Sulfhydryl Sites Research Articles

The role of cell surface sulfhydryl and amine binding sites in the removal of Cr(VI) from solution by Bacillus subtilis bacterial cells

A range of bacterial species are known to be capable of reducing soluble Cr(VI) to Cr(III), causing the precipitation of Cr(III) phases from solution, but the mechanism responsible for the initial interaction between anionic aqueous Cr(VI) and the negatively charged cell surface has not been identified. Our study examines Cr(VI) removal from solution by the Gram-positive soil bacterial species Bacillus subtilis. We measured the kinetics of Cr(VI) removal as a function of pH in B. subtilis suspensions with different Cr(VI):cell ratios, and we measured the effect of pH on the extent of Cr(VI) removal at a fixed time in experiments with B. subtilis biomass (cells plus exudates) and in experiments involving B. subtilis exudates alone. The roles of sulfhydryl and amine binding sites in Cr(VI) removal were constrained using site–specific blocking molecules, and the impact of the site blocking on Cr(VI) removal was studied as a function of pH. Our results indicate that Cr(VI) removal by B. subtilis cells under the experimental conditions is at least partially non-reversible and is dependent on binding site concentration, pH, and metal loading. Our results are consistent with a two step removal process: Cr(VI) first adsorbs reversibly onto a cell wall binding site, followed by Cr(VI) reduction to Cr(III) likely via electron transfer from cell wall electron transport chain molecules. B. subtilis exudates are capable of removing a relatively small fraction of Cr(VI) from solution, and hence our results indicate that the dominant mechanism of Cr(VI) removal requires interaction with bacterial biomass. The presence of either the sulfhydryl-specific blocking molecule or the amine-specific blocking molecule or both in the experimental systems dramatically reduces the extent of Cr(VI) reduction, especially under circumneutral pH conditions, strongly suggesting that both sulfhydryl and amine binding site types participate in the initial attachment of Cr(VI) onto the cell surface. The experiments with either sulfhydryl or amine sites blocked both exhibited a similar reduction in Cr(VI) removal to the experiments with both types of sites blocked, strongly suggesting that both types of sites are involved simultaneously in binding Cr(VI) species to the cell surface. For example, these sites could both be involved in forming a bidentate bond with Cr(VI) species, or the positive charge of an amine site in close proximity to a sulfhydryl site could be necessary in order to enable the Cr(VI) species to approach the cell wall and to bind to a sulfhydryl site. The results of this study are the first to propose a viable mechanism that can explain the binding of anionic Cr(VI) onto an overall negatively charged cell surface as a first step in the reduction to Cr(III) and subsequent removal of Cr from solution.

Characterization of Bacterial Exudates Through Potentiometric Titrations

Bacterial exudates are organic molecules released by bacteria that have proton- and metal-binding capacities, as well as the ability to facilitate redox reactions. However, the binding sites responsible for these functions are poorly characterized. In this work we examine the concentrations and types of proton active sites through potentiometric titrations of exudate solutions produced from several bacterial species. Additionally, we report the first measurement of sulfhydryl site concentrations of bacterial exudates. Finally, we examine the processes by which these exudates are produced in order to constrain the relative importance of active and passive production of the exudate molecules. Our results demonstrate that, while overall proton-binding behavior is primarily dependent on the amount of organic material produced, the concentration of sulfhydryl sites present on the exudate molecules is species-dependent and is not related to the amount of exudate produced. This result indicates that bulk proton-binding characteristics and total proton-active site concentrations on exudate molecules are relatively constant between species, but that the species of bacteria does impact the functionality of the exudates produced, particularly with respect to binding of chalcophile elements and redox reactions involving those elements.

Reduction of selenite by bacterial exudates

Bacterial reduction of Se(IV) is a significant component of the global selenium cycle, and hence affects the fate and transport of selenium in both natural and contaminated environments. However, it is unknown whether bacterially-produced exudates are capable of reducing selenium independent of bacterial cells. In this study, we measured the rate and extent of Se(IV) reduction by exudates from three bacterial species, and we determined the importance of exudate sulfhydryl sites by conducting parallel experiments after treatment of the exudates with a sulfhydryl-specific blocking molecule. We also conducted experiments with whole cell biomass for each of the three bacterial species to determine the importance of exudate-only reduction relative to whole biomass-promoted reduction. Under our experimental conditions, exudates from Bacillus subtilis and Pseudomonas putida remove Se(IV) from solution after an approximately 20–24 h lag period, but exudates from Shewanella oneidensis are ineffective at Se(IV) removal, except when the cells are grown in the presence of Se(IV). For both B. subtilis and P. putida, pretreatment blocking of the sulfhydryl sites on the exudate molecules dramatically decreases the rate and extent of Se(IV) removal, strongly suggesting that sulfhydryl groups on the exudate molecules play a key role in the Se(IV) reduction mechanism. The behavior of S. oneidensis exudates indicates that up-regulation of the Se(IV) reduction mechanism can occur in response to the Se content of the medium in which the cells grow. Our results demonstrate the capacity of some bacterial exudates to reduce Se(IV), and suggest that the activity of bacterial exudates should be accounted for when modeling selenium cycling in natural and engineered environments.

Sulfate induced surface modification of Chlorella for enhanced mercury immobilization

In this study, living Chlorella sorokiniana FK with an elevated concentration of cell envelop sulfur sites, induced by adding excess sulfate to the medium, was used as biosorbents for the removal of Hg(II) from aqueous solution. The cellular surface sulfur-based sites density of the obtained biosorbents was determined through selective sulfhydryl site-blocking and potentiometric titration. The sorption was conducted as terms of pH and initial Hg(II) concentration, and characterized by SEM, AFM, HRTEM, FT-IR, XRD and XPS. The results showed that Chlorella possessed the highest sulfhydryl site concentration and showed the largest capacity for Hg(II) detoxification in the 40 mg/L sulfur culture (termed B-40) with 74.2% sulfate consumption for the growth of algae. The equilibrium data for the studied biosorbents was adequately represented by the Langmuir isotherm and B-40 exhibited the highest sorption capacity for Hg(II), 2 times greater than that of no sulfate culture Chlorella at the optimized pH 6. Mercury biomineral particles (HgS, HgSO4 and Hg2SO5) were formed on the cellular envelop which induced mainly by sulfur-based groups (-SH, -OSO3-) for enhanced Hg(II) immobilization. These results suggested that surface sites concentration of algae can be manipulated by the available nutrients, and therefore cells with induced high densities of sulfhydryl sites represented a promising and low cost biosorbent for the effective removal and immobilization of Hg and possibly other heavy metals.

Controls on the Concentration of Bacterial Cell Surface Sulfhydryl Sites: Effects of Inorganic Nutrients

Understanding the controls on bacterial surface sulfhydryl site concentrations is crucial for modeling the effects of bacterial cells on the fate and transport of chalcophile metals in the environment. In this study, we investigate the effects of changing the concentrations of MgSO4, NaCl and NaNO3 in a growth medium on the concentration of cell surface sulfhydryl sites on the Gram-positive bacterial species Bacillus subtilis, as measured using a potentiometric titration approach. Our results show that the concentration of MgSO4 in a M9 minimal medium exerts a strong influence on bacterial surface sulfhydryl site concentrations under low MgSO4 concentrations but that this effect plateaus with increasing MgSO4 concentrations, indicating that a minimum amount of a sulfur source is required for bacteria to produce cell surface sulfhydryl sites. Although the sulfhydryl site concentration was below the detection limit on biomass samples that were grown in a M9 minimal medium containing 0.1 mM MgSO4, increasing the concentration of either NaCl or NaNO3 in the medium yielded biomass samples with measurable sulfhydryl site concentrations on the cell surface. In contrast, adding NaClO4 to the M9 medium did not increase cell surface sulfhydryl site concentrations. The NaCl and NaNO3 effects likely arise because increasing the concentration of NaCl or NaNO3 in a M9 minimal medium can promote the production of EPS by bacteria, and because the surface sulfhydryl sites of B. subtilis are located mostly on its EPS molecules. The results of this study suggest that bacterial nutrients and water chemistry composition are key controls on the abundance of bacterial surface sulfhydryl sites.

A novel covalent organic polymer with hierarchical pore structure for rapid and selective trace Hg(II) removal from drinking water

The deep purification of trace Hg(II) in drinking water has been being a challenge to water treatment owing to its mass transfer resistance and chronic harm to human body. Herein, a new covalent organic polymer material with hierarchical pore structure and ample thiol groups (hp-COP-SH) was successfully prepared by a facile and mild diazo-coupling reaction. The microporous-mesoporous structure of the as-prepared hp-COP-SH provides an excellent transport channel for low-concentration Hg(II), promoting its vortex diffusion extremely. Meanwhile, the ample sulfhydryl sites contribute to the strong affinity between the adsorbent and mercury, thereby greatly improving the adsorption speed and selectivity of the material to trace Hg(II). The adsorption kinetics results demonstrated that the effluent Hg(II) concentration decreased from initial 30 ppb to 0.72 ppb within 10 s when hp-COP-SH dosage was 10 mg/30 mL and 20 ppm Ca(II) coexisted, meeting drinking water standard (<1 ppb) of World Health Organization (WHO). Moreover, this material also showed a brilliant performance within a broad pH range (1.5–12.5), high selectivity to trace Hg(II) in the mixed solution of co-existing trace Pb(II) and high concentration K(I), Ca(II), Na(I), Mg(II), Cu(II), fast adsorption speed in actual tap water and excellent chemical stability, revealing excellent application prospect for rapid and selective removal of trace Hg(II) from drinking water.

Functional group diversity for the adsorption of lead(Pb) to bacterial cells and extracellular polymeric substances

Bacteria and their secreted extracellular polymeric substances (EPS) are widely distributed in ecosystems and have high capacity for heavy metal immobilization. The knowledge about the molecular-level interactions with heavy metal ions is essential for predicting the behavior of heavy metals in natural and engineering systems. This comprehensive study using potentiometric titration, Fourier-transform infrared (FTIR) spectroscopy, isothermal titration calorimetry (ITC) and X-ray absorption fine structure (XAFS) was able to reveal the functional diversity and adsorption mechanisms for Pb onto bacteira and the EPS in greater detail than ever before. We identified mono-carboxylic, multi-carboxylic, phosphodiester, phosphonic and sulfhydryl sites and found the partitioning of Pb to these functional groups varied between gram-negative and gram-positive bacterial strains, the soluble and cell-bound EPS and Pb concentrations. The sulfhydryl and phosphodiester groups preferentially complexed with Pb in P. putida cells, while multifunctional carboxylic groups promoted Pb adsorption in B. subtilis cells and the protein fractions in EPS. Though the functional site diversity, the adsorption of Pb to organic ligands occurred spontaneously through a universal entropy increase and inner-sphere complexation mechanism. The functional group scale knowledge have implications for the modeling of heavy metal behavior in the environment and application of these biological resources.

26P Structurally constrained tumor-specific epitope within the largest extracellular domain of sodium-dependent phosphate transporter NaPi2b

The search for tumor-specific targets among membrane proteins is important for the development for new effective anticancer drugs because membrane proteins represent 60% of drug targets. Transmembrane proteins in cancer cells exhibit an altered pattern of post-translational modifications and generally contain a large extracellular domains with multiple glycosylation and sulfhydryl sites with unknown function. We analyze the features of membrane protein which are required for recognition by monoclonal antibodies using the sodium-dependent phosphate transporter NaPi2b, containing a large extracellular domain 4 (EMD4, 250-360 a.a.) with 4 cysteines and 6 potential asparagine-linked glycosylation sites which can potentially contribute to foldability of conformationally exposed epitope corresponding to this domain.

Tellurite Adsorption onto Bacterial Surfaces.

Tellurium (Te) is an emerging contaminant and its chemical transformation in the environment is strongly influenced by microbial processes. In this study, we investigated the adsorption of tellurite [Te(IV), TeO32-] onto the common soil bacterium Bacillus subtilis. Thiol-blocking experiments were carried out to investigate the role of cell surface sulfhydryl sites in tellurite binding, and extended X-ray absorption fine structure (EXAFS) spectroscopy was performed to determine the chemical speciation of the adsorbed tellurite. The results indicate that tellurite reacts with sulfhydryl functional groups in the extracellular polymeric substances (EPS) produced by B. subtilis. Upon binding to sulfhydryl sites in the EPS, the Te changes from Te-O bonds to Te-S coordination. Further analysis of the surface-associated molecules shows that the EPS of B. subtilis contain proteins. Removal of the proteinaceous EPS dramatically decreases tellurite adsorption and the sulfhydryl surface site concentration. These findings indicate that sulfhydryl binding in EPS plays a key role in tellurite adsorption on bacterial surfaces.

Electron Donor Effects on Bacterial Surface Sulfhydryl Site Concentrations

In this study, we examined two effects on bacterial surface sulfhydryl site concentrations and distributions: (1) the effect of glucose concentration on the distribution of sulfhydryl sites between the bacterial cell surface and surface-associated extracellular polymeric substance (EPS) molecules, and (2) the effect of electron donor identity and concentration on sulfhydryl site concentrations on bacterial biomass. In each set of experiments, the total site concentration was measured using a potentiometric titration approach, and sulfhydryl site concentrations were determined using a site-specific blocking technique. The measurements were conducted with and without the removal of cellular EPS. For the first set of experiments, our results indicate that the two Gram-positive bacterial species, Bacillus subtilis, and Bacillus licheniformis, and one of the Gram-negative bacterial species studied, Pseudomonas putida, each have a greater concentration of sulfhydryl sites on their EPS molecules than is present on their cell surfaces, possibly serving to sequester toxic metals away from the cell surface. Conversely, for the Gram-negative bacterial species Shewanella oneidensis, the concentration of sulfhydryl sites on the cell surface is greater than that on its EPS molecules. S. oneidensis can gain metabolic energy through metal reduction, and hence enhancing the extent of metal binding to the cell wall through the formation of sulfhydryl binding sites may be more beneficial than the risk of metal toxicity. In the second set of experiments, increasing the concentration of two electron donors, pyruvate and glucose, in the growth medium of B. subtilis led to an increase in the percentage of total sites represented by sulfhydryl sites, but the concentration of the two other electron donors, glycerol and fumarate, had no effect on the percentage of sulfhydryl sites. Our results indicate that both the identity and the concentration of electron donors significantly influence the formation of sulfhydryl binding sites on bacterial cell surfaces. In addition, our results suggest that the total energy availability of a specific electron donor-bacterial species pairing affects both the ability of bacterial cells to produce sulfhydryl binding sites, and the distribution of those sites between the cell surface and its associated EPS molecules.

Effects of preheat treatment on the physicochemical and interfacial properties of cod proteins and its relation to the stability of subsequent emulsions

Interests in cod proteins (CPs) in food industry are growing due to their high nutritional attributes and the largest fishing amount worldwide. However, CPs are usually consumed as low value-added products, and limited attention has been paid to their emulsifying properties. In this study, the effects of preheat treatment at different temperatures (70–100 °C) and time (5–60 min) on the physicochemical and interfacial properties of CPs were investigated. Results revealed that increasing the preheating temperature and time led to smaller particle size, lower contents of sulfhydryl sites, and higher surface hydrophobicity of CPs. Preheat treatment further significantly increased the ratios of CPs-absorption at the oil-water interface of emulsions that presented smaller droplet size and lower interfacial tension due to promoted surface active properties of CPs. Non-reducing SDS-PAGE showed that CPs at the interface were mainly composed of tropomyosin, actin, as well as disulfide bonds cross-linked moieties, which formed a highly viscoelastic film. The interfacial physical barrier hampered the coalescence of adjacent droplets, as verified by lower creaming index, which demonstrated the enhanced stability of preheated CPs-coated emulsions. The findings expand the application potential of CPs as promising emulsifiers by a straightforward preheat treatment.

Cadmium and proton adsorption onto a halophilic archaeal species: The role of cell envelope sulfhydryl sites

The adsorption of metal onto archaea could affect metal behavior and bioavailability in hypersaline environments where archaea represent the dominant life form. However, the mechanism and thermodynamics of metal adsorption onto halophilic archaea are not well understood. In this study, Cd adsorption experiments and potentiometric titrations were conducted to model metal adsorption by the halophilic archaeon Halobacterium sp.. Cd adsorption onto the archaeal cells exhibits slow adsorption kinetics, taking 8 h to reach steady-state. The extent of Cd adsorption depends strongly on pCH+ and Cd loading. Desorption of Cd is even slower than adsorption, but exposure of the Cd-adsorbed biomass to high concentrations of cysteine promotes desorption and demonstrates that the distribution of Cd is controlled by reversible adsorption reactions, likely involving sulfhydryl binding of Cd on the cell surface. We conducted potentiometric titration experiments with and without sulfhydryl site blocking to yield a total sulfhydryl site concentration of 91 ± 28 μmol/g within the archaeal cell envelope. Using a combination of the X-ray absorption fine structure analysis results from Showalter et al. (2016) and the Cd adsorption measurements and titration data from this study, we construct a surface complexation model of the Cd adsorption behavior. Because the Cd adsorption behavior is likely similar to that of a range of chalcophilic micronutrients, the importance of Cd-sulfhydryl binding suggests that the production of high affinity sulfhydryl binding sites by the archaea may represent an adaptive strategy for halophiles to obtain metal nutrients in environments in which aqueous metal-chloride complexation dominates the speciation for many metals.

Role of bacterial cell surface sulfhydryl sites in cadmium detoxification by Pseudomonas putida

Understanding bacterial metal detoxification systems is crucial for determining the environmental impacts of metal pollution and for developing advanced bioremediation and water disinfection strategies. Here, we explore the role of cell surface sulfhydryl sites in bacterial detoxification of Cd, using Pseudomonas putida with surface sulfhydryl sites mostly on its EPS molecules as a model organism. Our results show that 5 and 20 ppm Cd in LB growth medium affects the lag phase of P. putida, but not the overall extent of cell growth at stationary phase, indicating that P. putida can detoxify Cd at these concentrations. EXAFS analysis of Cd bound to biomass from the different growth stages indicates that Cd binds to both sulfhydryl and non-sulfhydryl sites, but that the importance of Cd-sulfhydryl binding increases from early exponential to stationary phase. Cell growth is positively correlated to the measured sulfhydryl concentration on different biomass samples, but is independent of the measured non-sulfhydryl binding site concentration on the cell surfaces. Taken together, our results demonstrate that the sulfhydryl binding sites on EPS molecules can play an important role in binding and detoxifying toxic metals, significantly decreasing the bioavailability of the metal by sequestering it away from the bacterial cells.

A Mechanistic Study of Au(III) Removal from Solution by Bacillus subtilis

Bacteria–Au interactions control the fate of Au in a variety of geologic systems. Although previous studies have determined that non-metabolizing Bacillus subtilis cells can remove Au(III) from solution via cell surface adsorption reactions, and that upon removal Au(III) is rapidly reduced to Au(I) and remains bound to the cell surface, the mechanism of Au(III) removal by B. subtilis is poorly understood. This study provides further constraints on the mechanisms responsible for Au(III) removal by B. subtilis by conducting batch Au(III) removal experiments as a function of pH and Au loading (Au:biomass ratio) using biomass with and without two different types of treatment: (1) a treatment to remove extracellular polymeric substances (EPS) from the biomass, and (2) a treatment to irreversibly block surface sulfhydryl sites from Au binding. The experimental results suggest that Au(III) removal can be attributed primarily to Au complexation with bacterial sulfhydryl sites, but that Au–amino binding is also important under some conditions. Our experiments also suggest that Au–sulfhydryl binding occurs predominantly on EPS molecules produced by B. subtilis, and that Au–amino binding is also important and is located within the bacterial cell envelope. These findings are the first to constrain the location of sulfhydryl-binding sites for B. subtilis biomass, and they are the first to demonstrate the important role played by bacterial EPS in the process of Au adsorption and reduction by bacteria.

Influence of Extracellular Polymeric Substances on the Adsorption of Cadmium onto Three Bacterial Species

In this study, we measured the effect of EPS on Cd and proton adsorption behaviors by measuring the extent of adsorption onto biomass with and without the EPS removed via a cation exchange resin. We conducted both Cd adsorption experiments and potentiometric titrations of biomass using three common bacterial species: one Gram-positive (Bacillus subtilis) and two Gram-negative (Shewanella oneidensis, Pseudomonas putida) species. The Cd adsorption experiments were conducted as a function of metal loading in order to probe whether environmentally-low metal loadings lead to different adsorption mechanisms and roles for EPS than the higher metal loadings of most previous adsorption studies. We suspended each biomass sample in a solution of dissolved Cd in 0.01 M NaClO4 at metal loadings of 1, 2, 5, and 74 μmol/g. Surface complexation modeling (SCM) was used to determine stability constants for the important Cd-bacteria complexes, and the effect of metal loading on the resulting calculated stability constant values was determined.In general, the measured bulk Cd adsorption behavior is unaffected by EPS removal. However, our potentiometric titration results suggest that EPS removal does alter the distribution of site types, but not the mass-normalized total site concentration within the biomass. SCM suggests that high affinity sulfhydryl sites control Cd binding under low metal loading conditions for B. subtilis and P. putida, and that sulfhydryl sites are present both on the cells and within the EPS for these species. Conversely, the SCM results suggest that Cd-sulfhydryl binding is un-important on the EPS of S. oneidensis.

Adsorption of Methylmercury ontoGeobacter bemidijensisBem

The anaerobic bacterium Geobacter bemidijensis Bem has the unique ability to both produce and degrade methylmercury (MeHg). While the adsorption of MeHg onto bacterial surfaces can affect the release of MeHg into aquatic environments as well as the uptake of MeHg for demethylation, the binding of MeHg to the bacterial envelope remains poorly understood. In this study, we quantified the adsorption of MeHg onto G. bemidijensis and applied X-ray absorption spectroscopy (XAS) to elucidate the mechanism of MeHg binding. The results showed MeHg adsorption onto G. bemidijensis cell surfaces was rapid and occurred via complexation to sulfhydryl functional groups. Titration experiments yielded cell surface sulfhydryl concentrations of 3.8 ± 0.2 μmol/g (wet cells). A one-site adsorption model with MeHg binding onto sulfhydryl sites provided excellent fits to adsorption isotherms conducted at different cell densities. The log K binding constant of MeHg onto the sulfhydryl sites was determined to be 10.5 ± 0.4. These findings provide a quantitative framework to describe MeHg binding onto bacterial cell surfaces and elucidate the importance of bacterial cells as possible carriers of adsorbed MeHg in natural aquatic systems.

Adsorption of Selenite onto Bacillus subtilis: The Overlooked Role of Cell Envelope Sulfhydryl Sites in the Microbial Conversion of Se(IV).

Microbial activities play a central role in the global cycling of selenium. Microorganisms can reduce, methylate, and assimilate Se, controlling the transport and fate of Se in the environment. However, the mechanisms controlling these microbial activities are still poorly understood. In particular, it is unknown how the negatively charged Se(IV) and Se(VI) oxyanions that dominate the aqueous Se speciation in oxidizing environments bind to negatively charged microbial cell surfaces in order to become bioavailable. Here, we show that the adsorption of selenite onto Bacillus subtilis bacterial cells is controlled by cell envelope sulfhydryl sites. Once adsorbed onto the bacteria, selenite is reduced and forms reduced organo-Se compounds (e.g., R1S-Se-SR2). Because sulfhydryl sites are present within cell envelopes of a wide range of bacterial species, sulfhydryl-controlled adsorption of selenite likely represents a general mechanism adopted by bacteria to make selenite bioavailable. Therefore, sulfhydryl binding of selenite likely occurs in a wide range of oxidized Se-bearing environments, and because it is followed by microbial conversion of selenite to other Se species, the process represents a crucial step in the global cycling of Se.

Automated radiosynthesis of [18F]FBEM, a sulfhydryl site specific labeling agent for peptides and proteins

In the process of developing [18F]FBEM coupled target peptide, we have instituted a robust automated synthesis of [18F]FBEM, a sulfhydryl (-SH) site specific agent for radiolabeling of peptides and proteins. The radiosynthesis generated 1.67–3.89 GBq (45.1–105.1 mCi, 7.5–18.8% non-decay corrected yield) of [18F]FBEM from 22.2 GBq (600 mCi) of starting [18F]fluoride with molar activity of 31.8 ± 5.3 GBq/µmol (0.86 ± 0.14 mCi/nmol) (n = 3) at the end of synthesis. Radiochemical purity was greater than 98%, and total synthesis time was ~90 min.

Influence of sulfhydryl sites on palladium (II) adsorption by displaying EC20 on the surface of Escherichia coli

AbstractBACKGROUNDSulfhydryl binding dominates metal adsorption onto bacteria, however, their role in palladium adsorption under high metal loading conditions remains unclear. We investigated their influence on palladium adsorption by increasing (expression of EC20 protein) and decreasing (selective blocking of ‐SH) the number of sulfhydryl sites on the surface of Escherichia coli (E. coli).RESULTSKinetic and equilibrium studies confirmed that the sulfhydryl sites in the EC20 protein played an important role in Pd (II) adsorption, while blocking the sulfhydryl sites resulted in the decrease of adsorption capacity. The accumulation of palladium in the surface of E. coli BL21 cells was enhanced by displaying EC20 protein, and morphology properties of Pd‐loaded cells changed considerably. SEM and TEM analyses showed that palladium ions were absorbed both in the exterior and interior of cells. Pd‐S bond was detected by XPS in Pd‐loaded cells, and it was demonstrated that the unbound sulfhydryl site was a contributor in Pd (II) adsorption.CONCLUSIONSulfhydryl sites on the surface of E. coli are significant functional groups in Pd (II) adsorption, increasing their number by EC20 protein expression leads to the increase of Pd (II) adsorption capacity, and vice versa. © 2018 Society of Chemical Industry

Monothioarsenate Transformation Kinetics Determining Arsenic Sequestration by Sulfhydryl Groups of Peat.

In peatlands, arsenite was reported to be effectively sequestered by sulfhydryl groups of natural organic matter. To which extent porewater arsenite can react with reduced sulfur to form thioarsenates and how this affects arsenic sequestration in peatlands is unknown. Here, we show that, in the naturally arsenic-enriched peatland Gola di Lago, Switzerland, up to 93% of all arsenic species in surface and porewaters were thioarsenates. The dominant species, monothioarsenate, likely formed from arsenite and zerovalent sulfur-containing species. Laboratory incubations with sulfide-reacted, purified model peat showed increasing total arsenic sorption with decreasing pH from 8.5 to 4.5 for both, monothioarsenate and arsenite. However, X-ray absorption spectroscopy revealed no binding of monothioarsenate via sulfhydryl groups. The sorption observed at pH 4.5 was acid-catalyzed dissociation of monothioarsenate, forming arsenite. The lower the pH and the more sulfhydryl sites, the more arsenite sorbed which in turn shifted equilibrium toward further dissociation of monothioarsenate. At pH 8.5, monothioarsenate was stable over 41 days. In conclusion, arsenic can be effectively sequestered by sulfhydryl groups in anoxic, slightly acidic environments where arsenite is the only arsenic species. At neutral to slightly alkaline pH, monothioarsenate can form and its slow transformation into arsenite and low affinity to sulfhydryl groups suggest that this species is mobile in such environments.