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Cysteine thiols are susceptible to various oxidative posttranslational modifications (PTMs) due to their high chemical reactivity. Thiol‐based PTMs play a crucial role in regulating protein functions and are key contributors to cellular redox signaling. Although reversible thiol‐based PTMs, such as disulfide bond formation, S‐nitrosylation, and S‐glutathionylation, have been extensively studied for their roles in redox regulation, thiol sulfinic acid (–SO2H) modification is often perceived as irreversible and of marginal significance in redox signaling. Here, we revisit this narrow perspective and shed light on the redox regulatory roles of –SO2H in plant stress signaling. We provide an overview of protein sulfinylation in plants, delving into the roles of hydrogen peroxide‐mediated and plant cysteine oxidase‐catalyzed formation of –SO2H, highlighting the involvement of –SO2H in specific regulatory signaling pathways. Additionally, we compile the existing knowledge of the –SO2H reducing enzyme, sulfiredoxin, offering insights into its molecular mechanisms and biological relevance. We further summarize current proteomic techniques for detecting –SO2H and furnish a list of experimentally validated cysteine –SO2H sites across various species, discussing their functional consequences. This review aims to spark new insights and discussions that lead to further investigations into the functional significance of protein –SO2H‐based redox signaling in plants. Summary statement: This review expands upon the conventional understanding of cysteine thiol sulfinic acid modification and provides an updated overview of its redox‐dependent regulatory functions in the context of plant stress signaling pathways. [ABSTRACT FROM AUTHOR]

Sulfurous acid (H2SO3) is known to be thermodynamically instable decomposing into SO2 and H2O. All attempts to detect this elusive acid in solution failed up to now. Reported H2SO3 formation from an experiment carried out in a mass spectrometer as well as results from theoretical calculations, however, indicated a possible kinetic stability in the gas phase. Here, it is shown experimentally that H2SO3 is formed in the OH radical‐initiated gas‐phase oxidation of methanesulfinic acid (CH3S(O)OH) at 295±0.5 K and 1 bar of air with a molar yield of 53-17+7 ${{53}_{-17}^{+\ 7}}$ %. Further main products are SO2, SO3 and methanesulfonic acid. CH3S(O)OH represents an important intermediate product of dimethyl sulfide oxidation in the atmosphere. Global modeling predicts an annual H2SO3 production of ∼8 million metric tons from the OH+CH3S(O)OH reaction. The investigated H2SO3 depletion in the presence of water vapor results in k(H2O+H2SO3) <3×10−18 cm3 molecule−1 s−1, which indicates a lifetime of at least one second for atmospheric humidity. This work provides experimental evidence that H2SO3, once formed in the gas phase, is kinetically stable enough to allow its characterization and subsequent reactions. [ABSTRACT FROM AUTHOR]

Thiol dioxygenases (TDOs) participate in the metabolism of small thiol-containing molecules, and more recently they were discovered to contribute to cell signaling. A specific clade of TDOs in plants and metazoans regulates the oxygen-dependent degradation of regulatory proteins through the N-degron pathway. X-ray crystallography, spectroscopic analyses, and dynamic simulations have identified crucial features for the activity and specificity of TDOs. These techniques have also assisted in designing mutant variants and exploring their consequences in vitro and in vivo. Structural information, along with the design of activity assays in heterologous systems, has guided the identification of specific inhibitors. Thiol oxidation to dioxygenated sulfinic acid is catalyzed by an enzyme family characterized by a cupin fold. These proteins act on free thiol-containing molecules to generate central metabolism precursors and signaling compounds in bacteria, fungi, and animal cells. In plants and animals, they also oxidize exposed N-cysteinyl residues, directing proteins to proteolysis. Enzyme kinetics, X-ray crystallography, and spectroscopy studies prompted the formulation and testing of hypotheses about the mechanism of action and the different substrate specificity of these enzymes. Concomitantly, the physiological role of thiol dioxygenation in prokaryotes and eukaryotes has been studied through genetic and physiological approaches. Further structural characterization is necessary to enable precise and safe manipulation of thiol dioxygenases (TDOs) for therapeutic, industrial, and agricultural applications. [ABSTRACT FROM AUTHOR]

The expression of the phospholipase A₂ activity (aiPLA₂) of peroxiredoxin 6 (Prdx6) in the cell cytoplasm is physiologically relevant for the repair of peroxidized cell membranes, but aiPLA₂ assay in vitro indicates that, unlike assay at pH 4, activity at cytosolic pH is essentially absent with non-oxidized substrate. However, the addition of glutathione (GSH) to the assay medium significantly increased aiPLA₂ activity at cytosolic pH, while oxidized GSH (GSSG) and several other thiols had no effect. By mass spectroscopy (ESI MS), the addition of GSH to Prdx6 paradoxically led to oxidation of its conserved Cys47 residue to a sulfinic acid. The effect of GSH on PLA₂ activity was abolished by incubation under anaerobic conditions, confirming that auto-oxidation of the protein was the mechanism for the GSH effect. Analysis by circular dichroism (CD) and tryptophan fluorescence showed alterations of the protein structure in the presence of GSH. Independently of GSH, the oxidation of Prdx6 by exposure to H₂O₂ or the presence of oxidized phospholipid as substrate also significantly increased aiPLA₂ activity at pH 7. We conclude that the oxidation of the peroxidatically active Cys47 of Prdx6 results in an increase of aiPLA₂ activity at pH 7 without effect on the activity of the enzyme at pH 4.

Sulfinic acids and their salts are a useful source of sulfur-containing structures. Photocatalysis of these compounds with visible light enables to achieve various transformations under mild conditions. This review summarizes visible-light-induced reactions of sulfinic acids and their salts. It is organized by reaction type and brief discussions on plausible reaction mechanisms for typical transformations are presented. 1 Introduction 2 Sulfonylation Reactions 2.1 Sulfonylation of Alkenes 2.2 Sulfonylation of Alkynes 2.3 Sulfonylation of Arenes 2.4 sp3 C–H Functionalization 3 Desulfonylation Reactions 4 Sulfenylation Reactions 4.1 Sulfenylation of Heteroarenes 4.2 Sulfenylation of Carbonyl Chlorides 5 Conclusions [ABSTRACT FROM AUTHOR]

A new synthetic methodology has been developed for the synthesis of N-alkyl and N-aryl arenesulfinamides. The methodology involved reacting arenesulfinic acids (R = -Me, -H, -Cl) with 1,1'-carbonyldiimidazole (CDI) to form the reactive intermediate, an arenesulfinylimidazole. This intermediate was then reacted with both primary and secondary amines to yield the corresponding N-alkyl sulfinamides in yields up to 90%. While the overall yields ranged from 52% to 90%, the level of diastereoselection with racemic or enantiomerically enriched amines only reached a level of 54:46 favoring the major diastereomer as determined by analysis of the 500 MHz 1H NMR spectra. A series of aniline derivatives were also investigated as coupling partners and were found to form the N-aryl arenesulfinamide in good yield. [ABSTRACT FROM AUTHOR]

Decade advances of sulfinic acids (2010‐2021) were summarized via four categories by mechanisms: (1) sulfonyl radicals (2) RS radicals; (3) electrophilic intermediates; (4) nucleophilic S‐reagents. Their reactions with alkenes, alkynes, propiolic acids, cinnamic acids, allenes, ynones, azides, enynals, aldehydes, ketones, o‐aryl isocyanides, arenes, quinoline N‐oxides, disulfides, diselenides, thiophenol, P(O)‐H compounds, nitriles, alcohols, nitroso benzenes and dizo compounds will be discussed. [ABSTRACT FROM AUTHOR]

Human serum albumin (HSA) contains 17 disulfides and only one reduced cysteine, Cys34, which can be oxidized to a relatively stable sulfenic acid (HSA-SOH). This derivative has been previously detected and quantified. However, its properties are poorly understood. Herein, HSA-SOH formation from the exposure of HSA to hydrogen peroxide was confirmed using the sulfenic acid probe bicyclo [6.1.0]nonyne-biotin (BCN-Bio1), and by direct detection by whole protein mass spectrometry. The decay pathways of HSA-SOH were studied. HSA-SOH reacted with a thiol leading to the formation of a mixed disulfide. The reaction occurred through a concerted or direct displacement mechanism (S N 2) with the thiolate (RS−) as nucleophile towards HSA-SOH. The net charge of the thiolate affected the value of the rate constant. In the presence of hydrogen peroxide, HSA-SOH was further oxidized to sulfinic acid (HSA-SO 2 H) and sulfonic acid (HSA-SO 3 H). The rate constants of these reactions were estimated. Lastly, HSA-SOH spontaneously decayed in solution. Mass spectrometry experiments suggested that the decay product is a sulfenylamide (HSA-SN(R′)R″). Chromatofocusing analysis showed that the overoxidation with hydrogen peroxide predominates at alkaline pH whereas the spontaneous decay predominates at acidic pH. The present findings provide insights into the reactivity and fate of the sulfenic acid in albumin, which are also of relevance to numerous sulfenic acid-mediated processes in redox biology and catalysis. [Display omitted] • A sulfenic acid is formed in human serum albumin upon exposure to hydrogen peroxide. • Once formed, sulfenic acid can follow one of three possible routes. • Sulfenic acid reacts with thiols leading to the formation of mixed disulfides. • Sulfenic acid is further oxidized by hydrogen peroxide to sulfinic or sulfonic acid. • Sulfenic acid decays spontaneously, likely forming a sulfenylamide. [ABSTRACT FROM AUTHOR]

Sulfurous acid (H2SO3) is known to be thermodynamically instable decomposing into SO2 and H2O. All attempts to detect this elusive acid in solution failed up to now. Reported H2SO3 formation from an experiment carried out in a mass spectrometer as well as results from theoretical calculations, however, indicated a possible kinetic stability in the gas phase. Here, it is shown experimentally that H2SO3 is formed in the OH radical-initiated gas-phase oxidation of methanesulfinic acid (CH3S(O)OH) at 295 ± 0.5 K and 1 bar of air with a molar yield of [[EQUATION]] %. Further main products are SO2, SO3 and methanesulfonic acid. CH3S(O)OH represents an important intermediate product of dimethyl sulfide oxidation in the atmosphere. Global modeling predicts an annual H2SO3 production of ∼8 million metric tons from the OH + CH3S(O)OH reaction. The investigated H2SO3 depletion in the presence of water vapor results in k(H2O + H2SO3) < 3 × 10-18 cm3 molecule-1 s-1, which indicates a lifetime of at least one second for atmospheric humidity. This work provides experimental evidence that H2SO3, once formed in the gas phase, is kinetically stable enough to allow its characterization and subsequent reactions., (© 2024 Wiley‐VCH GmbH.)

Disulfide bonds play a key role in stabilizing proteins by cross-linking secondary structures. Whilst many disulfides are effectively unreactive, it is increasingly clear that some disulfides are redox active, participate in enzymatic reactions and/or regulate protein function by allosteric mechanisms. Previously (Karimi et al., Sci. Rep. 2016, 6, 38752) we have shown that some disulfides react rapidly with biological oxidants due to favourable interactions with available lone-pairs of electrons. Here we present data from kinetic, mechanistic and product studies for HOCl-mediated oxidation of a protected nine-amino acid model peptide containing a N - to C -terminal disulfide bond. This peptide reacts with HOCl with k 2 1.8 × 106 M−1 s−1, similar to other highly-reactive disulfide-containing compounds. With low oxidant excesses, oxidation yields multiple oxidation products from the disulfide, with reaction predominating at the N -terminal Cys to give sulfenic, sulfinic and sulfonic acids, and disulfide bond cleavage. Limited oxidation occurs, with higher oxidant excesses, at Trp and His residues to give mono- and di- (for Trp) oxygenated products. Site-specific backbone cleavage also occurs between Arg and Trp, probably via initial side-chain modification. Treatment of the previously-oxidised peptide with thiols (GSH, N -Ac-Cys), results in adduction of the thiol to the oxidised peptide, with this occurring at the original disulfide bond. This gives an open-chain peptide, and a new mixed disulfide containing GSH or N -Ac-Cys as determined by mass spectrometry. Disulfide bond oxidation may therefore markedly alter the structure, activity and function of disulfide-containing proteins, and provides a potential mechanism for protein glutathionylation. Image 1 • Disulfide bonds play a key role in stabilizing proteins by cross-linking secondary structure. • Kinetic, mechanistic and product data are presented for HOCl-induced disulfide oxidation. • Low HOCl levels give disulfide cleavage and oxy acids via reactive intermediates. • The intermediates react with thiols to give adducts at the former disulfide bond. • Disulfide bond oxidation provides a novel pathway to peptide glutathionylation. [ABSTRACT FROM AUTHOR]

Dimedone is a widely used reagent to assess the redox state of cysteine‐containing proteins as it will alkylate sulfenic acid residues, but not sulfinic acid residues. While it has been reported that dimedone can label selenenic acid residues in selenoproteins, we investigated the stability, and reversibility of this label in a model peptide system. We also wondered whether dimedone could be used to detect seleninic acid residues. We used benzenesulfinic acid, benzeneseleninic acid, and model selenocysteine‐containing peptides to investigate possible reactions with dimedone. These peptides were incubated with H2O2 in the presence of dimedone and then the reactions were followed by liquid chromatography/electrospray ionization mass spectrometry (LC/ESI‐MS). The native peptide, H‐PTVTGCUG‐OH (corresponding to the native amino acid sequence of the C‐terminus of mammalian thioredoxin reductase), could not be alkylated by dimedone, but could be carboxymethylated with iodoacetic acid. However the "mutant peptide," H‐PTVTGAUG‐OH, could be labeled with dimedone at low concentrations of H2O2, but the reaction was reversible by addition of thiol. Due to the reversible nature of this alkylation, we conclude that dimedone is not a good reagent for detecting selenenic acids in selenoproteins. At high concentrations of H2O2, selenium was eliminated from the peptide and a dimeric form of dimedone could be detected using LCMS and 1H NMR. The dimeric dimedone product forms as a result of a seleno‐Pummerer reaction with Sec‐seleninic acid. Overall our results show that the reaction of dimedone with oxidized cysteine residues is quite different from the same reaction with oxidized selenocysteine residues. [ABSTRACT FROM AUTHOR]

Sulfinate esters were prepared by the process of activating p-toluenesulfinic acid with either cyanuric chloride, methanesulfonyl chloride, or 1-ethyl-3-(3-dimethylaminopropyl)carbodiimide hydrochloride (EDC-HCl). Activation of p-toluenesulfinic acid with cyanuric chloride led to the formation of sulfinate esters that were accompanied by the formation of the corresponding sulfones. The use of methanesulfonyl chloride for activation via methanesulfonic p-toluenesulfinic anhydride afforded mixtures of sulfinate esters and methanesulfonates. The use of the carbodiimide EDC proved to yield the best results with the highly selective formation of the target sulfinate esters. The use of trimethylacetic p-toluenesulfinic anhydride or cyanuric chloride to achieve the synthesis of sulfinamides proved to be ineffective due to poor chemoselectivity of the nucleophilic attack on the activated p-toluenesulfinic acid anhydride. Ultimately, the use of EDC-HCl to form the sulfinamides proved to be the best pathway for synthesis. [ABSTRACT FROM AUTHOR]

The expression of the phospholipase A2 activity (aiPLA2) of peroxiredoxin 6 (Prdx6) in the cell cytoplasm is physiologically relevant for the repair of peroxidized cell membranes, but aiPLA2 assay in vitro indicates that, unlike assay at pH 4, activity at cytosolic pH is essentially absent with non-oxidized substrate. However, the addition of glutathione (GSH) to the assay medium significantly increased aiPLA2 activity at cytosolic pH, while oxidized GSH (GSSG) and several other thiols had no effect. By mass spectroscopy (ESI MS), the addition of GSH to Prdx6 paradoxically led to oxidation of its conserved Cys47 residue to a sulfinic acid. The effect of GSH on PLA2 activity was abolished by incubation under anaerobic conditions, confirming that auto-oxidation of the protein was the mechanism for the GSH effect. Analysis by circular dichroism (CD) and tryptophan fluorescence showed alterations of the protein structure in the presence of GSH. Independently of GSH, the oxidation of Prdx6 by exposure to H2O2 or the presence of oxidized phospholipid as substrate also significantly increased aiPLA2 activity at pH 7. We conclude that the oxidation of the peroxidatically active Cys47 of Prdx6 results in an increase of aiPLA2 activity at pH 7 without effect on the activity of the enzyme at pH 4.

Peroxiredoxin 2 (Prx2) is a redox enzyme that is abundantly expressed in red blood cells (RBCs) and has been the focus of clinical attention for monitoring the oxidative status. We previously developed a method to quantify the reduced and hyperoxidized forms of Prx2 in human RBCs using reverse-phase high-performance liquid chromatography (HPLC). In the present study, we investigated the hyperoxidative status of Prx2 at the molecular level in a post-translational modification analysis using a liquid chromatography-tandem mass spectrometry (LC-MS/MS) system. The LC-MS/MS analysis of the trypsin digests of Prx2 fractionated by reverse-phase HPLC demonstrated that the cysteine-51 residue (Cys-51) of the protein was modified with the hyperoxidative functional groups, sulfinic acid (-SOH) and sulfonic acid (-SOH), in RBCs treated with tert-butyl hydroperoxide ( t-BHP). Furthermore, a selected ion monitoring (SIM) analysis quantitatively showed that sulfinic acid- and sulfonic acid-induced modifications in Prx2 Cys-51 were increased by the treatment with the oxidant. It was demonstrated that the peroxidatic cysteine of Prx2 separated using our HPLC system for oxidative monitoring was hyperoxidized into sulfinic acid and sulfonic acid in RBCs under an oxidative stress condition. [ABSTRACT FROM AUTHOR]

A simple and mild Mn(OAc)3-promoted oxidative coupling of 1,4-dimethoxybenzenes with sodium and lithium sulfinates was developed. The reaction proceeded readily at room temperature in air, and various sulfones were synthesized in moderate to high yields. In addition, a straightforward approach for the conversion of organolithium reagents and sulfur dioxide into sulfonylated 1,4-dimethoxybenzenes was explored. [ABSTRACT FROM AUTHOR]

The reaction of thiols with H2O2 is central to many processes essential to life, from protein folding to redox signaling. The initial products are assumed to be sulfenic acids, but their observation, and the kinetic and mechanistic characterization of their subsequent reactions, has proven challenging. The introduction of a 9-fluorotriptycene substituent enabled the use of 19F NMR to directly monitor the reaction of a thiol with H2O2 to yield a sulfenic acid, and its subsequent oxidation to sulfinic and sulfonic acids. The oxidations are specific base catalyzed, as revealed by the lack of isotope effects and the dependence of the kinetics on pH but not buffer concentration. [ABSTRACT FROM AUTHOR]