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At least 1,153 records · Page 64Linked to original sources
PubMed2005

A synthetic breakthrough into an unanticipated stability regime: readily isolable complexes in which C16-C28 polyynediyl chains span two platinum atoms.

The oxidative cross-coupling of trans-(p-tol)(p-tol3P)2Pt(CC)2H (PtC4H) and excess H(CC)2SiEt3 (O2, cat. CuCl, TMEDA, acetone; Hay conditions) gives PtC8Si (29%), PtC12Si (30%), and PtC16Si (1%; Si = SiEt3). The less stable PtC6H is generated in situ from PtC6Si and n-Bu4N+F-; following the addition of ClSiMe3 (F- scavenger) and excess H(CC)2SiEt3, the Hay conditions afford PtC10Si (59%) and PtC14Si (7%). Analogous sequences can be conducted with PtC8Si and PtC10Si. When PtC6Si, PtC8Si, and PtC10Si are similarly reacted in the absence of H(CC)2SiEt3, homocouplings to PtC12Pt (88%), PtC16Pt (70%), and PtC20Pt (72%) occur. However, analogous reactions of PtC12Si and PtC14Si fail, presumably due to the rapid decomposition of PtC12H and PtC14H. Brønsted acidity trends suggest that (CC)n moieties should become better leaving groups with increasing chain length. Thus, when PtC12Si and PtC14Si are subjected to the Hay conditions alone, PtC24Pt (36%) and PtC28Pt (51%) are isolated, with substantial recovery of starting material from the former reaction. Presumably adventitious water and/or other nucleophiles effect desilylation (as also seen in the cross-couplings), after which homocoupling is rapid due to the simultaneous availability of an oxidizing agent. PtC24Pt and PtC28Pt are thermally stable to >/=140 degrees C, whereas all other known dodecaynes and tetradecaynes rapidly decompose at room temperature. There is every reason to believe that this series can be further extended. UV-visible spectra show progressively red-shifted and more intense bands with epsilon > 400 000 M-1 cm-1.

Journal Article↗
PubMed1998

Atomic structure of an alphabeta T cell receptor (TCR) heterodimer in complex with an anti-TCR fab fragment derived from a mitogenic antibody.

Each T cell receptor (TCR) recognizes a peptide antigen bound to a major histocompatibility complex (MHC) molecule via a clonotypic alphabeta heterodimeric structure (Ti) non-covalently associated with the monomorphic CD3 signaling components. A crystal structure of an alphabeta TCR-anti-TCR Fab complex shows an Fab fragment derived from the H57 monoclonal antibody (mAb), interacting with the elongated FG loop of the Cbeta domain, situated beneath the Vbeta domain. This loop, along with the partially exposed ABED beta sheet of Cbeta, and glycans attached to both Cbeta and Calpha domains, forms a cavity of sufficient size to accommodate a single non-glycosylated Ig domain such as the CD3epsilon ectodomain. That this asymmetrically localized site is embedded within the rigid constant domain module has implications for the mechanism of signal transduction in both TCR and pre-TCR complexes. Furthermore, quaternary structures of TCRs vary significantly even when they bind the same MHC molecule, as manifested by a unique twisting of the V module relative to the C module.

Amino Acid Sequence↗
PubMed1999

On the application of phase relationships to complex structures. XXXVI: some experiments with a small protein without heavy atoms.

The direct-methods program MULTAN88 has been applied to a known protein, ribonuclease (RNAP1), containing 808 non-H atoms, including five S atoms, plus 83 ordered solvent water molecules. Phase sets with mean phase errors between 69 and 75 degrees were selected by modified figures of merit for trials with the full data at 1.17 A resolution and also with restricted data at 1.25 and 1.5 A resolution. These figures of merit had previously only been applied to protein structures containing heavy atoms, and this is the first demonstration of their usefulness with no heavy atom present. An initial set of 1091 phases from a 1.17 A trial was developed by an objective procedure to give the full structure with a residual of 0. 21, which agrees well with the published structure.

Crystallography, X-Ray↗
PubMed2001

The tetra-manganese complex of photosystem II during its redox cycle - X-ray absorption results and mechanistic implications.

Using X-ray absorption spectroscopy (XAS), relevant information on structure and oxidation state of the water-oxidizing Mn complex of photosystem II has been obtained for all four semi-stable intermediate states of its catalytic cycle. We summarize our recent XAS results and discuss their mechanistic implications. The following aspects are covered: (a) information content of X-ray spectra (pre-edge feature, edge position, extended X-ray absorption fine-structure (EXAFS), dichroism in the EXAFS of partially oriented samples); (b) S(1)-state structure; (c) X-ray edge results on oxidation state changes; (d) EXAFS results on structural changes during the S-state cycle; (e) a structural model for the Mn complex in its S(3)-state; (f) XAS-based working model for the S(2)-S(3) transition; (g) XAS-based working model for the S(0)-S(1) transition; (h) potential role of hydrogen atom abstraction by the Mn complex. Finally, we present a specific hypothesis on the mechanism of dioxygen formation during the S(3)-(S(4))-S(0) transition. According to this hypothesis, water oxidation is facilitated by manganese reduction that is coupled to proton transfer from a substrate water to bridging oxides.

Electron Probe Microanalysis↗
PubMed2001

Reaction of hen egg white lysozyme with Fischer-type metallocarbene complexes. Characterization of the conjugates and determination of the metal complex binding sites.

The introduction of heavy atoms into protein crystals is sometimes rendered difficult and tedious because of the poor specificity of the available reagents for particular target residues. On the other hand, transition organometallic chemistry offers an almost untouched field for this purpose. In particular, Fischer-type metallocarbene complexes of the general formula (CO)5W=C(OR1)R2 may be attractive reagents because they contain the heavy element tungsten and specifically target amino groups to form stable, covalent aminocarbene adducts. With a small protein such as hen egg white lysozyme (HEWL) with a limited number of potential binding sites, it was possible to form protein-aminocarbene conjugates that have an average of one aminocarbene moiety per protein molecule. RP-HPLC combined with matrix-assisted laser desorption ionization time-of-flight (MALDI-TOF) MS analysis of the conjugates revealed that they were mixtures of the native protein, monoaminocarbenes and diaminocarbenes. Tryptic proteolysis experiments performed on the protein conjugates combined with MALDI-TOF-MS analysis of the aminocarbenic peptides allowed us to determine that lysines 13, 33, 97 and 116 were involved in the reaction of HEWL with (CO)5W=C(OMe)Me.

Animals↗
PubMed2003

A non-radical mechanism for methane hydroxylation at the diiron active site of soluble methane monooxygenase.

We propose a non-radical mechanism for the conversion of methane into methanol by soluble methane monooxygenase (sMMO), the active site of which involves a diiron active center. We assume the active site of the MMOH(Q) intermediate, exhibiting direct reactivity with the methane substrate, to be a bis(mu-oxo)diiron(IV) complex in which one of the iron atoms is coordinatively unsaturated (five-coordinate). Is it reasonable for such a diiron complex to be formed in the catalytic reaction of sMMO? The answer to this important question is positive from the viewpoint of energetics in density functional theory (DFT) calculations. Our model thus has a vacant coordination site for substrate methane. If MMOH(Q) involves a coordinatively unsaturated iron atom at the active center, methane is effectively converted into methanol in the broken-symmetry singlet state by a non-radical mechanism; in the first step a methane C-H bond is dissociated via a four-centered transition state (TS1) resulting in an important intermediate involving a hydroxo ligand and a methyl ligand, and in the second step the binding of the methyl ligand and the hydroxo ligand through a three-centered transition state (TS2) results in the formation of a methanol complex. This mechanism is essentially identical to that of the methane-methanol conversion by the bare FeO(+) complex and relevant transition metal-oxo complexes in the gas phase. Neither radical species nor ionic species are involved in this mechanism. We look in detail at kinetic isotope effects (KIEs) for H atom abstraction from methane on the basis of transition state theory with Wigner tunneling corrections.

Binding Sites↗
PubMed1988

Octahedral complexes of anti-cancer Pt(IV)(cyclohexyldiamine) agents with 9-methylguanine.

The compounds [Pt(IV)(dach)(9-methylguanine)2X2]2+ (X = Cl, OH) have been prepared and structurally characterized. Their existence shows that it is possible to accommodate two purine bases (in a cis configuration) and four other ligands around a Pt(IV) atom. The geometries of these complexes have very different orientations of the guanine rings as compared to their corresponding Pt(II) counterparts.

Antineoplastic Agents↗
PubMed2000

Oxo complexes of osmium(IV) formed via dioxygen activation. X-ray structures of [OsX(dcpe)2]PF6 (X = Cl, Br), [OsCl(eta 2-O2)(dcpe)2]BPh4, and [OsCl(O)(dcpe)2]BPh4 (dcpe = 1,2-bis(dicyclohexylphosphino)ethane).

Dioxygen addition to the 16-electron complexes [OsX(P-P)2]+ (3) gives the dioxygen adducts [OsCl(eta 2-O2)(P-P)2]+ (3), which in turn react with HCl gas to give the novel osmium(IV) oxo complexes trans-[OsX(O)(P-P)2]+ (5) (X = Cl, Br; P-P = 1,2-bis(dicyclohexylphosphino)ethane (dcpe), 1,2-bis(diethylphosphino)ethane (depe), 1,2-bis((2R,5R)-2,5-dimethylphospholano)benzene (Me-duphos)). The complexes [OsX(dcpe)2]+ (X = Cl, Br) (3) are studied by X-ray crystallography and are shown to have a "Y-shaped" coordination geometry in the equatorial plane. The X-ray structural analysis of [OsCl(eta 2-O2)(dcpe)2]+ (4a) reveals an exceptionally short O-O bond (1.315(5) A). trans-[OsCl(O)(dcpe)2]+ (5a), the first oxo complex of osmium(IV) investigated crystallographically, exhibits a long Os-O distance of 1.834(3) A. The reactivity of 4 and 5 as oxidants is described. The dioxygen complex 4a transfers one oxygen atom to PPh3 (to give Ph3PO) or oxidizes iodide ions to triiodide ions in the presence of anhydrous HCl. In both reactions, the corresponding oxo species 5a is quantitatively formed as the only metal-containing product. Oxo complexes 5 are surprisingly stable and unreactive toward standard reducing agents such as phosphines.

Journal Article↗
PubMed2006

Two-dimensional roughening of adsorbate islands in thermodynamic equilibrium.

Equilibrium fluctuations of islands of adsorbed O atoms on Ru(0001) were investigated by scanning tunneling microscopy (STM), density functional theory calculations (DFT) and Monte Carlo (MC) simulations. Very ramified (2 x 2)-O islands were observed by high-speed STM that point to complex interactions between the O atoms. The DFT calculations show that, in addition to pairwise attractive interactions between third-nearest neighbors, a repulsive three-body interaction exists between these. MC simulations that include three-body interactions reproduce the observed ordering behavior.

Journal Article↗
PubMed2000

Levels of control and closure in complex semiotic systems

It is natural to advance closures as atomic processes of universal evolution, and to analyze this concept specifically. Real complex systems like organisms and complex mechanisms cannot exist at either extreme of complete closure or lack of closure, nevertheless we should consider the properties of closures in general, the introduction of boundaries, a corresponding stability, the establishment of system autonomy and identity, and thereby the introduction of emergent new system of potentially new types. Our focus should move from simple physical closure of common objects and classical self-organizing systems to semiotically closed systems that maintain cyclic relations of perception, interpretation, decision, and action with their environments. Thus, issues arise concerning the use and interpretation of symbols, representations, and/or internal models (whether explicit or implicit) by the system; and the syntactic, semantic, and pragmatic relations among the sign tokens, their interpretations, and their use or function for the systems in question. Primitive semiotic closures are hypothesized as equivalent to simple control systems, and in turn equivalent to simple organisms. This leads us directly to the grand hierarchical control theories of Turchin, Powers, and Albus, which provide an explicit mechanism for the formation of new levels within complex semiotically closed systems.

Journal Article↗
PubMed1978

Influence of external potassium concentration on secretory responses to cholecystokinin-pancreozymin and ionophore A23187 in the pancreatic acinar cell.

1. The inhibitory effect of a diminution in [K+]o on the secretory processes of the pancreatic acinar cell was analysed in the isolated and perfused rat pancreas. 2. Partial replacement of KCl with NaCl produced falls in both amylase output and pancreatic juice flow which were induced by 5 m-u. cholecystokinin-pancreozymin (CCK-PZ)/ml., and the responses were slowly regained after reintroduction of a standard concentration of KCl. 3. A quantitative relation was found between the amount of amylase released by CCK-PZ and [K+]o over the range 1.0--5.6 mM. A linear relation was obtained between the ratio of E1K+]3(0)/amylase output and [K+]3(0). 4. A quantitative relation was also found between the pancreatic juice flow induced by CCK-PZ and [K+]o over the range 1.0--5.6 mM. A linear relation was obtained between the ratio of [K+]3(0)/juice flow and [K+]3(0). 5. The results obtained may be explained by a model as follows: (1) amylase output and juice flow induced by CCK-PZ are simply proportional to the amount of complex composed of a carrier molecule bearing one Ca and four Na atoms, (2) the inward movement of the complex is tightly linked to the CCK-PZ-induced activation of pumps which are postulated on both basal and apical sides of the membrane of the acinar cell, and (3) the Na:K coupling ratio of the pumps is close to 3:2. 6. A quantitative relation was also found between the amount of amylase released by Ca reintroduction after pre-treatment with Ca-ionophore A23187 and [K+]o. This may be explained by a model essentially similar to that mentioned above. 7. Total replacement of NaCl with NaBr slightly reduced the CCK-PZ-induced fluid and amylase secretion. Total replacement of NaCl with sodium isethionate or with sodium pyruvate, however, virtually abolished the CCK-PZ-induced fluid and amylase secretion.

Amylases↗
PubMed2003

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.

Algorithms↗
PubMed2006

Aqua(2,2'-bipyridyl)chloronitratocopper(II).

In a new hydrogen-bonded three-dimensional complex, [CuCl(NO3)(C10H8N2)(H2O)], the Cu atom has an elongated tetragonal octahedral environment, with two 2,2'-bipyridyl N atoms, one nitrate O atom and one Cl atom forming the equatorial plane, and a second O atom of the nitrate anion and a water molecule in the axial positions. The complex molecules are linked to form a three-dimensional supramolecular array by hydrogen-bonding interactions both between the water O atom and nitrate O atoms, and between the water O atom and the Cl atom of a neighboring molecule.

Journal Article↗
PubMed2004

Anionic Fischer-type carbene complexes as bidentate (N,O) ligands.

New polynuclear complexes, (L1)3M2 [M2 = Cr(III) (4a,4b), Fe(III) (5), Co(III) (8)], (L1)2M2(L2)2 [M2 = Co(II) (7), Ni(II) (9)], (L1)2M2(O)L2 [M2 = V(IV) (6)] and L1M2Cp2 [M2 = Ti(III) (10)] with L1 = (CO)5M1=C[C=NC(CH3)=CHS](O-)(M1 = Cr or W) and L2 = 4-methylthiazole or THF, are described. The molecular structures of these complexes determined by X-ray diffraction show that the Fischer-type carbene complexes act as bidentate ligands towards the second metal centre, coordinating through C(carbene)-attached O-atoms and imine N-atoms of the thiazolyl groups to form five-membered chelates with the oxygen atoms in the mer configuration. Isostructural complexes have similar characteristic band patterns in their far-IR spectra. Cyclic voltammetry of selected complexes reveals the oxidation of the carbene complex ligand between 1.01 and 1.29 V. Oxidation of the central metal (M2) takes place at 0.56 and 0.86 V for 7 and 9, respectively. Three stepwise reductions of Cr(III) to Cr(0) occur for 4a and 4b in the region -0.51 to -1.58 V. These new ligand types and other variants thereof should find application in ligand design with the first metal -- and other ligands attached thereto -- in the carbene complex ligand, playing an important role.

Journal Article↗