Molybdenum-Copper Antagonism In Metalloenzymes And Anti-Copper Therapy.

The connection between 3d (Cu) and 4d (Mo) via the "Mo-S-Cu" unit is called Mo-Cu antagonism. Biology offers case studies of such interactions in metalloproteins such as Mo/Cu-CO Dehydrogenases (Mo/Cu-CODH), and Mo/Cu Orange Protein (Mo/Cu-ORP). The CODH significantly maintains the CO level in the atmosphere below the toxic level by converting it to non-toxic CO2 for respiring organisms. Several models were synthesized to understand the structure-function relationship of these native enzymes. However, this interaction was first observed in ruminants, and they convert molybdate (MoO4 2- ) into tetrathiomolybdate (MoS4 2- ; TTM), reacting with cellular Cu to yield biological unavailable Mo/S/Cu cluster, then developing Cu-deficiency diseases. These findings inspire the use of TTM as a Cu-sequester drug, especially for treating Cu-dependent human diseases such as Wilson diseases (WD) and cancer. It is well known that a balanced Cu homeostasis is essential for a wide range of biological processes, but negative consequence leads to cell toxicity. Therefore, this review aims to connect the Mo-Cu antagonism in metalloproteins and anti-copper therapy.

Selenium-More than Just a Fortuitous Sulfur Substitute in Redox Biology.

Living organisms use selenium mainly in the form of selenocysteine in the active site of oxidoreductases. Here, selenium's unique chemistry is believed to modulate the reaction mechanism and enhance the catalytic efficiency of specific enzymes in ways not achievable with a sulfur-containing cysteine. However, despite the fact that selenium/sulfur have different physicochemical properties, several selenoproteins have fully functional cysteine-containing homologues and some organisms do not use selenocysteine at all. In this review, selected selenocysteine-containing proteins will be discussed to showcase both situations: (i) selenium as an obligatory element for the protein's physiological function, and (ii) selenium presenting no clear advantage over sulfur (functional proteins with either selenium or sulfur). Selenium's physiological roles in antioxidant defence (to maintain cellular redox status/hinder oxidative stress), hormone metabolism, DNA synthesis, and repair (maintain genetic stability) will be also highlighted, as well as selenium's role in human health. Formate dehydrogenases, hydrogenases, glutathione peroxidases, thioredoxin reductases, and iodothyronine deiodinases will be herein featured.

Cross-talk Between (Hydrogen)Sulfite and Metalloproteins: Impact on Human Health.

Sulfite is a potent toxic substance causing harm to multi-organ in human. Despite toxicity, it is widely used as preservative, anti-browning and anti-oxidant in foods, beverages, and pharmaceuticals, which cause easy admission of sulfite in human. Sulfite is also produced endogenously during the catabolism of cysteine and methionine. In vivo, the serum sulfite level at physiological range is strictly maintained by a molybdenum dependent sulfite oxidase (SO), which catalyzes sulfite to sulfate oxidation via a two-electron oxidation pathway. The loss of SO activity causes high serum sulfite level that fosters several diseases, including asthma, neurological dysfunction, birth defects, and heart diseases. The cytotoxicity of (bi)sulfite is implicated as sulfite radicals, which are generated by mainly heme-peroxidases via a one-electron oxidation pathway. On the other hand, the toxic sulfite radicals are neutralized to sulfite by heme-globins. The enzymatic reduction of sulfite to sulfide is catalyzed by sulfite reductase, which contains an unusual metal cofactor, siroheme-[4Fe4S]-cluster. Overall, the interaction of sulfite with various metalloproteins in vivo is a close relation with human health. Therefore, this review describes the metabolic conversion of (bi)sulfite to sulfate, sulfite radical or sulfide via oxidation or reduction pathways by various metalloproteins (specially SOs, peroxidases, heme-globins, and sulfite reductases), and the potential applications of sulfite in biosensors/biofuel cells, anti-browning, and advance oxidation process.

Bioengineered angiotensin-converting-enzyme-2: a potential therapeutic option against SARS-CoV-2 infection.

The recombinant soluble human angiotensin-converting enzyme 2 (rshACE2) is a promising therapy against SARS-CoV-2 infection, but it has some drawbacks that reduce the success of its clinical applications. The bioengineered ACE2 (Tag-sACE2 and probiotic-ACE2) as a way may overcome its therapeutic limitations against ongoing current pandemic.

Sulfide and transition metals - A partnership for life.

Sulfide and transition metals often came together in Biology. The variety of possible structural combinations enabled living organisms to evolve an array of highly versatile metal-sulfide centers to fulfill different physiological roles. The ubiquitous iron­sulfur centers, with their structural, redox, and functional diversity, are certainly the best-known partners, but other metal-sulfide centers, involving copper, nickel, molybdenum or tungsten, are equally crucial for Life. This review provides a concise overview of the exclusive sulfide properties as a metal ligand, with emphasis on the structural aspects and biosynthesis. Sulfide as catalyst and as a substrate is discussed. Different enzymes are considered, including xanthine oxidase, formate dehydrogenases, nitrogenases and carbon monoxide dehydrogenases. The sulfide effect on the activity and function of iron­sulfur, heme and zinc proteins is also addressed.

Designed Metal-ATCUN Derivatives: Redox- and Non-redox-Based Applications Relevant for Chemistry, Biology, and Medicine.

The designed "ATCUN" motif (amino-terminal copper and nickel binding site) is a replica of naturally occurring ATCUN site found in many proteins/peptides, and an attractive platform for multiple applications, which include nucleases, proteases, spectroscopic probes, imaging, and small molecule activation. ATCUN motifs are engineered at periphery by conjugation to recombinant proteins, peptides, fluorophores, or recognition domains through chemically or genetically, fulfilling the needs of various biological relevance and a wide range of practical usages. This chemistry has witnessed significant growth over the last few decades and several interesting ATCUN derivatives have been described. The redox role of the ATCUN moieties is also an important aspect to be considered. The redox potential of designed M-ATCUN derivatives is modulated by judicious choice of amino acid (including stereochemistry, charge, and position) that ultimately leads to the catalytic efficiency. In this context, a wide range of M-ATCUN derivatives have been designed purposefully for various redox- and non-redox-based applications, including spectroscopic probes, target-based catalytic metallodrugs, inhibition of amyloid-ß toxicity, and telomere shortening, enzyme inactivation, biomolecules stitching or modification, next-generation antibiotic, and small molecule activation.

Can Papain-like Protease Inhibitors Halt SARS-CoV-2 Replication?

SARS-CoV-2 encoded papain-like protease (PLpro) harbors a labile Zn site (Cys189-X-X-Cys192-X n -Cys224-X-Cys226) and a classic catalytic site (Cys111-His272-Asp286), which play key roles for viral replication and hence represent promising drug targets. In this Viewpoint, both sulfur-based drugs and peptides-based inhibitors may block Cys residues in the catalytic and/or Zn site of CoV-2-PLpro, leading to dysfunction of CoV-2-PLpro and thereby halting viral replication.

Heme/Hemeoxygenase-1 System Is a Potential Therapeutic Intervention for COVID-19 Patients with Severe Complications.

Acute respiratory distress syndrome (ARDS) is one of the critical stages of COVID-19, leading to lung injury and hemolysis. Dysfunctional hemoglobin (Hb) suffers low-level oxygenation, overloaded iron, and down-regulation of hemeoxygenase-1 (HO-1), representing potential therapeutic interventions. This Viewpoint outlines the Hb-HO-1 system as a host-cell target, and proposes possible therapies, including iron chelation and CO therapies, against COVID-19 with ARDS.

Potential Role of Peptide-Based Antiviral Therapy Against SARS-CoV-2 Infection.

Because of the uninterrupted spread of novel severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) infectious disease (COVID-19) with substantial illness and mortality rates, there is an urgent requirement of suitable antiviral agent/therapy to control this pandemic, but not yet established. The primary cause of SARS-CoV-2 infection is the crosstalk between the SARS-CoV-2 and host surface receptor protein, human angiotensin-converting enzyme 2 (hACE2), prior to cellular entry. Hence, blocking at the initial stage of virus entry could be a promising strategy/therapy to combat the SARS-CoV-2 infection. Many drugs as SARS-CoV-2 blocker have been proposed. Among them, peptide-based antivirals are one. This Viewpoint discusses the potential antiviral role and feasibility of two classes of peptides for prevention of SARS-CoV-2 infection, where (1) a designed peptide (replication of virus binding domain of hACE2), and (2) antimicrobial peptides (AMPs; natural and first line host defense peptide), both may reduce virus load into the host cell by blocking cellular surface receptors and/or disruption of virus cell membrane at the stage of virus entry. These finding may provide a novel antiviral therapy against COVID-19, which might control the current global health crisis.

NiII -ATCUN-Catalyzed Tyrosine Nitration in the Presence of Nitrite and Sulfite.

The nitration of tyrosine residues in proteins represents a specific footprint of the formation of reactive nitrogen species (RNS) in vivo. Here, the fusion product of orange protein (ATCUN-ORP) was used as an in vitro model system containing an amino terminal Cu(II)- and Ni(II)-binding motif (ATCUN) tag at the N-terminus and a native tyrosine residue in the metal-cofactor-binding region for the formation of 3-NO2 -Tyr (3-NT). It is shown that NiII -ATCUN unusually performs nitration of tyrosine at physiological pH in the presence of the NO2 - /SO3 2- /O2 system, which is revealed by a characteristic absorbance band at 430 nm in basic medium and 350 nm in acidic medium (fingerprint of 3-NT). Kinetics studies showed that the formation of 3-NT depends on sulfite concentration over nitrite concentration suggesting key intermediate products, identified as oxysulfur radicals, which are detected by spin-trap EPR study by using 5,5-dimethyl-1-pyrroline-N-oxide (DMPO). This study describes a new route in the formation of 3-NT, which is proposed to be linked with the sulfur metabolism pathway associated with the progression of disease occurrence in vivo.

Unusual Reduction Mechanism of Copper in Cysteine-Rich Environment.

Copper-cysteine interactions play an important role in Biology and herein we used the copper-substituted rubredoxin (Cu-Rd) from Desulfovibrio gigas to gain further insights into the copper-cysteine redox chemistry. EPR spectroscopy results are consistent with Cu-Rd harboring a CuII center in a sulfur-rich coordination, in a distorted tetrahedral structure ( g∥,⊥ = 2.183 and 2.032 and A∥,⊥ = 76.4 × 10-4 and 12 × 10-4 cm-1). In Cu-Rd, two oxidation states at Cu-center (CuII and CuI) are associated with Cys oxidation-reduction, alternating in the redox cycle, as pointed by electrochemical studies that suggest internal geometry rearrangements associated with the electron transfer processes. The midpoint potential of [CuI(S-Cys)2(Cys-S-S-Cys)]/[CuII(S-Cys)4] redox couple was found to be -0.15 V vs NHE showing a large separation of cathodic and anodic peaks potential (Δ Ep = 0.575 V). Interestingly, sulfur-rich CuII-Rd is highly stable under argon in dark conditions, which is thermodynamically unfavorable to Cu-thiol autoreduction. The reduction of copper and concomitant oxidation of Cys can both undergo two possible pathways: oxidative as well as photochemical. Under O2, CuII plays the role of the electron carrier from one Cys to O2 followed by internal geometry rearrangement at the Cu site, which facilitates reduction at Cu-center to yield CuI(S-Cys)2(Cys-S-S-Cys). Photoinduced (irradiated at λex = 280 nm) reduction of the CuII center is observed by UV-visible photolysis (above 300 nm all bands disappeared) and tryptophan fluorescence (∼335 nm peak enhanced) experiments. In both pathways, geometry reorganization plays an important role in copper reduction yielding an energetically compatible donor-acceptor system. This model system provides unusual stability and redox chemistry rather than the universal Cu-thiol auto redox chemistry in cysteine-rich copper complexes.

Small phospho-donors phosphorylate MorR without inducing protein conformational changes.

Phosphorylation is an essential mechanism of protein control and plays an important role in biology. The two-component system (TCS) is a bacterial regulation mechanism mediated by a response regulator (RR) protein and a kinase protein, which synchronize the regulatory circuit according to the environment. Phosphorylation is a key element in TCS function as it controls RR activity. In the present study, we characterize the behavior of MorR, an RR associated with Mo homeostasis, upon acetylphosphate and phosphoramidate treatment in vitro. Our results show that MorR was phosphorylated by both phospho-donors. Fluorescence experiments showed that MorR tryptophan emission is quenched by phosphoramidate. Furthermore, theoretical and computational results demonstrate that phosphorylation by phosphoramidate is more favorable than that by acetylphosphate. In conclusion, phosphorylated MorR is a monomeric protein and phosphorylation does not appear to induce observable conformational changes in the protein structure.

Insights into the Molybdenum/Copper Heterometallic Cluster Assembly in the Orange Protein: Probing Intermolecular Interactions with an Artificial Metal-Binding ATCUN Tag.

Orange protein (ORP) is a small bacterial protein, of unknown function, that contains a unique molybdenum/copper heterometallic cluster, [S2MoVIS2CuIS2MoVIS2]3- (Mo/Cu), non-covalently bound. The native cluster can be reconstituted in a protein-assisted mode by the addition of CuII plus tetrathiomolybdate to apo-ORP under controlled conditions. In the work described herein, we artificially inserted the ATCUN ("amino terminus Cu and Ni") motif in the Desulfovibrio gigas ORP (Ala1Ser2His3 followed by the native amino acid residues; modified protein abbreviated as ORP*) to increase our understanding of the Mo/Cu cluster assembly in ORP. The apo-ORP* binds CuII in a 1:1 ratio to yield CuII-ORP*, as clearly demonstrated by EPR (g||,⊥ = 2.183, 2.042 and ACu||,⊥ = 207 × 10-4 cm-1, 19 × 10-4 cm-1) and UV-visible spectroscopies (typical d-d transition bands at 520 nm, ε = 90 M-1 cm-1). The 1H NMR spectrum shows that His3 and His53 are significantly affected upon the addition of the CuII. The X-ray structure shows that these two residues are very far apart (Cα-Cα ≈ 27.9 Å), leading us to suggest that the metal-induced NMR perturbations are due to the interaction of two protein molecules with a single metal ion. Docking analysis supports the metal-mediated dimer formation. The subsequent tetrathiomolybdate binding, to yield the native Mo/Cu cluster, occurs only upon addition of dithiothreitol, as shown by UV-visible and NMR spectroscopies. Additionally, 1H NMR of AgI-ORP* (AgI used as a surrogate of CuI) showed that AgI strongly binds to a native methionine sulfur atom rather than to the ATCUN site, suggesting that CuII and CuI have two different binding sites in ORP*. A detailed mechanism for the formation of the Mo/Cu cluster is discussed, suggesting that CuII is reduced to CuI and transferred from the ATCUN motif to the methionine site; finally, CuI is transferred to the cluster-binding region, upon the interaction of two protein molecules. This result may suggest that copper trafficking is triggered by redox-dependent coordination properties of copper in a trafficking pathway.

Protein-Assisted Formation of Molybdenum Heterometallic Clusters: Evidence for the Formation of S2MoS2-M-S2MoS2 Clusters with M = Fe, Co, Ni, Cu, or Cd within the Orange Protein.

The Orange Protein (ORP) is a small bacterial protein, of unknown function, that harbors a unique molybdenum/copper (Mo/Cu) heterometallic cluster, [S2MoVIS2CuIS2MoVIS2]3-, noncovalently bound. The apo-ORP is able to promote the formation and stabilization of this cluster, using CuII- and MoVIS42- salts as starting metallic reagents, to yield a Mo/Cu-ORP that is virtually identical to the native ORP. In this work, we explored the ORP capability of promoting protein-assisted synthesis to prepare novel protein derivatives harboring molybdenum heterometallic clusters containing iron, cobalt, nickel, or cadmium in place of the "central" copper (Mo/Fe-ORP, Mo/Co-ORP, Mo/Ni-ORP, or Mo/Cd-ORP). For that, the previously described protein-assisted synthesis protocol was extended to other metals and the Mo/M-ORP derivatives (M = Cu, Fe, Co, Ni, or Cd) were spectroscopically (UV-visible and electron paramagnetic resonance (EPR)) characterized. The Mo/Cu-ORP and Mo/Cd-ORP derivatives are stable under oxic conditions, while the Mo/Fe-ORP, Mo/Co-ORP, and Mo/Ni-ORP derivatives are dioxygen-sensitive and stable only under anoxic conditions. The metal and protein quantification shows the formation of 2Mo:1M:1ORP derivatives, and the visible spectra suggest that the expected {S2MoS2MS2MoS2} complexes are formed. The Mo/Cu-ORP, Mo/Co-ORP, and Mo/Cd-ORP are EPR-silent. The Mo/Fe-ORP derivative shows an EPR S = 3/2 signal (E/D ≈ 0.27, g ≈ 5.3, 2.5, and 1.7 for the lower M= ±1/2 doublet, and g ≈ 5.7 and 1.7 (1.3 predicted) for the upper M = ±3/2 doublet), consistent with the presence of either one S = 5/2 FeIII antiferromagnetically coupled to two S = 1/2 MoV or one S = 3/2 FeI and two S = 0 MoVI ions, in both cases in a tetrahedral geometry. The Mo/Ni-ORP shows an EPR axial S = 1/2 signal consistent with either one S = 1/2 NiI and two S = 0 MoVI or one S = 1/2 NiIII antiferromagnetically coupled to two S = 1/2 MoV ions, in both cases in a square-planar geometry. The Mo/Cu-ORP and Mo/Cd-ORP are described as {MoVI-CuI-MoVI} and {MoVI-CdII-MoVI}, respectively, while the other derivatives are suggested to exist in at least two possible electronic structures, {MoVI-MI-MoVI} ↔ {MoV-MIII-MoV}.

The small iron-sulfur protein from the ORP operon binds a [2Fe-2S] cluster.

A linear cluster formulated as [S2MoS2CuS2MoS2](3-), a unique heterometallic cluster found in biological systems, was identified in a small monomeric protein (named as Orange Protein). The gene coding for this protein is part of an operon mainly present in strict anaerobic bacteria, which is composed (in its core) by genes coding for the Orange Protein and two ATPase proposed to contain Fe-S clusters. In Desulfovibrio desulfuricans G20, there is an ORF, Dde_3197 that encodes a small protein containing several cysteine residues in its primary sequence. The heterologously produced Dde_3197 aggregates mostly in inclusion bodies and was isolated by unfolding with a chaotropic agent and refolding by dialysis. The refolded protein contained sub-stoichiometric amounts of iron atoms/protein (0.5±0.2), but after reconstitution with iron and sulfide, high iron load contents were detected (1.8±0.1 or 3.4±0.2) using 2- and 4-fold iron excess. The visible absorption spectral features of the iron-sulfur clusters in refolded and reconstituted Dde_3197 are similar and resemble the ones of [2Fe-2S] cluster containing proteins. The refolded and reconstituted [2Fe-2S] Dde_3197 are EPR silent, but after reduction with dithionite, a rhombic signal is observed with gmax=2.00, gmed=1.95 and gmin=1.92, consistent with a one-electron reduction of a [2Fe-2S](2+) cluster into a [2Fe-2S](1+) state, with an electron spin of S=½. The data suggests that Dde_3197 can harbor one or two [2Fe-2S] clusters, one being stable and the other labile, with quite identical spectroscopic properties, but stable to oxygen.

Incorporation of molybdenum in rubredoxin: models for mononuclear molybdenum enzymes.

Molybdenum is found in the active site of enzymes usually coordinated by one or two pyranopterin molecules. Here, we mimic an enzyme with a mononuclear molybdenum-bis pyranopterin center by incorporating molybdenum in rubredoxin. In the molybdenum-substituted rubredoxin, the metal ion is coordinated by four sulfurs from conserved cysteine residues of the apo-rubredoxin and two other exogenous ligands, oxygen and thiol, forming a Mo((VI))-(S-Cys)4(O)(X) complex, where X represents -OH or -SR. The rubredoxin molybdenum center is stabilized in a Mo(VI) oxidation state, but can be reduced to Mo(IV) via Mo(V) by dithionite, being a suitable model for the spectroscopic properties of resting and reduced forms of molybdenum-bis pyranopterin-containing enzymes. Preliminary experiments indicate that the molybdenum site built in rubredoxin can promote oxo transfer reactions, as exemplified with the oxidation of arsenite to arsenate.

One electron reduced square planar bis(benzene-1,2-dithiolato) copper dianionic complex and redox switch by O2/HO(-).

The complex [Ph4P]2[Cu(bdt)2] (1(red)) was synthesized by the reaction of [Ph4P]2[S2MoS2CuCl] with H2bdt (bdt = benzene-1,2-dithiolate) in basic medium. 1(red) is highly susceptible toward dioxygen, affording the one electron oxidized diamagnetic compound [Ph4P][Cu(bdt)2] (1(ox)). The interconversion between these two oxidation states can be switched by addition of O2 or base (Et4NOH = tetraethylammonium hydroxide), as demonstrated by cyclic voltammetry and UV-visible and EPR spectroscopies. Thiomolybdates, in free or complex forms with copper ions, play an important role in the stability of 1(red) during its synthesis, since in its absence, 1(ox) is isolated. Both 1(red) and 1(ox) were structurally characterized by X-ray crystallography. EPR experiments showed that 1(red) is a Cu(II)-sulfur complex and revealed strong covalency on the copper-sulfur bonds. DFT calculations confirmed the spin density delocalization over the four sulfur atoms (76%) and copper (24%) atom, suggesting that 1(red) has a "thiyl radical character". Time dependent DFT calculations identified such ligand to ligand charge transfer transitions. Accordingly, 1(red) is better described by the two isoelectronic structures [Cu(I)(bdt2, 4S(3-,)*)](2-) ↔ [Cu(II)(bdt2, 4S(4-))](2-). On thermodynamic grounds, oxidation of 1(red) (doublet state) leads to 1(ox) singlet state, [Cu(III)(bdt2, 4S(4-))](1-).

Copper-substituted forms of the wild type and C42A variant of rubredoxin.

In order to gain insights into the interplay between Cu(I) and Cu(II) in sulfur-rich protein environments, the first preparation and characterization of copper-substituted forms of the wild-type rubredoxin (Rd) from Desulfovibrio vulgaris Hildenborough are reported, as well as those of its variant C42A-Rd. The initial products appear to be tetrahedral Cu(I)(S-Cys)n species for the wild type (n=4) and the variant C42A (n=3, with an additional unidentified ligand). These species are unstable to aerial oxidation to products, whose properties are consistent with square planar Cu(II)(S-Cys)n species. These Cu(II) intermediates are susceptible to auto-reduction by ligand S-Cys to produce stable Cu(I) final products. The original Cu(I) center in the wild-type system can be regenerated by reduction, suggesting that the active site can accommodate Cu(I)(S-Cys)2 and Cys-S-S-Cys fragments in the final product. The absence of one S-Cys ligand prevents similar regeneration in the C42A-Rd system. These results emphasize the redox instability of Cu(II)-(S-Cys)n centers.

Oxomolybdenum monodithiolene complexes linked with sulfur bridged iron: antiferromagnetically coupled Fe(III)Mo(V) systems.

Mo-Fe heterometallic complexes with Fe(X)(2) (X = Cl, SPh) moiety attached to monodithiolene oxomolybdenum via sulfur bridge, viz., [Ph(4)P](2)[Cl(2)FeS(2)MoOS(2)(DMED)] (2) (DMED, dimethylethylenedicarboxylate), [Ph(4)P](2)[Cl(2)FeS(2)MoO(tdt)] (3) (tdt, toluenedithiolate) and [Ph(4)P](2)[(SPh)(2)FeS(2)MoO(tdt)] (4) are reported. Mossbauer spectroscopy, magnetism, EPR, electrochemistry and electronic structure based on DFT and TD-DFT calculation show the transfer of electron from iron to molybdenum centre resulting antiferromagnetically coupled Fe(III)Mo(V) unit from the starting Fe(II) and Mo(VI) compounds. A net spin of S = 2 ground state arising from antiferromagnetically coupled Fe(III) and Mo(V) shows a rare X-band EPR in normal mode at g ~ 12 in the solid state. In addition, Mossbauer studies show that electron drifting is more pronounced upon substitution of the chloride ligand by thiophenolate. The changes in dithiolene periphery electronically affect the charge distribution between Mo-Fe in {OMo(µS)(2)Fe} core. DFT calculations indicate that the increasing stability of dative Fe → Mo hetero metal-metal bond in these complexes from 3 to 2 to 4 is related to the extent of electron transfer from the iron to molybdenum centre.

A straightforward and generalizable synthetic methodology for the synthesis of ruthenium(II) complexes with thioether ligands from either Ru(III) or Ru(0) precursors.

The reaction of Ru(3)(CO)(12) in chloroform with thioether ligands after 25 min of irradiation leads to the formation of mononuclear ruthenium(II) complexes of the general formula fac-[Ru(CO)(3)Cl(2)(S)] (S = thioether) if monodentate ligands are introduced. In the case of chelating bidentate thioethers a complex of the formula all-cis-[Ru(CO)(2)Cl(2)(S[intersection]S)] is produced. If irradiation time is prolonged to 2.5 h mononuclear complexes of the composition trans-[RuCl(2)(S)(4)] or trans-[RuCl(2)(S[intersection]S)(2)] are observed. The use of tetrahydrothiophene (THT) in the latter case produces a dinuclear ruthenium(ii) complex of the formula [Ru(2)(CO)(2)Cl(4)(THT)(2)(mu-THT)(2)] which is the first structurally characterized neutral ruthenium complex with a bridging thioether ligand. Regardless of the used ligand complexes trans-[RuCl(2)(S)(4)] or trans-[RuCl(2)(S[intersection]S)(2)] are formed from the reaction of RuCl(3).3H(2)O with the corresponding thioether in the presence of NEt(3) and formaldehyde in excellent yields. Six new ruthenium complexes are characterized by X-ray diffraction.