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PubMed1999

Probing the Molecular Geometry of Five-Coordinate Vanadyl Complexes with Pulsed ENDOR.

The pulsed electron paramagnetic resonance (EPR) technique of (51)V electron spin echo-electron nuclear double resonance (ESE-ENDOR) has been used to measure the nuclear quadrupole coupling constants of a series of five-coordinate vanadyl complexes containing Schiff base ligands with geometries ranging from distorted square pyramidal to distorted trigonal bipyramidal. Vanadium nuclear quadrupole coupling constants are sensitive to the coordination geometry of the vanadyl ion, and thus sensitive to the structural distortions within this series of complexes. (51)V ESE-ENDOR has been shown to be a probe of the coordination geometry of vanadyl complexes. Such a spectroscopic probe should prove useful in the investigation of vanadyl of unknown coordination geometry, such as may be found in the interaction of the vanadyl ion with biomolecules.

Journal Article↗
PubMed2005

Nitrogen activation via three-coordinate molybdenum complexes: comparison of density functional theory performance with wave function based methods.

Obtaining an accurate theoretical model for the activation of dinitrogen by three-coordinate molybdenum amide complexes (e.g. Mo(NH2)3) is difficult due to the interaction of various high- and low-spin open-shell complexes along the reaction coordinate which must be treated with comparable levels of accuracy in order to obtain reasonable potential energy surfaces. Density functional theory with present-day functionals is a popular choice in this situation; however, the dinitrogen activation reaction energetics vary substantially with the choice of functional. An assessment of the reaction using specialized wave function based methods indicates that although current density functionals in general agree qualitatively on the mechanistic details of the reaction, a variety of high-level electron correlation methods (including CCSD(T), OD(T), CCSD(2), KS-CCSD(T), and spin-flip CCSD) provide a consistent but slightly different representation of the system.

Journal Article↗
PubMed2004

The structural and spectroscopic characterisation of three actinyl complexes with coordinated and uncoordinated perrhenate: [UO2(ReO4)2(TPPO)3], [[(UO2)(TPPO)3]2(mu2-O2)][ReO4]2 and [NpO2(TPPO)4][ReO4].

The first structural characterization of an actinide complex with coordinated perrhenate is reported, [UO2(ReO4)2(TPPO)3] (1). In this [UO2]2+ complex two [ReO4]- anions and three TPPO (triphenylphosphine oxide) P=O donor ligands are coordinated in the equatorial plane in a cisoid arrangement. This bonding arrangement, and apparent strain observed in the equatorially bonded ligands, is attributed to the solid state packing in adjacent molecules in which hydrophobic TPPO ligands form an effective "shell" around a hydrophilic core of two UO2(ReO4)2 moieties. Solid state vibrational spectroscopy (infrared and Raman), 31P CP MAS NMR and elemental analysis are also consistent with the formula of 1. Solution state vibrational spectroscopy and 31P NMR measurements in EtOH indicate the lability of the TPPO and [ReO4]- groups. The photolytic generation of peroxide in EtOH solutions of 1 leads to the formation of trace quantities of [[(UO2)(TPPO)3]2(mu2-O2)][ReO4]2, 2, in which the coordinated [ReO4]- groups of 1 have been displaced by bridging O2(2-), derived from atmospheric O2. Finally, attempts to synthesise a [NpO2]+ analogue of have resulted only in the formation of [NpO2(TPPO)4][ReO4], 3, in which [ReO4]- acts solely as a counter anion. From these results it can be concluded that [ReO4]- will bond to [UO2]2+, but will be readily displaced by a more strongly coordinating ligand (e.g. peroxide) and will not coordinate to an actinyl cation with a lower charge, [NpO2]+, under the same reaction conditions.

Journal Article↗
PubMed2005

Molecular engineering of octupolar NLO molecules and materials based on bipyridyl metal complexes.

Coordination compounds are of particular interest for the design of new chromophores with large nonlinear optical (NLO) susceptibilities. They offer a wide range of metals with different oxidation states and ligands, which can give rise to tunable electronic properties. Metal ions are excellent templates to build 2D and 3D octupolar NLO-phores. Their associations with functionalized bipyridyl ligands give rise to octahedral and tetrahedral metal complexes with large first-order hyperpolarizabilities. The ability to functionalize these octupoles allows their incorporation into macromolecular architectures such as polymers and metallodendrimers, We also show in this Account two approaches to induce noncentrosymmetry at the macroscopic level, either by supramolecular organization or by the all optical poling procedure.

2,2'-Dipyridyl↗
PubMed2004

Electronic structure and spectra of linkage isomers of bis(bipyridine)(1,2-dihydroxy-9,10-anthraquinonato)ruthenium(ii) and their redox series.

Linkage isomers of bis(bipyridine)(1,2-dihydroxy-9,10-anthraquinonato)ruthenium(II), 1,2- and 1,9-coordinated complexes, and several of their oxidation products have been prepared chemically and/or electrochemically. For the 1,2-coordinated complex, the one- and two-electron oxidized species have been characterized, and for the 1,9-coordinated complex, the one-electron oxidized species has been characterized. The rich redox activity of these complexes leads to ambiguity in assessing the electronic structure. This paper reports EPR spectra of odd-electron species and detailed analyses of electronic spectra and structure of the complexes, based on INDO molecular orbital calculations. Results of calculations on the related 1-hydroxyanthraquinone complex and the free ligands,1,2-dihydroxy-9,10-anthraquinone (alizarin) and 1-hydroxyanthraquinone, are also briefly discussed.

Journal Article↗
PubMed2001

Specific recognition of chiral amino alcohols via lanthanide coordination chemistry: structural optimization of lanthanide tris(beta-diketonates) toward effective circular dichroism/fluorescence probing.

Lanthanide tris(beta-diketonates) formed stable, 1:1 highly coordinated complexes with amino alcohols, and the resulting complexes exhibited large enhanced fluorescence and intense induced circular dichroism (CD) signals. The stability constants of the highly coordinated complexes were determined for various combinations of lanthanide centers, beta-diketonate ligands, and organic substrates. These revealed that amino alcohol coordinated with the lanthanide center much more strongly than monoamine, monoalcohol, or diol derivative. On the basis of the highly coordinated complexation, several lanthanide tris(beta-diketonates) acted as CD/fluorescence probes specific for amino alcohols. Tris(6,6,7,7,8,8,8-heptafluoro-2,2-dimethyl-3,5-octadionato)europium(III) showed enhanced fluorescence in the presence of amino alcohols, while the corresponding ytterbium complex exhibited chirality-dependent CD signals for amino alcohols. In particular, the observed CD spectral profiles related well with the absolute configuration and optical purity of the bound amino alcohol, indicating that the structural optimization of lanthanide tris(beta-diketonates) offered specific sensing of amino alcohols and precise determination of their enantiomer excess percentages.

Journal Article↗
PubMed1989

Stereoselective ligand interactions of chicken liver phosphoenolpyruvate carboxykinase with fluorophosphoenolpyruvate.

The stereospecific interactions of chicken liver phosphoenolpyruvate carboxykinase (P-enolpyruvate carboxykinase) with the two geometric isomers of 3-fluorophosphoenolpyruvate (F-P-enolpyruvate) were examined. Previous studies have shown that the Z isomer of F-P-enolpyruvate is a substrate for P-enolpyruvate carboxykinase but the E isomer is a competitive inhibitor [T. H. Duffy and T. Nowak (1984) Biochemistry 23, 661-670]. The reasons for this substrate selectivity were investigated. Studies of the 1H, 19F, and 31P relaxation rates of the ligands in the binary Mn-ligand complexes indicate the formation of direct coordination complexes. The temperature and frequency dependence of the proton relaxation rates (PRR) of the respective enzyme-Mn-ligand complexes demonstrates that the perturbation of the electronic environment at the Mn(II) site on the enzyme is different upon binding of the inhibitor (E-F-P-enolpyruvate) in contrast to the binding of substrates (P-enolpyruvate or Z-F-P-enolpyruvate). Structural studies demonstrate that Z-F-P-enolpyruvate forms a second sphere coordination complex with enzyme-bound Mn(II). E-F-P-enolpyruvate exchanges slowly from the ternary complex and binds less than or equal to 10 A from the bound Mn(II). CD studies in the far-uv region demonstrate that the alpha-helical content of P-enolpyruvate carboxykinase is increased at the expense of antiparallel and parallel beta-sheet structure upon binding of Mn(II) and substrate (P-enolpyruvate or Z-F-P-enolpyruvate) to the apoenzyme, but show no such structural change upon binding of Mn(II) and E-F-P-enolpyruvate. Analogous results are observed from CD studies at the aromatic amino acid region (250-350 nm). The stereoselective catalytic activities of P-enolpyruvate carboxykinase with F-P-enolpyruvate analogs can be explained by different interactions of these ligands within the catalytic site of the enzyme.

Animals↗
PubMed1996

Spectral and kinetic studies on the activation of soluble guanylate cyclase by nitric oxide.

The soluble form of guanylate cyclase (sGC) is the only definitive receptor for the signaling agent nitric oxide (.NO). The enzyme is a heterodimer of homologous subunits in which each subunit binds 1 equiv of 5-coordinate high-spin heme. .NO increases the Vmax of sGC up to 400-fold and has previously been shown to bind to the heme to form a 5-coordinate complex. Using stopped-flow spectrophotometry, it is demonstrated that the binding of .NO to the heme of sGC is a complex process. .NO first binds to the heme to form a 6-coordinate nitrosyl complex, which then converts to a 5-coordinate nitrosyl complex through one of two ways. For 28 +/- 4% of the heme, the 6-coordinate nitrosyl complex rapidly (approximately 20 s-1) converts to the 5-coordinate complex. For the remaining 72 +/- 4% of the heme, the conversion of the 6-coordinate nitrosyl complex to a 5-coordinate nitrosyl complex is slow (0.1-1.0 s-1) and is dependent upon the interaction of .NO with an unidentified non-heme site on the protein. The heme (200 nM) was completely converted to the 5-coordinate state with as little as 500 nM .NO, and the equilibrium dissociation constant of .NO for activating the enzyme was determined to be < or = 250 nM. Gel-filtration analysis indicates that the binding of .NO to the heme has no effect on the native molecular mass of the protein. Correlation of electronic absorption spectra with activity measurements indicates that the 5-coordinate nitrosyl form of the enzyme is activated relative to the resting 5-coordinate ferrous form of the enzyme.

Animals↗
PubMed2003

Structural elements of metal selectivity in metal sensor proteins.

Staphylococcus aureus CzrA and Mycobacterium tuberculosis NmtR are homologous zinccobalt-responsive and nickelcobalt-responsive transcriptional repressors in vivo, respectively, and members of the ArsRSmtB superfamily of prokaryotic metal sensor proteins. We show here that Zn(II) is the most potent negative allosteric regulator of czr operatorpromoter binding in vitro with the trend Zn(II)>Co(II)Ni(II), whereas the opposite holds for the binding of NmtR to the nmt operatorpromoter, Ni(II)>Co(II)>Zn(II). Characterization of the metal coordination complexes of CzrA and NmtR by UVvisible and x-ray absorption spectroscopies reveals that metals that form four-coordinate tetrahedral complexes with CzrA [Zn(II) and Co(II)] are potent regulators of DNA binding, whereas metals that form five- or six-coordinate complexes with NmtR [Ni(II) and Co(II)] are the strongest allosteric regulators in this system. Strikingly, the Zn(II) coordination complexes of CzrA and NmtR cannot be distinguished from one another by x-ray absorption spectroscopy, with the best fit a His-3-carboxylate complex in both cases. Inspection of the primary structures of CzrA and NmtR, coupled with previous functional data, suggests that three conserved His and one Asp from the C-terminal alpha5 helix donate ligands to create a four-coordinate complex in both CzrA and NmtR, with NmtR uniquely capable of expanding its coordination number in the Ni(II) and Co(II) complexes by recruiting additional His ligands from a C-terminal extension of the alpha5 helix.

Amino Acid Sequence↗
PubMed2001

Clean oxidation of thiolates to sulfinates in a four-coordinate CoIII complex with a mixed carboxamido N-thiolato S donor set: relevance to nitrile hydratase.

A new [Co(N2(SO2)2)(CNtBu)2](Et4N) complex 6 was prepared from N,N'-(3-mercapto-3-methyl-butyryl)-o-phenylenediamine and completely characterized. While the starting square planar complex [Co(N2S2)](Et4N) 4 was destroyed by dioxirane, the Co ligated thiolates of the six-coordinate intermediate [Co(N2S2)(CNtBu)2](Et4N) complex 5 was readily oxidized to sulfinates with a stoichiometric amount of this oxidant. The resulting complex 6 crystallizes with an octahedral structure. The SO bonds of the SO2 groups are almost equivalent (approximately 1.483 and approximately 1.453 A). The isonitrile is linearly bonded to the cobalt with a Co-C-N angle of 177.5 degrees and a very short C-N(tBu) distance of 1.13 A, which has a triple bond character. As expected for six-coordinate CoIII complexes, 5 and 6 are diamagnetic in agreement with their 1H and 13C NMR spectra. The SO2 IR bands are located at 1210 cm(-1) (v(as)SO2) and 1070 cm(-1) (v(s)SO2), while the CN vibration of the isonitrile is observed at 2170 cm(-1) in 5 and 2210 cm(-1) in 6. Very recently, it has been reported in the literature that oxidation of the coordinated thiolates was required for activity of both Fe and Co nitrile hydratases. Complex 6, with two oxidized thiolates trans to two deprotonated carboxamido nitrogens, is the first to have an in-plane closely related to that of the Co-NHase active site.

Catalytic Domain↗
PubMed1999

DNA damage by copper(II) complexes: coordination-structural dependence of reactivities.

A variety of copper complexes with different structural features have been shown to bind double-helical DNA with binding constants of 10(4)-10(7) M-1 and to promote double-strand DNA damage upon reductant/H2O2 activation. The interaction of the Cu complex with DNA results in hyperchromism and shifts to longer wavelengths of the strongest transitions in the Cu complexes, as well as striking hypochromism or hyperchromism of DNA absorption at 260 nm. In the presence of DMPO as the spin trap, the solution of each copper complex exhibits typical four-line ESR spectra of the hydroxyl radical by adding 2-mercaptoethanol and H2O2 to the solution. Quantitation by 2-deoxy-D-ribose shows that the competence of hydroxyl radical generation by the copper complexes upon reductant and H2O2 activation decreases in order, that is, Cu(HTCD)(2+)-Cu(Im)4Cl2-Cu(IDB) (NO3)2 > Cu(IDB)Cl2 > Cu(IDBt)Cl2. The copper complex-mediated hydroxyl radical, a powerful oxidant that attacks the adjacent DNA, is responsible for the DNA oxidative damage. The lambda DNA damage chemistry illustrates that the competence and selectivity of double-strand lambda DNA damage by the copper complexes are dependent on their geometric structures and types of ligands. The decreasing order of the DNA damage capacity by the present complexes is Cu(Im)4Cl2-Cu(IDB)-(NO3)2 > Cu(HTCD)2+ > Cu(IDBt)Cl2 > Cu(IDB)Cl2.

Animals↗
PubMed1978

Effect of complexes of lidocaine with transient metals on histamine release from rat mast cells.

The effect of complexes of lidocaine with zinc, copper and cobalt on histamine release from peritoneal rat mast cells induced by various secretagogues was investigated. The complexes of lidocaine with metal ions inhibited histamine release induced by compound 48/80 and ionophoreous antibiotics: A23187 and X537A. The ionic complex of lidocaine with zinc was found to be the most potent against 48/80--induced histamine release and exerted the significant inhibition in the concentration of 10(-5) M. The action of coordination complex of lidocaine with zinc was somewhat weaker. The significant inhibitory effect on 48/80-induced histamine release of the copper and cobalt lidocaine complexes was observed in concentration of 10(-4) M. Histamine release by ionophores A23187 and X537A was also affected by zinc lidocaine complexes. However, coordination complex had more potent action on histamine release than the ionic one.

Animals↗
PubMed1996

Synthesis and Characterization of Six-Coordinate Nitrido Complexes of Vanadium(V), Chromium(V), and Manganese(V). Isolation of a Dinuclear, Mixed-Valent &mgr;-Nitrido Chromium(III)/Chromium(V) Species.

Photolysis of a series of octahedral monoazido complexes of the type [LM(III)(didentate ligand)(N(3))](n)(+)X(n) of vanadium(III), chromium(III), and manganese(III) in the solid state or in solution yields quantitatively the corresponding six-coordinate nitrido complexes [LM(V)(didentate ligand)(N)](n)(+)X(n) and 1 equiv of dinitrogen. L represents the macrocycle 1,4,7-triazacyclononane or its N-methylated derivative (L'), the didentate ligands are pentane-2,4-dionate (acac), 2,2,6,6-tetramethylheptane-3,5-dionate (tacac), picolinate (pic), phenanthroline (phen), and oxalate (ox), and X(-) represents perchlorate or hexafluorophosphate. The following nitrido complexes were prepared: [LV(V)(N)(acac)](ClO(4)) (6), [LCr(V)(N)(acac)](ClO(4)) (13), [LCr(V)(N)(tacac)](ClO(4)) (14), [LCr(V)(N)(pic)](ClO(4)) (15), [LCr(V)(N)(phen)](ClO(4))(2) (16), [LCr(V)(N)(ox)] (19), [L'Mn(V)(N)(acac)]PF(6) (21). Photolysis of [LCr(III)(N(3))(ox)] (17) in the solid state produces the &mgr;-nitrido-bridged mixed-valent species [L(2)Cr(2)(ox)(2)(&mgr;-N)](N(3)) (18). The structures of the precursor complex [L'Mn(acac)(N(3))]BPh(4) (20), of 13, and of [L'Mn(V)(N)(acac)]BPh(4) (21) have been determined by X-ray crystallography. Complex 13 crystallizes in the orthorhombic space group Pnma, with cell constants a = 27.187(5) Å, b = 9.228(2) Å, c = 7.070(1) Å, V = 1773.7(6) Å(3), and Z = 4; complex 20 crystallizes in the triclinic space group P&onemacr; with a = 14.769(5) Å, b = 16.83(1) Å, c = 16.96(1) Å, alpha = 108.19(5) degrees, beta = 105.06(4) degrees, gamma = 99.78(4) degrees, V = 3719(2) Å(3), and Z = 4; and complex 21 crystallizes in the monoclinic space group P2(1)/n with a = 10.443(3) Å, b = 16.035(4) Å, c = 21.463(5) Å, beta = 95.76(1) degrees, V = 3575.9(14) Å(3), and Z = 4. The Cr(V)&tbd1;N and Mn(V)&tbd1;N distances are short at 1.575(9) and 1.518(4) Å, respectively, and indicate a metal-to-nitrogen triple bond.

Journal Article↗
PubMed2006

Complexes of Ag(I), Hg(I) and Hg(II) with multidentate pyrazolyl-pyridine ligands: From mononuclear complexes to coordination polymers via helicates, a mesocate, a cage and a catenate.

The coordination chemistry of a series of di- and tri-nucleating ligands with Ag(I), Hg(I) and Hg(II) has been investigated. Most of the ligands contain two or three N,N'-bidentate chelating pyrazolyl-pyridine units pendant from a central aromatic spacer; one contains three binding sites (2 + 3 + 2-dentate) in a linear sequence. A series of thirteen complexes has been structurally characterised displaying a wide range of structural types. Bis-bidentate bridging ligands react with Ag(I) to give complexes in which Ag(I) is four-coordinate from two bidentate donors, but the complexes can take the form of one-dimensional coordination polymers, or dinuclear complexes (mesocate or helicate). A tris-bidentate triangular ligand forms a complicated two-dimensional coordination network with Ag(I) in which Ag...Ag contacts, as well as metal-ligand coordination bonds, play a significant role. Three dinuclear Hg(I) complexes were isolated which contain an {Hg2}2+ metal-metal bonded core bound to a single bis-bidentate ligand which can span both metal ions. Also characterised were a series of Hg(II) complexes comprising a simple mononuclear four-coordinate Hg(II) complex, a tetrahedral Hg(II)4 cage which incorporates a counter-ion in its central cavity, a trinuclear double helicate, and a trinuclear catenated structure in which two long ligands have spontaneously formed interlocked metallomacrocyclic rings thanks to cyclometallation of two of the Hg(II) centres.

Journal Article↗
PubMed2001

Elementary steps in the acquisition of Mn2+ by the fosfomycin resistance protein (FosA).

The fosfomycin resistance protein, FosA, catalyzes the Mn(2+)-dependent addition of glutathione to the antibiotic fosfomycin, (1R,2S)-epoxypropylphosphonic acid, rendering the antibiotic inactive. The enzyme is a homodimer of 16 kDa subunits, each of which contains a single mononuclear metal site. Stopped-flow absorbance/fluorescence spectrometry provides evidence suggesting a complex kinetic mechanism for the acquisition of Mn(2+) by apoFosA. The binding of Mn(H(2)O)(6)(2+) to apoFosA alters the UV absorption and intrinsic fluorescence characteristics of the protein sufficiently to provide sensitive spectroscopic probes of metal binding. The acquisition of metal is shown to be a multistep process involving rapid preequilibrium formation of an initial complex with release of approximately two protons (k(obsd) > or = 800 s(-1)). The initial complex either rapidly dissociates or forms an intermediate coordination complex (k > 300 s(-1)) with rapid isomerization (k > or = 20 s(-1)) to a set of tight protein-metal complexes. The observed bimolecular rate constant for formation of the intermediate coordination complex is 3 x 10(5) M(-1) s(-1). The release of Mn(2+) from the protein is slow (k approximately 10(-2) s(-1)). The kinetic results suggest a more complex chelate effect than is typically observed for metal binding to simple multidentate ligands. Although the addition of the substrate, fosfomycin, has no appreciable effect on the association kinetics of enzyme and metal, it significantly decreases the dissociation rate, suggesting that the substrate interacts directly with the metal center.

Bacterial Proteins↗
PubMed2003

Four-coordinate phosphinidene complexes of titanium prepared by alpha-H-migration: phospha-Staudinger and phosphaalkene-insertion reactions.

alpha-Hydrogen migration in the phosphide (Nacnac)Ti=CHtBu(PHR) (Nacnac- = [Ar]NC(Me)CHC(Me)N[Ar], Ar = 2,6-iPr2C6H3, R = C6H11, 2,4,6-iPr3C6H2, 2,4,6-tBuC6H2), prepared from salt metathesis of (Nacnac)Ti=CHtBu(PHR) with LiPHR, generates terminal and four-coordinate phosphinidene complexes (Nacnac)Ti=PR(CH2tBu), one of which was structurally characterized (R = 2,4,6-tBu3C6H2). Phosphinidene intermediate (Nacnac)Ti=PR(CH2tBu) (R = C6H11, 2,4,6-iPr3C6H2) transform to ([Ar]NC(Me)CHC(Me)P[R][CH2tBu])Ti=NAr(OEt2) through "phospha-Staudinger" and subsequent phosphaalkene-insertion reactions.

Journal Article↗