Trigonal Pyramidal Structure Research Articles

Protective effect of the perchlorophenyl group in organophosphorus chemistry

The homoleptic phosphine with the bulky perchlorophenyl group, (C6Cl5)3P (1), exhibits trigonal pyramidal structure (TPY-3), yet considerably flattened: Σ(C–P–C') = 321.0(1)°. Key steric and electronic properties of this simple organophosphorus species have been estimated by calculation. Attending to its characteristic features, 1 can be rated as a deactivated phosphine, where the less-basic P atom is sterically shielded by the bulky C6Cl5 groups. This marked inertness notwithstanding, it has been possible to obtain (under harsh conditions) derivatives with phosphorus in high oxidation state, namely the phosphine oxide (C6Cl5)3PO (2) and the difluorophosphorane (C6Cl5)3PF2 (3). These four- and five-substituted derivatives respectively exhibit trigonal pyramidal (TPY-4) and trigonal bipyramidal (TBPY-5) structures. The Σ(C–P–C') value steadily increases along the series 1–3, according to the referred structural variation. The P–C bond length is, in turn, invariably maintained at about 185 pm regardless of the different oxidation state, the increasing number of substituents around the P atom and the overall geometry. The hypervalent difluorophosphorane (C6Cl5)3PF2 (3) dissociates one of the axial fluorides in the gas phase giving rise to the fluorophosphonium cation [(C6Cl5)3PF]+, as detected by mass spectrometry. This cation is identified as a Lewis superacid.

Ligand Centered Reactivity of a Transition Metal Bound Geometrically Constrained Phosphine.

The electronic properties, coordination chemistry and reactivity of metal complexes of a planar (C2v symmetric) acridane-derived geometrically constrained phosphine, P(NNN), are described. On complexation to metal centers, the phosphine was found to adopt a distorted trigonal pyramidal structure with a high barrier to pyramidal inversion (22.3 kcal/mol at 298 K for Au[P(NNN)]Cl). Spectroscopic data and theoretical calculations carried out at the density functional level of theory indicate that P(NNN) is a moderate σ-donor, with significant π-acceptor properties. Despite the distortion undergone by the phosphorus atom on coordination to metal centers, the P(NNN) ligand retains its ability to react with small molecule substrates with polar E-H bonds (MeOH, NH2Ph, NH3). It does so in a concerted fashion across one of the P-N bonds, and reversibly in the case of amine substrates. This cooperative bond activation chemistry may ultimately prove beneficial in catalysis.

Hydrogen Storage Mechanism by Gas-Phase Vanadium Cluster Cations Revealed by Thermal Desorption Spectrometry.

The adsorption of hydrogen on gas-phase vanadium cluster cations, Vn+ (n = 3-14), at 300 K and desorption of hydrogen from hydride clusters, VnHm+, upon heating were observed experimentally by combined thermal desorption spectrometry and mass spectrometry analyses. The ratio m/n was approximately 1.3 for all n values at 300 K, which was reduced to approximately zero at 1000 K. For n = 4, stable cluster geometries of V4Hm+ (m = 0, 2, 4, and 6) were investigated by DFT calculations, revealing that V4 adopted a trigonal pyramidal structure and the H atoms adsorbed mainly on the μ2 bridge sites. The adsorption reaction pathway of one H2 molecule on V4+ was also investigated. The experimentally estimated desorption energies of the H2 molecules were consistent with their calculated binding energies. Among the observed hydride clusters, V6H8+ was found to be significantly thermally durable, probably because of its close-packed octahedral V6 core structure, with H atoms occupying all hollow sites.

Continuous Making, Stretch Breaking, and Exciton Dynamics of a Red-Emissive Organic Submicrometer-Sized Triangular Pyramid

Highly photoluminescent, single-component self-assembly being scalable and sustainable has a profound commercial significance. In this article, the challenges that we address in development of self-assembly of π-conjugated molecules are ─(i) feeble photoluminescence caused by poor molecular orientational ordering and (ii) costly fluorescent monomer owing to lack of an atom/step/energy economical synthetic method. We have discovered that a bright yellow fluorescent xanthene analog capable of instantly forming self-assembly can be synthesized by a home-built coil-in-spiral reactor using an inexpensive precursor, pyrogallol. Stimuli such as temperature (35 °C), acid fumes, and apolar solvent can trigger the formation of red-emissive self-assembly. In solution orientation of about 13 molecules yielding hydrogen-bonded self-assembly has a direct resemblance with a Coulomb-coupled J-aggregate highlighted in the Kasha model. After photoexcitation, the excited state dynamics of the monomer progress via three pathways: (i) relaxation (2 ± 0.3 ps), (ii) solvation (19.5 ± 3 ps), and (iii) decay (2.5 ns). On the other hand, self-assembly exciton evolves via two pathways: (i) relaxation (81 ± 25 ps) and (ii) decay (∼1 ns). In the solid state, the self-assembly retains the submicron-sized trigonal pyramidal structures by layer-by-layer stacking of triangular plates. This self-assembly doped in polymer even exhibits mechanofluorescence. Our study will pave way the use of this photoluminescent self-assembly for improved performance of organic/sustainable electronics and stretchable electronics.

Graphene, phosphorene and silicene coatings on the (0001) surfaces of hcp metals: Structural stability and hydrophobicity

Single atomic layer or monolayer (ML) 2D materials have unique features that can be relevant for protection and functionalization of high-performance surfaces. Here we report a combined density functional theory and ab initio molecular dynamics study on the stability of graphene (Gr), phosphorene (Pp) and silicene (Sl) coatings at the most stable (0001) surface of the metals that have hcp structure at room temperature: Hf, Mg, Os, Re, Ru, Sc, Ti, Zn and Zr. We found that the three 2D materials are stable at the surfaces of many of these metals: Gr is stable at Os, Re, Ti, Zn and Zr; Pp is stable in the blue form at Hf, Mg, Ti and Zr; Sl is stable in the 2D zigzag structure at Hf, Mg and Ti and in the planar form at the Zr surface. For the remaining metals Gr, Pp and Sl do not form van der Waals bonded MLs. In those cases we found structures such as 2D arrangements of mixed hexagons and pentagons, trigonal pyramidal structures and 2 half-monolayers. Many of these structures are very stable coatings covalently bound to the surfaces with considerable adsorption energies that can reach − 2 eV per atom of the coating. Gr imparts hydrophobicity to the surfaces of all materials here studied and increases the distance between these and the first H2O layer by as much as 75 % for Gr coated Ti. These H2O layers have increased rigidity despite being located further apart from the surfaces. These effects could be explored for the protection or functionalization of high-performance systems such as metallic electrodes for example.

Heteromeric three-stranded coiled coils designed using a Pb(II)(Cys)3 template mediated strategy.

Three-stranded coiled coils are peptide structures constructed from amphipathic heptad repeats. Here we show that it is possible to form pure heterotrimeric three-stranded coiled coils by combining three distinct characteristics: (1) a cysteine sulfur layer for metal coordination, (2) a thiophilic, trigonal pyramidal metalloid (Pb(II)) that binds to these sulfurs and (3) an adjacent layer of reduced steric bulk generating a cavity where water can hydrogen bond to the cysteine sulfur atoms. Cysteine substitution in an a site yields Pb(II)A2B heterotrimers, while d sites provide pure Pb(II)C2D or Pb(II)CD2 scaffolds. Altering the metal from Pb(II) to Hg(II) or shifting the relative position of the sterically less demanding layer removes heterotrimer specificity. Because only two of the eight or ten hydrophobic layers are perturbed, catalytic sites can be introduced at other regions of the scaffold. A Zn(II)(histidine)3(H2O) centre can be incorporated at a remote location without perturbing the heterotrimer selectivity, suggesting a unique strategy to prepare dissymmetric catalytic sites within self-assembling de novo-designed proteins.

Speciation of arsenic and antimony in basaltic magmas

This study applies X-ray absorption near edge structure (XANES) and extended X-ray absorption fine structures (EXAFS) spectroscopy at the K-edge to determine the speciation of arsenic and antimony in a suite of basaltic glasses synthesized over a range of oxygen fugacity (fO2) at 1200 °C and 0.1 MPa. Experiments were executed in evacuated fused silica ampoules using a variety of solid metal metal-oxide buffers to achieve fO2’s ranging from FMQ −3.3 to FMQ +5.7 (where FMQ is the fayalite magnetite quartz buffer). The oxidation state was calculated using linear combination fitting (LCF) to spectral reference material with known oxidation states using the XANES spectra. Speciation results were corrected for the quench effect of iron. Trivalent arsenic and antimony were determined to be the dominant oxidation state in the samples, with pentavalent arsenic contributing to less than 10% of the budget of arsenic unless the fO2 is greater than FMQ +5.3 ± 0.9, while pentavalent antimony was not observed. Additionally, no reduced oxidation states of arsenic or antimony were found in the glasses even at the lowest fO2 investigated. Structural parameters such as the coordination number and bond length were determined by fitting theoretical electron scattering paths to the EXAFS spectra. Arsenic is coordinated by three oxygens at 1.78 ± 0.01 Å forming AsIIIO3E (where E is the lone pair of electrons) trigonal pyramids. Antimony is coordinated by three oxygens at 1.98 ± 0.01 Å, interpreted to be in a trigonal pyramid structure similar to As3+. As both metalloids are primarily present in the trivalent state over the range of terrestrial fO2 (FMQ −3 to FMQ +5) it is expected that each will behave incompatibly during basaltic melting or crystallization, owing to (1) the tendency for these elements to form oxyanions, and (2) their poor match in ionic radius and charge for the major cation sites in mafic minerals.

Structure, acidity, and metal complexing properties of oxythioarsenites in hydrothermal solutions

In the pursuit of understanding the aqueous chemistry of oxythioarsenites (i.e., H3AsO2S and H3AsOS2) on a microscopic scale, first principles molecular dynamics (FPMD) simulations were performed to investigate the hydration structures, acidity constants, and metal-complexing properties of these species in aqueous solutions at temperatures up to 573K. The simulations showed that the oxythioarsenite species are stable and have trigonal pyramidal structures in both low- and high-temperature fluids. Their solvated structures were characterized in detail and it was found that the S or H atoms of the SH ligands form very weak H-bonds with the solvating water molecules and the OH ligands form H-bonds with water as both donors and acceptors. Furthermore, the dangling S or O sites act as strong acceptors to form H-bonds. Their acidity constants were calculated by using the FPMD-based vertical energy gap method. Together with our previous work (Liu et al., 2015, Chemical Geology, 411, 192–199), we found that in the arsenite-thioarsenite series (i.e., H3AsO3−xSx for x varying from 0 to 3), the pKa values of the oxythioarsenite species (H3AsO2S and H3AsOS2) were always located between those of the end members, H3AsO3 and H3AsS3, and decreased with the increasing temperature up to 573K. Based on the calculated acidity constants, the speciation versus pH distributions were obtained at temperatures ranging from ambient temperature to 573K. It was found that both H2AsO2S− and H2AsOS2− are the predominant species at near neutral pH for H3AsO2S and H3AsOS2 species, respectively. In addition, the structures and the dissociation free energy of the oxythioarsenite-metal complexes at 573K indicated that these As-S moieties are effective ligands for complexing with ore-forming metals in geological fluids.

Crystal growth and dislocation etch pits observation of chalcopyrite CdSiP2

CdSiP2 is the only crystal that can offer Non-critical Phase Matching (NCPM) for a 1064nm pumped optical parametric oscillation (OPO) with idler output in the 6μm range. In this paper, a large, crack-free CdSiP2 single crystal measuring 18mm in diameter and 65mm in length was successfully grown by the Vertical Bridgman method (MVB) with an explosion-proof quartz ampoule. The results of lattice parameters, element composition and IR transmittance of the as-grown crystal characterized by X-ray diffraction (XRD), energy dispersive X-ray spectrometer (EDS) and Fourier transformation infrared spectrometer (FTIR) showed the as grown crystal crystallized well and the absorption coefficients at 4878cm−1 and 2500cm−1 were 0.14cm−1 and 0.06cm−1. Moreover, a new etchant composed of Br2, HCl, HNO3, CH3OH and H2O (1:800:800:400:400 in volume ratio) was prepared and the dislocation etch pits on oriented faces of as-grown CdSiP2 crystal were observed for the first time. It is found the etch pits are in rectangular structure on the (101) face, but in trigonal pyramid structure on (312) face. According to the quantities of the etch pits, the average densities of dislocation were evaluated to be 2.28×105/cm2 and 1.4×105/cm2, respectively.

Lead(II) Binding in Natural and Artificial Proteins.

This article describes recent attempts to understand the biological chemistry of lead using a synthetic biology approach. Lead binds to a variety of different biomolecules ranging from enzymes to regulatory and signaling proteins to bone matrix. We have focused on the interactions of this element in thiolate-rich sites that are found in metalloregulatory proteins such as Pbr, Znt, and CadC and in enzymes such as δ-aminolevulinic acid dehydratase (ALAD). In these proteins, Pb(II) is often found as a homoleptic and hemidirectic Pb(II)(SR)3- complex. Using first principles of biophysics, we have developed relatively short peptides that can associate into three-stranded coiled coils (3SCCs), in which a cysteine group is incorporated into the hydrophobic core to generate a (cysteine)3 binding site. We describe how lead may be sequestered into these sites, the characteristic spectral features may be observed for such systems and we provide crystallographic insight on metal binding. The Pb(II)(SR)3- that is revealed within these α-helical assemblies forms a trigonal pyramidal structure (having an endo orientation) with distinct conformations than are also found in natural proteins (having an exo conformation). This structural insight, combined with 207Pb NMR spectroscopy, suggests that while Pb(II) prefers hemidirected Pb(II)(SR)3- scaffolds regardless of the protein fold, the way this is achieved within α-helical systems is different than in β-sheet or loop regions of proteins. These interactions between metal coordination preference and protein structural preference undoubtedly are exploited in natural systems to allow for protein conformation changes that define function. Thus, using a design approach that separates the numerous factors that lead to stable natural proteins allows us to extract fundamental concepts on how metals behave in biological systems.

Practical Synthesis and Properties of 2,5-Diarylarsoles.

2,5-Diarylarsoles were easily synthesized from nonvolatile arsenic precursors. Diiodoarsine was generated in situ and reacted with titanacyclopentadienes to give 2,5-diarylarsoles. The structures and optical properties were studied in comparison with those of 2,5-diarylphosphole. It was found that the arsoles were much more stable in the air than the phosphole. Single crystal X-ray diffraction revealed the arsenic atoms adopted a trigonal pyramidal structure, reflecting on the s-character of the lone pair. The obtained 2,5-diarylarsoles and 2,5-diarylphosphole showed intense emission in solutions and solid state. In addition, the optical properties were controlled by transition-metal coordination.

Vibrational Spectroscopy Reveals Varying Structural Motifs in Cu(+)(CH4)(n) and Ag(+)(CH4)(n) (n = 1-6).

Vibrational spectra are measured for Cu(+)(CH4)(Ar)2, Cu(+)(CH4)2(Ar), Cu(+)(CH4)n (n = 3-6), and Ag(+)(CH4)n (n = 1-6) in the C-H stretching region (2500-3100 cm(-1)) using photofragment spectroscopy. Spectra are obtained by monitoring loss of Ar or CH4. Interaction with the metal ion produces substantial red shifts in the C-H stretches of proximate hydrogens. The magnitude of the shift reflects the metal-methane distance and the coordination to the metal ion of the methane hydrogens (η(2) or η(3)). The structures of the complexes are determined by comparing the measured spectra with spectra calculated for candidate geometries using the B3LYP and CAM-B3LYP density functionals with 6-311++G(3df,3pd) and aug-cc-pVTZ-PP basis sets. Because of the d(10) electronic configuration of the metal ions, the complexes are expected to adopt symmetric structures, which is confirmed by the experiments. All of the complexes have η(2) hydrogen coordination in the first shell, in accord with theoretical predictions; second-shell ligands sometimes show η(3) hydrogen coordination. The vibrational spectrum of Cu(+)(CH4)(Ar)2 shows extensive structure due to Fermi resonance between the lowest-frequency C-H stretch and overtones of the H-C-H bends. The Cu(+)(CH4) cluster has a smaller red shift in the lowest-frequency C-H stretch than M(+)(CH4), M(+) = Co(+) (d(8)) and Ni(+) (d(9)). Although all three ions have similar binding energies, the metal-ligand electrostatic interaction is largest for Cu(+), while the contribution from covalent interactions is largest for Co(+). The larger ionic radius of Ag(+) leads to a larger metal-ligand distance and weaker interaction, resulting in substantially smaller red shifts than in the Cu(+) complexes. The Cu(+)(CH4)2 and Ag(+)(CH4)2 clusters have symmetrical structures, with the methanes on opposite sides of the metal, while Cu(+)(CH4)3 and Ag(+)(CH4)3 adopt symmetrical, trigonal planar structures with all M-C distances equal. For Cu(+)(CH4)4, the tetrahedral structure dominates the observed spectrum, although a trigonal pyramidal structure may contribute; however, only the tetrahedral structure is observed for Ag(+)(CH4)4. The structures of Cu(+)(CH4)n and Ag(+)(CH4)n differ for clusters with n > 4. For copper complexes, these are primarily formed by adding outer-shell methane ligand(s) to the tetrahedral n = 4 core. The observed spectra of the larger Ag(+) clusters are dominated by symmetrical structures in which all of the Ag-C distances are similar: Ag(+)(CH4)5 has a trigonal bipyramidal geometry and Ag(+)(CH4)6 is octahedral.

Synthesis, Structure and Thermal Decomposition of the Bismuth Silyltellurolate Bi(TeSitBu2Ph)3

AbstractThe compound Bi(TeSitBu2Ph)3 (1) was obtained from the reaction of BiCl3 with tBu2PhSiTeSiMe3 in diethyl ether. The single crystal structure analysis revealed a trigonal pyramidal structure of the BiTe3 core and weak Bi···C(arene) contacts. Compound 1 was characterized by multinuclear NMR spectroscopy, IR spectroscopy and elemental analysis. Thermogravimetric experiments show that compound 1 decomposes with formation of Bi2Te3 or a mixture of elemental bismuth and Bi4Te3, depending on the pressure conditions.

Cationic Hydridotriplatinum Complex with Bridging Germylene Ligands

Triplatinum complexes with bridging secondary germylene ligands, [{Pt(PMe3)}3(μ-GeR2)3] (R = Ph, nBu), were prepared from the reaction of H2GeR2 with [Pt(PMe3)4] in a 1:1 ratio. Protonation of the triplatinum complexes with HBF4 led to formation of the cationic hydride complexes [{Pt(PMe3)}3(μ-GeR2)3(μ3-H)]BF4 (R = Ph, nBu). The results of multinuclear NMR spectra and DFT calculations of the cationic complexes revealed that the Pt3H core has a trigonal-pyramidal structure with rapid exchange of the μ3-hydride ligand position between the two possible positions on the opposite side of the Pt3 plane.

Bonding analysis of trimethylenemethane (TMM) complexes [(CO)3M–TMM] (M = Fe, Ru, Os, Rh+). Absence of expected bond paths

Quantum chemical calculations using gradient-corrected DFT methods (BP86) using effective core potentials and TZP quality basis sets have been carried out for the complexes (CO)3M–TMM (M = Fe, Ru, Os, Rh+; TMM = trimethylenemethane). The calculated equilibrium geometries give trigonal pyramidal structures which are in good agreement with experimental results for the iron and ruthenium species. The TMM ligand adopts a pyramidal coordination of the central carbon atom which has a shorter distance to the metal atom than the terminal carbon atoms. The theoretically predicted bond dissociation energies of the TMM ligand are between 108.0 kcal/mol for the iron complex and 155.1 kcal/mol for the rhodium complex. The trend of the calculated TMM–M bond strength is Fe < Ru < Os < Rh+. The investigation of the electronic structure of the complexes (CO)3M–TMM using MO correlation diagrams and energy partitioning methods clearly shows that the metal atom is mainly bonded to the terminal carbon atoms of the TMM ligand while the bonding to the closer central carbon atom is much weaker. This is further supported by the AIM analysis which shows that the Laplacian distribution ∇2ρ(r) at the terminal carbon atoms possesses local area of charge concentration which points toward the metal atom while the Laplacian distribution at the central carbon atoms is nearly undistorted. In spite of the shape of the Laplacian distribution, there are bond paths between the metal atom and the central carbon atom but not to the terminal carbon atom which comes from the different interatomic distances. This clearly shows that the absence and the existence of a bond path are not reliable criteria for chemical bonding.

An unusual fan-type polyanion with a silver cation located at the axial center, [AgAsIII2(AsIIIAsVMo4O18(OH)2)3]11−

A new type of polyoxomolybdate containing mixed-valence arsenic, [AgAs(III)(2)(As(III)As(V)Mo(4)O(18)(OH)(2))(3)](11-) (1), has been synthesized. Single-crystal X-ray analysis was carried out on (NH(4))(5)(C(2)H(9)N(2))(4.5)H(1.5)[AgAs(III)(2)(As(III)As(V)Mo(4)O(18)(OH)(2))(3)]·14H(2)O (1a) and (NH(4))(11)[AgAs(III)(2)(As(III)As(V)Mo(4)O(18)(OH)(2))(3)]·16H(2)O (1b). The polyanion is composed of three unprecedented [As(III)As(V)Mo(4)O(18)(OH)(2)](6-) subunits linked together by two AsO(3) trigonal-pyramidal structures from both sides via sharing six bridging oxygen atoms, and a silver cation is enclosed in the center of the polyanion and coordinated by three As atoms with the As-Ag-As bond angle of 120°, to lead to a fan-like structure with an approximative C(3h) symmetry. The polyanion was also characterized by IR, X-ray photoelectron spectroscopy and thermogravimetry-differential scanning calorimetry. The stability of [AgAs(III)(2)(As(III)As(V)Mo(4)O(18)(OH)(2))(3)](11-) in an aqueous solution was investigated by electronic absorbance spectroscopy and electrospray ionization mass spectrometry, and the possible disaggregation mechanism of the polyanion is proposed.

SO2 Reaction on Cu(100): SO3 Structure and Formation –Density Functional Theory Investigation–

SO 3 structure and reaction on Cu(100) were investigated by using density functional theory-based calculation. SO 3 was found to adsorbed on the surface through its three oxygen atoms having a trigonal pyramid structure with its C 3 v axis lying nearly perpendicular to the surface. The interaction of SO 3 with the surface was found to occur at the d z 2 surface orbital that led to the delocalization of SO 3 molecular orbitals. This delocalization resulted in the elongation of one S–O bondlength. The barrier for the reaction SO 2 +O→SO 3 was found to be 0.2 eV, smaller than the one for reaction SO 2 →SO+O which indicate that the SO 2 +O→SO 3 should be more favorable to occur.

Ligand-to-Metal Charge-Transfer Dynamics in a Blue Copper Protein Plastocyanin: A Molecular Dynamics Study

Equilibrium and nonequilibrium dynamics of a blue copper protein plastocyanin in an oxidized state are studied by molecular dynamics (MD) simulation. Potential energy functions of the lowest seven electronic states, including ligand-to-metal charge-transfer (LMCT) and copper d --> d excited states, were taken from our previous work (Ando, K. J. Phys. Chem. B 2004, 108, 3940), which employed ab initio molecular orbital and density functional calculations on the active-site model. The equilibrium MD simulations in the ground state indicate that ligand motions coupled to transition from the ground state to the LMCT state are mostly represented by stretching and bending vibrations of the Cu-S(Cys) distance, Ndelta(His)-Cu-Ndelta(His) angle, and S(Cys)-Cu-[Ndelta(His)]2 trigonal pyramid structure. The nonequilibrium dynamics on the LMCT potential exhibit rapid decays in which surface crossings to the d --> d and the first excited states occur in 70-80 fs. The crossing dynamics mostly correlate with cleavage of the Cu-S(Cys) bond and the associated response in the Ndelta(His)-Cu-Ndelta(His) moiety. The average dynamics of the vertical energy gap coordinates exhibit an overdamped decay with a recurrence oscillation in 500 fs, which shows clear coherence surviving after the ensemble averaging. This oscillation stems mostly from the recoiling motion of the Ndelta(His)-Cu-Ndelta(His) part. The dynamics of the energy gaps after this coherent oscillation are randomized such that the ensemble average yields flat profiles along time, although each single trajectory exhibits fluctuations with amplitudes large enough to reach surface crossings. These indicate that the relaxation from the LMCT state first occurs via ballistic and coherent potential crossings in 70-80 and 500 fs, followed by thermally activated random transitions.

Use of Cu2+ as a metal ion probe for the EPR study of metal complexation sites in the double sulfite CuI 2SO3.CdII SO3 .2H2O

An evaluation was made of the presence of paramagnetic ions in the host lattice of CuI2SO3.CdII SO3.2H2O by the electron paramagnetic resonance (EPR) technique, using copper(II) ions as paramagnetic probes. The existing M I and M II complexation sites in the double sufite structure were successfully differentiated. The results indicate that the CuI2SO3.CdII SO3.2H2O structure was doped with CuII ions in the two metal ion substitutional positions. The low g<FONT FACE=Symbol>||</FONT>/A<FONT FACE=Symbol>||</FONT> ratio for the CuII probe in the distorted octahedral site resulted from the interaction of copper ions with oxygen donor atoms in a distorted tetragonal environment, with elongation of the molecule in the z direction. The very low value of the A33 hyperfine coupling constant was explained considering the admixture of the d x² -y² and d z² orbitals and the almost trigonal pyramidal structure of the other site.

Peptide Folding, Metal-Binding Mechanisms, and Binding Site Structures in Metallothioneins

This minireview specifically focuses on recent studies carried out on structural aspects of metal-free metallothionein (MT), the mechanism of metal binding for copper and arsenic, structural studies using x-ray absorption spectroscopy and molecular mechanics modeling, and speciation studies of a novel cadmium and arsenic binding algal MT. Molecular mechanics-molecular dynamics calculations of apo-MT show that significant secondary structural features are retained by the polypeptide backbone upon sequential removal of the metal ions, which is stabilized by a possible H-bonding network. In addition, the cysteinyl sulfurs were shown to rotate from within the domain core, where they are found in the metallated state, to the exterior surface of the domain, suggesting an explanation for the rapid metallation reactions that were measured. Mixing Cu6beta-MT with Cd4alpha-MT and Cu6alpha-MT with Cd3beta-MT resulted in redistribution of the metal ions to mixed metal species in each domain; however, the Cu+ ions preferentially coordinated to the beta domain in each case. Reaction of As3+ with the individual metal-free beta and alpha domains of MT resulted in three As3+ ions coordinating to each of the domains, respectively, in a proposed distorted trigonal pyramid structure. Kinetic analysis provides parameters that allow simulation of the binding of each of the As3+ ions. X-ray absorption spectroscopy provides detailed information about the coordination environment of the absorbing element. We have combined measurement of x-ray absorption near edge structure (XANES) and extended x-ray absorption fine structure (EXAFS) data with extensive molecular dynamics calculations to determine accurate metal-thiolate structures. Simulation of the XANES data provides a powerful technique for probing the coordination structures of metals in metalloproteins. The metal binding properties of an algal MT, Fucus vesiculosus, has been investigated by UV absorption and circular dichroism spectroscopy and electrospray ionization-mass spectrometry. The 16 cysteine residues of this algal MT were found to coordinate six Cd2+ ions in two domains with stoichiometries of a novel Cd3S7 cluster and a beta-like Cd3S9 cluster.