Reactive sulfur species: kinetics and mechanism of the equilibrium between cysteine sulfenyl thiocyanate and cysteine thiosulfinate ester in acidic aqueous solution

The kinetics and mechanism of the hydrolysis of cysteine sulfenyl thiocyanate (CySSCN) to give cysteine thiosulfinate ester (CyS(=O)SCy) have been investigated between pH 0 and 4. The reaction is reversible. The hydrolysis of CySSCN is second-order in [CySSCN] and inverse first-order in [H+] and [SCN-]. The following mechanism is proposed for the hydrolysis of CySSCN (where the charge depends upon the pH): CySSCN0/+ + H2O <==>CySOH0/+ + SCN- + H+, CySOH0/+ + CySSCN0/+ --> CyS(=O)SCy0/+/2+ + SCN- + H+; k1 = 3.36 +/- 0.01 x 10-3 s-1, K1k2 = 0.13 +/- 0.05 Ms-1 (which yields k2/k-1 = 39 M). The observed rate law rules out alternative mechanisms for 1 0.4 M). The following mechanism is proposed: CyS(=O)SCy2+ + H+ <==> CyS(OH)=SCy3+, Ka; CyS(OH)SCy3+ + SCN- --> CySOH+ + CySSCN+, k-2 = 0.239 +/- 0.007 M-2s-1/Ka M-1. Since cysteine sulfenic acids are known to play an important function in many enzymes, and SCN- exists in abundance in physiologic fluids, we discuss the possible role of sulfenyl thiocyanates in vivo.     

Reactive sulfur species: hydrolysis of hypothiocyanite to give thiocarbamate-S-oxide

Hypothiocyanite (OSCN-) hydrolyzes under alkaline conditions to give thiocarbamate-S-oxide (H2NC(=O)SO-, the conjugate base of carbamothioperoxoic acid) via a mechanism that involves rate-limiting nucleophilic attack of OH- on OSCN-, followed by fast protonation (with no net consumption of H+/OH- at pH 11.7). Thiocarbamate-S-oxide has been characterized by 13C NMR, 15N NMR, UV spectroscopy, and ion chromatography. It has also been independently synthesized by the reaction of thiocarbamate (H2NC(=O)S-) and hypochlorite (OCl-). The properties of thiocarbamate-S-oxide that is produced by hydrolysis of OSCN- and by oxidation of H2NC(=O)S- are the same. The possible relevance of thiocarbamate-S-oxide in human peroxidase defense mechanisms remains to be explored.     

Anion-exchange studies of metal thiocyanates in aqueous and mixed solvent systems

The anion-exchange behaviour of metal ions in aqueous NH(4)CNS and aqueous NH(4)CNS-organic solvent mixtures has been studied. The effect of the pH and the concentrations of thiocyanate and organic solvent on the distribution coefficients has been investigated. Fifteen binary metal ion separations are reported.     

Acyl thiocyanates—III: The synthesis of stable Aroyl thiocyanates

Two methods are devised for the synthesis of aroyl thiocyanates. One method is based on the thermal breakdown of 5-aroylthio-1,2,3,4-thiatriazoles to aroyl thiocyanates, nitrogen and sulfur. The other method is based on the reaction between thioacid salts and cyanogen halides. The latter synthesis also gives rise to diacyldisulfides. A hypothesis for the disulfide formation is advanced. These investigations have revealed that 2,4,6-tribromobenzoyl- and 2,4,6-tridobenzoyl thiocyanate are stable compounds which can be kept at room temperature for prolonged periods without decomposition, while 2,6-dimethyl-4-nitrobenzoyl and 3,4,5-trimethoxybenzoyl thiocyanate are of intermediate stability. The former thiocyanates are the first examples of stable acyl thiocyanates.     

The sulfur chemistry of shiitake mushroom

Allium herbs, such as Chinese chive, garlic, and onion, share a common sulfur biochemistry that occurs on cell breakage. Sulfoxide precursors are converted enzymatically to sulfenic acid intermediates and thence to a variety of pungent and in some cases noxious sulfur species that probably act to deter herbivores. Very similar biochemistry has been proposed to occur in shiitake mushrooms. Prior to the present work, our understanding of the sulfur biochemistry of these plants and fungi has been derived largely from conventional analysis procedures. We have used in situ sulfur K-edge X-ray absorption spectroscopy in intact and disrupted allium plants and shiitake mushroom. The expected changes in sulfur forms following cell breakage are indeed observed for the alliums, but no significant changes occur for the fungus. Thus, any changes involving the sulfur-containing compounds of shiitake mushroom following cell breakage occur to a far smaller extent than those involving allium plants, presumably reflecting the need in shiitake for action by multiple enzymes, namely a gamma-glutamyl transpeptidase and a C-S lyase. The shiitake C-S lyase occurs in far lower concentrations than the corresponding enzyme in garlic. Furthermore, cleavage of the flavorant precursor by the shiitake C-S lyase is reported to cease before cleavage of the precursor has been completed, presumably due to a product or suicide inhibition mechanism.     

Reactive sulfur species: kinetics and mechanisms of the oxidation of cysteine by hypohalous acid to give cysteine sulfenic acid

Cysteine sulfenic acid has been generated in alkaline aqueous solution by oxidation of cysteine with hypohalous acid (HOX, X = Cl or Br). The kinetics and mechanisms of the oxidation reaction and the subsequent reactions of cysteine sulfenic acid have been studied by stopped-flow spectrophotometry between pH 10 and 14. Two reaction pathways were observed: (1) below pH 12, the condensation of two sulfenic acids to give cysteine thiosulfinate ester followed by the nucleophilic attack of cysteinate on cysteine thiosulfinate ester and (2) above pH 10, a pH-dependent fast equilibrium protonation of cysteine sulfenate that is followed by rate-limiting comproportionation of cysteine sulfenic acid with cysteinate to give cystine. The observation of the first reaction suggests that the condensation of cysteine sulfenic acid to give cysteine thiosulfinate ester can be competitive with the reaction of cysteine sulfenic acid with cysteine.     

Polarographic reduction of aqueous sulfur dioxide

Direct- and alternating-current polarograms of aqueous SO₂ · OH₂ solutions show four reduction waves, more than previously reported. Waves I and II probably result from the electroreduction of SO₂ · OH₂ and HSO⁻₃, respectively; these two waves completely overlap at pH 1 but are partially resolved at higher pH values due to different pH dependence. Reduction of SO₂ · OH₂ involves two electrons and two H⁺ ions and the initial product is probably sulfoxylic acid, H₂SO₂. This product can disproportionate to S⁰ and SO₂ · OH₂ in very acidic media (pH ≤ 1) and, in the limit, double the reduction current of SO₂ · OH₂. Reduction of HSO⁻₃ appears to occur via two paths: one is a two-electron three-H⁺ ion path and the other is a one-electron one-H⁺ ion path. The former dominates at pH ≤ 3 and probably produces H₂SO₂; the latter dominates at pH > 4 and may produce SO⁻₂. H₂SO₂ in less acidic media can react with HSO⁻₃ to yield dithionite species (such as H₂S₂O₄, HS₂O⁻₄ and S₂O²⁻₄) and HSO₂ and SO⁻₂ by dissociation of the dithionite species. Waves III and IV are believed to result from reduction of HSO₂ and SO⁻₂, respectively, to H₂SO₂ species.     

Reactive halogen chemistry in the troposphere

Halogen chemistry is well known for ozone destruction in the stratosphere, however reactive halogens also play an important role in the chemistry of the troposphere. In the last two decades, an increasing number of reactive halogen species have been detected in a wide range of environmental conditions from the polar to the tropical troposphere. Growing observational evidence suggests a regional to global relevance of reactive halogens for the oxidising capacity of the troposphere. This critical review summarises our current understanding and uncertainties of the main halogen photochemistry processes, including the current knowledge of the atmospheric impact of halogen chemistry as well as open questions and future research needs.     

Reactive sulfur species: kinetics and mechanisms of the reaction of cysteine thiosulfinate ester with cysteine to give cysteine sulfenic acid

The kinetics and mechanisms of the reaction of cysteine with cysteine thiosulfinate ester in aqueous solution have been studied by stopped-flow spectrophotometry between pH 6 and 14. Two reaction pathways were observed for pH > 12: (1) an essentially pH-independent nucleophilic attack of cysteinate on cysteine thiosulfinate ester, and (2) a pH-dependent fast equilibrium protonation of cysteine sulfenate that is followed by rate-limiting comproportionation of cysteine sulfenic acid with cysteinate to give cystine. For 6 < pH < 12, the rate-determining reaction between cysteinate and cysteine thiosulfinate ester becomes pH-dependent due to the protonation of their amine groups. Hydrolysis of cysteine thiosulfinate ester does not play a role in the aforementioned mechanisms because the rate-determining nucleophilic attack by hydroxide is relatively slow.     

Revisiting nucleophilic substitution reactions: microwave-assisted synthesis of azides, thiocyanates, and sulfones in an aqueous medium

A practical, rapid, and efficient microwave (MW) promoted synthesis of various azides, thiocyanates, and sulfones is described in an aqueous medium. This general and expeditious MW-enhanced nucleophilic substitution approach uses easily accessible starting materials such as halides or tosylates in reaction with alkali azides, thiocyanates, or sulfinates in the absence of any phase-transfer catalyst, and a variety of reactive functional groups are tolerated.     

Galvanochemical oxidation of thiocyanates

Method for galvanochemical purification of thiocyanate-containing wastewater, with hydrogen peroxide as oxidizing agent, was suggested. The optimal conditions were found and kinetic parameters of the process were calculated.     

Atmospheric oxidation of reduced sulfur species

The kinetics of elementary gas phase reactions involved in the oxidation of reduced sulfur species, H₂S, CS₂, OCS, CH₃SH, CH₃SCH₃, and CH₃SSCH₃, to SO₂ (or other products) are reviewed. The reactions with OH and NO₃ which are the processes that initiate the degradation of the above compounds have been evaluated. Reactions of key intermediates, HS, HSO, CH₃S, and CH₃SO, are discussed. Whenever possible, recommendations for the rate coefficients are made and the need for further work indicated. The review has been carried out with the atmospheric chemistry in mind by looking at the laboratory based kinetics data. This review also provides information that will help model the Earth's sulfur cycle.     

活性氧刺激响应纳米载体

纳米载体一般是由天然高分子或人工合成高分子组成的、纳米级范畴的运输系统,具有减少药物毒性、提高药物的靶向性、增加药物有效性等优点。随着生物医学技术的进步,有研究表明,作为氧化代谢产物的活性氧(ROS)在疾病部位常常伴随着过表达的异常现象。基于此,近年来ROS刺激响应纳米载体获得了关注和发展,以不同响应机制的ROS响应基团为基础,发展了一系列的ROS响应纳米载体,实现了疾病部位ROS刺激下的药物特异性可控释放。该文聚焦于近年来常用于纳米载体的ROS响应基团,依据元素划分为两大类:硫族元素类响应基团(硫醚、缩硫酮、硒化物、二硒化物、碲化物)和其他元素类响应基团(芳香硼酸酯、过氧草酸酯、二茂铁);通过不同的设计理念将其引入纳米载体,根据ROS响应纳米载体的不同响应机制(疏水-亲水相变、断裂),探讨了载体各自的ROS响应情况、体外药物释放情况,以及在活体中的应用情况。     

Chemical precipitation of sulfur nanoparticles from aqueous solutions

Process of formation and coarsening of sulfur nanoparticles deposited from aqueous solutions of calcium and sodium polysulfides and sodium thiosulfate and from solutions of elementary sulfur in a mixture of hydrazine hydrate and monoethanolamine was considered. The sulfur nanoparticles were characterized with a laser analyzer of particle sizes, X-ray diffraction analysis, and electron probe microscopy. It was shown that the sizes of primary particles strongly depend on the concentration of an acid and on the molar ratio between the sulfur-containing solutions and the acid. The antifungal effects of sulfur with nano- and micrometer dimensions were compared. The experimental results demonstrated a strong antifungal effect of sulfur nanoparticles on various kinds of fungi.     

2-Trimethylsilylethyl sulfides in the von Braun cyanogen bromide reaction: selective preparation of thiocyanates and application to nucleoside chemistry

Mixed 2-(trimethylsilyl)ethyl sulfides were synthesized and used in the von Braun cyanogen bromide reaction for preparing selectively thiocyanates in high yield. We show here that this cleavage reaction is highly selective in methanol in comparison with the reaction of the corresponding non-silyl sulfide analogues. This reaction was applied to the synthesis of nucleosidic thiocyanates such as the new nucleosides 14 and 18 in the search for mechanism-based inhibitors of ribonucleoside diphosphate reductase and bioactive molecules. The selective cleavage is possible for sulfides bearing hydroxyl functions and aromatic rings. The reactions of cyanogen bromide as cyanating and brominating agent were observed for the first time under the same conditions with the naphthoxyhexyl 2-trimethylsilylethyl sulfide 7, which, treated with cyanogen bromide in dichloromethane, led selectively to the p-bromonaphthoxyhexyl thiocyanate 10 in 89% yield. Another reaction induced by cyanogen bromide was observed in dichloromethane with the 2-(trimethylsilylethyl)thio nucleoside 13, which gives the corresponding symmetrical disulfide 21 in good yield.     

Acid-base chemistry of omeprazole in aqueous solutions

Omeprazole is a potent anti-acid drug. Its absorption and mode of action are closely related to its prototropic behavior. In the present study, omeprazole samples from different sources and in different forms were studied spectrophotometrically to obtain pK_a values. In the neutral to alkaline pH region, two consistent pK_a values of 7.1 and 14.7 were obtained from various samples. The assignment of these pK_a values was realized by comparison with the prototropic properties of N(1)-methylated omeprazole substituted on the nitrogen at the 1-position of the benzimidazole ring, which was found to have a pK_a of 7.5. The omeprazole pK_a of 14.7 is assigned to the dissociation of the hydrogen from the 1-position of the benzimidazole ring and the pK_a of 7.1 is assigned to the dissociation from the protonated pyridine nitrogen of omeprazole. The results presented are at variance with those of earlier work.