salt in stablizing the aggregates. With respect to the catalytic activity the optimum polycation/catalyst ratio, expressed as the N+/Co ratio, decreased with increasing chain lenght and reached a constant level of 50 at a 2,4‐ionene nonamer. For the trimers and pentamers this ratio is affected by the type of end‐group. All synthesized 2,4‐ionene oligomers exhibited excellent co‐catalytic properties at the optimum N+/Co ratio, with maximum turnover frequencies of 4600 mol thiol/(mol Co s), i.e. 50 times higher than those obtained for the polymer‐free system. In the case of monodisperse 2,4‐ionene pentamer with bromo end‐groups, double Michaelis–Menten kinetics were observed, as also was exhibited by high molecular weight ionene.

Latex‐supported catalysts for the autoxidation of thiols, 1. Use of ionene‐based macromonomers

2,4-Ionene was prepd. from 1,4-dibromobutane and N,N,N',N'-tetramethylethylenediamine, and then treated with (chloromethyl)styrene to give styryl-terminated macromonomer. The macromonomer was polymd. with styrene to give latexes that had high catalytic activity in the autoxidn. of 2-mercaptoethanol

Autoxidation of 2‐mercaptoethanol catalyzed by cobalt(II) phthalocyaninetetrasulfonate on colloidal particles

AbstractCobalt(II) phthalocyaninetetrasulfonate (CoPcTS) is a highly active catalyst in aqueous media for the autoxidation of thiols to disulfides, and cationic polymers further enhance the activity. In this work we tested a variety of cationic polymer supports for the CoPcTS‐catalyzed autoxidation of 2‐mercaptoethanol in aqueous dispersions at 25°C. The order of activities is cationic polyelectrolytes > cationic polymer colloids > cationic colloidal silica > aqueous solution > ion exchange resins. Spectroscopic studies indicate that the high activity of the polyelectrolytes correlates with their ability to aggregate the CoPcTS. © 1992 John Wiley & Sons, Inc.

Catalytic activity of iron and cobalt phthalocyanines wrapped in poly(peptide) for autoxidation of thiol

The autoxidation of 2-mercaptoethanol was studied by using poly(peptide)-wrapped unsubstituted metal phthalocyanine as a polymer catalyst, where the poly(peptide)s consisted of L-lysine, L-glutamic acid and β-alanine and the metal was iron or cobalt. In spite of the lack of catalytic activity of poly(peptide) or metal phthalocyanine alone, the poly(peptide)-wrapped metal phthalocyanine possessed catalytic activity for the autoxidation of 2-mercaptoethanol. In particular, higher catalytic activity was observed in the polymer catalyst with the higher content of L-lysine residue. The catalytic system was found to obey Michaelis-Menten kinetics with respect to substrate concentration. The rate constant k 2 and the Michaelis-Menten constant K m were determined from a Lineweaver-Burk plot. The overall activation energy E a for the autoxidation was also determined from an Arrhenius plot over the temperature range 10–40°C. Overall, the present polymer catalysts exhibited enzyme-like behavior for the autoxidation of 2-mercaptoethanol.

Effects of ionenes on structure and catalytic activity of cobalt phthalocyanine: Part 7. Dependence of reaction kinetics on ph, type of polymer and type of thiol

The cobalt phthalocyanine ( 1)-catalyzed autoxidation of thiols in the presence of ionenes ( 2) was investigated kinetically, using the two-substrate Michaelis-Menten model. Effects of pH, type of ionene and type of thiol on the overall reaction rate were established, as well as their effects on the individual kinetic parameters. Overall reaction rate appeared to be mainly determined by k 1 the rate constant for complexation of the thiolate anion with the catalyst, and especially k 3, the rate constant of the product formation step (see Scheme 1). Since k 1 is related to the local thiolate anion concentration, it could be increased by increasing the pH, the effective ionene charge density and the negative charge on the R-group of the thiol. Regarding k 3, it was found that this rate constant decreases at pH > 11 or when negatively-charged thiols are used. From the results obtained for different ionenes, it was concluded that the degree of phthalocyanine aggregation is an important factor, presumably affecting the ease of oxidation of the cobalt complex. A detailed reaction mechanism, combining all observed dependencies is proposed and discussed.

Superoxide-driven NAD(P)H oxidation induced by EDTA-manganese complex and mercaptoethanol

A purely chemical system for NAD(P)H oxidation to biologically active NAD(P) + has been developed and characterized. Suitable amounts of EDTA, manganous ions and mercaptoethanol, combined at physiological pH, induce nucleotide oxidation through a chain length also involving molecular oxygen, which eventually undergoes quantitative reduction to hydrogen peroxide. Mn 2+ is specifically required for activity, while both EDTA and mercaptoethanol can be replaced by analogs. Optimal molar ratios of chelator/metal ion (2 : 1) yield an active coordination compound which catalyzes thiol autoxidation to thiyl radical. The latter is further oxidized to disulfide by molecular oxygen whose one-electron reduction generates superoxide radical. Superoxide dismutase (SOD) inhibits both thiol oxidation and oxygen consumption as well as oxidation of NAD(P)H if present in the mixture. A tentative scheme for the chain length occurring in the system is proposed according to stoichiometry of reactions involved. Two steps appear of special importance in nucleotide oxidation: (a) the supposed transient formation of NAD(P) • from the reaction between NAD(P)H and thiyl radicals; (b) the oxidation of the reduced complex by superoxide to keep thiol oxidation cycling.