Presence Of Excess Thiol Research Articles

Characterizing rheological properties and microstructure of thioester networks during degradation.

Covalent adaptable networks are designed for applications including cell and drug delivery and tissue regeneration. These applications require network degradation at physiological conditions and on a physiological timescale with microstructures that can: (1) support, protect and deliver encapsulated cells or molecules and (2) provide structure to surrounding tissue. Due to this, the evolving microstructure and rheological properties during scaffold degradation must be characterized. In this work, we characterize degradation of covalent adaptable poly(ethylene glycol) (PEG)-thioester networks with different amounts of excess thiol. Networks are formed between PEG-thiol and PEG-thioester norbornene using photopolymerization. These networks are adaptable because of a thioester exchange reaction that takes place in the presence of excess thiol. We measure degradation of PEG-thioester networks with L-cysteine using multiple particle tracking microrheology (MPT). MPT measures the Brownian motion of fluorescent probe particles embedded in a material and relates this motion to rheological properties. Using time-cure superposition (TCS), we characterize the microstructure of these networks at the gel-sol phase transition by calculating the critical relaxation exponent, n, for each network with different amounts of excess thiol. Based on the measured n values, networks formed with 0% and 50% excess thiol are tightly cross-linked and elastic in nature. While networks formed with 100% excess are similar to ideal, percolated networks, which have equal viscous and elastic components. MPT measurements during degradation of these networks also measure a non-monotonic increase in probe motility. We hypothesize that this is network rearrangement near the phase transition. We then measure macroscopic material properties including the equilibrium modulus and stress relaxation. We measure a trend in bulk network properties that agrees with the values of n. Elastic modulus and stress relaxation measurements show that networks with 50% excess thiol are more elastic compared to the other two networks. As the amount of excess thiol is increased from 0% to 50%, the networks become more elastic. Further increasing excess thiol to 100% reduces the elastically effective cross-links. We hypothesize that these properties are due to network non-idealities, resulting in networks with 50% excess thiol that are more elastic. This work characterizes dynamic rheological properties during degradation, which mimics processes that could occur during implantation. This work provides information that can be used in the future design of implantable materials enabling both the rheological properties and timescale of degradation to be specified.

Synthesis and Hydrogen Sulfide Releasing Properties of Diaminodisulfides and Dialkoxydisulfides.

Heterosubstituted disulfides are an understudied class of molecules that have been used in biological studies, but they have not been investigated for their ability to release hydrogen sulfide (H2S). The synthesis of two sets of chemicals with the diaminodisulfide (NSSN) and dialkoxydisulfide (OSSO) functional groups was reported. These chemicals were synthesized from commercially available sulfur monochloride or a simple disulfur transfer reagent. Both the diaminodisulfide and dialkoxydisulfide functional groups were found to have rapid rates of H2S release in the presence of excess thiol. The release of H2S was complete with 10 min, and the only byproducts were conversion of the thiols into disulfides and the amines or alcohols originally used in the synthesis of the diaminodisulfide or dialkoxydisulfide functional groups. These results will allow the design of H2S releasing chemicals that also release natural, biocompatible alcohols or amines. Chemicals with the diaminodisulfide and dialkoxydisulfide functional groups may find applications in medicine where a controlled, burst release of H2S is needed.

Independent Generation and Reactivity of Thymidine Radical Cations.

Thymidine radical cation (1) is produced by ionizing radiation and has been invoked as an intermediate in electron transfer in DNA. Previous studies on its structure and reactivity have utilized thymidine as a precursor, which limits quantitative product analysis because thymidine is readily reformed from 1. In this investigation, radical cation 1 is independently generated via β-heterolysis of a pyrimidine radical generated photochemically from an aryl sulfide. Thymidine is the major product (33%) from 1 at pH 7.2. Diastereomeric mixtures of thymidine glycol and the corresponding 5-hydroxperoxides resulting from water trapping of 1 are formed. Significantly lower yields of products such as 5-formyl-2'-deoxyuridine that are ascribable to deprotonation from the C5-methyl group of 1 are observed. Independent generation of the N3-methyl analogue of 1 (NMe-1) produces considerably higher yields of products derived from water trapping, and these products are formed in much higher yields than those attributable to the C5-methyl group deprotonation in NMe-1. N3-Methyl-thymidine is, however, the major product and is produced in as high as 70% yield when the radical cation is produced in the presence of excess thiol. The effects of exogenous reagents on product distributions are consistent with the formation of diffusively free radical cations (1, NMe-1). This method should be compatible with producing radical cations at defined positions within DNA.

Expanding the Scope of Hypervalent Iodine Reagents for Perfluoroalkylation: From Trifluoromethyl to Functionalized Perfluoroethyl.

A series of new hypervalent iodine reagents based on the 1,3-dihydro-3,3-dimethyl-1,2-benziodoxole and 1,2-benziodoxol-3-(1H)-one scaffolds, which contain a functionalized tetrafluoroethyl group, have been prepared, characterized, and used in synthetic applications. Their corresponding electrophilic fluoroalkylation reactions with various sulfur, oxygen, phosphorus, and carbon-centered nucleophiles afford products that feature a tetrafluoroethylene unit, which connects two functional moieties. A related λ(3) -iodane that contains a fluorophore was shown to react with a cysteine derivative under mild conditions to give a thiol-tagged product that is stable in the presence of excess thiol. Therefore, these new reagents show a significant potential for applications in chemical biology as tools for fast, irreversible, and selective thiol bioconjugation.

Tuning the Magic Size of Atomically Precise Gold Nanoclusters via Isomeric Methylbenzenethiols.

Toward controlling the magic sizes of atomically precise gold nanoclusters, herein we have devised a new strategy by exploring the para-, meta-, ortho-methylbenzenethiol (MBT) for successful preparation of pure Au130(p-MBT)50, Au104(m-MBT)41 and Au40(o-MBT)24 nanoclusters. The decreasing size sequence is in line with the increasing hindrance of the methyl group to the interfacial Au-S bond. That the subtle change of ligand structure can result in drastically different magic sizes under otherwise similar reaction conditions is indeed for the first time observed in the synthesis of thiolate-protected gold nanoclusters. These nanoclusters are highly stable as they are synthesized under harsh size-focusing conditions at 80-90 °C in the presence of excess thiol and air (i.e., without exclusion of oxygen).

Reactivity of HNO‐Induced Modifications and Its Relevance to Phospholamban Function

HNO, a potential heart failure therapeutic, is known to post-translationally modify cysteine residues. Among reactive nitrogen oxide species, the modification of cysteine residues to sulfinamides [RS(O)NH2] is unique to HNO. Because this modification can alter protein structure and function, we have examined the reactivity of sulfinamides in several systems, including small organic molecules, peptides, and a protein. At physiological pH and temperature, relevant reactions of sulfinamides involve reduction to free thiols in the presence of excess thiol and hydrolysis to form sulfinic acids [RS(O)OH]. In addition to utilizing ESI-MS and other spectroscopic methods to study sulfinamide reduction, we have applied 15N-edited 1H-NMR techniques to sulfinamide detection and used this method to explore sulfinamide hydrolysis. We have also investigated the effect of local environment on the reactivity of HNO with C-terminal cysteines. Our findings indicate that the nature of HNO-derived modifications of C-terminal cysteines is influenced by the C-terminal carboxylate. HNO has been shown to enhance cardiac sarcoplasmic reticulum Ca2+ cycling independent of the β-adrenergic pathway. In a collaborative project, the effects of HNO on the cardiac protein, phospholamban (PLN) are investigated using 15N-edited NMR methods and biochemistry protocols. This research was supported by the National Science Foundation (CHE-1213438) and Cardioxyl Pharmaceuticals.

One-phase controlled synthesis of Au25 nanospheres and nanorods from 1.3 nm Au : PPh3 nanoparticles: the ligand effects.

We report the controlled synthesis of [Au25(PPh3)10(SR1)5X2](2+) nanorods (H-SR1: alkyl thiol, H-SC2H4Ph and H-S(n-C6H13)) and Au25(SR2)18 nanospheres (H-SR2: aromatic thiol, H-SPh and H-SNap) under the one-phase thiol etching reaction of the polydisperse Aun(PPh3)m parent-particles (core diameter: 1.3 ± 0.4 nm, 20 < n < 50). These as-obtained gold nanoclusters are identified by UV-vis spectroscopy and matrix-assisted laser desorption ionization mass spectrometry. Furthermore, the conversion process, from Aun(PPh3)m nanoparticles to Au25(SNap)18 nanospheres, is monitored by UV-vis spectroscopy. It is observed that the Au25(PPh3)10(SR1)5X2 nanorods cannot convert to Au25(SR)18 nanospheres in the presence of excess thiol (both the alkyl and aromatic thiol) even under thermal conditions (e.g., 55 and 80 °C), indicating that both the Au25 nanorods and nanospheres are in a stable state during the alkyl and aromatic thiol etching reactions, respectively. The two different conversion pathways (i.e., to Au25(PPh3)10(SR1)5X2 nanorods and Au25(SR2)18 nanospheres) mainly are attributed to the different electronic properties and the steric effects of the alkyl and aromatic thiol ligands. The significant ligand effect also is observed in catalytic CO oxidation. The Au25(SC2H4Ph)18/CeO2 catalyst shows catalytic activity at 80 °C and reaches up to 80.7% and 98.5% (based on CO conversion) at 100 and 150 °C, while Au25(SNap)18/CeO2 and Au25(PPh3)10(SC2H4Ph)5X2/CeO2 give rise to a low activity at 100 °C with only 3.3% and 10.2% CO conversion and 98.0% and 94.6% at 150 °C.

Au99(SPh)42 nanomolecules: aromatic thiolate ligand induced conversion of Au144(SCH2CH2Ph)60.

A new aromatic thiolate protected gold nanomolecule Au99(SPh)42 has been synthesized by reacting the highly stable Au144(SCH2CH2Ph)60 with thiophenol, HSPh. The ubiquitous Au144(SR)60 is known for its high stability even at elevated temperature and in the presence of excess thiol. This report demonstrates for the first time the reactivity of the Au144(SCH2CH2Ph)60 with thiophenol to form a different 99-Au atom species. The resulting Au99(SPh)42 compound, however, is unreactive and highly stable in the presence of excess aromatic thiol. The molecular formula of the title compound is determined by high resolution electrospray mass spectrometry (ESI-MS) and confirmed by the preparation of the 99-atom nanomolecule using two ligands, namely, Au99(SPh)42 and Au99(SPh-OMe)42. This mass spectrometry study is an unprecedented advance in nanoparticle reaction monitoring, in studying the 144-atom to 99-atom size evolution at such high m/z (∼12k) and resolution. The optical and electrochemical properties of Au99(SPh)42 are reported. Other substituents on the phenyl group, HS-Ph-X, where X = -F, -CH3, -OCH3, also show the Au144 to Au99 core size conversion, suggesting minimal electronic effects for these substituents. Control experiments were conducted by reacting Au144(SCH2CH2Ph)60 with HS-(CH2)n-Ph (where n = 1 and 2), bulky ligands like adamantanethiol and cyclohexanethiol. It was observed that conversion of Au144 to Au99 occurs only when the phenyl group is directly attached to the thiol, suggesting that the formation of a 99-atom species is largely influenced by aromaticity of the ligand and less so on the bulkiness of the ligand.

Core Size Conversion: Route for Exclusive Synthesis of Au38 or Au40 Nanomolecules

Gold nanomolecules with a precise number of gold atoms and ligands have promise for catalytic, optical, and biomedical applications. For practical applications, it is essential to develop synthetic protocols to prepare monodisperse gold nanomolecules. A typical synthesis yields a number of nanomolecules with discrete numbers of core atoms. Thermochemical treatment in the presence of excess thiol, etching, is known to narrow down the number of discrete nanomolecules, by selective degradation of sizes with lower stability. Au38(SR)24 and Au40(SR)24 are abundantly formed in these etching reactions due to their extraordinary stability to chemical etching. These nanomolecules are of high interest due in part to its stability, X-ray crystallographic structure availability (Au38), and intrinsic chirality arising from the arrangement of the Au-SR interface. However, the synthetic routes typically yield a mixture of Au38 and Au40, demanding extensive separation protocols. Here, we present a synthetic route to prepare either Au38 or Au40 exclusively in the product of etching. This was made possible by conducting a comprehensive mechanistic study starting from single-sized reactant. Au67 on etching yields Au40 exclusively. Au(103-105)(SR)(45-46) on etching also yields Au40 exclusively. Clusters of various sizes smaller than Au67 on etching yield Au38 exclusively. This is the first direct evidence for the exclusive formation of Au38 and Au40 nanomolecules by core size conversion. Mass spectrometry was used to study the core size conversion reactions to understand the mechanism. Au38 and Au40 nanomolecules form via different intermediates, as observed in the mass spectrometry data.

NMR Detection and Study of Hydrolysis of HNO-Derived Sulfinamides

Nitroxyl (HNO), a potential heart failure therapeutic, is known to post-translationally modify cysteine residues. Among reactive nitrogen oxide species, the modification of cysteine residues to sulfinamides [RS(O)NH2] is unique to HNO. We have applied (15)N-edited (1)H NMR techniques to detect the HNO-induced thiol to sulfinamide modification in several small organic molecules, peptides, and the cysteine protease, papain. Relevant reactions of sulfinamides involve reduction to free thiols in the presence of excess thiol and hydrolysis to form sulfinic acids [RS(O)OH]. We have investigated sulfinamide hydrolysis at physiological pH and temperature. Studies with papain and a related model peptide containing the active site thiol suggest that sulfinamide hydrolysis can be enhanced in a protein environment. These findings are also supported by modeling studies. In addition, analysis of peptide sulfinamides at various pH values suggests that hydrolysis becomes more facile under acidic conditions.

Antioxidant generation and regeneration in lipid bilayers: the amazing case of lipophilic thiosulfinates and hydrophilic thiols

We demonstrate that the garlic-derived chemopreventive agent allicin and the related anamu-derived petivericin are poor radical-trapping antioxidants in lipid bilayers, but that the in situ reaction of a lipophilic analog and a hydrophilic thiol yields an extremely potent radical-trapping antioxidant that can be recycled in the presence of excess thiol.

Reactivity of Nitroxyl-Derived Sulfinamides

Sulfinamide [RS(O)NH(2)] formation is known to occur upon exposure of cysteine residues to nitroxyl (HNO), which has received recent attention as a potential heart failure therapeutic. Because this modification can alter protein structure and function, we have examined the reactivity of sulfinamides in several systems, including a small organic molecule, peptides, and a protein. Although it has generally been assumed that this thiol to sulfinamide modification is irreversible, we show that sulfinamides can be reduced back to the free thiol in the presence of excess thiol at physiological pH and temperature. We have examined this sulfinamide reduction both in peptides, where a cyclic intermediate analogous to that proposed for asparagine deamidation reactions potentially can contribute, and in a small organic molecule, where the mechanism is restricted to a direct thiolysis. These studies suggest that the contribution from the cyclic intermediate becomes more important in environments with lower dielectric constants. In addition, although sulfinic acid [RS(O)OH] formation is observed upon prolonged incubations in water, reduction of sulfinamides is found to dominate in the presence of thiols. Finally, studies with the cysteine protease, papain, suggest that the reduction of sulfinamide to the free thiol is viable in a protein environment.

Antioxidant Activity of the Anti‐Inflammatory Compound Ebselen: A Reversible Cyclization Pathway via Selenenic and Seleninic Acid Intermediates

A revised mechanism that accounts for the glutathione peroxidase (GPx)-like catalytic activity of the organoselenium compound ebselen is described. It is shown that the reaction of ebselen with H(2)O(2) yields seleninic acid as the only oxidized product. The X-ray crystal structure of the seleninic acid shows that the selenium atom is involved in a noncovalent interaction with the carbonyl oxygen atom. In the presence of excess thiol, the Se--N bond in ebselen is readily cleaved by the thiol to produce the corresponding selenenyl sulfide. The selenenyl sulfide thus produced undergoes a disproportionation in the presence of H(2)O(2) to produce the diselenide, which upon reaction with H(2)O(2), produces a mixture of selenenic and seleninic acids. The addition of thiol to the mixture containing selenenic and seleninic acids leads to the formation of the selenenyl sulfide. When the concentration of the thiol is relatively low in the reaction mixture, the selenenic acid undergoes a rapid cyclization to produce ebselen. The seleninic acid, on the other hand, reacts with the diselenide to produce ebselen as the final product. DFT calculations show that the cyclization of selenenic acids to the corresponding selenenyl amides is more favored than that of sulfenic acids to the corresponding sulfenyl amides. This indicates that the regeneration of ebselen under a variety of conditions protects the selenium moiety from irreversible inactivation, which may be responsible for the biological activities of ebselen.

Lack of correlation between the observed stability and pharmacological properties of S-nitroso derivatives of glutathione and cysteine-related peptides

S-Nitrosothiols (RSNOs) have been widely studied as donors of nitric oxide. In general, RSNOs are considered to be somewhat unstable; however, they are both potent vasodilators and inhibitors of platelet aggregation. In order to improve our understanding of the factors that determine the biological activity of RSNOs, the chemical stability and pharmacological activity of a series of RSNOs was determined. Results show that millimolar solutions of S-nitrosocysteine (SNOCys) and S-nitroso- l-cysteinylglycine (SNOCysGly) were the least stable, whereas S-nitroso-3-mercaptopropionic acid (SNOPROPA) and S-nitroso- N-acetyl- l-cysteine (SNONAC) were the most stable of the compounds tested. Recent evidence suggests that RSNOs, such as SNONAC, are as unstable as SNOCys at micromolar concentrations. The decomposition of certain RSNOs is catalysed by trace amounts of copper (II) ions, with this phenomenon being particularly evident for SNOCys and SNOCysGly. The decomposition of the more stable RSNOs, including S-nitroso- l-glutathione (SNOGSH) and l-γ-glutamyl- l-cysteine (SNOGluCys), were not as sensitive to copper ions. The decomposition of the stable RSNO, SNOGSH, was more rapid in the presence of excess thiol, whereas the decay of the unstable RSNO, SNOCys, was reduced with added thiol. All RSNOs tested inhibited platelet aggregation, relaxed vascular smooth muscle, and inhibited cell growth in the nanomolar range, but their order of potency did not correlate with their chemical stability of millimolar solutions. It is apparent that the potency of an RSNO in a physiological situation will depend on the concentration of the compound present, the presence of trace metal ions such as copper, and the occurrence of transnitrosation reactions.

On the mechanism of reactions of nitrosoarenes with thiols. Formation of a common intermediate "semimercaptal".

To get more insight into the reactions of nitrosoarenes with thiols which may be responsible for cytotoxic effects, the reaction mechanism was studied with nitrosobenzene and 1-thioglycerol as model compounds. A transient intermediate was isolated by high performance liquid chromatography and identified as 2,3-dihydroxypropanesulfenic N-hydroxyphenylamide ("semimercaptal") by UV, 13C-NMR, and FAB mass spectroscopy. In aqueous solution this labile compound reassembles into 2,3-dihydroxypropanesulfinic phenylamide in a first order reaction. In the presence of excess thiol or ascorbic acid the "semimercaptal" is reduced to 2,3-dihydroxypropanesulfenic phenylamide without transient formation of a complete "mercaptal". Hydrolysis rather than thiolysis liberates aniline from the sulfenic phenylamide. Both the sulfinic and sulfenic phenylamides were obtained in crystalline form and identified by NMR and FAB mass spectroscopy. A scheme is presented of the known reactions of nitrosoarenes with thiols.

Biological activity of synthetic tetranuclear iron-sulphur analogues of the active sites of ferredoxins

Two water-soluble tetranuclear iron-sulphur clusters have been synthesised which are stable under anaerobic conditions in the presence of excess thiol in aqueous solution. In such a solution, in the presence of sodium dithionite, they undergo a reversible one electron reduction to the trianion. These two clusters can replace ferredoxins in a hydrogen evolving system using Clostridium pasteurianum hydrogenase with dithionite as the electron donor; we believe this is the first demonstration of such biological activity.

ChemInform Abstract: KINETIC, ELECTRON SPIN RESONANCE, AND PRODUCT STUDIES OF THE REACTION BETWEEN 2,2‐DIPHENYL‐1‐PICRYLHYDRAZYL AND 2,4,6‐TRI‐T‐BUTYLBENZENETHIOL

AbstractBei der Reaktion des Hydrazyls (I) mit dem Mercaptan (II) in Benzol entstehen hauptsächlich das Hydrazin (III) und das Disulfid (IV).

Kinetic, Electron Spin Resonance, and Product Studies of the Reaction Between 2,2-Diphenyl-1-picrylhydrazyl and 2,4,6- Tri-t-butylbenzenethiol

The reaction of 2,2-diphenyl-1-picrylhydrazyl (DPPH) with 2,4,6-tri-t-butylbenzenethiol (TBBT) in benzene solution has been studied. The two major products, 2,2-diphenyl-1-picrylhydrazine (DPPH-H) and di-(2,4,6-tri-t-butylphenyl) disulfide, are those anticipated for a reaction in which DPPH abstracts hydrogen from the thiol yielding thiyl radicals which undergo coupling to produce the disulfide. About 30% of the DPPH is converted into a derivative in which a nitro group has been displaced by a 2,4,6-tri-t-butylphenylthiyl group. Displacement of NO2 from DPPH by the tri-t-butylphenylthiyl radical is thought to constitute an important secondary reaction.In the presence of excess thiol, the rate constant for disappearance of DPPH, determined spectrophotometrically, is 1.23 ± 0.04 M−1 s−1 at 25 °C, and the activation energy is 5.8 ± 0.5 kcal mol−1. The rate of reaction is halved when the thiol is deuterated at the SH group, confirming that the primary step involves hydrogen transfer. Addition of DPPH-H retards the overall reaction indicating that this primary step is reversible.An e.s.r. study of the reacting solutions shows that the concentration of thiyl radical is relatively low until most of the DPPH disappears; it also shows the slow buildup of the 2,4,6-tri-t-butylphenylthiyl derivative of DPPH. The e.s.r. spectrum of the latter derivative has been analyzed and shown to be consistent with nitrogen coupling constants A1 = 10.4 ± 0.2 G and A2 = 6.45 ± 0.2 G.

Arginase isozymes and their detection by catalytic staining in starch gel

A method for the separation by starch gel and the in situ detection of the isozymes of arginase is presented. The method is capable of detecting 0.2 units of activity; it is specific, giving no staining in the absence of either arginine or urease from the developing solution. For purposes of the present study all parameters have been optimized for the separation and localization of arginase isozymes in tissue extracts. The method ought to be applicable, however, with appropriate modifications, to other enzymes yielding either alkaline reaction products (detectable in the presence of excess thiol) or yielding free thiol (such as reduced Coenzyme A) in alkaline medium.

Biochemical properties of anti-inflammatory drugs—IX: Uncoupling of oxidative phosporylation and inhibition of a thiol enzyme (papain) by some cyclic β-diones and ninhydrin

Some novel 2-acyl-indan-1,3-diones were synthesized and characterized. These and other acyl-indandiones were tested for uncoupling activity using rat liver mitochondria (oxidizing succinate). o.Hydroxybenzoyl-indandiones, which readily cyclized to yield chromones during their preparation, also uncoupled oxidative phosphorylation. The uncoupling potency of acyl-indandiones, pyrazolidinediones and certain acidic pyrazolones generally paralleled (a) their lipophilic character and (b) their avidity for protein ϵ-lysyl amino groups (as indicated by the compound's ability to inhibit the 2,4,6-trinitrobenzaldehyde-albumin reaction). p. p′-Dichlorophenylbutazone, the 4-butyryl analogue of phenylbutazone and the following indandiones = 2-isoheptanoyl-2- p. chlorphenyl-, 2-(3′, 4′) dichlorobenzoyl-, were the most active uncoupling agents found in this limited survey. The anomalous (low) uncoupling activity and lysyl-binding activity of certain ortho substituted benzoyl-indandiones could be attributed to steric hindrance. Some limitations of the trinitrobenzaldehyde-albumin reaction for screening compounds for potential drug activity are disclosed, notably (i) reaction of the aldehyde with compounds containing an active methylene group (e.g. unsubstituted β-diones like indan-1,3-dione itself), and (ii) where the drug under investigation may bind to plasma albumin at sites other thsn lysine amino groups (e.g. sulphinpyrazone). Sulphamethazine potentiates the reaction between trinitrobenzaldehyde and bovine plasma albumin. Ninhydrin, hydrindantin and some related non-acidic aromatic 1,2 and 1,4-diones also uncoupled oxidative phosphorylation, but only in the absence of added thiols, and did not inhibit the trinitrobenzaldehyde-albumin reaction (in those instances where these diones did not react with either the albumin or the trinitrobenzaldehyde). These compounds, and also phenylbutazone and some of its analogues, were potent inhibitors of the esterase activity of crystalline papain in the presence of excess thiol. It is postulated that non-acidic diones may uncouple oxidative phosphorylation by reacting with essential thiol groups involved in mitochondrial energy conservation, whereas acidic uncoupling drugs primarily inhibit mitochondrial phosphorylation by “neutralizing” essential amino groups.