Organometallic trinuclear niobium cluster complex in aqueous solution: synthesis and characterization of niobium complexes containing Nb(3)(mu-eta(2):eta(2)( perpendicular)-NCCH(3))(mu(2)-O)(3)(6+) cluster core.

The synthesis and characterization of an organometallic trinuclear oxo-bridged niobium cluster complex with perpendicularly coordinated mu(3)-eta(2):eta(2)( perpendicular)-acetonitrile ligand in aqueous solution is reported. Reaction of NbCl(5) in acetonitrile with aluminum under argon followed by reaction with aqueous hydrochloric acid affords, after suitable workup, the isolation of the organometallic [Nb(3)(mu-eta(2):eta(2)-NCCH(3))O(3)(H(2)O)(9)](6+) aqua ion by cation-exchange chromatography. The purple niobium aqua ion in 2 M HCl shows a small peak at 365 nm (epsilon approximately 511 M(-1) cm(-1) per Nb) and a broad peak at 565 nm (epsilon approximately 335 M(-1) cm(-1)) in the UV-visible region. It is electron paramagnetic resonance (EPR)-active (g = 1.98), but no hyperfine interaction with the (93)Nb nuclear spin (I = 9/2) was observed. The cyclic voltammogram of [Nb(3)(mu-eta(2):eta(2)-NCCH(3))O(3)(H(2)O)(9)](6+) in 4 M HCl on edge-plane pyrolytic graphite electrode at 50 mV s(-1) in the potential range -1.2 V to +1.1 V (vs SCE) exhibits three anodic peaks at -0.12, +0.53, and +0.85 V and a large cathodic peak at -0.91 V with a slight shoulder at about -0.8 V. The purple aqua ion reacted with potassium thiocyanate to give the green thiocyanate derivative, which was crystallized as ((CH(3))(3)NH)(3)[Nb(3)(mu-eta(2):eta(2)-NCCH(3))O(3)(NCS)(9)].2.5H(2)O (1) and subjected to X-ray structure analysis. Density-functional theory (DFT) calculations fully supported the structure of the cluster.

Synthesis and characterization of mixed chalcogen triangular complexes with new Mo3(mu3-S)(mu2-Se2)3(4+) and M3(mu3-S)(mu2-Se)3(4+) (M = Mo, W) cluster cores.

In our pursuit of mixed chalcogen-bridged cluster complexes, solids of the compositions Mo(3)SSe(6)Br(4) and W(3)SSe(6)Br(4) were prepared using high-temperature synthesis from the elements. Treatment of Mo(3)SSe(6)Br(4) with Bu(4)NBr in a vibration mill yielded (Bu(4)N)(3){[Mo(3)(mu(3)-S)(mu(2)-Se(2))(3)Br(6)]Br} (I). Its all-selenide analogue (Bu(4)N)(3){[Mo(3)(mu(3)-Se)(mu(2)-Se(2))(3)Br(6)]Br} (II) was prepared from Mo(3)Se(7)Br(4) in a similar way. Both compounds were characterized by IR, Raman, and (77)Se NMR spectroscopy. The structure of II was determined by X-ray single-crystal analysis. Compound I is isostructural with II and contains the new Mo(3)(mu(3)-S)Se(6)(4+) cluster core. By treatment of a 4 M Hpts solution of I with PPh(3) followed by cation-exchange chromatography, the new mixed chalcogenido-molybdenum aqua ion, [Mo(3)(mu(3)-S)(mu(2)-Se)(3)(H(2)O)(9)](4+), was isolated and characterized using UV-vis spectroscopy and, after derivatization into [Mo(3)(mu(3)-S)(mu(2)-Se)(3)(acac)(3)(py)(3)](+), electrospray ionization mass spectrometry. From HCl solutions of the aqua ion, a supramolecular adduct with cucurbit[6]uril (CB[6]), {[Mo(3)(mu(3)-S)(mu(2)-Se)(3)(H(2)O)(6)Cl(3)](2)CB[6]}Cl(2) x 11 H(2)O (III), was isolated and its structure determined using X-ray crystallography. W(3)SSe(6)Br(4) upon reaction with H(3)PO(2) gave a mixture of all of the [W(3)S(x)Se(4-x)(H(2)O)(9)](4+) species. After repeated chromatography, crystals of {[W(3)(mu(3)-S)(mu(2)-Se)(3)(H(2)O)(7)Cl(2)](2)CB[6]}Cl(4) x 12 H(2)O (IV) were crystallized from the fraction rich in [W(3)(mu(3)-S)Se(3)(H(2)O)(9)](4+) and structurally characterized.

Computational chemistry of modified [MFe3S4] and [M2Fe2S4] clusters: assessment of trends in electronic structure and properties.

The aim of this work is to understand the molecular evolution of iron-sulfur clusters in terms of electronic structure and function. Metal-substituted models of biological [Fe(4)S(4)] clusters in oxidation states [M(x)Fe(4-x)S(4)](3+/2+/1+) have been studied by density functional theory (M = Cr, Mn, Fe, Co, Ni, Cu, Zn, and Pd, with x = 1 or 2). Most of these clusters have not been characterized before. For those that have been characterized experimentally, very good agreement is obtained, implying that also the predicted structures and properties of new clusters are accurate. Mean absolute errors are 0.024 A for bond lengths ([Fe(4)S(4)], [NiFe(3)S(4)], [CoFe(3)S(4)]) and 0.09 V for shifts in reduction potentials relative to the [Fe(4)S(4)] cluster. All structures form cuboidal geometries similar to the all-iron clusters, except the Pd-substituted clusters, which instead form highly distorted trigonal and tetragonal local sites in compromised, pseudocuboidal geometries. In contrast to other electron-transfer sites, cytochromes, blue copper proteins, and smaller iron-sulfur clusters, we find that the [Fe(4)S(4)] clusters are very insensitive to metal substitution, displaying quite small changes in reorganization energies and reduction potentials upon substitution. Thus, the [Fe(4)S(4)] clusters have an evolutionary advantage in being robust to pollution from other metals, still retaining function. We analyze in detail the electronic structure of individual clusters and rationalize spin couplings and redox activity. Often, several configurations are very close in energy, implying possible use as spin-crossover systems, and spin states are predicted accurately in all but one case ([CuFe(3)S(4)]). The results are anticipated to be helpful in defining new molecular systems with catalytic and magnetic properties.

Accurate computation of reduction potentials of 4Fe-4S clusters indicates a carboxylate shift in Pyrococcus furiosus ferredoxin.

This work describes the computation and accurate reproduction of subtle shifts in reduction potentials for two mutants of the iron-sulfur protein Pyrococcus furiosus ferredoxin. The computational models involved only first-sphere ligands and differed with respect to one ligand, either acetate (aspartate), thiolate (cysteine), or methoxide (serine). Standard procedures using vacuum optimization gave qualitatively wrong results and errors up to 0.07 V. Using electrostatically screened geometries and large basis sets for expanding the wave functions gave quantitatively correct results, with errors of only 0.03 V. Correspondingly, only this approach predicted a change in the coordination mode of aspartate (i.e., a carboxylate shift) accompanying the reduction of the wild-type cluster, confirming results from synthetic models and explaining why electrostatic screening is necessary. Hence, the carboxylate shift appears to occur in the proteins from which data were collected. The results represent the most accurate predictions of shifts in reduction potentials for modified proteins, the success in part being due to the similar nature of the three amino acid ligands involved. The predicted carboxylate shift is expected to tune aspartate's degree of electron donation to the cluster's two oxidation states, thus making the reversible redox reaction feasible.

Assembled monolayers of Mo3S4(4+) clusters on well-defined surfaces.

A class of inorganic monolayers formed by assembling the molybdenum-sulfur cluster, Mo3S4(4+), onto a well-defined Au(111) surface is presented. The monolayers have been comprehensively characterized by electrochemistry, X-ray photoelectron spectroscopy (XPS), and in situ scanning tunneling microscopy (in situ STM). The voltammetric data show strong reductive and oxidative desorption signals from Au-S interactions, supported by the presence of both S and Mo signals in XPS. In situ STM shows many small pits in the dense adlayers uniformly spread over the surface, which is a typical feature of self-assembled monolayers (SAMs) of alkanethiols. The density of the pits is ca. 23 (+/-5)% and is significantly higher than for straight-chain alkanethiol SAMs with a single -SH group. The pit shapes are irregular, suggesting multiple Au-S interactions from Mo3S4(4+). High resolution images disclose bright round spots of ca. 8 A diameter representing individual molecules in the SAM. This is the first example of in situ monolayer formation by a metal-chalcogenide cluster directly anchored onto the gold surface through core ligands and offers a simple way to prepare a new class of functionalized inorganic monolayers.

Preparation, properties, and reactivities of unprecedented oxo-sulfido Nb(IV) aqua ions and crystal structure of (Me2NH2)6[Nb5(mu3-S)2(mu3-O)2(mu2-O)2(NCS)14].3.5H2O.

By treatment of Zn-reduced ethanolic solutions of NbCl5 with HCl in the presence of sulfide followed by cation-exchange chromatography, two oxo-sulfido niobium aqua ions, the red [Nb4(mu4-S)(mu2-O)5(H2O)10]4+ and the yellow-brown [Nb5(mu3-S)2(mu3-O)2(mu2-O)2(H2O)14]8+, were isolated. Both readily form their respective thiocyanate complexes, for which the structure for the former has been previously reported. Brown crystals of (Me2NH2)6[Nb5S2O4(NCS)14].3.5H2O (1) were isolated in the case of the latter, and the structure was determined by X-ray crystallography (space group: a = 15.4018(5) A, b = 21.1932(8) A, c = 22.0487(8) A, alpha=gamma = 90 degrees , beta = 103.4590(10) degrees , and R(1) = 0.0659). An unprecedented pentanuclear Nb5S2O48+ core is revealed in which short Nb-Nb distances (2.7995(8)-2.9111(8) A) are consistent with metal-metal bonding. A stopped-flow kinetic study of the 1:1 equilibration of NCS- with [Nb4(mu4-S)(mu2-O)5(H2O)10]4+ has been carried out. Equilibration rate constants are independent of [H(+)] in the range investigated (0.5-2.0 M) and at 25 degrees C; kf= 9.5 M(-1) s(-1), kaq = 2.6 x 10(-2) s(-1), and K = 365 M1). Conditions with first NCS- and then [Nb4(mu4-S)(mu2-O)5(H2O)10]4+ in excess revealed a statistical factor of 4, suggesting the presence of four kinetically equivalent Nb atoms. Attempts to study the 1:1 substitution of NCS- with [Nb5(mu3-S)2(mu3-O)2(mu2-O)2(H2O)14]8+ showed signs of saturation kinetics. Quantum chemical calculations using the density functional theory (DFT) approach were performed on both the Nb4O5S4+ and Nb5O4S28+ naked clusters. The highest occupied and lowest unoccupied molecular orbitals have dominant Nb(4d) character. The HOMO for Nb4O5S4+ is a nondegenerate fully filled MO, whereas for Nb5O4S28+, it is a nondegenerate partially filled MO with one unpaired electron. EPR spectroscopy on [Nb5(mu3-S)2(mu3-O)2(mu2-O)2(H2O)14]8+ shows that the molecule has total anisotropy (C2v), with all three tensors, gx= 2.399, gy= 1.975, and gz= 1.531, resolved. No hyperfine interaction expected from the nuclear moment of I = 9/2 for 93Nb was observed.

An unprecedented tetranuclear niobium aqua ion with a capping mu4-sulfido ligand.

The aqueous solution chemistry of niobium is rather unexplored, and well characterized aqua complexes are scarce. In this work, a new niobium aqua ion is obtained upon treatment of Zn-reduced ethanolic solutions of NbCl5 with HCl in the presence of a sulfide source. The red aqua ion, obtained upon cation-exchange chromatography, forms readily the thiocyanate complex which has been crystallized as Cs(4.26)Na(1.74)[Nb4SO5(NCS)10] . 0.33H2O. X-ray crystallography revealed an unprecedented metal-metal bonded tetranuclear Nb4(mu4-S)(mu2-O)5(4+) core with a capping mu4-S ligand.

In situ STM imaging and direct electrochemistry of Pyrococcus furiosus ferredoxin assembled on thiolate-modified Au111 surfaces.

We have addressed here electron transfer (ET) of Pyrococcus furiosus ferredoxin (PfFd, 7.5 kDa) in both homogeneous solution using edge plane graphite (EPG) electrodes and in the adsorbed state by electrochemistry on surface-modified single-crystal Au111 electrodes, This has been supported by surface microscopic structures of PfFd monolayers, as revealed by scanning tunneling microscopy under potential control (in situ STM). Direct ET between PfFd in phosphate buffer solution, pH 7.9, and EPG electrodes is observed in the presence of promoters. Neomycin gives rise to a pair of redox peaks with a formal potential of ca -430 mV (vs SCE), corresponding to [3Fe-4S]1+/0. The presence of an additional promoter, which can be propionic acid, alanine, or cysteine, induces a second pair of redox peaks at approximately -900 mV (vs SCE) arising from [3Fe-4S]0/1-. A robust neomycin-PfFd complex was detected by mass spectrometry. The results clearly favor an ET mechanism in which the promoting effect of small organic molecules is through formation of promoter-protein complexes. The interaction of PfFd with small organic molecules in homogeneous solution offers clues to confine the protein on the electrode surface modified by the same functional group monolayer and to address diffusionless direct electrochemistry, as well as surface microstructures of the protein monolayer. PfFd molecules were found to assemble on either mercaptopropionic acid (MPA) or cysteine-modified Au111 surfaces in stable monolayers or submonolayers. Highly ordered (2 radical 3 x 5)R30 degrees cluster structures with six MPA molecules in each cluster were found by in situ STM. Individual PfFd molecules on the MPA layer are well resolved by in situ STM. Under Ar protection reversible cyclic voltammograms were obtained on PfFd-MPA/Au111 and PfFd-cysteine/Au111 electrodes with redox potentials of -220 and -201 mV (vs SCE), respectively, corresponding to the [Fe3S4]1+/0 couple. These values are shifted positively by 200 mV relative to homogeneous solution due to interactions between the promoting layers and the protein molecules. Possible mechanisms for such interactions and their ET patterns are discussed.

The 1.5 A resolution crystal structure of [Fe3S4]-ferredoxin from the hyperthermophilic archaeon Pyrococcus furiosus.

The structure of [Fe(3)S(4)]-ferredoxin from the hyperthermophilic archaeon Pyrococcus furiosus has been determined to 1.5 A resolution from a crystal belonging to space group P2(1) with two molecules in the asymmetric unit. The structure has been solved with molecular replacement by use of the ferredoxin from Thermotoga maritima. The fold is similar to that of related monocluster ferredoxins and contains two double-stranded antiparallel beta-sheets and two alpha-helices. The hydrophobic interaction between Trp2 and Tyr46 is confirmed, linking the C-terminus to the longer alpha-helix. The structure contains a double-conformation disulfide bond existing in a left-handed and a right-handed spiral conformation. The crystal packing reveals a beta-sheet interaction, which supports the suggestion that P. furiosus ferredoxin is a functional dimer. The extraordinary thermostability of P. furiosus ferredoxin is further discussed.

Cytoplasmic expression of the Achromobacter xylosoxidans blue copper nitrite reductase in Escherichia coli and characterisation of the recombinant protein.

The gene of the Achromobacter xylosoxidans (DSM 2402) blue copper-containing nitrite reductase was amplified using the polymerase chain reaction. DNA sequence analysis reveals that the amino acid sequence is identical to those of the GIFU1051 and the NCIMB11015 A. xylosoxidans nitrite reductases. The gene encoding the mature coding region for DSM 2402 nitrite reductase was cloned into a pET-vector, overexpressed in the cytoplasm of Escherichia coli BL21(DE3), and the expressed holoprotein was purified to apparent homogeneity by cation-exchange chromatography. The recombinant blue copper-containing nitrite reductase was obtained in high yields of 70mgL(-1) of culture. The specific catalytic activity as well as the electronic absorption and electron paramagnetic resonance spectra agree with corresponding data for the native protein. Mass spectroscopic analysis of the recombinant nitrite reductase gave a molecular weight of 36659.1Da for the apo-protein monomer, in agreement with the expected molecular mass based on the amino acid sequence.