Sulfur Transfer Reactions Research Articles

One-Pot Telescoping S-Transfer and Trifluoromethylation for the Synthesis of 2-CF3S-Imidazoles with N-Oxides as Convenient Precursors.

Readily available 2-unsubstituted imidazole N-oxides were examined as starting materials for the preparation of fully substituted 1,4,5-aryl/alkyl 2-trifluoromethylsulfanyl-imidazoles. Whereas activation of the N-oxide function followed by attempted nucleophilic addition of the -SCF3 was in vain, the alternative approach involving "sulfur transfer reaction" and subsequent electrophilic trifluoromethylation with Togni reagent provided target products in high yield via a one-pot procedure. The structure of representative enantiomerically pure imidazol-2-yl trifluoromethyl sulfide was confirmed by X-ray analysis.

Structural and biochemical characterization of an encapsulin-associated rhodanese from Acinetobacter baumannii.

Rhodanese-like domains (RLDs) represent a widespread protein family canonically involved in sulfur transfer reactions between diverse donor and acceptor molecules. RLDs mediate these transsulfuration reactions via a transient persulfide intermediate, created by modifying a conserved cysteine residue in their active sites. RLDs are involved in various aspects of sulfur metabolism, including sulfide oxidation in mitochondria, iron-sulfur cluster biogenesis, and thio-cofactor biosynthesis. However, due to the inherent complexity of sulfur metabolism caused by the intrinsically high nucleophilicity and redox sensitivity of thiol-containing compounds, the physiological functions of many RLDs remain to be explored. Here, we focus on a single domain Acinetobacter baumannii RLD (Ab-RLD) associated with a desulfurase encapsulin which is able to store substantial amounts of sulfur inside its protein shell. We determine the 1.6 Å x-ray crystal structure of Ab-RLD, highlighting a homodimeric structure with a number of unusual features. We show through kinetic analysis that Ab-RLD exhibits thiosulfate sulfurtransferase activity with both cyanide and glutathione acceptors. Using native mass spectrometry and in vitro assays, we provide evidence that Ab-RLD can stably carry a persulfide and thiosulfate modification and may employ a ternary catalytic mechanism. Our results will inform future studies aimed at investigating the functional link between Ab-RLD and the desulfurase encapsulin.

Structural and Biochemical Characterization of Mycobacterium tuberculosis Zinc SufU-SufS Complex.

Iron-sulfur (Fe-S) clusters are inorganic prosthetic groups in proteins composed exclusively of iron and inorganic sulfide. These cofactors are required in a wide range of critical cellular pathways. Iron-sulfur clusters do not form spontaneously in vivo; several proteins are required to mobilize sulfur and iron, assemble and traffic-nascent clusters. Bacteria have developed several Fe-S assembly systems, such as the ISC, NIF, and SUF systems. Interestingly, in Mycobacterium tuberculosis (Mtb), the causative agent of tuberculosis (TB), the SUF machinery is the primary Fe-S biogenesis system. This operon is essential for the viability of Mtb under normal growth conditions, and the genes it contains are known to be vulnerable, revealing the Mtb SUF system as an interesting target in the fight against tuberculosis. In the present study, two proteins of the Mtb SUF system were characterized for the first time: Rv1464(sufS) and Rv1465(sufU). The results presented reveal how these two proteins work together and thus provide insights into Fe-S biogenesis/metabolism by this pathogen. Combining biochemistry and structural approaches, we showed that Rv1464 is a type II cysteine-desulfurase enzyme and that Rv1465 is a zinc-dependent protein interacting with Rv1464. Endowed with a sulfurtransferase activity, Rv1465 significantly enhances the cysteine-desulfurase activity of Rv1464 by transferring the sulfur atom from persulfide on Rv1464 to its conserved Cys40 residue. The zinc ion is important for the sulfur transfer reaction between SufS and SufU, and His354 in SufS plays an essential role in this reaction. Finally, we showed that Mtb SufS-SufU is more resistant to oxidative stress than E. coli SufS-SufE and that the presence of zinc in SufU is likely responsible for this improved resistance. This study on Rv1464 and Rv1465 will help guide the design of future anti-tuberculosis agents.

Rhodanese-Fold Containing Proteins in Humans: Not Just Key Players in Sulfur Trafficking.

The Rhodanese-fold is a ubiquitous structural domain present in various protein subfamilies associated with different physiological functions or pathophysiological conditions in humans. Proteins harboring a Rhodanese domain are diverse in terms of domain architecture, with some representatives exhibiting one or several Rhodanese domains, fused or not to other structural domains. The most famous Rhodanese domains are catalytically active, thanks to an active-site loop containing an essential cysteine residue which allows for catalyzing sulfur transfer reactions involved in sulfur trafficking, hydrogen sulfide metabolism, biosynthesis of molybdenum cofactor, thio-modification of tRNAs or protein urmylation. In addition, they also catalyse phosphatase reactions linked to cell cycle regulation, and recent advances proposed a new role into tRNA hydroxylation, illustrating the catalytic versatility of Rhodanese domain. To date, no exhaustive analysis of Rhodanese containing protein equipment from humans is available. In this review, we focus on structural and biochemical properties of human-active Rhodanese-containing proteins, in order to provide a picture of their established or putative key roles in many essential biological functions.

Synthesis and Selected Transformations of 2-Unsubstituted Imidazole N-Oxides Using a Ball-Milling Mechanochemical Approach

Synthetically relevant 2-unsubstituted imidazole N-oxides were obtained by using the ball-milling mechanochemical method. The presented approach extended the scope of the known method and enabled the preparation of hitherto little known N(1)-aryl-substituted derivatives, which are of interest as starting materials for the synthesis of more complex imidazole-based organic materials, generally in good to excellent yields. In addition, selected one-pot mechanochemical transformations including N- and O-alkylations as well as sulfur transfer reactions based on either (3+2)-cycloaddition reaction with 2,2,4,4-tetramethylcyclobutane-1,3-dithione or sulfurization of the transient imidazol-2-ylidenes, generated from corresponding imidazolium salts, were studied. The reported results can be considered as a continuation of long-term studies focused on the synthesis and applications of 2-unsubstituted imidazole N-oxides.

Biosynthesis of Chuangxinmycin Featuring a Deubiquitinase-like Sulfurtransferase.

The knowledge on sulfur incorporation mechanism involved in sulfur-containing molecule biosynthesis remains limited. Chuangxinmycin is a sulfur-containing antibiotic with a unique thiopyrano[4,3,2-cd]indole (TPI) skeleton and selective inhibitory activity against bacterial tryptophanyl-tRNA synthetase. Despite the previously reported biosynthetic gene clusters and the recent functional characterization of a P450 enzyme responsible for C-S bond formation, the enzymatic mechanism for sulfur incorporation remains unknown. Here, we resolve this central biosynthetic problem by in vitro biochemical characterization of the key enzymes and reconstitute the TPI skeleton in a one-pot enzymatic reaction. We reveal that the JAMM/MPN+ protein Cxm3 functions as a deubiquitinase-like sulfurtransferase to catalyze a non-classical sulfur-transfer reaction by interacting with the ubiquitin-like sulfur carrier protein Cxm4GG. This finding adds a new mechanism for sulfurtransferase in nature.

Coordination Properties of Non-Rigid Phosphinoyldithioformate Complexes of the [Mo2O2(µ-S)2]2+ Cation in Catalytic Sulfur Transfer Reactions with Thiiranes

Two “Mo2O2S2”-based complexes with phosphinoyldithioformate ligands were synthesized from the metathesis reaction of [R2P(O)CS2]− with (Me4N)2[Mo2O2(µ-S)2(Cl)4] to give [Mo2O2(µ-S)2{R2P(O)CS2}2] (1; R = Ph, 2; R = Bn). The complexes were fully characterized, including the X-ray crystal structure for 1. Variable temperatures 31P NMR of 1 and 2 exhibit non-rigid behavior in solution where three and two coordination isomers were present, respectively. The organic substituent on the P atom greatly impacts the complex non-rigid properties and behavior. The catalytic activity of 1 and 2 towards sulfur atom transfer (SAT) using propylene sulfide and cyclohexene sulfide was explored, employing homogeneous reaction conditions at an ambient temperature on the NMR scale. The complexes showed distinctly different properties along with high conversions in short reaction times. A catalytic cycle consistent with the results is proposed.

Molecular Mechanism of ISC Iron-Sulfur Cluster Biogenesis Revealed by High-Resolution Native Mass Spectrometry.

Iron-sulfur (Fe-S) clusters are ubiquitous protein cofactors that are required for many important biological processes including oxidative respiration, nitrogen fixation, and photosynthesis. Biosynthetic pathways assemble Fe-S clusters with different iron-to-sulfur stoichiometries and distribute these clusters to appropriate apoproteins. In the ISC pathway, the pyridoxal 5'-phosphate-dependent cysteine desulfurase enzyme IscS provides sulfur to the scaffold protein IscU, which templates the Fe-S cluster assembly. Despite their functional importance, mechanistic details for cluster synthesis have remained elusive. Recent advances in native mass spectrometry (MS) have allowed proteins to be preserved in native-like structures and support applications in the investigation of protein structure, dynamics, ligand interactions, and the identification of protein-associated intermediates. Here, we prepared samples under anaerobic conditions and then applied native MS to investigate the molecular mechanism for Fe-S cluster synthesis. This approach was validated by the high agreement between native MS and traditional visible circular dichroism spectroscopic assays. Time-dependent native MS experiments revealed potential iron- and sulfur-based intermediates that decay as the [2Fe-2S] cluster signal developed. Additional experiments establish that (i) Zn(II) binding stabilizes IscU and protects the cysteine residues from oxidation, weakens the interactions between IscU and IscS, and inhibits Fe-S cluster biosynthesis; and (ii) Fe(II) ions bind to the IscU active site cysteine residues and another lower affinity binding site and promote the intermolecular sulfur transfer reaction from IscS to IscU. Overall, these results support an iron-first model for Fe-S cluster synthesis and highlight the power of native MS in defining protein-associated intermediates and elucidating mechanistic details of enzymatic processes.

Sulfur transfer reactions of a zinc tetrasulfanido complex.

A zinc tetrasulfanido complex supported by a bis(carboxamide)pyridine ligand framework has been synthesized by the insertion of elemental sulfur into the zinc-S(thiolate) bond of a zinc dithiolate complex ([LZn]2-). This paper reports on sulfur transfer reactions of this polysulfanido complex ([1]2-) and compares this behavior to known reactions of metal polysulfido complexes. Complex [1]2- was demonstrated to be in exchange with [LZn]2- and free elemental sulfur in solution. Although triphenylphosphine abstracts sulfur from [1]2- to form [LZn]2-, complex [LZn]2- can abstract sulfur from the zinc polysulfido complex (TMEDA)ZnS6 (TMEDA = N,N,N',N'-tetramethylethylenediamine). The tetrasulfanido complex [1]2- can also transfer sulfur to dimethyl acetylenedicarboxylate to form a zinc dithiolene complex. These studies demonstrate that the zinc complex with a tetrasulfanido moiety can undergo similar reactions as metal complexes with purely inorganic polysulfido groups, although the final metal-containing products are different.

Synthesis and characterization of asymmetric [Mo2O2(μ-S)2(S2)(L)] complexes (L = bipy, en, dien) and their heterogeneous reaction with propylene sulfide

Crystalline and amorphous molybdenum sulfide “MoSx” catalysts are well established in sulfur removal from petrochemicals in heterogeneous reactions. Accessibility of sulfido ligands on the catalyst surface is generally accepted as pre-requisite for catalytic initiation. Interest in study of discrete molecules with similar active sites for sulfur transfer reactions in organic reactions prompted synthesis of the asymmetric dinuclear complexes, [Mo2O2(μ-S)2(S2)(L)] (1, L = bipyridine (bipy); 2, L = ethylene diamine (en), 3, L = diethylenediamine (dien)) in facile ligand exchange reaction. A single crystal of 1 was obtained from DMF/ether solution as dark red plates and analyzed by X-ray diffraction. The complexes remove sulfur from propylene sulfide in a heterogeneous reaction at ambient temperature on the NMR scale. Reactivity expected to reflect the donation properties of L with the largest LMCT contribution rendering the most reactive sulfido ligand yielded surprising results. DFT calculations of the optimized minimal energy structures of all three complexes were performed and several possible isomeric structures evaluated. The optimal energy form of 1 compared well with its crystal structure and results from calculated structures of 2 and 3 agree with spectroscopic observations. The modest reactivity is in agreement with poor access to the disulfide as inferred by DFT structures of the complexes and the crystal structure of 1 where strong SS interactions were found along its b-axis.

X-Ray Structure of Human Sulfide:Quinone Oxidoreductase: Insights into the Mechanism of Mitochondrial Hydrogen Sulfide Oxidation

Hydrogen sulfide (H2S) is a gasotransmitter exhibiting pivotal functions in diverse biological processes, including activation of multiple cardioprotective pathways. Sulfide:quinone oxidoreductase (SQOR) is an integral membrane flavoprotein that catalyzes the first step in the mitochondrial metabolism of H2S. As such, it plays a critical role in controlling physiological levels of the gasotransmitter and has attracted keen interest as a potential drug target. We report the crystal structure of human SQOR, unraveling the molecular basis for the enzyme's ability to catalyze sulfane sulfur transfer reactions with structurally diverse acceptors. We demonstrate that human SQOR contains unique features: an electropositive surface depression implicated as a binding site for sulfane sulfur acceptors and postulated to funnel negatively charged substrates to a hydrophilic H2S-oxidizing active site, which is connected to a hydrophobic internal tunnel that binds coenzyme Q. These findings support a proposed model for catalysis and open the door for structure-based drug design.

Same but different: Comparison of two system-specific molecular chaperones for the maturation of formate dehydrogenases

The maturation of bacterial molybdoenzymes is a complex process leading to the insertion of the bulky bis-molybdopterin guanine dinucleotide (bis-MGD) cofactor into the apo-enzyme. Most molybdoenzymes were shown to contain a specific chaperone for the insertion of the bis-MGD cofactor. Formate dehydrogenases (FDH) together with their molecular chaperone partner seem to display an exception to this specificity rule, since the chaperone FdhD has been proven to be involved in the maturation of all three FDH enzymes present in Escherichia coli. Multiple roles have been suggested for FdhD-like chaperones in the past, including the involvement in a sulfur transfer reaction from the l-cysteine desulfurase IscS to bis-MGD by the action of two cysteine residues present in a conserved CXXC motif of the chaperones. However, in this study we show by phylogenetic analyses that the CXXC motif is not conserved among FdhD-like chaperones. We compared in detail the FdhD-like homologues from Rhodobacter capsulatus and E. coli and show that their roles in the maturation of FDH enzymes from different subgroups can be exchanged. We reveal that bis-MGD-binding is a common characteristic of FdhD-like proteins and that the cofactor is bound with a sulfido-ligand at the molybdenum atom to the chaperone. Generally, we reveal that the cysteine residues in the motif CXXC of the chaperone are not essential for the production of active FDH enzymes.

Sulfur Radical Transfer and Coupling Reaction to Benzoxazine Groups: A New Reaction Route for Preparation of Polymeric Materials Using Elemental Sulfur as a Feedstock.

A novel approach to preparing polymeric materials using elemental sulfur as a feedstock through the newly developed sulfur radical transfer and coupling (SRTC) reaction is reported herein. Polybenzoxazines with high sulfur contents are prepared using the SRTC reaction with benzoxazine compounds as the radical acceptors. The reactions between elemental sulfur and benzoxazine rings are analyzed with Fourier transform infrared (FTIR), 1 H NMR, and 13 C DEPT spectroscopies to elucidate the SRTC reaction mechanism. Moreover, the prepared polybenzoxazine-sulfur hybrid materials show attractive repairing properties based on the dynamic S-S linkages. An effective reaction mechanism and the prepared repairable sulfur-possessing polymeric materials are demonstrated.

Mechanism of Sulfur Transfer Across Protein–Protein Interfaces: The Cysteine Desulfurase Model System

CsdA cysteine desulfurase (the sulfur donor) and the CsdE sulfur acceptor are involved in biological sulfur trafficking and in iron–sulfur cluster assembly in the model bacterium Escherichia coli. CsdA and CsdE form a stable complex through a polar interface that includes CsdA Cys328 and CsdE Cys61, the two residues known to be involved in the sulfur transfer reaction. Although mechanisms for the transfer of a sulfur moiety across protein–protein interfaces have been proposed based on the IscS–IscU and IscS–TusA structures, the flexibility of the catalytic cysteine loops involved has precluded a high resolution view of the active-site geometry and chemical environment for sulfur transfer. Here, we have used a combination of X-ray crystallography, solution NMR and SAXS, isothermal calorimetry, and computational chemistry methods to unravel how CsdA provides a specific recognition platform for CsdE and how their complex organizes a composite functional reaction environment. The X-ray structures of persulfur...

Recent Progress in the Chemistry of 2-Unsubstituted 1H-Imidazole 3-Oxides

The 2-unsubstituted 1H-imidazole 3-oxides, considered as ‘nitrone-like’ azaheterocycles, are well known as versatile building blocks, useful for the preparation of more complex systems containing an imidazole moiety. Their applications in diverse reactions, such as isomerization to corresponding imidazol-2-ones, [3+2]-cycloadditions, sulfur transfer reactions leading to imidazole-2-thiones, heterocyclizations leading to fused heterocyclic systems, as well as cross coupling reactions are discussed. New methods for the preparation of enantiomerically pure 1H-imidazole 3-oxides and their applications in asymmetric reactions, synthesis of room temperature ionic liquids and biologically active compounds are also reviewed.

Functional investigation of Bacillus subtilis YrkF’s involvement in sulfur transfer reactions

AbstractSulfur incorporation into the molybdenum cofactor (Moco) in the Gram-negative bacterium Escherichia coli involves six enzymes. The initial reaction includes the cysteine desulfurase IscS, the sulfurtransferase TusA, and the rhodanese domaincontaining protein YnjE. The Gram-positive bacterium Bacillus subtilis contains no direct homologs for IscS, but rather four distinct cysteine desulfurases (YrvO, NifS, NifZ, SufS) and YrkF, a two-domain rhodanese protein with an N-terminal domain similar to TusA. Bioinformatic analysis was used to identify potential enzymes involved in the B. subtilis Moco thiolation pathway and in vitro reactions demonstrated that YrkF can accept sulfur from and enhance the activity of YrvO.

Carbon-, sulfur-, and phosphorus-based charge transfer reactions in inductively coupled plasma–atomic emission spectrometry

In this work, the influence of carbon-, sulfur-, and phosphorus-based charge transfer reactions on the emission signal of 34 elements (Ag, Al, As, Au, B, Ba, Be, Ca, Cd, Co, Cr, Cu, Fe, Ga, Hg, I, In, Ir, K, Li, Mg, Mn, Na, Ni, P, Pb, Pd, Pt, S, Sb, Se, Sr, Te, and Zn) in axially viewed inductively coupled plasma–atomic emission spectrometry has been investigated. To this end, atomic and ionic emission signals for diluted glycerol, sulfuric acid, and phosphoric acid solutions were registered and results were compared to those obtained for a 1% w w−1 nitric acid solution. Experimental results show that the emission intensities of As, Se, and Te atomic lines are enhanced by charge transfer from carbon, sulfur, and phosphorus ions. Iodine and P atomic emission is enhanced by carbon- and sulfur-based charge transfer whereas the Hg atomic emission signal is enhanced only by carbon. Though signal enhancement due to charge transfer reactions is also expected for ionic emission lines of the above-mentioned elements, no experimental evidence has been found with the exception of Hg ionic lines operating carbon solutions. The effect of carbon, sulfur, and phosphorus charge transfer reactions on atomic emission depends on (i) wavelength characteristics. In general, signal enhancement is more pronounced for electronic transitions involving the highest upper energy levels; (ii) plasma experimental conditions. The use of robust conditions (i.e. high r.f. power and lower nebulizer gas flow rates) improves carbon, sulfur, and phosphorus ionization in the plasma and, hence, signal enhancement; and (iii) the presence of other concomitants (e.g. K or Ca). Easily ionizable elements reduce ionization in the plasma and consequently reduce signal enhancement due to charge transfer reactions.

Polymorphic Variants of Human Rhodanese Exhibit Differences in Thermal Stability and Sulfur Transfer Kinetics

Rhodanese is a component of the mitochondrial H2S oxidation pathway. Rhodanese catalyzes the transfer of sulfane sulfur from glutathione persulfide (GSSH) to sulfite generating thiosulfate and from thiosulfate to cyanide generating thiocyanate. Two polymorphic variations have been identified in the rhodanese coding sequence in the French Caucasian population. The first, 306A→C, has an allelic frequency of 1% and results in an E102D substitution in the encoded protein. The second polymorphism, 853C→G, has an allelic frequency of 5% and leads to a P285A substitution. In this study, we have examined differences in the stability between wild-type rhodanese and the E102D and P285A variants and in the kinetics of the sulfur transfer reactions. The Asp-102 and Ala-285 variants are more stable than wild-type rhodanese and exhibit kcat/Km,CN values that are 17- and 1.6-fold higher, respectively. All three rhodanese forms preferentially catalyze sulfur transfer from GSSH to sulfite, generating thiosulfate and glutathione. The kcat/Km,sulfite values for the variants in the sulfur transfer reaction from GSSH to sulfite were 1.6- (Asp-102) and 4-fold (Ala-285) lower than for wild-type rhodanese, whereas the kcat/Km,GSSH values were similar for all three enzymes. Thiosulfate-dependent H2S production in murine liver lysate is low, consistent with a role for rhodanese in sulfide oxidation. Our studies show that polymorphic variations that are distant from the active site differentially modulate the sulfurtransferase activity of human rhodanese to cyanide versus sulfite and might be important in differences in susceptibility to diseases where rhodanese dysfunction has been implicated, e.g. inflammatory bowel diseases.

Synthesis of optically active polycyclic N-heterocycles derived from l-prolinamine

The N-Boc-protected (S)-prolinamine reacts readily with formaldehyde and diverse α-hydroxyimino ketones to give imidazole N-oxides with an enantiomerically pure N-protected (pyrrolidin-2-yl)methyl substituent. Subsequent deprotection yields the corresponding NH derivatives. Upon treatment of these products with Ac2O at room temperature, a cascade of reactions leads to optically active tricyclic products. In all these processes, the stereogenic center is preserved. In one case, the bis-heterocyclic imidazole N-oxide was transformed into the corresponding optically active imidazole-2-thione via a sulfur-transfer reaction with 2,2,4,4-tetramethylcyclobutane-1,3-dithione.

ChemInform Abstract: A Highly Efficient Cu‐Catalyzed S‐Transfer Reaction: From Amine to Sulfide.

AbstractA copper‐catalyzed sulfur transfer reaction between arylamines, alkyl halides, and sodium thiosulfate is described to convert aromatic amines to a variety of alkyl aryl sulfides in a one‐pot manner.