Sulfinic Acid Research Articles

Backbone resonance assignment of human DJ-1 in the reduced state and in the cysteine sulfinic acid state.

DJ-1 is a highly conserved soluble protein that is associated to several cellular pathways. In humans, DJ-1 has been implicated in several pathologies such as cancer, Parkinson's disease and amyotrophic lateral sclerosis. Several roles have been attributed to DJ-1, including defense against oxidative stress, chaperone activity and proteasome regulation. The recent finding that DJ-1 acts as a protein and DNA deglycase further confirms the protective function of DJ-1 and suggests a common mechanism of action in the various pathways in which DJ-1 is involved. Cysteine 106, located in the putative active site of DJ-1, is critical for the biological activity of DJ-1 and is easily oxidized to cysteine-sulfinate. While such oxidation modulates DJ-1 activity, the underlying molecular mechanism has not yet been elucidated. Cysteine oxidation does not perturb the protein structure, therefore changes in protein dynamics in solution could modulate its function. Here, we report a revised and completed (98%) backbone assignment of reduced DJ-1, together with the backbone assignment of oxidized DJ-1. Chemical shift perturbation is observed in several regions across the sequence, while no changes in secondary structure are observed. These data will provide the starting point for further characterization of the changes in the backbone dynamics of DJ-1 upon oxidation in solution at physiological temperature.

A straightforward bioprocess for a cleaner paper decolorization

Abstract A new biotechnological sequence for decolorizing red and black colored paper was developed to reduce the environmental impact of the chemicals used in paper recycling processes. Commercially available low-redox potential laccase from Myceliophthora thermophila, which operates optimally under alkaline conditions, was used in combination with natural mediators to make the process even greener. Based on the optical properties of the resulting decolorized paper, red and black dyes were efficiently removed by all laccase–mediator systems. The best results were provided by the laccase–methyl syringate combination, followed by the laccase–acetosyringone system and the laccase–syringaldehyde system. The decolorization rate for red paper achieved with the laccase–methyl syringate treatment exceeded that obtained with ozone. Red was removed by about 98% by combining two enzymatic stages and hydrogen peroxide stage, and black by 65%, without altering the physical properties of the colored paper in either case. A sequence combining oxidative and reductive (formamidine sulfinic acid) chemical treatments led to comparable optical and physical properties for the two types of paper. The effects of Myceliophthora thermophila laccase and methyl syringate were similar to those of high-redox potential laccase from Trametes villosa combined with either methyl syringate or the synthetic mediator violuric acid.

Conserved N-terminal cysteine dioxygenases transduce responses to hypoxia in animals and plants.

Organisms must respond to hypoxia to preserve oxygen homeostasis. We identify a thiol oxidase, previously assigned as cysteamine (2-aminoethanethiol) dioxygenase (ADO), as a low oxygen affinity (high-K mO2) amino-terminal cysteine dioxygenase that transduces the oxygen-regulated stability of proteins by the N-degron pathway in human cells. ADO catalyzes the conversion of amino-terminal cysteine to cysteine sulfinic acid and is related to the plant cysteine oxidases that mediate responses to hypoxia by an identical posttranslational modification. We show in human cells that ADO regulates RGS4/5 (regulator of G protein signaling) N-degron substrates, modulates G protein-coupled calcium ion signals and mitogen-activated protein kinase activity, and that its activity extends to other N-cysteine proteins including the angiogenic cytokine interleukin-32. Identification of a conserved enzymatic oxygen sensor in multicellular eukaryotes opens routes to better understanding and therapeutic targeting of adaptive responses to hypoxia.

Oxygen sensing across kingdoms

Biochemistry The ability to sense and respond to changes in oxygen levels is critical for most forms of life. To date, mechanistic studies of this process in mammals have focused on the oxygen-sensitive stability of a transcription factor called hypoxia-inducible factor. Masson et al. discovered an enzymatic oxygen sensor in humans that is functionally identical to plant cysteine oxidases, enzymes that control responses to hypoxia in plants. The human and plant enzymes convert the N-terminal cysteine in substrate proteins to cysteine sulfinic acid, a modification that ultimately targets the proteins for degradation. Oxygen sensing is impaired in many human diseases, and further study of the human enzyme could help in the development of strategies for therapeutic intervention. Science , this issue p. [65][1] [1]: /lookup/doi/10.1126/science.aaw0112

Structural basis for the modulation of plant cytosolic triosephosphate isomerase activity by mimicry of redox-based modifications.

Reactive oxidative species (ROS) and S-glutathionylation modulate the activity of plant cytosolic triosephosphate isomerases (cTPI). Arabidopsis thaliana cTPI (AtcTPI) is subject of redox regulation at two reactive cysteines that function as thiol switches. Here we investigate the role of these residues, AtcTPI-Cys13 and At-Cys218, by substituting them with aspartic acid that mimics the irreversible oxidation of cysteine to sulfinic acid and with amino acids that mimic thiol conjugation. Crystallographic studies show that mimicking AtcTPI-Cys13 oxidation promotes the formation of inactive monomers by reposition residue Phe75 of the neighboring subunit, into a conformation that destabilizes the dimer interface. Mutations in residue AtcTPI-Cys218 to Asp, Lys, or Tyr generate TPI variants with a decreased enzymatic activity by creating structural modifications in two loops (loop 7 and loop 6) whose integrity is necessary to assemble the active site. In contrast with mutations in residue AtcTPI-Cys13, mutations in AtcTPI-Cys218 do not alter the dimeric nature of AtcTPI. Therefore, modifications of residues AtcTPI-Cys13 and AtcTPI-Cys218 modulate AtcTPI activity by inducing the formation of inactive monomers and by altering the active site of the dimeric enzyme, respectively. The identity of residue AtcTPI-Cys218 is conserved in the majority of plant cytosolic TPIs, this conservation and its solvent-exposed localization make it the most probable target for TPI regulation upon oxidative damage by reactive oxygen species. Our data reveal the structural mechanisms by which S-glutathionylation protects AtcTPI from irreversible chemical modifications and re-routes carbon metabolism to the pentose phosphate pathway to decrease oxidative stress.

Polymerization of Oxidized DJ-1 via Noncovalent and Covalent Binding: Significance of Disulfide Bond Formation.

The reactive cysteine residue at position 106 (Cys106) of DJ-1 is preferentially oxidized under oxidative stress, generating oxidized DJ-1 (oxDJ-1). Oxidation of Cys106 to sulfinic acid changes the biologic action of DJ-1 and increases its cytoprotective properties. The similar activation step is known in peroxiredoxins (Prxs), in which oxidation of reactive Cys to sulfinic acid induces polymerization of Prxs and changes its enzyme characteristic from peroxidase to molecular chaperone. In the present study, oxDJ-1 was prepared and its polymerization and related amino acid residues were investigated. We found that oxDJ-1 formed a characteristic polymer with disulfide bonds and with noncovalent and covalent binding other than disulfide. The physiological concentration of glutathione resolved the polymer form of oxDJ-1, and glutathionylation of other two Cys residues, such as Cys 46 and 53, was detected. Mutant analysis indicated the necessity not only of Cys106 but also of Cys46 for the polymer formation. The cellular experiment demonstrated that the electrophilic quinone treatment induced a high-molecular-weight complex containing oxDJ-1. Dynamic polymerization of oxDJ-1 with a ring and a stacked structure was observed by an atomic force microscope. Collectively, these results clearly demonstrated the characteristic polymer formation of oxDJ-1 with a disulfide bond and noncovalent and covalent binding other than disulfide, which might be related to the biologic function of oxDJ-1.

Taurine metabolism and effects of inclusion levels in rotifer (Brachionus rotundiformis, Tschugunoff, 1921) on Atlantic bluefin tuna (Thunnus thynnus, L.) larvae

Taurine appears to be a crucial nutrient for teleosts, especially top predator species such as Atlantic bluefin tuna (Thunnus thynnus, L.; ABT). While dietary taurine supplementation has been highly recommended, there is a lack of studies on taurine assimilation and biosynthesis for this iconic species. The present study aims to provide insight into the molecular mechanisms involved in taurine biosynthesis and transport in ABT by studying tissue distribution and ontogenetic development of expression of cysteine dioxygenase (cdo), cysteine sulfinic acid decarboxylase (csad), 2-aminoethanethiol dioxygenase (ado) and taurine transporter (tauT) in response to graded levels of dietary taurine supplementation. The full open reading frame (ORF) for cdo and partial sequences for csad, ado and tauT were obtained, with the translated polypeptides being 202, 176, 166 and 324 amino acids, respectively. All three showed characteristics such as cupin motifs in Cdo and predicted N-glycosylation sites in Taut that are common to these genes in other species. Phylogenetic analysis showed that the ABT sequences clustered with sequences of other teleosts, and separately from mammals and molluscs. Tissue distribution varied, with adipose tissue, kidney, white muscle and testis/brain showing highest expression of cdo, csad, ado and tauT, respectively. Whole larvae expression of csad peaked at 15 dah, whereas the other genes generally increased throughout development to show highest expression at 25 dah. The nutritional trial was carried out by feeding ABT larvae from mouth opening to 14 days after hatching (dah) with rotifers (Brachionus rotundiformis) enriched with 4 different levels of taurine: 0.0 (tau0), 0.5 (tau0.5), 1.0 (tau1), and 2.0 g taurine per 106 rotifers (tau2). Rotifers effectively accumulated taurine with ABT larvae fed on treatment tau2 attaining the highest concentration of taurine. However, ABT larvae fed tau1 displayed higher growth and survival, and flexion index at 14 dah, than larvae fed the other taurine levels. Larvae fed tau1 also showed generally higher expression of tauT and cdo and digestive and antioxidant enzyme genes. While this study showed that larval ABT express taurine metabolism genes, suggesting possible synthesis that could contribute to the taurine pool in the fish, larval performance was enhanced by a level of dietary taurine (3.7 mg taurine g−1 rotifer) supplied by enrichment of rotifers at 1 g taurine per 106 rotifers.

Palladium(II) Ion Mediated Disulfide/Thiolate Interconversion: Predicting the Disulfide Group State from First Principles.

Different reactivity of homologous disulfides toward Pd2+ was previously reported: stepwise complexation to Pd2+ for l-cystine and cystamine ligands, while for dl-homocystine and 3,3'-dithiodipropionic acid, disulfide's disproportionation toward thiolate and sulfinic acid complexes is observed. The disulfide/thiolate interconversion of four different disulfide ligands in the presence of nonredox metal cation Pd2+ in aqueous solution has been computationally investigated. We see this different reactivity in different capacities of considered homologous disulfides to stabilize forming S,S'-binuclear complexes, which are believed to be key intermediates toward interconversion products. We thus devise a theoretical model that rationalizes experimentally observed phenomenon of disulfides different reactivity toward nonredox transition metal cation Pd2+.

Cysteine dioxygenase 1 is a metabolic liability for non-small cell lung cancer.

NRF2 is emerging as a major regulator of cellular metabolism. However, most studies have been performed in cancer cells, where co-occurring mutations and tumor selective pressures complicate the influence of NRF2 on metabolism. Here we use genetically engineered, non-transformed primary murine cells to isolate the most immediate effects of NRF2 on cellular metabolism. We find that NRF2 promotes the accumulation of intracellular cysteine and engages the cysteine homeostatic control mechanism mediated by cysteine dioxygenase 1 (CDO1), which catalyzes the irreversible metabolism of cysteine to cysteine sulfinic acid (CSA). Notably, CDO1 is preferentially silenced by promoter methylation in human non-small cell lung cancers (NSCLC) harboring mutations in KEAP1, the negative regulator of NRF2. CDO1 silencing promotes proliferation of NSCLC by limiting the futile metabolism of cysteine to the wasteful and toxic byproducts CSA and sulfite (SO32-), and depletion of cellular NADPH. Thus, CDO1 is a metabolic liability for NSCLC cells with high intracellular cysteine, particularly NRF2/KEAP1 mutant cells.

Identifying Reducing and Capping Sites of Protein-Encapsulated Gold Nanoclusters.

The reducing and capping sites along with their local structure impact photo properties of the red bovine serum albumin-capped Au nanocluster (BSA-AuNC), however, they are hard to identify. We developped a workflow and relevant techniques using mass spectrometry (MS) to identify the reducing and capping sites of BSA-AuNCs involved in their formation and fluorescence. Digestion without disulfide cleavages yielded an Au core fraction exhibiting red fluorescence and [AunSm] ion signals and a non-core fraction exhibiting neither of them. The core fraction was identified to mainly be comprised of peptides containing cysteine residues. The fluorescence and [AunSm] signals were quenched by tris(2-carboxyethyl)phosphine, confirming that disulfide groups were required for nanocluster stabilization and fluorescence. By MS sequencing, the disulfide pairs, C75–C91/C90–C101 in domain IA, C315–C360/C359–C368 in domain IIB, and C513–C558/C557–C566 in domain IIIB, were identified to be main capping sites of red AuNCs. Peptides containing oxidized cysteines (sulfinic or cysteic acid) were identified as reducing sites mainly in the non-core fraction, suggesting that disulfide cleavages by oxidization and conformational changes contributed to the subsequent growth of nanoclusters at nearby intact disulfide pairs. This is the first report on precise identification of the reducing and capping sites of BSA-AuNCs.

Recent Advances in Electrochemical Oxidative Cross-Coupling for the Construction of C–S Bonds

With the importance of sulfur-containing organic molecules, developing methodologies toward C–S bond formation is a long-standing goal, and, to date, considerable progress has been made in this area. Recent electrochemical oxidative cross-coupling reactions for C–S bond formation allow the synthesis of sulfur-containing molecules from more effective synthetic routes with high atom economy under mild conditions. In this review, we highlight the vital progress in this novel research arena with an emphasis on the synthetic and mechanistic aspects of the organic electrochemistry reactions.1 Introduction2 Electrochemical Oxidative Sulfonylation for the Formation of C–S Bonds2.1 Applications of Sulfinic Acid Derivatives for the Formation of C–S Bonds2.2 Applications of Sulfonylhydrazide Derivatives for the Formation of C–S Bonds3 Electrochemical Oxidative Thiolation for the Formation of C–S Bonds3.1 Applications of Disulfide Derivatives for the Formation of C–S Bonds3.2 Applications of Thiophenol Derivatives for the Formation of C–S Bonds4 Electrochemical Oxidative Thiocyanation for the Formation of C–S Bonds5 Electrochemical Oxidative Cyclization for the Formation of C–S Bonds6 Conclusion

A synthetic cdo/csad taurine pathway in the green unicellular alga Chlamydomonas reinhardtii

Synthetic taurine is an important animal feed supplement critical for fish and cat nutrition. However, there is a demand for natural and renewable sources of this nutraceutical additive. The objective of this study was to increase the production of taurine using genetically engineered Chlamydomonas reinhardtii modified to have the cysteine dioxygenase/cysteine sulfinic acid decarboxylase (CDO/CSAD) taurine synthesis pathway from animals. Because unicellular green microalgae produce taurine at low levels, methods to increase taurine content may qualify the use of these microbes as a feed supplement. Here, increased microalgal taurine content was accomplished using a synthetic approach based on addition of the CDO/CSAD taurine biosynthetic pathway from animals into the chloroplasts of Chlamydomonas reinhardtii. To optimize synthesis, genes were codon optimized and transformed as either separate genes (cdo::csad) or as a fusion protein (cdo-csad). Genotyping confirmed the identity of stable homoplasmic cdo::sad and cdo-csad transformants while HPLC-MS was used to identify pathway intermediates and the end product taurine. While the cdo-csad fusion transformants produced only the first taurine intermediate, cysteine sulfinic acid (CSA), the cdo::csad transformants produced CSA, hypotaurine, and taurine indicating successful synthetic expression of the animal taurine pathway in microalgae. Unsuccessful expression of the fusion construct explained the lack of end product formation and resulted from premature transcript cleavage as indicated by Northern and Western blot analysis. Furthermore, culture supplementation with the pathway precursors, cysteine and methionine, improved production of CDO/CSAD pathway intermediates. These results support the feasibility of using engineered microalgae coupled with culture supplementation to meet taurine requirements for use as an animal feed supplement.

Impact of Mutations in the DEXD‐motif of the Yeast RNA‐dependent ATPase Sub2

Pre‐mRNA splicing by the spliceosome is an essential part of eukaryotic gene expression. Errors in splicing can be detrimental to an organism, which is why it is extremely important to understand the spliceosome's mechanisms of assembly and action. Key features of splicing include the numerous RNA‐protein remodeling and RNA unwinding reactions that aid in spliceosome assembly. Some of these reactions require one of three, well‐conserved DEAD‐motif proteins that are part of the splicing machinery (Prp5/DDX46, Prp28/DDX23, and Sub2/UAP56). While Prp5 and Prp28 contain the canonical DEAD tetrapeptide motif within their active site, Sub2 is unusual in that it instead contains a conserved DECD motif. The goal of my research is to understand the function of the cysteine within Sub2's DECD motif in vitro and in vivo. Contrary to previous reports, I have shown that yeast containing a DECD to DEAD mutation in Sub2 are viable with little impact on yeast proliferation. To explain the conservation of Sub2's DECD sequence, we looked for changes in yeast phenotypes under stress conditions. I exposed yeast containing either DEAD‐Sub2 or DECD‐Sub2 to hydrogen peroxide and found that yeast containing DECD‐Sub2 died at lower oxidative stress conditions than the DEAD mutant. This suggests that Sub2's DECD‐motif cysteine is capable of being oxidized and that this results in a disruption in gene expression. Since one cysteine oxidation product (sulfinic acid) is isosteric with aspartate, I am now testing a DEDD mutant of Sub2 for function as well as attempting to chemically characterize the amino acid modifications resulting from Sub2 oxidation. Together my results will provide insight into a potential function for Sub2's conserved DECD‐motif cysteine.This abstract is from the Experimental Biology 2019 Meeting. There is no full text article associated with this abstract published in The FASEB Journal.

Examining the Metabolic Role of the Pseudomonas aeruginosa MDO Operon

Mammalian cysteine dioxygenase (CDO) is a mononuclear iron enzyme that catalyzes the oxidation of L‐cysteine to L‐cysteine sulfinic acid. It contains an amino acid‐derived cofactor formed between residues Cys93‐Tyr157. Putative CDO enzymes have been identified in several bacteria; however, all known bacterial CDO enzyme homologs lack the Cys‐Tyr amino acid‐derived cofactor. 1Although the MDO enzyme has been proposed to convert 3‐mercaptopropionate (3‐MPA) to 3‐sulfinopropionate (3‐SPA), the physiological function of either 3‐MPA or 3‐SPA remains unclear. 2,3In numerous bacteria, the MDO gene is on the same operon as an annotated sulfurtransferase, and the operon is expressed during bacterial sulfur limitation. The sulfurtransferase has amino acid sequence similarity to mercaptopyruvate sulfurtransferase enzymes.Different thiol‐containing substrates were evaluated to support a defined metabolic pathway involving both MDO and the sulfurtransferase. MDO gave similar catalytic parameters with both 3‐MPA and mercaptopyruvate, suggesting that both are viable substrates for the enzyme. However, the specificity of MDO for mercaptopyruvate correlates with the classification of the coexpressed sulfurtransferase. Therefore, MDO and the sulfurtransferase enzymes may be metabolically linked, and optimal catalytic function could involve protein‐protein interactions between the two enzymes. Potential protein‐protein interactions between MDO and the sulfurtransferase were identified by surface plasmon resonance, and hydrogen‐deuterium exchange was performed to further identify where the interaction sites of MDO and the sulfurtransferase are located. Coupling of MDO with the sulfurtransferase was also investigated to understand the functional role of the enzymes in bacteria. The proposed overall mechanism for both enzymes would result in sulfite release that could be utilized in the sulfur assimilation pathway. Based on these findings, the MDO/sulfurtransferase system represents an additional pathway for bacteria to adapt to sulfur limiting conditions that had not been previously identified.This abstract is from the Experimental Biology 2019 Meeting. There is no full text article associated with this abstract published in The FASEB Journal.

Rapid and Chemodivergent Synthesis of N-Heterocyclic Sulfones and Sulfides: Mechanistic and Computational Details of the Persulfate-Initiated Catalysis

N-Heterocyclic sulfones and sulfides are key functional motifs in medicinal and agricultural chemistry and important building blocks in organic synthesis. Currently available methods produce N-heterocyclic sulfones in low yields and under harsh conditions. Here, we describe a rapid sulfone synthesis under ambient conditions that is initiated by persulfate with water as a cosolvent. The reaction has a broad scope and is readily expanded to the chemodivergent synthesis of symmetrical N-heterocyclic sulfones and sulfides. The products can be isolated by simple filtration. Our combined experimental and computational study suggests that the remarkable persulfate-initiated acceleration of the sulfone formation in the biphasic system that enables the otherwise sluggish reaction under ambient conditions is due to the combination of the previously unknown rapid acidification of the persulfate–sulfinate system, the self-catalysis of the reaction of sulfinic acid with halo-N-heterocycles, and the substantial acceleration of the reaction in the aqueous phase.

Cysteine Dioxygenase Regulates the Epithelial Morphogenesis of Mammary Gland via Cysteine Sulfinic Acid

SummaryEpithelial morphogenesis is a common feature in various organs and contributes to functional formation. However, the molecular mechanisms behind epithelial morphogenesis remain largely unknown. Mammary gland is an excellent model system to investigate the molecular mechanisms of epithelial morphogenesis. In this study, we found that cysteine dioxygenase (CDO), a key enzyme in cysteine oxidative metabolism, was involved in mammary epithelial morphogenesis. CDO knockout (KO) females exhibited severe defects in mammary branching morphogenesis and ductal elongation, resulting in poor lactation. CDO contributes to the luminal epithelial cell differentiation, proliferation, and apoptosis mainly through its downstream product cysteine sulfinic acid (CSA). Exogenous supplementation of CSA not only rescued the defects in CDO KO mouse but also enhanced ductal growth in wild-type mouse. It suggests that CDO regulates luminal epithelial differentiation and regeneration via CSA and consequently contributes to mammary development, which raises important implications for epithelial morphogenesis and pathogenesis of breast cancer.

Hyperoxidation of Peroxiredoxin 6 Induces Alteration from Dimeric to Oligomeric State.

Peroxiredoxins(Prdx), the family of non-selenium glutathione peroxidases, are important antioxidant enzymes that defend our system from the toxic reactive oxygen species (ROS). They are thiol-based peroxidases that utilize self-oxidation of their peroxidatic cysteine (Cp) group to reduce peroxides and peroxidized biomolecules. However, because of its high affinity for hydrogen peroxide this peroxidatic cysteine moiety is extremely susceptible to hyperoxidation, forming peroxidase inactive sulfinic acid (Cys-SO2H) and sulfonic acid (Cys-SO3H) derivatives. With the exception of peroxiredoxin 6 (Prdx6), hyperoxidized sulfinic forms of Prdx can be reversed to restore peroxidase activity by the ATP-dependent enzyme sulfiredoxin. Interestingly, hyperoxidized Prdx6 protein seems to have physiological significance as hyperoxidation has been reported to dramatically upregulate its calcium independent phospholipase A2 activity. Using biochemical studies and molecular dynamic (MD) simulation, we investigated the roles of thermodynamic, structural and internal flexibility of Prdx6 to comprehend the structural alteration of the protein in the oxidized state. We observed the loosening of the hydrophobic core of the enzyme in its secondary and tertiary structures. These changes do not affect the internal dynamics of the protein (as indicated by root-mean-square deviation, RMSD and root mean square fluctuation, RMSF plots). Native-PAGE and dynamic light scattering experiments revealed the formation of higher oligomers of Prdx6 under hyperoxidation. Our study demonstrates that post translational modification (like hyperoxidation) in Prdx6 can result in major alterations of its multimeric status.

Visible-light-induced deoxygenative C2-sulfonylation of quinoline N-oxides with sulfinic acids

The first example of direct synthesis of 2-sulfonylquinolines through visible-light-induced deoxygenative C2-sulfonylation of quinoline N-oxides with organic dye as the catalyst and ambient air as the sole oxidant was developed.

Kinetics and Mechanism of Oxidation of N-acetyl-d-penicillamine in Acidified lodate and Aqueous Iodine

The oxidation of the biologically-active thiol, N-acetyl-d-penicillamine (NDPen) by acidified iodate and aqueous iodine has been studied. The stoichiometry of the reaction is 1:1 in which the thiol (RSH) is oxidized to its sulfonic acid (RSO3H) without cleavage of the C-S bond. In excess acidified iodate the reaction displayed a short induction period, followed by formation of aqueous iodine. Overall stoichiometry in excess iodate was 6:5:6IO3- + 5RSH + 6H+ ® 5 RSO3H + 3I2(aq) + 3 H2O. The direct reaction of aqueous iodine and was relatively fast, over within 100 ms and had a stoichiometry of 1:3:3 I2(aq) + RSH + 3H2O ® RSO3H + 6 I- + 6H+. The reaction is essentially diffusion-controlled and our stopped-flow with a mixing time limitation of 1.00 ms could only catch the reaction of the last iodine molecule. This reaction is, however, strongly inhibited by the product of reaction, I-. This is due to the formation of the relatively inert triiodide anion: I2(aq) + I- ® I3-. Mass spectrometry results showed that the reaction proceeds through the sulfinic acid with negligible stabilization of the sulfenic acid. In excess of reductant, the dimeric species is the favoured product due to a rapid condensation-type reaction between the electrophilic unstable sulfenic acid and unreacted thiol. Keywords: Biological thiols, bioactivation, oxidations, oxyhalogen chemistry.

Oscillations in the Electro-Oxidation of Thiourea on Platinum Electrode Studied by On-Line HPLC

The oscillations in the electro-oxidation of thiourea on platinum electrode in perchloric acid were studied by the cyclic voltammetry and potentiostatic method combined with on-line HPLC. The effects of different scanning speed, initial concentration of thiourea and sampling on the current oscillations of thiourea were investigated, results showed that: (1) the lower scanning speed and higher initial concentration of thiourea facilitated the oscillations; (2) the sampling increased the oscillation current and oxidation peak current; (3) the intermediates formamidine disulphide (TU22+) and formamidine sulphinic acid (TUO2) were detected at different potentials on the surface of platinum electrode and concentration of TU, TU22+ and TUO2 all exhibited oscillations in the potentiostatic eletrolysis. The existence of a closed feedback loop formed by a series of reactions leaded to electrochemical oscillations of thiourea. This paper provides a new perspective and method for the study of electrochemical oscillations.