Chloride- and Hydrosulfide-Bound 2Fe Complexes as Models of the Oxygen-Stable State of [FeFe] Hydrogenase

[FeFe] hydrogenases demonstrate remarkable catalytic efficiency in hydrogen evolution and oxidation processes. However, susceptibility of these enzymes to oxygen-induced degradation impedes their practical deployment in hydrogen-production devices and fuel cells. Recent investigations into the oxygen-stable (Hinact) state of the H-cluster revealed its inherent capacity to resist oxygen degradation. Herein, we present findings on Cl- and SH-bound [2Fe-2S] complexes, bearing relevance to the oxygen-stable state within a biological context. A characteristic attribute of these complexes is the terminal Cl-/SH- ligation to the iron center bearing the CO bridge. Structural analysis of the t-Cl demonstrates a striking resemblance to the Hinact state of DdHydAB and CbA5H. The t-Cl/t-SH exhibit reversible oxidation, with both redox species, electronically, being the first biomimetic analogs to the Htrans and Hinact states. These complexes exhibit notable resistance against oxygen-induced decomposition, supporting the potential oxygen-resistant nature of the Htrans and Hinact states. The swift reductive release of the Cl-/SH-group demonstrates its labile and kinetically controlled binding. The findings garnered from these investigations offer valuable insights into properties of the enzymatic O2-stable state, and key factors governing deactivation and reactivation conversion. This work contributes to the advancement of bio-inspired molecular catalysts and the integration of enzymes and artificial catalysts into H2-evolution devices and fuel-cell applications.

Synthesis, Characterization, and Reactivity of a Synthetic End-On Cobalt(II) Alkyl Persulfide Complex as a Model Platform for Thiolate Persulfidation

Persulfides (RSS-) are ubiquitous source of sulfides (S2-) in biology, and interactions between RSS- and bioinorganic metal centers play critical roles in biological hydrogen sulfide (H2S) biogenesis, signaling, and catabolism. Here, we report the use of contact-ion stabilized [Na(15-crown-5)][tBuSS] (1) as a simple synthon to access rare metal alkyl persulfide complexes and to investigate the reactivity of RSS- with transition metal centers to provide insights into metal thiolate persulfidation, including the fundamental difference between alkyl persulfides and alkyl thiolates. Reaction of 1 with [CoII(TPA)(OTf)]+ afforded the η1-alkyl persulfide complex [CoII(TPA)(SStBu)]+ (2), which was characterized by X-ray crystallography, UV-vis spectroscopy, and Raman spectroscopy. RSS- coordination to the Lewis acidic Co2+ center provided additional stability to the S-S bond, as evidenced by a significant increase in the Raman stretching frequency for 2 (vS-S = 522 cm-1, ΔvS-S = 66 cm-1). The effect of persulfidation on metal center redox potentials was further elucidated using cyclic voltammetry, in which the Co2+ → Co3+ oxidation potential of 2 (Ep,a = +89 mV vs SCE) is lowered by nearly 700 mV when compared to the corresponding thiolate complex [CoII(TPA)(StBu)]+ (3) (Ep,a = +818 mV vs SCE), despite persulfidation being generally seen as an oxidative post-translational modification. The reactivity of 2 toward reducing agents including PPh3, BH4-, and biologically relevant thiol reductant DTT led to different S2- output pathways, including formation of a dinuclear 2Co-2SH complex [CoII2(TPA)2(μ2-SH)2]2+(4).

Reversible Hydrosulfide (HS-) Binding Using Exclusively C-H Hydrogen-Bonding Interactions in Imidazolium Hosts

H2S is a physiologically important signaling molecule with complex roles in biology and exists primarily as HS- at physiological pH. Despite this anionic character, few investigations have focused on the molecular recognition and reversible binding of this important biological anion. Using a series of imidazole and imidazolium host molecules, we investigate the role of preorganization and charge on HS- binding. Using a macrocyclic bis-imidazolium receptor, we demonstrate the unexpected 2:1 host-guest binding of HS-, which was characterized both in solution and by X-ray crystallography. To the best of our knowledge, this is the first example of this binding stoichiometry for HS- binding. Moreover, the short C-H···S distances of 2.53, 2.54, 2.76, and 2.79 Å are well within the sum of the van der Waals radii of the interacting atoms, which is consistent with strong C-H···S interactions.

Synthesis of Terminal Bis(hydrosulfido) and Disulfido Complexes of Ni(II) from a Geometrically Frustrated Tetrahedral Ni(II) Chloride Complex

Recent studies have highlighted how reactive sulfur species (RSS) can be regulated and transported by metal-sulfur coordination compounds. We report herein the reactivity of ^PhB(^tBuIm)₃NiCl (1) with RSS, including the hydrosulfide anion ([Bu₄N][SH]) and a reduced tetrasulfide ([K18-C-6]₂[S₄]). The strongly donating tris(carbene) ligand in 1 is geometrically constrained to a tetrahedral geometry, and the energetically preferable square planar geometry is not achievable with the [^PhB(^tBuIm)₃]^- ligand. Upon reaction of 1 with [Bu₄N][SH] and [K18-C-6]₂[S₄], the square planar complexes ^PhB(^tBuIm)₂(^tBuImH)Ni(SH)₂ (2) and ^PhB(^tBuIm)₂(^tBuImH)Ni(η²-S₂) (3) are formed, respectively, via the protonation of one carbene ligand donor atom. Mechanistic investigation suggest that protonation occurs either from decomposition of 1 during the reaction progress, reactions with advantageous [Bu₄N]⁺/[K18-C-6]⁺ countercations or from the generation of transient unidentified RSS that facilitate proton transfer reactions.

Hydrosulfide-selective ChemFETs for aqueous H2S/HS- measurement

We have prepared and characterized hydrosulfide-selective ChemFET devices based on a nitrile butadiene rubber membrane containing tetraoctylammonium nitrate as a chemical recognition element that is applied to commercially available field-effect transistors. The sensors have fast (120 s) reversible responses, selectivity over other biologically relevant thiol-containing species, detection limits of 8 mM, and a detection range from approximately 5 to 500 mM. Sensitivities are shown to be 53 mV per decade at pH 8. Use of this compact, benchtop sensor platform requires little training – only the ability to measure DC voltage, which can be accomplished with a conventional multimeter or a simple analog data acquisition device paired with a personal computer. To the best of our knowledge, this report describes the first example of direct potentiometric measurement of the hydrosulfide ion in water.

Chalcogenido-Dimethylgallates and -Indates DMPyr2 [Me2 M(μ2 -E)]2 (M=Ga, In; E=S, Se): Building Blocks for Higher and Lower Order Chalcogenidoindates

Metalation of the anions in the ionic liquids DMPyr[SH] and DMPyr[SeH] (DMPyr=1,1-dimethylpyrrolidinium) by trimethylgallium and trimethylindium is investigated. The reaction proceeds via pre-coordination of [EH]- , methane elimination and formation of an unprecedented series of chalcogenido metalates DMPyr2 [Me2 M(μ2 -E)]2 (M=Ga, In; E=S, Se). These show the presences of dinuclear dianions with four-membered ring structures displaying highly nucleophilic bridging chalcogenide ligands in their crystallographically determined molecular structures. Some representative reactions of these building blocks with amphoteric electrophiles were studied: Addition of two equivalents of E(SiMe3 )2 (E=S, Se) to the indates DMPyr2 [Me2 In(μ2 -S)]2 and DMPyr2 [Me2 In(μ2 -Se)]2 leads to a cleavage of the ring, E silylation and formation of mononuclear, monoanionic indates DMPyr[Me2 In(SSiMe3 )2 ], DMPyr[Me2 In(SeSiMe3 )2 ], and even a mixed sulfido-selenido dimethylindate DMPyr[Me2 In(SSiMe3 )(SeSiMe3 )]. Reaction of DMPyr2 [Me2 In(μ2 -S)]2 with two equivalents of Lewis acid Me3 In leads to charge delocalization, ring expansion and formation of six-membered ring DMPyr3 [Me2 In(μ2 -S-InMe3 )]3 . The latter is a key intermediate in the formation of dianionic sulfidoindate DMPyr2 [(Me2 In)6 (μ3 -S)4 ] displaying an unusual inverse heteroadamantane cage structure with four capping sulfido ligands.

A two-component charge transfer hydrogel with excellent sensitivity towards the microenvironment: a responsive platform for biogenic thiols

A two-component charge transfer (CT) hydrogel has been derived from a supramolecular heteroassembly of a pyrene amino acid conjugate (PyHisOH, donor) with a 4-chloro-7-nitrobenzofurazan (NBD-Ox, acceptor) derivative in aqueous medium. The mechanical stiffness, as well as the thermal stability of the CT hydrogels largely depend on the relative ratios of donor and acceptor units as well as on their overall concentration. Moreover, the gel-to-sol transition is found to be susceptible to various external stimuli such as heat, pH, metal ions, etc. Circular dichroism and morphological investigation reveal the formation of left-handed helical fibers in the CT gel network. XRD studies show the lamellar packing of the interactive units in the 3D network of the CT hydrogel. The determination of different rheological parameters confirms the viscoelastic as well as the thixotropic nature of the CT gel. Furthermore, the CT gel is employed for turn-on sensing of biogenic thiols, cyan fluorescence was observed with cysteine/homocysteine, while blue fluorescence with glutathione. Nucleophilic attack at the NBD moiety leads to the formation of thermodynamically stable amino-linked derivatives for cysteine or homocysteine and kinetically controlled thiol-linked adduct for glutathione. Thus, the current system presents a unique opportunity, where a CT hydrogel sample is involved for discriminating biogenic thiols via specific chemodosimetric interactions.

Tuning Supramolecular Selectivity for Hydrosulfide: Linear Free Energy Relationships Reveal Preferential C-H Hydrogen Bond Interactions

Supramolecular anion receptors can be used to study the molecular recognition properties of the reactive yet biologically critical hydrochalcogenide anions (HCh^-). Achieving selectivity for HCh^- over the halides is challenging but necessary for not only developing future supramolecular probes for HCh^- binding and detection, but also for understanding the fundamental properties that govern these binding and recognition events. Here we demonstrate that linear free energy relationships (LFERs)-including Hammett and Swain-Lupton plots-reveal a clear difference in sensitivity to the polarity of an aryl C-H hydrogen bond (HB) donor for HS^- over other HCh^- and halides. Analysis using electrostatic potential maps highlights that this difference in sensitivity results from a preference of the aryl C-H HB donor for HS^- in this host scaffold. From this study, we demonstrate that LFERs are a powerful tool to gain interpretative insight into motif design for future anion-selective supramolecular receptors and highlight the importance of C-H HB donors for HS^- recognition. From our results, we suggest that aryl C-H HB donors should be investigated in the next generation of HS^- selective receptors based on the enhanced HS^- selectivity over other competing anions in this system.

Hydrosulfide complexes of the transition elements: diverse roles in bioinorganic, cluster, coordination, and organometallic chemistry

Molecules containing transition metal hydrosulfide linkages are diverse, spanning a variety of elements, coordination environments, and redox states, and carrying out multiple roles across several fields of chemistry.

Speciation of reactive sulfur species and their reactions with alkylating agents: do we have any clue about what is present inside the cell?

BACKGROUND AND PURPOSE: Posttranslational modifications of cysteine residues represent a major aspect of redox biology, and their reliable detection is key in providing mechanistic insights. The metastable character of these modifications and cell lysis-induced artifactual oxidation render current state-of-the-art protocols to rely on alkylation-based stabilization of labile cysteine derivatives before cell/tissue rupture. An untested assumption in these procedures is that for all cysteine derivatives, alkylation rates are faster than their dynamic interchange. However, when the interconversion of cysteine derivatives is not rate limiting, electrophilic labelling is under Curtin-Hammett control; hence, the final alkylated mixture may not represent the speciation that prevailed before alkylation.

EXPERIMENTAL APPROACH

Buffered aqueous solutions of inorganic, organic, cysteine, GSH and GAPDH polysulfide species were used. Additional experiments in human plasma and serum revealed that monobromobimane can extract sulfide from the endogenous sulfur pool by shifting speciation equilibria, suggesting caution should be exercised when interpreting experimental results using this tool.

KEY RESULTS

In the majority of cases, the speciation of alkylated polysulfide/thiol derivatives depended on the experimental conditions. Alkylation perturbed sulfur speciation in both a concentration- and time-dependent manner and strong alkylating agents cleaved polysulfur chains. Moreover, the labelling of sulfenic acids with dimedone also affected cysteine speciation, suggesting that part of the endogenous pool of products previously believed to represent sulfenic acid species may represent polysulfides.

CONCLUSIONS AND IMPLICATIONS

We highlight methodological caveats potentially arising from these pitfalls and conclude that current derivatization strategies often fail to adequately capture physiological speciation of sulfur species.

LINKED ARTICLES

This article is part of a themed section on Chemical Biology of Reactive Sulfur Species. To view the other articles in this section visit http://onlinelibrary.wiley.com/doi/10.1111/bph.v176.4/issuetoc.

Systematic study on anion–cation interactions via doubly ionic H-bonds in 1,3-dimethylimidazolium salts comprising chalcogenolate anions MMIm [ER] (E = S, Se; R = H, tBu, SiMe3)

We present convenient syntheses of so far inaccessible, crystalline and highly pure 1,3-dialkylimidazolium salts with extremely nucleophilic thiolate and selenolate anions [ER]⁻ (R = H, tBu, SiMe₃).

The role of polysulfide dianions and radical anions in the chemical, physical and biological sciences, including sulfur-based batteries

Polysulfide dianions and radical anions play a crucial role in biological chemistry, geochemical processes, alkali metal–sulfur batteries, organic syntheses, coordination chemistry, and materials sciences.

Expanding reversible chalcogenide binding: supramolecular receptors for the hydroselenide (HSe-) anion

Synthetic supramolecular receptors have been widely used to study reversible solution binding of anions; however, few systems target highly-reactive species. In particular, the hydrochalcogenide anions hydrosulfide (HS^-) and hydroselenide (HSe^-) have been largely overlooked despite their critical roles in biological systems. Herein we present the first example of reversible HSe^- binding in two distinct synthetic supramolecular receptors, using hydrogen bonds from N-H and aromatic C-H moieties. The arylethynyl bisurea scaffold 1 ^{t Bu} achieved a binding affinity of 460 ± 50 M^-1 for HSe^- in 10% DMSO-d ₆/CD₃CN, whereas the tripodal-based receptor 2^CF₃ achieved a binding affinity of 290 ± 50 M^-1 in CD₃CN. Association constants were also measured for HS^-, Cl^-, and Br^-, and both receptors favored binding of smaller, more basic anions. These studies contribute to a better understanding of chalcogenide hydrogen bonding and provide insights into further development of probes for the reversible binding, and potential quantification, of HSe^- and HS^-.

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.

A practical guide to working with H2S at the interface of chemistry and biology

Hydrogen sulfide (H₂S) is the most recently accepted endogenously produced gasotransmitter and is now implicated in a variety of physiological functions. In this tutorial review, our goal is to provide researchers new to the field of H₂S chemical biology with practical considerations, pitfalls, and best practices to enable smooth entry into investigations focused on biological H₂S. We present practical handling and safety considerations for working with this reactive biomolecule, and cover basic roles of H₂S biogenesis and action. Experimental methods for modulating H₂S levels, including enzymatic knockout, RNA silencing, enzymatic inhibition, and use of small molecule H₂S donors are highlighted. Complementing H₂S modulation techniques, we also highlight current strategies for H₂S detection and quantification.

Detection of sulfide ion and gaseous H2S using a series of pyridine-2,6-dicarboxamide based scaffolds

This work presents a series of pyridine-2,6-dicarboxamide based scaffolds with different appendages and their roles as chemosensors for the selective detection of S²⁻ ion, as well as gaseous H₂S, in primarily aqueous media.

Computational Study of H2S Release in Reactions of Diallyl Polysulfides with Thiols

Hydrogen sulfide (H₂S) is a gasotransmitter molecule recognized for its role in cell signaling. Garlic-derived polysulfides including diallyl disulfide (DADS) and diallyl trisulfide (DATS) have been shown to release H₂S. We investigated the mechanism of the reaction of DADS and DATS with biological thiols, including cysteine (Cys) and glutathione (GSH), using density functional theory. We propose that Cys and GSH react with DADS and DATS in their anionic forms. Thiol anions are much more likely to attack the sulfur atoms of DADS and DATS than the α-carbon of allyl groups. We found that nucleophilic attack of thiol anions on the peripheral sulfur of DATS is kinetically and thermodynamically more favorable than that on the central sulfur atom, resulting in the formation of allyl perthiol anion (ASS^-). In the presence of Cys or GSH, H₂S is released by proton-shuffle from the thiol to ASS^-, followed by another nucleophilic attack by thiol anion on ASSH. Our computed potential energy surfaces revealed that GSH and Cys are capable of releasing H₂S from DATS and that DADS is a much poorer H₂S donor than DATS.

Spectroscopic investigation of the reaction of metallo-protoporphyrins with hydrogen sulfide

Hydrogen sulfide (H₂S) is the most recently discovered gasotransmitter molecule joining nitric oxide and carbon monoxide. In addition to being biologically important gases, these gasotransmitters also provide distinct modes of reactivity with biomimetic metal complexes. The majority of previous investigations on the reactivity of H₂S with bioinorganic models have focused on Fe-based porphyrin systems, whereas investigations with other metals remains underinvestigated. To address this gap, we report here an examination of the reactions of H₂S, HS^-, and S₈ with Mg^II, Cu^II, Co^II, Zn^II, Cr^II, Sn^IV, and Mn^II/III protoporphyrins.

Fluorescent detection of multiple ions by two related chemosensors: structural elucidations and logic gate applications

Two related chemosensors L1 and L2 display selective detection of multiple ions (Cu²⁺, Al³⁺, Cd²⁺ and S²⁻) as a result of minor variation of functional groups at a remote arene ring.

The Intersection of NO and H2S: Persulfides Generate NO from Nitrite through Polysulfide Formation

Hydrogen sulfide (H₂S) and nitric oxide (NO) are important biosignaling molecules, and their biochemistries are increasingly recognized to be intertwined. Persulfides are an oxidized product of biological H₂S and have emerged as important species involved in the biological action of reactive sulfur species. Using isolated persulfides, we employed a combination of experimental and computational methods to investigate the contribution of persulfides to H₂S/NO crosstalk. Our studies demonstrate that isolated persulfides react with nitrite to produce NO via polysulfide and perthionitrite intermediates. These results highlight the importance of persulfides, polysulfides, and perthionitrite as intertwined reactive nitrogen and sulfur species.

A Synthetic Supramolecular Receptor for the Hydrosulfide Anion

Hydrogen sulfide (H2 S) has emerged as a crucial biomolecule in physiology and cellular signaling. Key challenges associated with developing new chemical tools for understanding the biological roles of H2 S include developing platforms that enable reversible binding of this important biomolecule. The first synthetic small molecule receptor for the hydrosulfide anion, HS(-) , using only reversible, hydrogen-bonding interactions in a series of bis(ethynylaniline) derivatives, is reported. Binding constants of up to 90 300±8700 m(-1) were obtained in MeCN. The fundamental science of reversible sulfide binding, in this case featuring a key CH⋅⋅⋅S hydrogen bond, will expand the possibility for discovery of sulfide protein targets and molecular recognition agents.

Stabilization of a Zn(ii) hydrosulfido complex utilizing a hydrogen-bond accepting ligand

Inclusion of a hydrogen bond accepting motif in the secondary coordination sphere of a pyridinediimine ligand enables formation of a stable Zn–SH adduct. We report here reversible coordination of HS⁻ to Zn(didpa)Cl₂ to form [Zn(didpa)Cl₂SH]⁻, which is stabilized by an intramolecular hydrogen bond.

Spectroscopic investigations into the binding of hydrogen sulfide to synthetic picket-fence porphyrins

The picket-fence porphyrin system is used a model for a sterically-constrained, protected binding environment to study H₂S and HS⁻ligation.

Mechanistic Insights into the H₂S-Mediated Reduction of Aryl Azides Commonly Used in H₂S Detection

Hydrogen sulfide (H2S) is an important biological mediator and has been at the center of a rapidly expanding field focused on understanding the biogenesis and action of H2S as well as other sulfur-related species. Concomitant with this expansion has been the development of new chemical tools for H2S research. The use of H2S-selective fluorescent probes that function by H2S-mediated reduction of fluorogenic aryl azides has emerged as one of the most common methods for H2S detection. Despite this prevalence, the mechanism of this important reaction remains under-scrutinized. Here we present a combined experimental and computational investigation of this mechanism. We establish that HS(-), rather than diprotic H2S, is the active species required for aryl azide reduction. The hydrosulfide anion functions as a one-electron reductant, resulting in the formation of polysulfide anions, such as HS2(-), which were confirmed and trapped as organic polysulfides by benzyl chloride. The overall reaction is first-order in both azide and HS(-) under the investigated experimental conditions with ΔS(⧧) = -14(2) eu and ΔH(⧧) = 13.8(5) kcal/mol in buffered aqueous solution. By using NBu4SH as the sulfide source, we were able to observe a reaction intermediate (λ(max) = 473 nm), which we attribute to formation of an anionic azidothiol intermediate. Our mechanistic investigations support that this intermediate is attacked by HS(-) in the rate-limiting step of the reduction reaction. Complementing our experimental mechanistic investigations, we also performed DFT calculations at the B3LYP/6-31G(d,p), B3LYP/6-311++G(d,p), M06/TZVP, and M06/def2-TZVPD levels of theory applying the IEF-PCM water and MeCN solvation models, all of which support the experimentally determined reaction mechanism and provide cohesive mechanistic insights into H2S-mediated aryl azide reduction.