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At least 1,027 records · Page 57Linked to original sources

PubMed2005

Atomic scale conductance induced by single impurity charging.

A scanning tunneling microscope was used to probe electron transport through an alkali doped C(60) monolayer crystal on Al(2)O(3) grown by the oxidation of NiAl(110). Each individual alkali atom forms a localized complex with the neighboring C(60) molecules. Charging of the complex induces a substantial rise in the current that persists outside the physical dimensions of the complex. This induction of the current rise is characterized by spatially resolved spectroscopy and mapping of the differential conductance (dI/dV) in the vicinity of the complex.

Journal Article↗

PubMed1979

Inactivation of Hydrogenase in Cell-free Extracts and Whole Cells of Chlamydomonas reinhardi by Oxygen.

O(2) irreversibly inactivates hydrogenase from Chlamydomonas reinhardi. The mechanism for the inactivation involves the reaction of one molecule of hydrogenase with one molecule of O(2) (or two oxygen atoms) in the transition complex of the rate-limiting step. The second order rate constant for this reaction is 190 atmospheres(-1) minute(-1) (1.4 x 10(5) molar(-1) minute(-1)). At levels above 0.01 atmosphere O(2), the increased numbers of O(2) molecules may compete for the site of inactivation hindering the proper orientation for inactivation of any one O(2) molecule and resulting in lowered rates of inactivation.CO is a reversible inhibitor of hydrogenase acting competitively against H(2). The K(i) for CO is 0.0010 atmosphere. CO antagonizes O(2) inactivation. In a period when complete inactivation by O(2) would usually occur, the presence of CO greatly reduces the inactivation rate.After 3 hours of adaptation in whole cells, the presence of H(2) lowers the rate of deadaptation of hydrogenase. Inasmuch as H(2) promotes increased O(2) uptake the cellular concentration of O(2) is likely to be lower. After 48 hours of adaptation O(2) uptake is reduced even when H(2) is present and the pattern of deadaptation under O(2) with and without H(2) and CO is qualitatively the same as observed for the inactivation of cell-free hydrogenase. The mechanism of inactivation of cell-free hydrogenase by O(2) may be the same as the mechanism for loss of hydrogenase during deadaptation in whole algal cells.

Journal Article↗

PubMed2000

Pentacarbonyl(di-2-pyridylamine)tungsten(0).

In the title molecular complex, (I), the W atom is in an octahedral environment with four equatorial carbonyl ligands and a fifth in an axial position trans to the monodentate dipyridylamine ligand. The long dimension of this last bisects the angle between two of the equatorial carbonyl groups and while the non-bonded pyridyl N atom is directed away from the W atom, the bridging amine group is directed towards it. Thus, in addition to the N atom to which it is attached, the amino H has two nearest neighbour C atoms of equatorial carbonyl groups but does not participate in hydrogen bonding in any real or usual sense. The W-C bond distance for the axial carbonyl group is notably less than those of the equatorial groups.

Journal Article↗

PubMed2003

[Bis[(S)-2-(4-tert-butyl-4,5-dihydrooxazol-2-yl)phenyl]phenylphoshine-kappa(2)N,P](1,3-dimethyl-pi-allyl-kappa(3)C)palladium(II) hexafluorophosphate and (S)(P)-[bis[(S)-2-(4-tert-butyl-4,5-dihydrooxazol-2-yl)phenyl]phenylphosphine-kappa(2)N,P]dichloropalladium(II).

In the structures of the title compounds, [Pd(C(5)H(9))(C(32)H(37)N(2)O(2)P)]PF(6) and [PdCl(2)(C(32)H(37)N(2)O(2)P)], the bis(dihydrooxazolyl)phosphine ligand is N,P-bidentate, with S chirality on the P atom. In the allyl complex, the pi-allyl ligand ligates in a syn-syn-kappa(3)C manner.

Journal Article↗

PubMed2004

Poly[[bis(1H-benzimidazole-kappaN3)cadmium(II)]-mu-aqua-mu-succinato-kappa2O1:O4].

The title compound, [Cd(C(4)H(4)O(4))(C(7)H(6)N(2))(2)(H(2)O)](n), is a three-dimensional polymeric complex. The Cd(II) atom is located on an inversion centre and assumes an elongated octahedral coordination geometry, with a long Cd-O distance of 2.5381 (5) A to the coordinated bridging water molecule. The succinate dianion, located on another inversion centre, bridges adjacent Cd atoms to form succinate-bridged polymeric chains. The coordinated water molecule is located on a twofold axis and links adjacent succinate-bridged chains to form a water-bridged polymeric chain.

Journal Article↗

PubMed2004

catena-Poly[1,4-diazoniabicyclo[2.2.2]octane [silver(I)-tri-mu-thiocyanato-kappa6S:S]].

The title compound, [(C6H14N2)[Ag(NCS)3]]n, is a polymeric silver(I) complex. The Ag(I) atom is hexacoordinated by the S atoms of six thiocyanate anions, with each thiocyanate S atom acting in a bridging mode to link the Ag atoms together. The unique Ag(I) atom lies at a cell origin and has crystallographically imposed 3 symmetry. The diazonia[2.2.2]octane molecule lies about a site with 6 imposed symmetry with the unique N atom on a threefold axis. The S and N atoms of the thiocyanate ligands sit on a mirror plane and a threefold axis, respectively. The crystal structure consists of one-dimensional chains, which are stabilized by N-H...N hydrogen bonds to form a three-dimensional network.

Journal Article↗

PubMed1990

Structures of the meridional and facial isomers of triamminechromium pyrophosphate dihydrate.

The meridional and facial isomers of triamminechromium pyrophosphate were separated by chromatography on Dowex-50-H+, and crystallized as isoionic species. The meridional isomer crystallized as a monomer [Cr(HP2O7)(NH3)3(H2O)].2H2O but the facial isomer crystallized as a centrosymmetric dimer of [Cr(HP2O7)(NH3)3]. 2H2O, in which the remaining water in the coordination sphere of each chromium is replaced by a phosphate oxygen from the other monomer unit. Meridional isomer: monoaquatriammine(pyrophosphato)chromium(III) dihydrate, Mr = 332.1, monoclinic, P21/c, a = 7.825 (2), b = 10.107 (3), c = 15.322 (5) A, beta = 103.92 (5) degrees, V = 1176 (1) A 3, Z = 4, Dx = 1.875 g cm-3 lambda(Mo K alpha) = 0.71073 A, mu = 12.6 cm-1, F(000) = 684, final R = 0.050 for 1828 reflections. The most notable difference between this compound and the corresponding tetraammine complex reported previously is a significant shortening of the metal-ligand bond lengths not only for the water ligand but also for the ammonia ligands. The dimer: mu-(pyrophosphato-O,O',O")-bis[triamminechromium(III)] dihydrate, Mr = 314.0, monoclinic, P21/c, a = 8.695 (2), b = 10.327 (3), c = 11.913 (4) A, beta = 97.81 (5) degrees, V = 1060 (1) A3, Z = 4, Dx = 1.969 g cm-3, lambda(Cu K alpha) = 1.5418 A, mu = 125.7 cm-1, F(000) = 644, final R = 0.047 for 1389 reflections. This structure, which sits on a center of inversion, forms a tricyclic complex involving two Cr atoms. The structure is characterized by reciprocal coordination between the metal ions and anionic O atoms of the pyrophosphate moieties.(ABSTRACT TRUNCATED AT 250 WORDS)

Molecular Structure↗

PubMed1997

trans-diaquabis(2,2'-bipyridine-N,N')-ruthenium(II) bis(hexafluorophosphate).

The Ru atom of the title complex, [Ru(C10H8N2)2-(H2O)2](PF6)2, is located on a crystallographic inversion center; the bipyridine ligands are bowed [dihedral angle between C5N planes is 162.68 (12) degrees] due to the interligand steric interactions of the trans bipyridyl units. The Ru--O distance is 2.116 (2) A and the mean Ru--N distance is 2.074 (2) A.

Crystallography, X-Ray↗

PubMed2000

Neutron Rietveld refinement of the incommensurate phase of the ordered perovskite Pb2CoWO6.

The incommensurate structure of lead cobalt tungstate has been refined by the Rietveld method on neutron data collected at 250 K. The space group is planar monoclinic 12/m(alpha0gamma)0s [a = 7.9602 (4), b = 5.6779 (3), c = 5.6967 (3) A, beta = 90.047 (5)degrees, q(inc) = 0.9000 (9)a* + 0.1735 (6)c*]. The use of powder diffraction techniques to investigate ferroelastic modulated phases is discussed and compared with a previous polydomain single-crystal structural analysis. The modulated displacements of light atoms have been determined, allowing an accurate description of the modulation of both the cations and the O-atom framework. The refinement suggests a displacive model for the phase transition, involving significant atomic shifts for Pb atoms and a quite complex mixing of tilt and deformation of the oxygen octahedra. The average character of this modulated structure is antiferroelectric.

Journal Article↗

PubMed1994

Wild-type and E106Q mutant carbonic anhydrase complexed with acetate.

The molecular structures of the acetate complexes of wild-type human carbonic anhydrase II (HCAII) and of E106Q mutant human carbonic anhydrase II were solved with high completeness (89-91%) to 2.1 and 1.9 A resolution, respectively. Both wild-type and mutant enzyme crystallize in space group P2(1) with cell dimensions a = 42.7, b = 41.7, c = 73.0 A and beta = 104.6 degrees. The altered active-site hydrogen-bond network caused by the mutation results in a different binding of the inhibitor in the two complexes. In the mutant, but not in the wild-type complex, a carboxylate O atom is within hydrogen-bond distance of Thr199 Ogamma1. In the wild-type enzyme ligand hydrogen bonding to this atom is normally only found for hydrogen-bond donors. The importance of this discrimination on catalysis by the enzyme is discussed briefly.

Journal Article↗

PubMed2006

Computational modeling of a new fluorescent biosensor for caspase proteolytic activity improves dynamic range.

The class of fluorescence resonance energy transfer (FRET) protein biosensors that are useful for measuring protease activity is composed of a tandem fusion of yellow fluorescent protein (YFP), a cleavage recognition sequence, and cyan fluorescent protein (CFP). The dynamic range of these FRET-based protein biosensors is often weak, but applications such as high throughput drug screening require stronger dynamic ranges. Using the biosensor for the caspase-3 protease as an example, here we showed a computational approach to improve the FRET dynamic range based on the atomic structure of caspase-3 bound to its inhibitor. This result was verified from our experiments where the FRET dynamic range improved by at least 60% on average in both in vitro and in vivo contexts. In concept, the same strategy can be applied to improve dynamic range of other FRET-based protein biosensors for protease activity where there exist solved atomic structures for protein complexes.

Caspase 3↗

PubMed2004

Molecular mechanisms of DNA end-loop formation by TRF2.

In the telomere region of human chromosomes, the (TTAGGG)n sequence stretches over several kilobases and forms a distinct higher-order structure with various proteins. Telomere repeat binding factors (TRFs) bind specifically to this sequence and play critical roles in the maintenance of telomere structure and function. Here, we prepared a series of linear DNA carrying a stretch of telomeric sequence ((TTAGGG)n, approximately 1.8 (kb) with different end-structures and observed their higher-order complexes with TRFs by atomic force microscopy. TRF2 molecules exclusively bound to the telomeric DNA region at several different places simultaneously mainly as a dimer, and often mediated DNA loop formation by forming a tetramer at the root. These multiple-binding, multimerization and DNA loop formation by TRF2 were observed regardless of the DNA-end structure (blunt, 3'-overhanging, telomeric, non-telomeric). However, when the DNA end carried the telomeric-3'-overhanging region, the loop was frequently formed at the end of the DNA. Namely, the TRF2-mediated DNA loop formation is independent of the end-structure and the 3'-overhanging TTAGGG sequence is responsible for the stabilization of the loop. TRF1 also bound to the telomeric DNA as a dimer, but did not mediate DNA loop formation by itself. These results provide a new insight into the molecular mechanism of DNA end-loop formation by TRFs.

Binding Sites↗

PubMed2002

Noble gas-actinide compounds: complexation of the CUO molecule by Ar, Kr, and Xe atoms in noble gas matrices.

The CUO molecule, formed from the reaction of laser-ablated U atoms with CO in a noble gas, exhibits very different stretching frequencies in a solid argon matrix [804.3 and 852.5 wave numbers (cm(-1))] than in a solid neon matrix (872.2 and 1047.3 cm(-1)). Related experiments in a matrix consisting of 1% argon in neon suggest that the argon atoms are interacting directly with the CUO molecule. Relativistic density functional calculations predict that CUO can bind directly to one argon atom (U-Ar = 3.16 angstroms; binding energy = 3.2 kilocalories per mole), accompanied by a change in the ground state from a singlet to a triplet. Our experimental and theoretical results also suggest that multiple argon atoms can bind to a single CUO molecule.

Journal Article↗

PubMed1991

Atomic structure of FKBP-FK506, an immunophilin-immunosuppressant complex.

The structure of the human FK506 binding protein (FKBP), complexed with the immunosuppressant FK506, has been determined to 1.7 angstroms resolution by x-ray crystallography. The conformation of the protein changes little upon complexation, but the conformation of FK506 is markedly different in the bound and unbound forms. The drug's association with the protein involves five hydrogen bonds, a hydrophobic binding pocket lined with conserved aromatic residues, and an unusual carbonyl binding pocket. The nature of this complex has implications for the mechanism of rotamase catalysis and for the biological actions of FK506 and rapamycin.

Anti-Bacterial Agents↗

PubMed1996

Membrane-associated methane monooxygenase from Methylococcus capsulatus (Bath).

An active preparation of the membrane-associated methane monooxygenase (pMMO) from Methylococcus capsulatus Bath was isolated by ion-exchange and hydrophobic interaction chromatography using dodecyl beta-D-maltoside as the detergent. The active preparation consisted of three major polypeptides with molecular masses of 47,000, 27,000, and 25,000 Da. Two of the three polypeptides (those with molecular masses of 47,000 and 27,000 Da) were identified as the polypeptides induced when cells expressing the soluble MMO are switched to culture medium in which the pMMO is expressed. The 27,000-Da polypeptide was identified as the acetylene-binding protein. The active enzyme complex contained 2.5 iron atoms and 14.5 copper atoms per 99,000 Da. The electron paramagnetic resonance spectrum of the enzyme showed evidence for a type 2 copper center (g perpendicular = 2.057, g parallel = 2.24, and magnitude of A parallel = 172 G), a weak high-spin iron signal (g = 6.0), and a broad low-field (g = 12.5) signal. Treatment of the pMMO with nitric oxide produced the ferrous-nitric oxide derivative observed in the membrane fraction of cells expressing the pMMO. When duroquinol was used as a reductant, the specific activity of the purified enzyme was 11.1 nmol of propylene oxidized.min-1.mg of protein-1, which accounted for approximately 30% of the cell-free propylene oxidation activity. The activity was stimulated by ferric and cupric metal ions in addition to the cytochrome b-specific inhibitors myxothiazol and 2-heptyl-4-hydroxyquinoline-N-oxide.

Alkenes↗

PubMedDate not supplied

Reactions in glass-ionomer cements: IV. Effect of chelating comonomers on setting behavior.

The oscillating rheometer is a valuable instrument for studying the effects of additives on the setting behavior of a cement system. Using this instrument, it was found that certain chelating comonomers, the hydroxycarboxylic acids, could improve the setting characteristics of the glass-ionomer cement system when added to the PAA solution. The acid chelates probably assign the extraction of metal ions from the glass and also tend to hold them in solution, preventing premature ion binding of the polyanion chains. The effect is to increase the rate of hardening without reducing the working time, which may indeed by slightly increased. Tartaric acid, the most effective of the comonomers, can form a chelate bridge between aluminum atoms, and this metal complex probably acts as a flexible bridge structure linking polyanion chains. This mechanism offers some steric advantages over a simple salt bridge.

Acrylic Resins↗

PubMed2002

Visualization of complexes of Hoechst 33258 and DNA duplexes in solution by atomic force microscopy.

Tertiary structure changes in DNA duplexes, induced by Hoechst 33258 binding, have been examined by the use of atomic force microscopy. Besides minor groove binding, which is an established mode of binding for this drug, Hoechst 33258 has now been found to show another binding mode, which causes an unwinding of the duplex. When the drug concentration is as high as 0.5 microg/ml, the Hoechst 33258 molecule seems to function as a clamp for two DNA chains and forms a condensate. The condensate was found to have a toroidal shape. By surveying more than 100 microscopic images of such condensates formed in I microg/ml drug solution, a mechanism of toroidal condensate formation has been proposed.

Bisbenzimidazole↗

PubMed2006

Interferences removal for cadmium determination in samples with complex matrices by hydride generation coupled with non-dispersive atomic fluorescence spectrometry.

An intermittent on-line concentration and separation system coupled with HG-AFS was developed to eliminate serious interferences from Cu2+, Pb2+ and Zn2+ on the determination of cadmium. In the present method, the interferences from common coexisting ions, such as Cu2+, Pb2+, Zn2+, Fe3+ and Ni2+, were greatly reduced. Under the optimized conditions, a detection limit of 3 pg ml(-1) (3sigma, n=11) and a precision of 1.9% RSD for 1 ng ml(-1) of Cd were obtained. The method was successfully applied to the determination of cadmium in a series of Chinese Geological Reference Materials (SRMs) and GBW01621 ferronickel alloy using simple aqueous standard calibration technique. The results obtained were in good agreement with the certified values.

Cadmium↗