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PubMed2006

Crystal structures of the Nicotiana glutinosa ribonuclease NT in complex with nucleoside monophosphates.

Ribonuclease NT (RNase NT), induced upon tobacco mosaic virus (TMV) infection in Nicotiana glutinosa leaves, has a broad base specificity. The crystal structures of RNase NT in complex with either 5'-AMP, 5'-GMP, or 2'-UMP were determined at 1.8 A resolutions by molecular replacement. RNase NT consists of seven helices and seven beta strands, and the structure is highly similar to that of RNase NW, a guanylic acid preferential RNase from the N. glutinosa leaves, showing root mean square deviation (rmsd) of 1.1 A over an entire length of two molecules for Calpha atoms. The complex structures revealed that Trp42, Asn44, and Trp50 are involved in interactions with bases at B1 site (primary site), whereas Gln12, Tyr17, Ser78, Leu79, and Phe89 participate in recognition of bases at B2 site (subsite). The 5'-GMP and 5'-AMP bind both B1 and B2 sites in RNase NT, while 2'-UMP predominantly binds B1 site in the complex. The nucleotide binding modes in these complexes would provide a clue to elucidation of structural basis for the broad base specificity for RNase NT.

Base Sequence↗
PubMed2003

Nickel complexes of o-amidochalcogenophenolate(2-)/o-iminochalcogenobenzosemiquinonate(1-) pi-radical: synthesis, structures, electron spin resonance, and x-ray absorption spectroscopic evidence.

The preparation of complexes trans-[Ni(-SeC(6)H(4)-o-NH-)(2)](-) (1), cis-[Ni(-TeC(6)H(4)-o-NH-)(2)](-) (2), trans-[Ni(-SC(6)H(4)-o-NH-)(2)](-) (3), and [Ni(-SC(6)H(4)-o-S-)(2)](-) (4) by oxidative addition of 2-aminophenyl dichalcogenides to anionic [Ni(CO)(SePh)(3)](-) proves to be a successful approach in this direction. The cis arrangement of the two tellurium atoms in complex 2 is attributed to the intramolecular Te.Te contact interaction (Te.Te contact distance of 3.455 A). The UV-vis electronic spectra of complexes 1 and 2 exhibit an intense absorption at 936 and 942 nm, respectively, with extinction coefficient epsilon > 10000 L mol(-)(1) cm(-)(1). The observed small g anisotropy, the principal g values at g(1) = 2.036, g(2) = 2.062, and g(3) = 2.120 for 1 and g(1) = 2.021, g(2) = 2.119, and g(3) = 2.250 for 2, respectively, indicates the ligand radical character accompanied by the contribution of the singly occupied d orbital of Ni(III). The X-ray absorption spectra of all four complexes show L(III) peaks at approximately 854.5 and approximately 853.5 eV. This may indicate a variation of contribution of the Ni(II)-Ni(III) valence state. According to the DFT calculation, the unpaired electron of complex 1 and 2 is mainly distributed on the 3d(xz)() orbital of the nickel ion and on the 4p(z)() orbital of selenium (tellurium, 5p(z)()) as well as the 2p(z)() orbital of nitrogen of the ligand. On the basis of X-ray structural data, UV-vis absorption, electron spin resonance, magnetic properties, DFT computation, and X-ray absorption (K- and L-edge) spectroscopy, the monoanionic trans-[Ni(-SeC(6)H(4)-o-NH-)(2)](-) and cis-[Ni(-TeC(6)H(4)-o-NH-)(2)](-) complexes are appositely described as a resonance hybrid form of Ni(III)-bis(o-amidochalcogenophenolato(2-)) and Ni(II)-(o-amidochalcogenophenolato(2-))-(o-iminochalcogenobenzosemiquinonato(1-) pi-radical; i.e., complexes 1 and 2 contain delocalized oxidation levels of the nickel ion and ligands.

Journal Article↗
PubMed2004

Diorganotin(IV) derivatives of substituted benzohydroxamic acids with high antitumor activity.

A series of diorganotin(IV) and dichlorotin(IV) derivatives of 4-X-benzohydroxamic acids, [HL(1) (X = Cl) or HL(2) (X = OCH(3))] formulated as [R(2)SnL(2)] (R = Me, Et, nBu, Ph or Cl; L = L(1) or L(2)), along with their corresponding mixed-ligand complexes [R(2)Sn(L(1))(L(2))] have been prepared and characterized by FT-IR, (1)H, (13)C, and (119)Sn NMR spectroscopy, mass spectrometry, elemental analysis, and melting points. In addition, single-crystal X-ray diffraction analyses were carried out for [Me(2)SnL(2)] (L = L(1) or L(2)), which show coordination structures intermediate between distorted octahedra and bicapped tetrahedra. The hydroxamate ligands are asymmetrically coordinated by the oxygen atoms, the carbonyl oxygen atom is further away from the metal center than the other oxygen atom. The complexes are stable monomeric species; most of them are soluble not only in chlorohydrocarbon solvents, but also in alcohols and hydroalcoholic solutions. In polar solvents, the mixed-ligand complexes gradually decompose into the corresponding single-ligand complex couples. The complexes exhibit in vitro antitumor activities (against a series of human tumor cell lines) which, in some cases, are identical to, or even higher than, that of cisplatin. For the dialkyltin complexes, the activity increases with the length of the carbon chain of the alkyl ligand and is higher in the case of the chloro-substituted benzohydroxamato ligand. The [nBu(2)Sn(L(1))(2)] complex displays a high in vivo activity against H22 liver and BGC-823 gastric tumors, and has a relatively low toxicity.

Antineoplastic Agents↗
PubMed2006

Synthesis, structural characterization, and properties of chromium(III) complexes containing amidinato ligands and eta2-pyrazolato, eta(2)-1,2,4-triazolato, or eta1-tetrazolato ligands.

Treatment of anhydrous chromium(III) chloride with 2 or 3 equivalents of 1,3-di-tert-butylacetamidinatolithium or 1,3-diisopropylacetamidinatolithium in tetrahydrofuran at ambient temperature afforded Cr(tBuNC(CH3)NtBu)2(Cl)(THF) and Cr(iPrNC(CH3)NiPr)3 in 78% and 65% yields, respectively. Treatment of Cr(tBuNC(CH3)NtBu)2(Cl)(THF) with the potassium salts derived from pyrazoles and 1,2,4-triazoles afforded Cr(tBuNC(CH3)NtBu)2(X), where X=3,5-disubstituted pyrazolato or 3,5-disubstituted 1,2,4-triazolato ligands, in 65-70% yields. X-Ray crystal structure analyses of Cr(tBuNC(CH3)NtBu)2(Me2pz) (Me2pz=3,5-dimethylpyrazolato) and Cr(tBuNC(CH3)NtBu)2(Me2trz) (Me2trz=3,5-dimethyl-1,2,4-triazolato) revealed eta2-coordination of the Me2pz and Me2trz ligands. Treatment of Cr(tBuNC(CH3)NtBu)2(Cl)(THF) with trifluoromethyltetrazolatosodium (NaCF3tetz) in the presence of 4-tert-butylpyridine afforded Cr(tBuNC(CH3)NtBu)2(CF3tetz)(4-tBupy) in 30% yield. An X-ray crystal structure determination showed eta1-coordination of the tetrazolato ligand through the 2-nitrogen atom. The complexes Cr(iPrNC(CH3)NiPr)3 and Cr(tBuNC(CH3)NtBu)2(X) are volatile and sublime with <1% residue between 120 and 165 degrees C at 0.05 Torr. In addition, these complexes are thermally stable at >300 degrees C under an inert atmosphere such as nitrogen or argon. Due to the good volatility and high thermal stability, these new compounds are promising precursors for the growth of chromium-containing thin films using atomic layer deposition.

Journal Article↗
PubMed1999

Structural and Spectroscopic Studies of 16-Electron, Unsaturated Derivatives of Low-Valent, Group 6 Carbonyl Complexes Containing pi-Donor Ligands.

Several eighteen- and sixteen-electron derivatives of tungsten(0), molybdenum(0), and chromium(0) carbonyl complexes, including [PPN](2)[Cr(CO)(3)(O,S-C(6)H(4))] (2c), [PPN](2)[W(CO)(3)(NH,S-C(6)H(4))] (5c), [PPN](2)[W(CO)(3)(O,S-C(6)H(4))] (6c), [PPN](2)[W(CO)(4)(S,S-C(6)H(4))] (7b) have been synthesized from the reaction of photochemically generated M(CO)(5)THF with a series of doubly deprotonated 1,2-disubstituted benzene rings with the appropriate oxygen, nitrogen, and sulfur donor atoms. These complexes have been characterized in the solid state by X-ray crystallography and in solution by IR and (13)C NMR spectroscopies. The crystal of 2c (C(84)H(71)N(3)O(5)P(4)SCr) is triclinic P&onemacr;, a =13.869(3) Å, b = 23.128(5) Å, c = 12.056(2) Å, alpha = 104.84(3) degrees, beta = 106.91(3) degrees, gamma = 95.29(3) degrees, Z = 2; that of 5c (C(89)H(77)N(7)O(3)P(4)SW) is monoclinic P2(1), a = 11.054(2) Å, b = 28.140(6) Å, c = 12.566(2) Å, beta = 90.58(1) degrees, Z = 2; that of 6c (C(85)H(70)N(4)O(4)P(4)SW) is triclinic P&onemacr;, 12.236(2) Å, b = 14.419(2) Å, c = 22.748(4) Å, alpha = 76.44(1) degrees, beta = 75.98(2) degrees, gamma = 70.98(1) degrees, Z = 2; that of 7b (C(82)H(64)N(2)O(4)P(4)S(2)W) is triclinic P&onemacr;, a = 12.650(1) Å, b = 14.810(1) Å, c = 21.053(2) Å, alpha = 77.182(7) degrees, beta = 78.334(7) degrees, gamma = 66.579(7), Z = 2; and that of 8 (C(10)H(8)O(4)P(2)W) is monoclinic P2(1)/c, a = 11.582(1) Å, b = 10.791(1) Å, c = 10.449(1) Å, beta = 100.867(7) degrees, Z = 2. The average nu(CO) frequencies for each tricarbonyl species reported are compared to those related dianions previously reported in order to gauge the pi-donor character of the different ligands. The (13)C NMR spectrum for each tricarbonyl derivative consists of a single sharp peak for the three inequivalent carbonyls as a result of a low-energy, fast intramolecular exchange process. Both inter- and intramolecular CO-exchange processes have been probed via variable temperature (13)C NMR. In the case of the 16-electron species the geometry of the metal dianion is that of a distorted trigonal bipyramid consisting of three carbonyl ligands and a five-membered chelate ring bound through the pi-donor atoms at an equatorial and an axial position, with the stronger pi-donor atom in the equatorial site. The equatorial site for the most effective pi-donor is preferred over the axial position because the unoccupied d(xy)() orbital lies in the equatorial plane, and may be stabilized via a pi-donor ligand in the equatorial position. The axial position exhibits a filled/filled repulsion as both orbitals available for pi-bonding are filled.

Journal Article↗
PubMed2002

Crystal structure of [Cu(N-quinolin-8-yl-p-toluenesulfonamidate)(2)]: study of its interaction with DNA and hydrogen peroxide.

A new copper complex with N-quinolin-8-yl-p-toulenesulfonamide has been prepared and characterised. The compound crystallises in the triclinic system, space group P1, with a=13.457(3), b=15.067(5), c=18.589(3) A; alpha=112.05(2), beta=93.92(2), gamma=108.30(2) degrees and Z=4. The geometry of the Cu(II) ion is distorted square planar. The N-quinolin-8-yl-p-toulenesulfonamidate anion behaves as a bidentate ligand through the N(sulfonamidate)and N(quinoline) atoms. The complex does not cleave DNA in the presence of hydrogen peroxide.

Copper↗
PubMed1997

Synthesis and antitumor activity of the metformin platinum (IV) complex. Crystal structure of the tetrachloro(metformin)platinum (IV) dimethylsulfoxide solvate.

The synthesis of (metformin) tetrachloroplatinum (IV) was investigated (metformin is N,N-dimethylbiguanide). It crystallizes with one dimethylsulfoxide molecule as solvate in the monoclinic system, space group P2(1)/n (No. 14) with Z = 4. The cell dimensions are: a = 13.136(7), b = 9.424(2), c = 14.009(8) A, beta = 111.96(4) degrees, V = 1608.4(2) A3. Of the 4269 independent nonzero reflections collected, 1979 with I > 3 sigma (I) were considered and used in the calculations. The structure was refined to R = 0.043 and wR = 0.045. The platinum coordination is octahedral, built up from four chloride anions and one bidentate chelating ligand via the two imine nitrogen atoms cis position. The distances and angles are typical of six-membered rings that have similar donor atoms. The complex was evaluated in vitro and in vivo on murine P388 leukemia. It was found to be as potent as cis-dichlorodiammine platinum (II), CDDP, in inhibiting the proliferation of the sensitive P388 cells. However the resistant P388/CDDP cells were threefold more sensitive to the compound than to CDDP. The two compounds induced a similar perturbation in the G2+M phases of the cell-cycle. The complex was less active than CDDP in vivo on P388 leukemia when administered i.p. (intra peritoneal) on day 1.

Animals↗
PubMed1996

Proteasome: from structure to function.

During the past two years, significant progress has been made in understanding the structure and function of the proteasome. Recent work has revealed the three-dimensional structure of the 700 kDa proteolytic complex at atomic resolution and elucidated its novel catalytic mechanism. Close relationships to a number of other amino-terminal hydrolases have emerged, making the proteasomal subunits the prototype of this newly discovered structural superfamily.

Animals↗
PubMed1994

Synthesis and spectroscopy studies of copper(II) nitrate of sulfacetamide drug. Crystal structure of [Cu(sulfacetamide)2(NO3)2]. Antibacterial studies.

The structural spectroscopic, and thermal properties of a complex of sulfacetamide (Hsacm) with Cu(II) have been investigated. The complex [Cu(Hsacm)2(NO3)2] crystallizes in the monoclinic system, space group P2(1)/n. The cell dimensions are a = 7.696(7) A, b = 8.017(7) A, c = 19.230(10), beta = 110.80(1) degree, V = 1109(1) A3, Z = 2, and Dx = 1.84 g/cm3. The structure was refined to R = 0.0776. Cu(Hsacm)2(NO3)2 molecules form a long polymeric chain extended along the b-axis. The copper(II) coordinated geometry is tetragonally distorted octahedral with two amino nitrogens from Hsacm and two oxygens from nitrato anions in the basal plane and two acetamido oxygens from neighbor Hsacm molecules in the apical position. Each sulfacetamide, acting as a bidentate ligand, links two Cu(II) ions as a bridge through the Namino and the Oacetamido atoms. The complex proved to possess higher bacteriostatic activity than the corresponding ligand.

Chemical Phenomena↗
PubMed1996

113Cd Shielding Tensors of Monomeric Cadmium Compounds Containing Nitrogen Donor Atoms. 3. CP/MAS Studies on Five-Coordinate Cadmium Complexes Having N(3)X(2) (X = H, N, O, and S) Donor Atoms.

The principal elements of the (113)Cd shielding tensor for a set of five- coordinate compounds having mixed donor atoms coordinating to the cadmium were determined via CP/MAS NMR experiments. The first complex, [HB(3,5-Me(2)pz)(3)]CdBH(4) (where pz = pyrazolyl), has a CdN(3)H(2) inner coordination sphere. The isotropic chemical shift in the solid state is 355.1 ppm, and its chemical shift anisotropy (CSA, Deltasigma) is -596 ppm with an asymmetry parameter (eta) of 0.64. The second complex, [HB(3,5-Me(2)pz)(3)]Cd[H(2)B(pz)(2)], has five nitrogen donor atoms bonded to the cadmium. This N(5) or N(3)N(2) compound was the only material of this study to manifest dipolar splitting of the cadmium resonance from the quadrupolar (14)N. The isotropic chemical shift, CSA, and the value of eta for this material were therefore determined at higher field where the dipolar splitting was less than the linewidth, yielding values of 226.6 ppm, -247 ppm, and 0.32, respectively. A second N(5) material, [HB(3-Phpz)(3)]Cd[H(2)B(3,5-Me(2)pz)(2)], was also investigated and has an isotropic shift of 190.2 ppm, a CSA of 254 ppm, and an eta of 0.86. Also studied was [HB(3-Phpz)(3)]Cd[(Bu(t)CO)(2)CH], which has an CdN(3)O(2) inner core. The isotropic chemical shift of this complex is 173.6 ppm, and the values of Deltasigma and eta were determined to be -258 ppm and 0.38, respectively. The final compound, [HB(3,5-Me(2)pz)(3)]Cd[S(2)CNEt(2)], with N(3)S(2) donor atoms, has an isotropic shift of 275.8 ppm, an eta of 0.51, and a CSA of +375 ppm. Utilizing previous assignments, the most shielded tensor element was determined to be oriented normal to the plane of the tridentate ligand. The shielding tensor information is used to speculate on the coordination geometry of the CdN(3)O(2) inner core complex.

Journal Article↗
PubMed2005

Polymeric networks of copper(II) phenylmalonate with heteroaromatic n-donor ligands: synthesis, crystal structure, and magnetic properties.

Two new phenylmalonate-bridged copper(II) complexes with the formulas [Cu(4,4'-bpy)(Phmal)](n).2nH(2)O (1) and [Cu(2,4'-bpy)(Phmal)(H(2)O)](n)() (2) (Phmal = phenylmalonate dianion, 4,4'-bpy = 4,4'-bipyridine, 2,4'-bpy = 2,4'-bipyridine) have been synthesized and characterized by X-ray diffraction. Complex 1 crystallizes in monoclinic space group P2(1), Z = 4, with unit cell parameters of a = 9.0837(6) Angstroms, b = 9.3514(4) Angstroms, c = 11.0831(8) Angstroms, and beta = 107.807(6) degrees , whereas complex 2 crystallizes in orthorhombic space group C2cb, Z = 8, with unit cell parameters of a = 10.1579(7) Angstroms, b = 10.3640(8) Angstroms, and c = 33.313(4) Angstroms. The structures of 1 and 2 consist of layers of copper(II) ions with bridging bis-monodentate phenylmalonate (1 and 2) and 4,4'-bpy (1) ligands and terminal monodentate 2,4'-bpy (2) groups. Each layer in 1 contains rectangles with dimensions of 11.08 x 4.99 Angstroms(2), the edges being defined by the Phmal and 4,4'-bpy ligands. The intralayer copper-copper separations in 1 through the anti-syn equatorial-apical carboxylate-bridge and the 4,4'-bpy molecule are 4.9922(4) and 11.083(1) Angstroms, respectively. The anti-syn equatorial-equatorial carboxylate bridge links the copper(II) atoms in complex 2 within each layer with a mean copper-copper separation of 5.3709(8) Angstroms. The presence of 2,4'-bpy as a terminal ligand accounts for the large interlayer separation of 15.22 Angstroms. The copper(II) environment presents a static pseudo-Jahn-Teller disorder which has been studied by EPR and low-temperature X-ray diffraction. Magnetic susceptibility measurements of both compounds in the temperature range 2-290 K show the occurrence of weak antiferromagnetic [J = -0.59(1) cm(-1) (1)] and ferromagnetic [J = +0.77(1) cm(-1) (2)] interactions between the copper(II) ions. The conformation of the phenylmalonate-carboxylate bridge and other structural factors, such as the planarity of the exchange pathway in 1, account for the different nature of the magnetic interaction.

Journal Article↗
PubMed1998

Approaches to Alkaline Earth Metal-Organic Chemical Vapor Deposition Precursors. Synthesis and Characterization of Barium Fluoro-beta-ketoiminate Complexes Having Appended Polyether "Lariats"

The synthesis and characterization of a family of beta-ketoimines derived from 1,1,1,5,5,5-hexafluoro-2,4-pentanedione (hfa) and the corresponding volatile barium beta-ketoiminate-polyether complexes having the general formula Ba[CF(3)COCHC(NR)CF(3)](2) where R = (CH(2)CH(2)O)(2)CH(3), (CH(2)CH(2)O)(2)CH(2)CH(3), and (CH(2)CH(2)O)(3)CH(2)CH(3) is reported. These complexes can be transported in the vapor phase at 160 degrees C/0.05 Torr without decomposition. The beta-ketoiminate ligands are synthesized by condensation of the appropriate amine-terminated poly(ethylene oxide)s with the trimethylsilyl enol ether derivative of hfa and converted to barium beta-ketoiminate-polyether complexes by reaction with BaH(2). The poly(ethylene oxide) amines are in turn synthesized by triphenylphosphine-mediated reduction of the corresponding poly(ethylene oxide) azides (synthesized via the tosylates) to afford the amines in good yields and analytical purity. The amines, beta-ketoimines, and barium complexes were characterized by elemental analysis, (1)H, (19)F, and (13)C NMR spectroscopy, mass spectroscopy, and thermogravimetric analysis. The eight- and ten-coordinate Ba(2+) complexes having the formula Ba[CF(3)COCHC(NR)CF(3)](2) where R = (CH(2)CH(2)O)(2)CH(2)CH(3) [C(22)H(28)N(2)F(12)O(6)Ba; space group = monoclinic, P2(1); a = 12.1175(2) Å, b = 14.9238(2) Å, c = 16.9767(3) Å, alpha = gamma = 90 degrees, beta = 90.0840(10) degrees, Z = 4] and R = (CH(2)CH(2)O)(3)CH(2)CH(3) [C(26)H(36)N(2)F(12)O(8)Ba; space group = triclinic, P&onemacr; (#2); a = 10.971(2) Å, b = 12.134(2) Å, c = 15.280(4) Å, alpha = 89.94(2) degrees, beta = 110.00(2) degrees, gamma = 116.75(2) degrees, Z = 2] were characterized by single-crystal X-ray diffraction. Both analyses reveal monomeric structures with the beta-ketoiminate ligands coordinated to the Ba(2+) center through all available oxygen and nitrogen atoms. These complexes are substantially more volatile than Ba(2,2,6,6,-tetramethyl-3,5-heptanedionate)(2) but less so than the most volatile Ba(1,1,1,5,5,5-hexafluoro-2,4-pentanedionate)(2).polyether complexes.

Journal Article↗
PubMed2006

Diiron amido-imido complex [(CpFe)2(mu2-NHPh)(mu2-NPh)]: synthesis and a net hydrogen atom abstraction reaction to form a bis(imido) complex.

A reaction between [CpFeCl]x and LiNHPh (1 equiv to Fe) produces a new paramagnetic Fe(II)-Fe(III) mu2-amido-mu2-imido complex [(CpFe)2(mu2-NHPh)(mu2-NPh)] (1), which, upon interaction with 2,2'-azobis(2,4-dimethylvaleronitrile), undergoes a net N-H hydrogen atom abstraction reaction to give a diamagnetic Fe(III)-Fe(III) mu2-imido dimer [CpFe(mu2-NPh)]2 (2). The molecular structures of 1 and 2 have been determined by single-crystal X-ray diffraction.

Journal Article↗
PubMed2004

Dynamics of water molecules buried in cavities of apolipoprotein E studied by molecular dynamics simulations and continuum electrostatic calculations.

Molecular dynamics (MD) simulations of several nanoseconds each were used to monitor the dynamic behavior of the five crystal water molecules buried in the interior of the N-terminal domain of apolipoprotein E. These crystal water molecules are fairly well conserved in several apolipoprotein E structures, suggesting that they are not an artifact of the crystal and that they may have a structural and/or functional role for the protein. All five buried crystal water molecules leave the protein interior in the course of the longest simulations and exchange with water molecules from the bulk. The free energies of binding evaluated from the electrostatic binding free energy computed using a continuum model and estimates of the binding entropy changes represent shallow minima. The corresponding calculated residence times of the buried water molecules range from tens of picoseconds to hundreds of nanoseconds, which denote rather short times as for buried water molecules. Several water exchanges monitored in the simulations show that water molecules along the exit/entrance pathway use a relay of H bonds primarily formed with charged residues which helps either the exit or the entrance from or into the buried site. The exit/entrance of water molecules from/into the sites is permitted essentially by local motions of, at most, two side chains, indicating that, in these cases, complex correlated atomic motions are not needed to open the buried site toward the surface of the protein. This provides a possible explanation for the short residence times.

Apolipoproteins E↗
PubMed2003

C-C alpha insertion: insertion of an alkyne into the C-C single bond between the carbene-carbon atom and the alpha-carbon atom of a Fischer carbene complex by an unprecedented metalla(di-pi-methane) skeletal rearrangement.

The first examples of insertion of a C(triple bond)C bond of an alkyne into a C(carbene)-Calpha single bond of a carbene complex (C-Calpha insertion) are reported. (prim-Alkyl)carbene complexes [(OC)(5)M=C(OEt)CH(2)R] (1 a-f; M=Cr, W; R=nPr, C(7)H(7), Ph) undergo C-Calpha insertion of electron-deficient alkynes [PhC(triple bond)CC(XEt)NMe(2)]BF(4) (5 a,b; X=O, S) to give zwitterionic carbiminium carbonylmetalates 3 a-g, which are thermally transformed into (CO)(4)M chelate carbene complexes 4 a-g by elimination of CO. The overall reaction is highly regio- and stereoselective. It involves an unprecedented metalla(di-pi-methane) rearrangement as the key step.

Journal Article↗
PubMed2005

Nitrogen fixation under mild ambient conditions: part I--the initial dissociation/association step at molybdenum triamidoamine complexes.

In several recent studies Schrock and collaborators demonstrated for the first time how molecular dinitrogen can be catalytically transformed under mild and ambient conditions to ammonia by a molybdenum triamidoamine complex. In this work, we investigate the geometrical and electronic structures involved in this process of dinitrogen activation with quantum chemical methods. Density functional theory (DFT) has been employed to calculate the coordination energies of ammonia and dinitrogen relevant for the dissociation/association step in which ammonia is substituted by dinitrogen. In the DFT calculations the triamidoamine chelate ligand has been modeled by a systematic hierarchy of increasingly complex substituents at the amide nitrogen atoms. The most complex ligand considered is an experimentally known ligand with an HMT = 3,5-(2,4,6-Me3C6H2)2C6H3 substituent. Several assumptions by Schrock and collaborators on key reaction steps are confirmed by our calculations. Additional information is provided on many species not yet observed experimentally. Particular attention is paid to the role of the charge of the complexes. The investigation demonstrates that dinitrogen coordination is enhanced for the negatively charged metal fragment, that is, coordination is more favorable for the anionic metal fragment than for the neutral species. Coordination of N2 is least favorable for the cationic metal fragment. Furthermore, ammonia abstraction from the cationic complex is energetically unfavorable, while NH3 abstraction is less difficult from the neutral and easily feasible from the anionic low-spin complex.

Journal Article↗
PubMed1998

The three-dimensional structure of a helix-less variant of intestinal fatty acid-binding protein.

Intestinal fatty acid-binding protein (I-FABP) is a cytosolic 15.1-kDa protein that appears to function in the intracellular transport and metabolic trafficking of fatty acids. It binds a single molecule of long-chain fatty acid in an enclosed cavity surrounded by two five-stranded antiparallel beta-sheets and a helix-turn-helix domain. To investigate the role of the helical domain, we engineered a variant of I-FABP by deleting 17 contiguous residues and inserting a Ser-Gly linker (Kim K et al., 1996, Biochemistry 35:7553-7558). This variant, termed delta17-SG, was remarkably stable, exhibited a high beta-sheet content and was able to bind fatty acids with some features characteristic of the wild-type protein. In the present study, we determined the structure of the delta17-SG/palmitate complex at atomic resolution using triple-resonance 3D NMR methods. Sequence-specific 1H, 13C, and 15N resonance assignments were established at pH 7.2 and 25 degrees C and used to define the consensus 1H/13C chemical shift-derived secondary structure. Subsequently, an iterative protocol was used to identify 2,544 NOE-derived interproton distance restraints and to calculate its tertiary structure using a unique distance geometry/simulated annealing algorithm. In spite of the sizable deletion, the delta17-SG structure exhibits a backbone conformation that is nearly superimposable with the beta-sheet domain of the wild-type protein. The selective deletion of the alpha-helical domain creates a very large opening that connects the interior ligand-binding cavity with exterior solvent. Unlike wild-type I-FABP, fatty acid dissociation from delta17-SG is structurally and kinetically unimpeded, and a protein conformational transition is not required. The delta17-SG variant of I-FABP is the only wild-type or engineered member of the intracellular lipid-binding protein family whose structure lacks alpha-helices. Thus, delta17-SG I-FABP constitutes a unique model system for investigating the role of the helical domain in ligand-protein recognition, protein stability and folding, lipid transfer mechanisms, and cellular function.

Animals↗