三核钼原子簇化合物的EPR和电子结构

本文报道一个新的三核钼原子簇化合物在室温和77K固态和液态的EPR谱,这些谱均呈现出a,b两套谱线的叠加,分析谱线的强度比,线宽和谱参数,认为g值较大的a谱归属于三核钼原子簇化合物,而b谱归属于单核钼杂质。从由X射线晶体结构方法确定的空间结构出发,三核钼原子簇化合物可能有两种分子形式:Mo3(μ3-S)(μ2-S)3Cl[S2P(OEt)2]4 1Mo3(μ3-S)(μ2-S)(μ2-Cl)2Cl[S2P(OEt)2]4 2分别用简单量子化学理论和EHMO法计算出未配对电子所处的分子轨道,求出g1和g1,并与EPR实验值相比较,认为该化合物的分子式应为2,簇骼{Mo3}属七电子体系,在77K温度下,其未配对电子主要局域在三个钼原子所组成的近似等腰三角形簇骼的顶点钼原子周围。     

Spectroscopic and density functional studies of the red copper site in nitrosocyanin: role of the protein in determining active site geometric and electronic structure

The electronic structure of the red copper site in nitrosocyanin is defined relative to that of the well understood blue copper site of plastocyanin by using low-temperature absorption, circular dichroism, magnetic circular dichroism, resonance Raman, EPR and X-ray absorption spectroscopies, combined with DFT calculations. These studies indicate that the principal electronic structure change in the red copper site is the sigma rather than the pi donor interaction of the cysteine sulfur with the Cu 3d(x2-y2) redox active molecular orbital (RAMO). Further, MCD data show that there is an increase in ligand field strength due to an increase in coordination number, whereas resonance Raman spectra indicate a weaker Cu-S bond. The latter is supported by the S K-edge data, which demonstrate a less covalent thiolate interaction with the RAMO of nitrosocyanin at 20% relative to plastocyanin at 38%. EXAFS results give a longer Cu-S(Cys) bond distance in nitrosocyanin (2.28 A) compared to plastocyanin (2.08 A) and also show a large change in structure with reduction of the red copper site. The red copper site is the only presently known blue copper-related site with an exogenous water coordinated to the copper. Density functional calculations reproduce the experimental properties and are used to determine the specific protein structure contributions to exogenous ligand binding in red copper. The relative orientation of the CuNNS and the CuSC(beta) planes (determined by the protein sequence) is found to be key in generating an exchangeable coordination position at the red copper active site. The exogenous water ligation at the red copper active site greatly increases the reorganization energy (by approximately 1.0 eV) relative to that of the blue copper protein site, making the red site unfavorable for fast outer-sphere electron transfer, while providing an exchangeable coordination position for inner-sphere electron transfer.     

Spectroscopic and kinetic studies of perturbed trinuclear copper clusters: the role of protons in reductive cleavage of the O-O bond in the multicopper oxidase Fet3p

The multicopper oxidase Fet3p couples four 1e(-) oxidations of substrate to the 4e(-) reduction of O2 to H2O. Fet3p uses four Cu atoms to accomplish this reaction: the type 1, type 2, and coupled binuclear type 3 sites. The type 2 and type 3 sites together form a trinuclear Cu cluster (TNC) which is the site of O2 reduction. This study focuses on mutants of two residues, E487 and D94, which lie in the second coordination sphere of the TNC and defines the role that each plays in the structural integrity of the TNC, its reactivity with O2, and in the directional movement of protons during reductive cleavage of the O-O bond. The E487D, E487A, and D94E mutants have been studied in the holo and type 1 depleted (T1D) forms. Residue E487, located near the T3 center, is found to be responsible for donation of a proton during the reductive cleavage of the O-O bond in the peroxide intermediate and an inverse kinetic solvent isotope effect, which indicates that this proton is already transferred when the O-O bond is cleaved. Residue D94, near the T2 site, plays a key role in the reaction of the reduced TNC with O2 and drives electron transfer from the T2 Cu to cleave the O-O bond by deprotonating the T2 Cu water ligand. A mechanism is developed where these second sphere residues participate in the proton assisted reductive cleavage of the O-O bond at the TNC.     

Spectroscopic characterization of the Leu513His variant of fungal laccase: effect of increased axial ligand interaction on the geometric and electronic structure of the type 1 Cu site

A variety of spectroscopic techniques, combined with density functional calculations, are used to describe the electronic structure of the Leu513His variant of the type 1 Cu site in Myceliophthora thermophila laccase. This mutation changes the type 1 Cu from a blue to a green site. Electron paramagnetic resonance (EPR), optical absorption, circular dichroism, and magnetic circular dichroism (MCD) spectroscopies reveal that, relative to the trigonal planar blue type 1 Cu site in wild-type fungal laccase, the covalency and the ligand field strength at the Leu513His green type 1 Cu center decrease. Additionally, there is a significant reorientation of the d(x)()()2(-)(y)()()2( )singly occupied MO, such that the overlap with the Cys sulfur valence orbital changes from pi to sigma. A density functional study in which internal coordinates are systematically altered reveals that these changes are due to the increased strength of the axial ligand (none to His), leading to a tetragonal distortion and elongation of the equatorial Cu-ligand bonds. These calculations provide insight into the experimental differences in the EPR parameters, charge-transfer absorption spectrum, and ligand-field MCD spectrum between the axial-His variant and blue Cu centers (plastocyanin and the type 1 site in fungal laccase). There are also significant differences between the green site in the Leu513His variant and other naturally occurring, green type 1 Cu sites such as in nitrite reductase, which have short axial Cu-S(Met) bonds. The large difference in EPR parameters between these green type 1 sites derives from a change in ligand field excitation energies observed by MCD, which reflects a decrease in ligand field strength. This is associated with different steric interactions of a His vs an axial Met ligand in a tetragonally distorted type 1 site. Changes in the electronic structure of the Cu site correlate with the difference in reactivity of the green His variant relative to blue wild-type fungal laccase.     

Geometrical control of the active site electronic structure of pyranopterin enzymes by metal-dithiolate folding: aldehyde oxidase

Density functional calculations on geometry-optimized oxidized (Mo(VI)) and reduced (Mo(IV)) analogues of the isolated active site of aldehyde oxidase (MOP), a member of the xanthine oxidase family of pyranopterindithiolate enzymes, show that fold angle changes of the dithiolate ligand modulate the relative metal and dithiolate contributions to the frontier redox orbitals. Proton abstraction from the equatorial aqua ligand of the oxidized Mo(VI) site also flattens the metal dithiolate fold angle. It is proposed that static and/or dynamic changes in the structure of the protein surrounding the active site can induce changes in the dithiolate fold angle and thereby provide a mechanism for electronic buffering of the redox orbital, for fine-tuning the nucleophilicity of the equatorial aqua/hydroxide ligand, and for modulating the electron-transfer regeneration of the active sites of molybdenum and tungsten enzymes via a "dithiolate folding effect".     

Spectroscopic properties and coordination structure studies on the [Cu(l-Glu)(Im)] complex

Summary The electronic absorption spectrum and photoacoustic absorption spectrum of (l-glutamato)(imidazole)copper(II) were recorded and the results are discussed quantitatively with respect to ligand field theory and the radial scaling wave function of a non-free copper(II) ion. The axial interactions are discussed by comparing experimental with calculated values according to CuN(2)O(2) and CuN(2)O(3) chromophores, respectively, and by the bond-strength analysis.Author to whom all correspondence should be directed.     

Ground-state electronic and magnetic properties of a mu3-oxo-bridged trinuclear Cu(II) complex: correlation to the native intermediate of the multicopper oxidases

The ground-state electronic and magnetic properties of one of the possible structures of the trinuclear Cu(II) site in the native intermediate (NI) of the multicopper oxidases, the mu(3)-oxo-bridged structure, are evaluated using the C(3)-symmetric Cu(3)(II) complex, mu(3)O. mu(3)O is unique in that no ligand, other than the oxo, contributes to the exchange coupling. However, mu(3)O has a ferromagnetic ground state, inconsistent with that of NI. Therefore, two perturbations have been considered: protonation of the mu(3)-oxo ligand and relaxation of the mu(3)-oxo ligand into the Cu(3) plane. Notably, when the oxo ligand is sufficiently close to the Cu(3) plane (<0.3 Angstroms), the ground state of mu(3)O becomes antiferromagnetic and can be correlated to that of NI. In addition, the ferromagnetic (4)A ground state of mu(3)O is found from variable-temperature EPR to undergo a zero-field splitting (ZFS) of 2D = -5.0 cm(-1), which derives from the second-order anisotropic exchange. This allows evaluation of the sigma-to-pi excited-state exchange pathways and provides experimental evidence that the orbitally degenerate (2)E ground state of the antiferromagnetic mu(3)O would also undergo a ZFS by the first-order antisymmetric exchange that has the same physical origin as the anisotropic exchange. The important contribution of the mu(3)-oxo bridge to the ground-to-ground and ground-to-excited-state superexchange pathways that are responsible for the isotropic, antisymmetric, and anisotropic exchanges are discussed.     

Nature of the intermediate formed in the reduction of O(2) to H(2)O at the trinuclear copper cluster active site in native laccase

The multicopper oxidases contain at least four copper atoms and catalyze the four-electron reduction of O(2) to H(2)O at a trinuclear copper cluster. An intermediate, termed native intermediate, has been trapped by a rapid freeze-quench technique from Rhus vernicifera laccase when the fully reduced form reacts with dioxygen. This intermediate had been described as an oxygen-radical bound to the trinuclear copper cluster with one Cu site reduced. XAS, however, shows that all copper atoms are oxidized in this intermediate. A combination of EXAFS, multifrequency EPR, and VTVH MCD has been used to understand how this fully oxidized trinuclear Cu cluster relates to the fully oxidized resting form of the enzyme. It is determined that in the native intermediate all copper atoms of the cluster are bridged by the product of full O(2) reduction. In contrast, the resting form has one copper atom of the cluster (the T2 Cu) magnetically isolated from the others. The native intermediate decays to the resting oxidized form with a rate that is too slow to be in the catalytic cycle. Thus, the native intermediate appears to be the catalytically relevant fully oxidized form of the enzyme, and its role in catalysis is considered.     

Theoretical studies on electronic structure and magnetic properties of mixed‐valence uteroferrin active site

The electronic structure and magnetic interaction of the active site of pig purple acid phosphatase (PAP, uteroferrin) were investigated using pure DFT (UBLYP) and hybrid DFT methods (UB3LYP and UB2LYP). Uteroferrin catalyzes the hydrolysis of a phosphate ester under acidic conditions and contains a binuclear iron center. The mammalian PAPs are expected to be targets for drug design of osteoporosis. Their active sites are typical examples of the Fe(II)‐Fe(III) mixed‐valence system. We studied double exchange interaction of the mixed‐valence system, using the potential energy difference between the Fe(II)‐Fe(III) and the Fe(III)‐Fe(II) states. The pathway of the antiferromagnetic coupling between Fe(III) and Fe(II) were also discussed by using chemical indices, which are evaluated by the occupation numbers of singly occupied natural orbitals. © 2009 Wiley Periodicals, Inc. Int J Quantum Chem, 2010     

Spectroscopic and electronic structure studies of protocatechuate 3,4-dioxygenase: nature of tyrosinate-Fe(III) bonds and their contribution to reactivity.

The geometric and electronic structure of the high-spin ferric active site of protocatechuate 3,4-dioxygenase (3,4-PCD) has been examined by absorption (Abs), circular dichroism (CD), magnetic CD (MCD), and variable-temperature-variable-field (VTVH) MCD spectroscopies. Density functional (DFT) and INDO/S-CI molecular orbital calculations provide complementary insight into the electronic structure of 3,4-PCD and allow an experimentally calibrated bonding scheme to be developed. Abs, CD, and MCD indicate that there are at least seven transitions below 35 000 cm(-1) which arise from tyrosinate ligand-to-metal-charge transfer (LMCT) transitions. VTVH MCD spectroscopy gives the polarizations of these LMCT bands in the principal axis system of the D-tensor, which is oriented relative to the molecular structure from the INDO/S-CI calculations. Three transitions are associated with the equatorial tyrosinate and four with the axial tyrosinate. This large number of transitions per tyrosinate is due to the pi and importantly the sigma overlap of the two tyrosinate valence orbitals with the metal d orbitals and is governed by the Fe-O-C angle and the Fe-O-C-C dihedral angles. The previously reported crystal structure indicates that the Fe-O-C angles are 133 degrees and 148 degrees for the equatorial and axial tyrosinate, respectively. Each tyrosinate has transitions at different energies with different intensities, which correlate with differences in geometry that reflect pseudo-sigma bonding to the Fe(III) and relate to reactivity. These factors reflect the metal-ligand bond strength and indicate that the axial tyrosinate-Fe(III) bond is weaker than the equatorial tyrosinate-Fe(III) bond. Furthermore, it is found that the differences in geometry, and hence electronic structure, are imposed by the protein. The consequences to catalysis are significant because the axial tyrosinate has been shown to dissociate upon substrate binding and the equatorial tyrosinate in the enzyme-substrate complex is thought to influence asymmetric binding of the chelated substrate moiety via a strong trans influence which activates the substrate for reaction with O2.     

Covalent linkage of the type-2 and type-3 structural mimics to model the active site structure of multicopper oxidases: synthesis and magneto- structural properties of two angular trinuclear copper(II) complexes

Two new angular trinuclear copper(II) complexes of formulation [Cu(3)(HL)LL'](ClO(4)), where L' is imidazole (Him, 1) or 1-methylimidazole (1-MeIm, 2) and H(3)L is a Schiff base obtained from the condensation of salicylaldehyde and 1,3-diaminopropan-2-ol (2:1 mole ratio), are prepared from a reaction of [Cu(2)L(mu-Br)] and [Cu(HL)] in the presence of L' and isolated as perchlorate salts. The crystal structures of 1 and 2 consist of a trinuclear copper(II) unit formed by the covalent linkage of monomeric type-2 mimic and dimeric type-3 mimic precursor complexes to give an angular arrangement of the metal atoms in the core which is a model for the active site structure of blue multicopper oxidases. In 1 and 2, the coordination geometry of two terminal copper atoms is distorted square-planar. The central copper has a distorted square-pyramidal (4 + 1) geometry. The mean Cu...Cu distance is approximately 3.3 A. The complex has a diphenoxo-bridged dicopper(II) unit with the phenoxo oxygen atoms showing a planar geometry. In addition, the complex has an endogenous alkoxo-bridged dicopper(II) unit showing a pyramidal geometry for the oxygen atom. The 1:1 electrolytic complexes show a d-d band at 607 nm. Cyclic voltammetry of the complexes in MeCN containing 0.1 M TBAP using a glassy carbon working electrode displays a Cu(3)(II)/Cu(2)(II)Cu(I) couple near -1.0 V (vs SCE). The variable temperature magnetic susceptibility measurements in the range 300-18 K show antiferromagnetic coupling in the complexes giving magnetic moments of approximately 3.0 mu(B) at 300 K and approximately 2.1 mu(B) at 18 K for the tricopper(II) unit. The experimental susceptibility data are theoretically fitted using a model with Heisenberg spin-(1)/(2) Hamiltonian for a trimer of spin-(1)/(2) copper(II) ions having two exchange parameters involving the alkoxo-bridged dicopper(II) (J1) and the diphenoxo-bridged dicopper(II) (J2) units, giving J1 and J2 values of -82.7, -73 cm(-1) for 1 and -98.3, -46.1 cm(-1) for 2, respectively. The structural features indicate a higher magnitude of anitiferromagnetic coupling in the alkoxo-bridged unit based on the greater value of the Cu-O-Cu angle in comparison to the diphenoxo-bridged unit. The core structures of 1 and 2 compare well with the first generation model complexes for the active site structure of multicopper oxidases in the oxidized form. The crystal structure of 1 exhibits a lamellar structure with a gap of approximately 7 A containing water molecules in the interlamellar space. Complex 2 forms a hexanuclear species due to intermolecular hydrogen bonding interactions involving two trimeric units. The crystal packing diagram of 2 displays formation of a three-dimensional framework with cavities containing the perchlorate anions.     

Spectroscopic and electronic structure studies of phenolate Cu(II) complexes: phenolate ring orientation and activation related to cofactor biogenesis

A combination of spectroscopies and DFT calculations have been used to define the electronic structures of two crystallographically defined Cu(II)-phenolate complexes. These complexes differ in the orientation of the phenolate ring which results in different bonding interactions of the phenolate donor orbitals with the Cu(II), which are reflected in the very different spectroscopic properties of the two complexes. These differences in electronic structures lead to significant differences in DFT calculated reactivities with oxygen. These calculations suggest that oxygen activation via a Cu(I) phenoxyl ligand-to-metal charge transfer complex is highly endergonic (>50 kcal/mol), hence an unlikely pathway. Rather, the two-electron oxidation of the phenolate forming a bridging Cu(II) peroxoquinone complex is more favorable (11.3 kcal/mol). The role of the oxidized metal in mediating this two-electron oxidation of the coordinated phenolate and its relevance to the biogenesis of the covalently bound topa quinone in amine oxidase are discussed.     

The adjacent Fe oxidation greatly enhancing OER activity on the Ni active site: S plays the role in optimizing local coordination and electronic structure

Oxygen evolution reaction (OER) is the bottleneck process of water splitting, and finding efficient, durable, low-cost, and earth-abundant electrocatalysts remains a major challenge. Here, FeNi₂-400-S is to be a promising OER electrocatalyst which exhibits a low overpotential of 214 mV at a current density of 10 mA/cm². X-ray analysis indicates that the introduction of S leads to a mismatch in bond distance between the metal-sulfur bond and the metal-metal bond, which can change the local electronic structure and favorably control the electronic oxidation. The active site position of FeNi₂-400-S has been further confirmed by DFT, which the 1OOH can stably adsorb on the Ni site of the oxidized Fe-Ni-S benefitting from the synergetic effect of the Ni site and the adjacent oxidized O on the Fe atom. Our findings demonstrate that the internal reconstruction of catalyst can make the optimization of local coordination and electronic structure, in which the in-situ generated vacancy can enable the outstanding OER performance.