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At least 1,171 records · Page 65Linked to original sources
PubMed2006

Interplay between structure and magnetism in Mo12S9I9 nanowires.

We investigate the equilibrium geometry and electronic structure of Mo12S9I9 nanowires using ab initio density functional calculations. The skeleton of these unusually stable nanowires consists of rigid, functionalized Mo octahedra, connected by flexible, bistable sulfur bridges. This structural flexibility translates into a capability to stretch up to approximately 20% at almost no energy cost. The nanowires change from conductors to narrow-gap magnetic semiconductors in one of their structural isomers.

Journal Article↗
PubMed2006

Interplay between thermodynamics and kinetics in the capping of InAs/GaAs(001) quantum dots.

A microscopic picture for the GaAs overgrowth of self-organized quantum dots is developed. Scanning tunneling microscopy measurements reveal two capping regimes: the first being characterized by a dot shrinking and a backward pyramid-to-dome shape transition. This regime is governed by fast dynamics resulting in island morphologies close to thermodynamic equilibrium. The second regime is marked by a true overgrowth and is controlled by kinetically limited surface diffusion processes. A simple model is developed to describe the observed structural changes which are rationalized in terms of energetic minimization driven by lattice mismatch and alloying.

Journal Article↗
PubMed2006

Low-energy charge-density excitations in MgB2: Striking interplay between single-particle and collective behavior for large momenta.

A sharp feature in the charge-density excitation spectra of single-crystal MgB2, displaying a remarkable cosinelike, periodic energy dispersion with momentum transfer (q) along the c* axis, has been observed for the first time by high-resolution nonresonant inelastic x-ray scattering (NIXS). Time-dependent density-functional theory calculations show that the physics underlying the NIXS data is strong coupling between single-particle and collective degrees of freedom, mediated by large crystal local-field effects. As a result, the small-q collective mode residing in the single-particle excitation gap of the B pi bands reappears periodically in higher Brillouin zones. The NIXS data thus embody a novel signature of the layered electronic structure of MgB2.

Journal Article↗
PubMed2006

Interplay between phase defects and spin polarization in the specific heat of the spin-density-wave compound (TMTTF)2Br in a magnetic field.

Equilibrium heat relaxation experiments provide evidence that the ground state of the commensurate spin-density-wave compound (TMTTF)2Br after the application of a sufficient magnetic field is different from the conventional ground state. The experiments are interpreted on the basis of the local model of strong pinning as the deconfinement of soliton-antisoliton pairs triggered by the Zeeman coupling to spin degrees of freedom, resulting in a magnetic-field-induced density-wave glass for the spin carrying the phase configuration.

Journal Article↗
PubMed1993

The interplay of light and the circadian clock. Independent dual regulation of clock-controlled gene ccg-2(eas).

Ambient light is the major agent mediating entrainment of circadian rhythms and is also a major factor influencing development and morphogenesis. We show that in Neurospora crassa the expression of clock-controlled gene 2 (ccg-2), a gene under the control of the circadian clock and allelic to the developmental gene easy wettable (eas), is regulated by light in wild-type strains. Light elicits a direct and important physiological effect on ccg-2(eas) expression as demonstrated using several mutant Neurospora strains. In white collar mutants (wc-1 and wc-2) that are "blind" to blue light, ccg-2(eas) mRNA shows no variation following illumination with saturating light. By contrast, ccg-2(eas) mRNA is photoinduced in clock-null strains such as frequency (bd;frq). The results in the clock mutants show that an intact circadian oscillator is not required for light induction of ccg-2(eas). Thus, ccg-2(eas) is subject to a dual regulation that involves separable regulation by light and circadian rhythm.

Alleles↗
PubMed2004

The transcriptional innate immune response to flg22. Interplay and overlap with Avr gene-dependent defense responses and bacterial pathogenesis.

Animals and plants carry recognition systems to sense bacterial flagellin. Flagellin perception in Arabidopsis involves FLS2, a Leu-rich-repeat receptor kinase. We surveyed the early transcriptional response of Arabidopsis cell cultures and seedlings within 60 min of treatment with flg22, a peptide corresponding to the most conserved domain of flagellin. Using Affymetrix microarrays, approximately 3.0% of 8,200 genes displayed transcript level changes in flg22 elicited suspension cultures and seedlings. FLARE (Flagellin Rapidly Elicited) genes mostly encode signaling components, such as transcription factors, protein kinases/phosphatases, and proteins that regulate protein turnover. Approximately 80% of flg22-induced genes were also up-regulated in Arabidopsis seedlings treated with cycloheximide. This suggests that many FLARE genes are negatively regulated by rapidly turned-over repressor proteins. Twenty-one tobacco Avr9/Cf-9 rapidly elicited (ACRE) cDNA full-length sequences were used to search for their Arabidopsis orthologs (AtACRE). We identified either single or multiple putative orthologs for 17 ACRE genes. For 13 of these ACRE genes, at least one Arabidopsis ortholog was induced in flg22-elicited Arabidopsis suspension cells and seedlings. This result revealed a substantial overlap between the Arabidopsis flg22 response and the tobacco Avr9 race-specific defense response. We also compared FLARE gene sets and genes induced in basal or gene-for-gene interactions upon different Pseudomonas syringae treatments, and infer that Pseudomonas syringae pv tomato represses the flagellin-initiated defense response.

Arabidopsis↗
PubMed2006

Interplay between Arabidopsis activating factors E2Fb and E2Fa in cell cycle progression and development.

Eukaryotic E2Fs are conserved transcription factors playing crucial and antagonistic roles in several pathways related to cell division, DNA repair, and differentiation. In plants, these processes are strictly intermingled at the growing zone to produce postembryonic development in response to internal signals and environmental cues. Of the six AtE2F proteins found in Arabidopsis (Arabidopsis thaliana), only AtE2Fa and AtE2Fb have been clearly indicated as activators of E2F-responsive genes. AtE2Fa activity was shown to induce S phase and endoreduplication, whereas the function of AtE2Fb and the interrelationship between these two transcription factors was unclear. We have investigated the role played by the AtE2Fb gene during cell cycle and development performing in situ RNA hybridization, immunolocalization of the AtE2Fb protein in planta, and analysis of AtE2Fb promoter activity in transgenic plants. Overexpression of AtE2Fb in transgenic Arabidopsis plants led to striking modifications of the morphology of roots, cotyledons, and leaves that can be ascribed to stimulation of cell division. The accumulation of the AtE2Fb protein in these lines was paralleled by an increased expression of E2F-responsive G1/S and G2/M marker genes. These results suggest that AtE2Fa and AtE2Fb have specific expression patterns and play similar but distinct roles during cell cycle progression.

Arabidopsis↗
PubMed2006

Discrimination in the dark. Resolving the interplay between metabolic and physical constraints to phosphoenolpyruvate carboxylase activity during the crassulacean acid metabolism cycle.

A model defining carbon isotope discrimination (delta13C) for crassulacean acid metabolism (CAM) plants was experimentally validated using Kalanchoe daigremontiana. Simultaneous measurements of gas exchange and instantaneous CO2 discrimination (for 13C and 18O) were made from late photoperiod (phase IV of CAM), throughout the dark period (phase I), and into the light (phase II). Measurements of CO2 response curves throughout the dark period revealed changing phosphoenolpyruvate carboxylase (PEPC) capacity. These systematic changes in PEPC capacity were tracked by net CO2 uptake, stomatal conductance, and online delta13C signal; all declined at the start of the dark period, then increased to a maximum 2 h before dawn. Measurements of delta13C were higher than predicted from the ratio of intercellular to external CO2 (p(i)/p(a)) and fractionation associated with CO2 hydration and PEPC carboxylations alone, such that the dark period mesophyll conductance, g(i), was 0.044 mol m(-2) s(-1) bar(-1). A higher estimate of g(i) (0.085 mol m(-2) s(-1) bar(-1)) was needed to account for the modeled and measured delta18O discrimination throughout the dark period. The differences in estimates of g(i) from the two isotope measurements, and an offset of -5.5 per thousand between the 18O content of source and transpired water, suggest spatial variations in either CO2 diffusion path length and/or carbonic anhydrase activity, either within individual cells or across a succulent leaf. Our measurements support the model predictions to show that internal CO2 diffusion limitations within CAM leaves increase delta13C discrimination during nighttime CO2 fixation while reducing delta13C during phase IV. When evaluating the phylogenetic distribution of CAM, carbon isotope composition will reflect these diffusive limitations as well as relative contributions from C3 and C4 biochemistry.

Carbon Dioxide↗
PubMed2003

The Arabidopsis cax1 mutant exhibits impaired ion homeostasis, development, and hormonal responses and reveals interplay among vacuolar transporters.

The Arabidopsis Ca(2+)/H(+) transporter CAX1 (Cation Exchanger1) may be an important regulator of intracellular Ca(2+) levels. Here, we describe the preliminary localization of CAX1 to the tonoplast and the molecular and biochemical characterization of cax1 mutants. We show that these mutants exhibit a 50% reduction in tonoplast Ca(2+)/H(+) antiport activity, a 40% reduction in tonoplast V-type H(+)-translocating ATPase activity, a 36% increase in tonoplast Ca(2+)-ATPase activity, and increased expression of the putative vacuolar Ca(2+)/H(+) antiporters CAX3 and CAX4. Enhanced growth was displayed by the cax1 lines under Mn(2+) and Mg(2+) stress conditions. The mutants exhibited altered plant development, perturbed hormone sensitivities, and altered expression of an auxin-regulated promoter-reporter gene fusion. We propose that CAX1 regulates myriad plant processes and discuss the observed phenotypes with regard to the compensatory alterations in other transporters.

Alleles↗
PubMed2001

N-(2-Amino-1,6-dihydro-5-nitroso-6-oxopyrimidin-4-yl)-L-isoleucine-water (4/1): interplay of molecular and supramolecular structures.

In the title compound, 2C10H15N5O4-0.5H2O, there are two independent molecules of the pyrimidinylisoleucine in general positions and a water molecule lying on a twofold rotation axis. The bond lengths within the organic moieties demonstrate significant polarization of the electronic structure. Each of the organic molecules participates in 12 intermolecular hydrogen bonds, of O-H...O and N-H...O types, while the water molecule acts as a double donor and as a double acceptor of O-H...O hydrogen bonds. The organic components are linked by the hydrogen bonds into a single three-dimensional framework, reinforced by the water molecules.

Journal Article↗
PubMed2001

Triclinic and orthorhombic polymorphs of 2-iodo-4-nitroaniline: interplay of hydrogen bonds, nitro...I interactions and aromatic pi-pi-stacking interactions.

In the triclinic polymorph of 2-iodo-4-nitroaniline, C(6)H(5)IN(2)O(2), space group P-1, the molecules are linked by paired N-H...O hydrogen bonds into C(8)[R(2)(2)(6)] chains of rings. These chains are linked into sheets by nitro...I interactions, and the sheets are pairwise linked by aromatic pi-pi-stacking interactions. In the orthorhombic polymorph, space group Pbca, the molecules are linked by single N-H...O hydrogen bonds into spiral C(8) chains; the chains are linked by nitro...O interactions into sheets, each of which is linked to its two immediate neighbours by aromatic pi-pi-stacking interactions, so producing a continuous three-dimensional structure.

Aniline Compounds↗
PubMed2001

Ethyl 6-amino-2-methoxypyridine-3-carboxylate, interplay of molecular and supramolecular structure.

The title compound, C(9)H(12)N(2)O(3), crystallizes with two molecules in the asymmetric unit. There is extensive hydrogen bonding which results in the formation of a two-dimensional corrugated sheet. This supramolecular structure is determined by the formation of hydrogen-bonded chains resulting from the presence of a 6-amino group and an ethoxycarbonyl group as substituents on a pyridine ring in relative para positions which constitute a pi-electron 'push-pull' system.

Journal Article↗
PubMed2002

Tetrameric triphenylsilanol, (Ph(3)SiOH)(4), and the adduct (Ph(3)SiOH)(2)-dimethyl sulfoxide, both at 120 K, and the adduct (Ph(3)SiOH)(4)-1,4-dioxan at 150 K: interplay of O[bond]H...O and C[bond]H...pi(arene) interactions.

The structure of tetrameric triphenylsilanol, C(18)H(16)OSi, (I), has been re-investigated at 120 (2) K. The hydroxyl H atoms were readily located and one of the arene rings is disordered over two closely positioned sets of sites. The molecules are linked into cyclic tetramers, having approximate 4 (S(4)) symmetry, via O[bond]H...O hydrogen bonds [H...O 1.81-1.85 A, O...O 2.634 (3)-2.693 (3) A and O[bond]H...O 156-166 degrees ]. At ambient temperature, there are indications of multiple disorder of the phenyl-ring sites. In bis(triphenylsilanol) dimethyl sulfoxide solvate, 2C(18)H(16)OSi x C(2)H(6)OS, (II), the dimethyl sulfoxide component is disordered across a twofold rotation axis in C2/c, and the molecular components are linked by a single O[bond]H...O hydrogen bond [H...O 1.85 A, O...O 2.732 (2) A and O[bond]H...O 172 degrees ] into three-molecule aggregates, which are themselves linked into a single three-dimensional framework by two C[bond]H...pi(arene) interactions. In tetrakis(triphenylsilanol) 1,4-dioxan solvate, 4C(18)H(16)OSi x C(4)H(8)O(2), (III), the 1,4-dioxan component lies across an inversion centre in space group P-1 and centrosymmetric five-molecule aggregates are linked by paired C[bond]H...pi(arene) interactions to form molecular ladders.

Journal Article↗
PubMed2002

2,6-Diiodo-4-nitrophenol, 2,6-diiodo-4-nitrophenyl acetate and 2,6-diiodo-4-nitroanisole: interplay of hydrogen bonds, iodo-nitro interactions and aromatic pi-pi-stacking interactions to give supramolecular structures in one, two and three dimensions.

In 2,6-diiodo-4-nitrophenol, C(6)H(3)I(2)NO(3), the molecules are linked, by an O-H...O hydrogen bond and two iodo-nitro interactions, into sheets, which are further linked into a three-dimensional framework by aromatic pi-pi-stacking interactions. The molecules of 2,6-diiodo-4-nitrophenyl acetate, C(8)H(5)I(2)NO(4), lie across a mirror plane in space group Pnma, with the acetyl group on the mirror, and they are linked by a single iodo-nitro interaction to form isolated sheets. The molecules of 2,6-diiodo-4-nitroanisole, C(7)H(5)I(2)NO(3), are linked into isolated chains by a single two-centre iodo-nitro interaction.

Journal Article↗
PubMed2004

The interplay between experiment and theory in charge-density analysis.

The comparison of theory and experiment remains a cornerstone of scientific inquiry. Various levels of such comparison applicable to charge-density analysis are discussed, including static and dynamic electron densities, topological properties, d-orbital occupancies and electrostatic moments. The advantages and drawbacks of the pseudoatom multipole are discussed, as are the experimentally constrained wavefunctions introduced by Jayatilaka and co-workers, which combine energy minimization with the requirement to provide a reasonable fit to the X-ray structure factors. The transferability of atomic densities can be exploited through construction of a pseudoatom databank, which may be based on analysis of ab initio molecular electron densities, and can be used to evaluate a host of physical properties. Partitioning of theoretical energies with the Morokuma-Ziegler energy decomposition scheme allows direct comparison with electrostatic interaction energies obtained from electron densities represented by the pseudoatom formalism. Compared with the Buckingham expression for the interaction between non-overlapping densities, the agreement with theory is much improved when a newly developed hybrid EP/MM (exact potential/multipole model) method is employed.

Models, Chemical↗
PubMed2000

N-(6-Amino-3,4-dihydro-3-methyl-5-nitroso-4-oxopyrimidin-2-yl) derivatives of glycine, valine, serine, threonine and methionine: interplay of molecular, molecular-electronic and supramolecular structures.

In each of N-(6-amino-3, 4-dihydro-3-methyl-5-nitroso-4-oxopyrimidin-2-yl)valine, C(10)H(15)N(5)O(4) (3) (orthorhombic, P2(1)2(1)2(1)), N-(6-amino-3, 4-dihydro-3-methyl-5-nitroso-4-oxopyrimidin-2-yl)serine monohydrate, C(8)H(11)N(5)O(5).H(2)O (4) (orthorhombic, P2(1)2(1)2(1)), and N-(6-amino-3, 4-dihydro-3-methyl-5-nitroso-4-oxopyrimidin-2-yl)threonine, C(9)H(13)N(5)O(5)(5) (monoclinic, P2(1)), the C-nitroso fragments exhibit almost equal C-N and N-O bond lengths: the C-N range is 1. 315 (3)-1.329 (3) A and the N-O range is 1.293 (3)-1.326 (3) A. In each compound there are also very short intermolecular O-H.O hydrogen bonds, in which carboxyl groups act as hydrogen-bond donors to the nitrosyl O atoms: the O.O distances range from 2.440 (2) to 2. 504 (4) A and the O-H.O angles lie between 161 and 163 degrees. An interpretation of the relationship between the unusual intramolecular bond lengths and the very short intermolecular hydrogen bonds has been developed based on database analysis and computational modelling. In each of (3)-(5) there is an extensive network of intermolecular hydrogen bonds, generating three-dimensional frameworks in (3) and (5), and two-dimensional sheets in (4).

Journal Article↗
PubMed2000

Crystal engineering in the gem-alkynol family: interplay between strong and weak interactions in structures of 2,3,5,6-tetrahalo

Structures of the title compounds are all mediated by strong cooperative arrangements of O-H.O hydrogen bonds, supported by a variety of weaker interactions which affect the type of O-H.O synthon that is formed. The tetrafluoro compound contains hexameric O-H.O synthons in a supramolecular chair conformation, together with C identical withC-H.F interactions. However, the tetrachloro and tetrabromo compounds both form tetrameric O-H.O synthons. This dominant pattern is supported by halogen.halogen interactions having one C-Cl[Br].Cl[Br] angle close to 180 degrees and the other close to 90 degrees, and by C identical withC-H.Cl[Br] interactions.

Journal Article↗
PubMed2001

Hydrated metal complexes of N-(6-amino-3,4-dihydro-3-methyl-5-nitroso-4-oxopyrimidin-2-yl)glycinate: interplay of molecular, molecular-electronic and supramolecular structures.

The title anion, (C(7)H(8)N(5)O(4))(-), L(-), forms hydrated metal complexes with a range of metal ions M(+) and M(2+). Lithium and manganese(II) form finite molecular aggregates [Li(L)(H(2)O)(3)] (1) and [Mn(L)(2)(H(2)O)(4)].6H(2)O (4) in which the molecular aggregates are linked into three-dimensional frameworks by extensive hydrogen bonding. The sodium and potassium derivatives, [Na(2)(L)(2)(H(2)O)(3)] (2) and [K(L)(H(2)O)] (3) both form organic-inorganic hybrid sheets in which metal-oxygen ribbons are linked by strips containing only organic ligands: these sheets are linked by hydrogen bonds into three-dimensional frameworks. In (2) the metal-oxygen ribbon is built from pairs of edge-shared trigonal bipyramids linked by water molecules, while in (3) it consists of a continuous chain of vertex-sharing octahedra. The nitroso group in the anion acts as an eta(1) ligand towards Na(+) and as an eta(2) ligand towards K(+). In all cases the anion L(-) shows the same unusual pattern of interatomic distances as the neutral parent LH.

Journal Article↗