A realistic in silico model for structure/function studies of molybdenum–copper CO dehydrogenase

CO dehydrogenase (CODH) is an environmentally crucial bacterial enzyme that oxidizes CO to CO2 at a Mo–Cu active site. Despite the close to atomic resolution structure (1.1 Å), significant uncertainties have remained with regard to the protonation state of the water-derived equatorial ligand coordinated at the Mo-center, as well as the nature of intermediates formed during the catalytic cycle. To address the protonation state of the equatorial ligand, we have developed a realistic in silico QM model (~179 atoms) containing structurally essential residues surrounding the active site. Using our QM model, we examined each plausible combination of redox states (MoVI–CuI, MoV–CuII, MoV–CuI, and MoIV–CuI) and Mo-coordinated equatorial ligands (O2−, OH−, H2O), as well as the effects of second-sphere residues surrounding the active site. Herein, we present a refined computational model for the Mo(VI) state in which Glu763 acts as an active site base, leading to a MoO2-like core and a protonated Glu763. Calculated structural and spectroscopic data (hyperfine couplings) are in support of a MoO2-like core in agreement with XRD data. The calculated two-electron reduction potential (E = −467 mV vs. SHE) is in reasonable agreement with the experimental value (E = −558 mV vs. SHE) for the redox couple comprising an equatorial oxo ligand and protonated Glu763 in the MoVI–CuI state and an equatorial water in the MoIV–CuI state. We also suggest a potential role of second-sphere residues (e.g., Glu763, Phe390) based on geometric changes observed upon exclusion of these residues in the most plausible oxidized states

Dithiolate complexes of manganese and rhenium: X-ray structure and properties of an unusual mixed valence cluster Mn-3(CO)(6)(mu-eta(2)-SCH2CH2CH2S)(3)

Treatment of Mn-2(CO)(10) with 3,4-toluenedithiol and 1,2-ethanedithiol in the presence of Me(3)NO(.)2H(2)O in CH2CI2 at room temperature afforded the dinuclear complexes Mn-2(CO)(6)(mu-eta(4)-SC6H3(CH3)S-SC6H3(CH3)S) (1), and Mn-2(CO)(6)(mu-eta(4)-SCH2CH2S-SCH2CH2S) (2), respectively. Similar reactions of Re-2(CO)(10) with 3,4-toluenedithiol, 1,2benzenedithiol, and 1,2-ethanedithiol yielded the dirhenium complexes Re-2(CO)(6)(mu-eta(4)-SC6H3(CH3)S-SC6H3(CH3)S) (3), Re-2(CO)(6)(mu-eta(4)-SCH2SC6H4S) (4), and Re-2(CO)(6)(SCH2CH2S-SCH2CH2S) (5), respectively. In contrast, treatment of Mn2(CO)10 with 1,3-propanedithiol afforded the trimanganese compound Mn-3(CO)(6)(mu-eta(2)-SCH2CH2CH2S)(3) (6), whereas Re2(CO)10 gave only intractable materials. The molecular structures of 1, 3, and 6 have been determined by single-crystal X-ray diffraction studies. The dimanganese and dirhenium carbonyl compounds 1-5 contain a binucleating disulfide ligand, formed by interligand disulfide bond formation between two dithiolate ligands identical in structure to that of the previously reported dimanganese complex Mn-2(CO)(6)(mu-eta(4)-SC6H4S-SC6H4S). Complex 6, on the other hand, forms a unique example of a mixed-valence trimangenese carbonyl compound containing three bridging 1,3-propanedithiolate ligands. The solution properties of 6 have been investigated by UV-vis and EPR spectroscopies as well as electrochemical techniques

Degradation of Metal Ions with Electricity Generation by Using Fruit Waste as an Organic Substrate in the Microbial Fuel Cell

A potential and developing green technology for producing renewable energy and treating wastewater is the microbial fuel cell (MFC). Despite several advancements, there are still several serious problems with this approach. In the present work, we addressed the problem of the organic substrate in MFC, which is necessary for the degradation of metal ions in conjunction with the production of energy. The utilization of fruit waste as a carbon source was strongly suggested in earlier research. Hence, the mango peel was used as a substrate in the current study. Within 25 days of operation, a 102-mV voltage was achieved in 13 days, while the degradation efficiency of Cr3+ was 69.21%, Co2+ was 72%, and Ni2+ was 70.11%. The procedure is carried out in the batch mode, and there is no continuous feeding of the organic substrate. In addition, a detailed explanation of the hypothesized mechanism for this investigation is provided, which focuses on the process of metal ion degradation. Lastly, future and concluding remarks are also enclosed

Sulfanyl stabilization of copper-bonded phenoxyls in model complexes and galactose oxidase

Integrating sulfanyl substituents into copper-bonded phenoxyls significantly alters their optical and redox properties and provides insight into the influence of cysteine modification of the tyrosine cofactor in the enzyme galactose oxidase. The model complexes [1SR2]+ are class II mixed-valent CuII-phenoxyl-phenolate species that exhibit intervalence charge transfer bands and intense visible sulfur-aryl π → π∗ transitions in the energy range, which provides a greater spectroscopic fidelity to oxidized galactose oxidase than non-sulfur-bearing analogs. The potentials for phenolate-based oxidations of the sulfanyl-substituted 1SR2 are lower than the alkyl-substituted analogs by up to ca. 150 mV and decrease following the steric trend: -StBu > -Si Pr > -SMe. Density functional theory calculations suggest that reducing the steric demands of the sulfanyl substituent accommodates an in-plane conformation of the alkylsulfanyl group with the aromatic ring, which stabilizes the phenoxyl hole by ca. 8 kcal mol-1 (1 kcal = 4.18 kJ; 350 mV) through delocalization onto the sulfur atom. Sulfur K-edge X-ray absorption spectroscopy clearly indicates a contribution of ca. 8–13% to the hole from the sulfur atoms in [1SR2]+. The electrochemical results for the model complexes corroborate the ca. 350 mV (density functional theory) contribution of hole delocalization on to the cysteine–tyrosine cross-link to the stability of the phenoxyl radical in the enzyme, while highlighting the importance of the in-plane conformation observed in all crystal structures of the enzyme

Structural and Spectroscopic Characterization of Iron(II), Cobalt(II), and Nickel(II) <i>ortho</i>-Dihalophenolate Complexes: Insights into Metal–Halogen Secondary Bonding

Metal complexes incorporating the tris­(3,5-diphenylpyrazolyl)­borate ligand (Tp^Ph2) and <i>ortho</i>-dihalophenolates were synthesized and characterized in order to explore metal–halogen secondary bonding in biorelevant model complexes. The complexes Tp^Ph2ML were synthesized and structurally characterized, where M was Fe­(II), Co­(II), or Ni­(II) and L was either 2,6-dichloro- or 2,6-dibromophenolate. All six complexes exhibited metal–halogen secondary bonds in the solid state, with distances ranging from 2.56 Å for the Tp^Ph2Ni­(2,6-dichlorophenolate) complex to 2.88 Å for the Tp^Ph2Fe­(2,6-dibromophenolate) complex. Variable temperature NMR spectra of the Tp^Ph2Co­(2,6-dichlorophenolate) and Tp^Ph2Ni­(2,6-dichlorophenolate) complexes showed that rotation of the phenolate, which requires loss of the secondary bond, has an activation barrier of ∼30 and ∼37 kJ/mol, respectively. Density functional theory calculations support the presence of a barrier for disruption of the metal–halogen interaction during rotation of the phenolate. On the other hand, calculations using the spectroscopically calibrated angular overlap method suggest essentially no contribution of the halogen to the ligand-field splitting. Overall, these results provide the first quantitative measure of the strength of a metal–halogen secondary bond and demonstrate that it is a weak noncovalent interaction comparable in strength to a hydrogen bond. These results provide insight into the origin of the specificity of the enzyme 2,6-dichlorohydroquinone 1,2-dioxygenase (PcpA), which is specific for <i>ortho</i>-dihalohydroquinone substrates and phenol inhibitors