Thermal and photochemical reactions of sulfhydryl radicals. Implications for colloid photocorrosion

Chemistry of Hydrogen Sulfide-Pathological and Physiological Functions in Mammalian Cells

Hydrogen sulfide (H2S) was recognized as a gaseous signaling molecule, similar to nitric oxide (-NO) and carbon monoxide (CO). The aim of this review is to provide an overview of the formation of hydrogen sulfide (H2S) in the human body. H2S is synthesized by enzymatic processes involving cysteine and several enzymes, including cystathionine-β-synthase (CBS), cystathionine-γ-lyase (CSE), cysteine aminotransferase (CAT), 3-mercaptopyruvate sulfurtransferase (3MST) and D-amino acid oxidase (DAO). The physiological and pathological effects of hydrogen sulfide (H2S) on various systems in the human body have led to extensive research efforts to develop appropriate methods to deliver H2S under conditions that mimic physiological settings and respond to various stimuli. These functions span a wide spectrum, ranging from effects on the endocrine system and cellular lifespan to protection of liver and kidney function. The exact physiological and hazardous thresholds of hydrogen sulfide (H2S) in the human body are currently not well understood and need to be researched in depth. This article provides an overview of the physiological significance of H2S in the human body. It highlights the various sources of H2S production in different situations and examines existing techniques for detecting this gas.

Autocatalytic Mechanism in the Anaerobic Reduction of Metmyoglobin by Sulfide Species

The mechanism of the metal centered reduction of metmyoglobin (MbFeIII) by sulfide species (H2S/HS-) under an argon atmosphere has been studied by a combination of spectroscopic, kinetic, and computational methods. Asymmetric S-shaped time-traces for the formation of MbFeII at varying ratios of excess sulfide were observed at pH 5.3 < pH < 8.0 and 25 °C, suggesting an autocatalytic reaction mechanism. An increased rate at more alkaline pHs points to HS- as relevant reactive species for the reduction. The formation of the sulfanyl radical (HS•) in the slow initial phase was assessed using the spin-trap phenyl N-tert-butyl nitrone. This radical initiates the formation of S-S reactive species as disulfanuidyl/ disulfanudi-idyl radical anions and disulfide (HSSH•-/HSS•2- and HSS-, respectively). The autocatalysis has been ascribed to HSS-, formed after HSSH•-/HSS•2- disproportionation, which behaves as a fast reductant toward the intermediate complex MbFeIII(HS-). We propose a reaction mechanism for the sulfide-mediated reduction of metmyoglobin where only ferric heme iron initiates the oxidation of sulfide species. Beside the chemical interest, this insight into the MbFeIII/sulfide reaction under an argon atmosphere is relevant for the interpretation of biochemical aspects of ectopic myoglobins found on hypoxic tissues toward reactive sulfur species.

Improving removal rate and efficiency of As(V) by sulfide from strongly acidic wastewater in a modified photochemical reactor

Employing ultraviolet light to enhance the removal of As(V) by sulfide (S(-II)) from strongly acidic wastewater is a potential method. However, we found the arsenic trisulfide (As₂S₃) and elemental sulfur (S₈) particles formed in this method not only vastly hinder light transmission in the wastewater but also undergo light-induced redissolution, leading to a decrease in removal rate and efficiency of As(V). Herein, As(V) removal by sulfide from strongly acidic wastewater was performed in a modified photochemical reactor to weaken the effect of the formed particles on As(V) removal. It was found that in this study, the formed particles could be efficiently removed from the photoreactor by three operations, i.e. circulation-filtration, septum setting, and lamp sleeve cleaning. The removal of As(V) was approximately 11-fold faster than that without three operations, saving 90.9% of the reaction time and 89.4% of energy consumption. The removal efficiency of As(V) also increased through weakening the light-induced redissolution of the formed particles. This study facilitates the practical application of the UV light promoted As(V) removal technology and also provides a new method to lessen the light-blocking effect in the particle-forming photochemical reaction systems.

Physiological roles of hydrogen sulfide in mammalian cells, tissues and organs

H2S belongs to the class of molecules known as gasotransmitters, which also includes nitric oxide (NO) and carbon monoxide (CO). Three enzymes are recognized as endogenous sources of H2S in various cells and tissues: cystathionine g-lyase (CSE), cystathionine β-synthase (CBS) and 3-mercaptopyruvate sulfurtransferase (3-MST). The current article reviews the regulation of these enzymes as well as the pathways of their enzymatic and non-enzymatic degradation and elimination. The multiple interactions of H2S with other labile endogenous molecules (e.g. NO) and reactive oxygen species are also outlined. The various biological targets and signaling pathways are discussed, with special reference to H2S and oxidative posttranscriptional modification of proteins, the effect of H2S on channels and intracellular second messenger pathways, the regulation of gene transcription and translation and the regulation of cellular bioenergetics and metabolism. The pharmacological and molecular tools currently available to study H2S physiology are also reviewed, including their utility and limitations. In subsequent sections, the role of H2S in the regulation of various physiological and cellular functions is reviewed. The physiological role of H2S in various cell types and organ systems are overviewed. Finally, the role of H2S in the regulation of various organ functions is discussed as well as the characteristic bell-shaped biphasic effects of H2S. In addition, key pathophysiological aspects, debated areas, and future research and translational areas are identified A wide array of significant roles of H2S in the physiological regulation of all organ functions emerges from this review.

Abiotic stress-triggered oxidative challenges: Where does H2S act?

Hydrogen sulfide (H₂S) was once principally considered the perpetrator of plant growth cessation and cell death. However, this has become an antiquated view, with cumulative evidence showing that the H₂S serves as a biological signaling molecule notably involved in abiotic stress response and adaptation, such as defense by phytohormone activation, stomatal movement, gene reprogramming, and plant growth modulation. Reactive oxygen species (ROS)-dependent oxidative stress is involved in these responses. Remarkably, an ever-growing body of evidence indicates that H₂S can directly interact with ROS processing systems in a redox-dependent manner, while it has been gradually recognized that H₂S-based posttranslational modifications of key protein cysteine residues determine stress responses. Furthermore, the reciprocal interplay between H₂S and nitric oxide (NO) in regulating oxidative stress has significant importance. The interaction of H₂S with NO and ROS during acclimation to abiotic stress may vary from synergism to antagonism. However, the molecular pathways and factors involved remain to be identified. This review not only aims to provide updated information on H₂S action in regulating ROS-dependent redox homeostasis and signaling, but also discusses the mechanisms of H₂S-dependent regulation in the context of oxidative stress elicited by environmental cues.

Hydrogen Sulfide and the Kidney: Physiological Roles, Contribution to Pathophysiology, and Therapeutic Potential

Significance: Hydrogen sulfide (H₂S), the third member of the gasotransmitter family, has a broad spectrum of biological activities, including antioxidant and cytoprotective actions, as well as vasodilatory, anti-inflammatory and antifibrotic effects. New, significant aspects of H₂S biology in the kidney continue to emerge, underscoring the importance of this signaling molecule in kidney homeostasis, function, and disease. Recent Advances: H₂S signals via three main mechanisms, by maintaining redox balance through its antioxidant actions, by post-translational modifications of cellular proteins (S-sulfhydration), and by binding to protein metal centers. Important renal functions such as glomerular filtration, renin release, or sodium reabsorption have been shown to be regulated by H₂S, using either exogenous donors or by the endogenous-producing systems. Critical Issues: Lower H₂S levels are observed in many renal pathologies, including renal ischemia-reperfusion injury and obstructive, diabetic, or hypertensive nephropathy. Unraveling the molecular targets through which H₂S exerts its beneficial effects would be of great importance not only for understanding basic renal physiology, but also for identifying new pharmacological interventions for renal disease. Future Directions: Additional studies are needed to better understand the role of H₂S in the kidney. Mapping the expression pattern of H₂S-producing and -degrading enzymes in renal cells and generation of cell-specific knockout mice based on this information will be invaluable in the effort to unravel additional roles for H₂S in kidney (patho)physiology. With this knowledge, novel targeted more effective therapeutic strategies for renal disease can be designed. Antioxid. Redox Signal. 36, 220-243.

Recovery of Re(VII) from strongly acidic wastewater using sulphide: Acceleration by UV irradiation and the underlying mechanism

Strongly acidic wastewater generated from the molybdenum and copper smelting process is of great value for recycling sulfuric acid and valuable metals, such as rhenium (Re). Herein, a high Re(VII) (HReO₄) recovery efficiency of 99% within 35 min from strongly acidic wastewater was successfully achieved by using sulphide coupled with ultraviolet (UV) light, and soluble Re(VII) precipitated as Re₂S₇ in this process. Mechanistic experiments showed that the intermediate Re-S species (i.e., HReO₃S) was the dominant limitation responsible for Re(VII) precipitation in the dark, and UV irradiation dramatically accelerated the generation and conversion of HReO₃S by inducing the formation of HS• and H•. The H• produced from the photodissociation of H₂S promoted HReO₄ transformation to H₂ReO₄•, which rapidly reacted with HS• to produce HReO₃S, accelerating the conversion of HReO₄. The radical-induced acceleration can also take place during the HReO₃S conversion by slowly introducing H₂S into the strongly acidic wastewater to continuously produce H• and HS•. This work offers an insight into the improvement of Re(VII) recovery by UV light, which can be potentially applied into resource recovery from strongly acidic wastewater.

Specific H2S Release from Thiosulfate Promoted by UV Irradiation for Removal of Arsenic and Heavy Metals from Strongly Acidic Wastewater

The removal of arsenic and heavy metals (HMs) from strongly acidic wastewater by hydrogen sulfide (H₂S) is an efficient method. However, traditional sulfuration reagents (Na₂S, FeS, CaS, etc.) rapidly release H₂S under acidic conditions via spontaneous hydrolysis, leading to serious H₂S pollution. Herein, a H₂S release process employing thiosulfate as a sulfuration reagent was proposed to eliminate H₂S pollution. We found that thiosulfate can release H₂S with specificity both in the dark and under ultraviolet (UV) irradiation under acidic conditions. In the absence of arsenic/HMs, H₂S is not released because the formed H₂S is consumed by a thiosulfate decomposition product, sulfite, or by its photolysis. In the presence of arsenic/HMs, H₂S is released because the formed H₂S immediately reacts with arsenic/HMs to generate sulfide precipitates rather than being consumed. The efficiency of transforming thiosulfate to H₂S under UV irradiation is 2.5-fold the efficiency in the dark, because UV irradiation promotes the transformation of "effective sulfur" in thiosulfate molecules to form H₂S through the transformation of HS· and S₂O₃^• - radicals. Moreover, more than 99.9% of arsenic/HMs were removed from strongly acidic wastewater without producing H₂S pollution under UV irradiation. This thiosulfate-based H₂S-specific release process solves the problem of H₂S pollution under acidic conditions.

On a heavy path - determining cold plasma-derived short-lived species chemistry using isotopic labelling

Cold atmospheric plasmas (CAPs) are promising medical tools and are currently applied in dermatology and epithelial cancers. While understanding of the biomedical effects is already substantial, knowledge on the contribution of individual ROS and RNS and the mode of activation of biochemical pathways is insufficient. Especially the formation and transport of short-lived reactive species in liquids remain elusive, a situation shared with other approaches involving redox processes such as photodynamic therapy. Here, the contribution of plasma-generated reactive oxygen species (ROS) in plasma liquid chemistry was determined by labeling these via admixing heavy oxygen 18O2 to the feed gas or by using heavy water H2 18O as a solvent for the bait molecule. The inclusion of heavy or light oxygen atoms by the labeled ROS into the different cysteine products was determined by mass spectrometry. While products like cysteine sulfonic acid incorporated nearly exclusively gas phase-derived oxygen species (atomic oxygen and/or singlet oxygen), a significant contribution of liquid phase-derived species (OH radicals) was observed for cysteine-S-sulfonate. The role, origin, and reaction mechanisms of short-lived species, namely hydroxyl radicals, singlet oxygen, and atomic oxygen, are discussed. Interactions of these species both with the target cysteine molecule as well as the interphase and the liquid bulk are taken into consideration to shed light onto several reaction pathways resulting in observed isotopic oxygen incorporation. These studies give valuable insight into underlying plasma-liquid interaction processes and are a first step to understand these interaction processes between the gas and liquid phase on a molecular level.

Production of a Surface-Localized Oxidant during Oxygenation of Mackinawite (FeS)

The oxygenation of mackinawite (FeS) frequently occurs at anoxic-oxic interfaces in both natural and engineered systems such as intertidal sediment, in activated sludge in water treatment processes, and during sulfidized zero-valent iron particle corrosion. During reoxygenation events, FeS may drive a Fenton-like process leading to the production of strong oxidants though the details of this process are poorly understood. In this study, benzoic acid (BA) has been used to probe both the magnitude and identity of these strong oxidants under circumneutral pH conditions. The major product of BA oxidation during FeS oxygenation was found to be 2,5-dihydroxybenzoic acid (2,5-DHBA) rather than monohydroxybenzoic acids identified to be the major products in a range of hydroxyl radical (HO^·)-dominated systems. Based upon relative reactivity with other competitive probes and nature of the hydroxybenzoate product distribution, it is hypothesized that the strong oxidant must be a surface-localized entity such as high-valent iron or surface-associated hydroxyl or sulfur-based radicals with reactivity differing from those formed in free solution. The importance of both the reactivity of the oxidant and adsorption of the substrate to the surface is demonstrated.

UV light irradiation improves the aggregation and settling performance of metal sulfide particles in strongly acidic wastewater

Tiny metal sulfide particles are usually generated in the metal sulfide precipitation process, and residual dissolved sulfide (H₂S, HS^- and S^2-) generally makes the aggregation and settling performance of these particles worse. In this work, under strongly acidic conditions with 0.102 M H₂SO₄, the characteristics of CuS, CdS and mixed metal sulfide particles (CuS and CdS) produced in strongly acidic wastewater by H₂S were initially studied. Then, UV light irradiation was employed to improve the aggregation and settling performance of the metal sulfide particles, and relevant mechanisms were investigated. The results showed that the residual H₂S in suspension can deteriorate the settling performance of metal sulfide particles. After H₂S was removed, CuS particles exhibited good settling performance, but the settling performance of CdS and mixed sulfide particles remained poor. Subsequent studies showed that UV light irradiation effectively improved the aggregation and settling performance of CdS and mixed metal sulfide particles in the presence of H₂S. After 30 min of UV irradiation, the average hydrodynamic diameter of CdS particles increased approximately 166-fold in the presence of 131.40 mg/L H₂S, and that of mixed sulfide particles increased approximately 105-fold with the existence of 129.30 mg/L H₂S. Further study revealed that H₂S in suspensions played an important role in the aggregation of metal sulfide particles under UV irradiation. HS• generated by the photolysis of H₂S oxidized a portion of S^2- on the surface of CdS particles and a portion of S^2- and S_n^2- on the surface of mixed sulfide particles to S₂^2- and S⁰, which resulted in a decrease of negative charge and an enhanced aggregation of these particles. Finally, this study showed that the settling performance of CdS and mixed sulfide particles was effectively improved at high initial H₂S concentrations under UV irradiation. This study provides a clean and effective method to improve the metal sulfide precipitation process for strongly acidic wastewater treatment.

Hydrogen Sulfide and Persulfides Oxidation by Biologically Relevant Oxidizing Species

Hydrogen sulfide (H₂S/HS^–) can be formed in mammalian tissues and exert physiological effects. It can react with metal centers and oxidized thiol products such as disulfides (RSSR) and sulfenic acids (RSOH). Reactions with oxidized thiol products form persulfides (RSSH/RSS^–). Persulfides have been proposed to transduce the signaling effects of H₂S through the modification of critical cysteines. They are more nucleophilic and acidic than thiols and, contrary to thiols, also possess electrophilic character. In this review, we summarize the biochemistry of hydrogen sulfide and persulfides, focusing on redox aspects. We describe biologically relevant one- and two-electron oxidants and their reactions with H₂S and persulfides, as well as the fates of the oxidation products. The biological implications are discussed.

UV-Light-Induced Aggregation of Arsenic and Metal Sulfide Particles in Acidic Wastewater: The Role of Free Radicals

The removal of arsenic and metals by sulfide (S(-II)) from acidic wastewater is an efficient method. However, the small sulfide particles formed in such a process make solid-liquid separation difficult, which greatly hinders its application. This study investigated the aggregation behavior of different sulfide particles (As₂S₃, CuS and CdS) under ultraviolet (UV) irradiation. In the dark, the aggregation rate of the arsenic sulfide (As₂S₃) particles was extremely slow. However, under UV irradiation, the growth of the As₂S₃ particles was significantly enhanced. A possible mechanism of UV-light-induced aggregation of As₂S₃ particles was proposed. The HS· and ·OH radicals formed by a series of photochemical reactions can efficiently attack the S(-II) in the As₂S₃ particle, leading to the formation of an intermediate species, [As₂S₂-S·]⁺. Then, two [As₂S₂-S·]⁺ species combine to form [As₂S₂-S-S-S₂As₂]²⁺. The formation of [As₂S₂-S-S-S₂As₂]²⁺ results in the attenuation of the electronegativity and the rapid aggregation of the sulfide particles. In addition, the small S⁰ particles generated in irradiated As₂S₃ system can efficiently coalesce into As₂S₃ particles. The CuS and CdS particles should have similar aggregation mechanisms. This study proposed a potential method for sulfide particle aggregation and provided a theoretical foundation for the development and application of UV-light-induced sulfide particle aggregation technology.

Removal of Arsenic from Strongly Acidic Wastewater Using Phosphorus Pentasulfide As Precipitant: UV-Light Promoted Sulfuration Reaction and Particle Aggregation

Strongly acidic wastewater (H₂SO₄) with a high arsenic concentration is produced by many industries. The removal of arsenic by traditional sulfide (e.g., Na₂S, FeS) from strongly acidic wastewater introduces cations (Na⁺ and Fe²⁺) to the solution, which may prevent the recycle of acid. In this study, a new sulfuration agent, phosphorus pentasulfide (P₂S₅) was employed, and its feasibility in arsenic removal from strongly acidic wastewater was investigated. In the dark, As(III) was efficiently removed, but the removal rate of As(V) was rather slow, which was the crucial defect for this method. We found that this defect can be efficiently overcome by UV irradiation through accelerating the formation and transformation of an intermediate species, monothioarsenate (H₃AsO₃S) in the As(V) removal process. In addition, the hydrolysis of P₂S₅ was enhanced under UV irradiation, which resulted in the increase of the arsenic removal efficiencies. Besides, the aggregation of the formed particles was also promoted. Different from FeS and Na₂S, P₂S₅ introduces H₃PO₄ instead of cations to the solution, which can facilitate the recycle and reuse of arsenic and acid in strongly acidic wastewater.

Chemical Biology of H2S Signaling through Persulfidation

Signaling by H2S is proposed to occur via persulfidation, a posttranslational modification of cysteine residues (RSH) to persulfides (RSSH). Persulfidation provides a framework for understanding the physiological and pharmacological effects of H2S. Due to the inherent instability of persulfides, their chemistry is understudied. In this review, we discuss the biologically relevant chemistry of H2S and the enzymatic routes for its production and oxidation. We cover the chemical biology of persulfides and the chemical probes for detecting them. We conclude by discussing the roles ascribed to protein persulfidation in cell signaling pathways.

Mechanisms of UV-Light Promoted Removal of As(V) by Sulfide from Strongly Acidic Wastewater

Strongly acidic wastewater with a high arsenic concentration is produced by a number of industries. The removal of As(V) (H₃AsO₄) by sulfide from strongly acidic wastewater remains a difficult issue. This study proposed a UV-assisted method to efficiently remove As(V) by sulfide, and the involved mechanisms were systematically investigated. In the dark, the low removal efficiency of As(V) by sulfide was attributed to the slow formation and transformation of an intermediate species, i.e., monothioarsenate (H₃AsO₃S), in the As(V) sulfuration reaction, which were the rate-controlling steps in this process. However, UV irradiation significantly promoted the removal efficiency of As(V) not only by promoting the formation of H₃AsO₃S through light-induced HS^• and •H radicals but also by enhancing the transformation of H₃AsO₃S through a charge-transfer process between S(-II) and As(V) in the H₃AsO₃S complex, leading to the reduction of As(V) to As(III) and the oxidation of S(-II) to S(0). The formed As(III) species immediately precipitated as As₂S₃ under excess S(-II). Kinetic modeling offered a quantitative explanation of the results and verified the proposed mechanisms. This study provides a theoretical foundation for the application of light-promoted As(V) sulfuration removal, which may facilitate the recycling and reuse of arsenic and acid in strongly acidic wastewater.

H2S and its role in redox signaling

Hydrogen sulfide (H2S) has emerged as an important gaseous signaling molecule that is produced endogenously by enzymes in the sulfur metabolic network. H2S exerts its effects on multiple physiological processes important under both normal and pathological conditions. These functions include neuromodulation, regulation of blood pressure and cardiac function, inflammation, cellular energetics and apoptosis. Despite the recognition of its biological importance and its beneficial effects, the mechanism of H2S action and the regulation of its tissue levels remain unclear in part owing to its chemical and physical properties that render handling and analysis challenging. Furthermore, the multitude of potential H2S effects has made it difficult to dissect its signaling mechanism and to identify specific targets. In this review, we focus on H2S metabolism and provide an overview of the recent literature that sheds some light on its mechanism of action in cellular redox signaling in health and disease. This article is part of a Special Issue entitled: Thiol-Based Redox Processes.

Chemical characterization of the smallest S-nitrosothiol, HSNO; cellular cross-talk of H2S and S-nitrosothiols

Dihydrogen sulfide recently emerged as a biological signaling molecule with important physiological roles and significant pharmacological potential. Chemically plausible explanations for its mechanisms of action have remained elusive, however. Here, we report that H(2)S reacts with S-nitrosothiols to form thionitrous acid (HSNO), the smallest S-nitrosothiol. These results demonstrate that, at the cellular level, HSNO can be metabolized to afford NO(+), NO, and NO(-) species, all of which have distinct physiological consequences of their own. We further show that HSNO can freely diffuse through membranes, facilitating transnitrosation of proteins such as hemoglobin. The data presented in this study explain some of the physiological effects ascribed to H(2)S, but, more broadly, introduce a new signaling molecule, HSNO, and suggest that it may play a key role in cellular redox regulation.

Small molecule signaling agents: the integrated chemistry and biochemistry of nitrogen oxides, oxides of carbon, dioxygen, hydrogen sulfide, and their derived species

Several small molecule species formally known primarily as toxic gases have, over the past 20 years, been shown to be endogenously generated signaling molecules. The biological signaling associated with the small molecules NO, CO, H₂S (and the nonendogenously generated O₂), and their derived species have become a topic of extreme interest. It has become increasingly clear that these small molecule signaling agents form an integrated signaling web that affects/regulates numerous physiological processes. The chemical interactions between these species and each other or biological targets is an important factor in their roles as signaling agents. Thus, a fundamental understanding of the chemistry of these molecules is essential to understanding their biological/physiological utility. This review focuses on this chemistry and attempts to establish the chemical basis for their signaling functions.

Understanding the role of the sulfide redox couple (S2-/S(n)2-) in quantum dot-sensitized solar cells

The presence of sulfide/polysulfide redox couple is crucial in achieving stability of metal chalcogenide (e.g., CdS and CdSe)-based quantum dot-sensitized solar cells (QDSC). However, the interfacial charge transfer processes play a pivotal role in dictating the net photoconversion efficiency. We present here kinetics of hole transfer, characterization of the intermediates involved in the hole oxidation of sulfide ion, and the back electron transfer between sulfide radical and electrons injected into TiO(2) nanoparticles. The kinetic rate constant (10(7)-10(9) s(-1)) for the hole transfer obtained from the emission lifetime measurements suggests slow hole scavenging from CdSe by S(2-) is one of the limiting factors in attaining high overall efficiency. The presence of the oxidized couple, by addition of S or Se to the electrolyte, increases the photocurrent, but it also enhances the rate of back electron transfer.

Reactivity of hydrogen sulfide with peroxynitrite and other oxidants of biological interest

Hydrogen sulfide (H(2)S) is an endogenously generated gas that can also be administered exogenously. It modulates physiological functions and has reported cytoprotective effects. To evaluate a possible antioxidant role, we investigated the reactivity of hydrogen sulfide with several one- and two-electron oxidants. The rate constant of the direct reaction with peroxynitrite was (4.8±1.4)×10(3)M(-1) s(-1) (pH 7.4, 37°C). At low hydrogen sulfide concentrations, oxidation by peroxynitrite led to oxygen consumption, consistent with a one-electron oxidation that initiated a radical chain reaction. Accordingly, pulse radiolysis studies indicated that hydrogen sulfide reacted with nitrogen dioxide at (3.0±0.3)×10(6)M(-1) s(-1) at pH 6 and (1.2±0.1)×10(7)M(-1) s(-1) at pH 7.5 (25°C). The reactions of hydrogen sulfide with hydrogen peroxide, hypochlorite, and taurine chloramine had rate constants of 0.73±0.03, (8±3)×10(7), and 303±27M(-1) s(-1), respectively (pH 7.4, 37°C). The reactivity of hydrogen sulfide was compared to that of low-molecular-weight thiols such as cysteine and glutathione. Considering the low tissue concentrations of endogenous hydrogen sulfide, direct reactions with oxidants probably cannot completely account for its protective effects.

Hydrosulfide oxidation pathways in oxic solutions containing iron(III) chelates

The role of dissolved oxygen (DO2) on the oxidation of hydrosulfide ions (HS-; C(HS-)0 = 50-150 micromol/L) into polysulfides (S(n)2-; n = 2-9), colloidal sulfur, and oxysulfur species with iron(III) trans-1,2-diaminocyclohexanetetraacetate (iron(III)-cdta; C(Fe(III)0 = 50-300 micromol/L) complexes in alkaline solutions (pH 9-10.2) was investigated at 25 +/- 1 degree C. At higher pH, oxygen was seen to slow down the hydrosulfide conversion rate. For instance, the HS- half-life was 24.8 min in a DO2-saturated iron(III)-cdta solution compared to 11.3 min in the corresponding anoxic solution (pH 10.2, C(HS-)0 = 80 micromol/L, C(Fe(III))0 = 200 micromol/L). In anoxia, HS- oligomerizes into chain-like polysulfides which behave as autocatalysts on the HS- conversion rates. The presence of DO2 disrupts the HS- oligomerization process by generating thiosulfate precursors from polysulfides, a pathway that impedes the HS- uptake. At lower alkaline pH where the hydroxide-free Fe(3+)cdta(4-) is the prevailing iron(III)-cdta species, the "iron(II)-cdta + DO2" oxidative reaction becomes crucial. Oxidative regeneration of iron(III) as Fe(3+)cdta(4-) (being more reactive than Fe(3+)OH(-)cdta(4-)) offsets to some extent the restrictive role of oxygen on the accumulation of polysulfides. Thiosulfate and sulfate were the main end-products for the current experimental conditions to the detriment of colloidal sulfur, which did not form in DO2-saturated solutions.

The reaction of Cl with CH(4): a connection between kinetics and dynamics

This paper presents an analysis of the reaction of Cl with CH(4) by combining measurements of thermal rate constants and state-dependent reaction cross sections. State-dependent measurements have shown that the reaction probability is enhanced by vibrational excitation of CH(4). Measured thermal rate constants were fit with a model incorporating this information. The results provide estimates of rate constants at extreme temperatures and information about the temperature and collision energy dependence of the vibrational enhancement.