Effect of Te addition on the optical properties of As(2)S(3) thin film

Bilayer thin films of Te/As(2)S(3) were prepared from Te and As(2)S(3) by thermal technique under high vacuum. Optical constants were calculated by analysing the transmission spectrum in the spectral range 400-1100 nm. The optical band gap decreases with the addition of Te to As(2)S(3). The decrease of optical band gap has been explained on the basis of density of states and the increase in disorder in the system. We have irradiated the as-deposited films using a diode pumped solid state laser of 532 nm wavelength to study photo-diffusion of Te into As(2)S(3). The changes were characterised by Fourier Transform Infrared and X-ray Photoelectron Spectroscopy (XPS). The optical band gap is found to be decreased with the light irradiation which is proposed due to homopolar bond formation. The core level peaks in XPS spectra give information about different bond formation. (C) 2011 Elsevier B.V. All rights reserved.

Branching out at C-2 of septanosides. Synthesis of 2-deoxy-2-C-alkyl/aryl septanosides from a bromo-oxepine

This paper deals with the synthesis of 2-deoxy-2-C-alkyl/aryl septanosides. A range of such septanoside derivatives was synthesized by using a common bromo-oxepine intermediate, involving C-C bond forming organometallic reactions. Unsaturated, seven-membered septanoside vinyl bromides or bromo-oxepines, obtained through a ring expansion methodology of the cyclopropane derivatives of oxyglycals, displayed a good reactivity towards several acceptor moieties in C-C bond forming Heck, Suzuki and Sonogashira coupling reactions, thus affording 2-deoxy-2-C-alkyl/aryl septanosides. Whereas Heck and Sonogashira coupling reactions afforded 2-deoxy-2-C-alkenyl and -alkynyl derivatives, respectively, the Suzuki reaction afforded 2-deoxy-2-C-aryl septanosides. Deprotection and reduction of the 2-deoxy-2-alkenyl derivative afforded the corresponding 2-deoxy-2-C-alkyl septanoside free of protecting groups. The present study illustrates the reactivity of bromo-oxepine in the synthesis of hitherto unknown septanosides, branching out at C-2, through C-C bond formation with alkyl and aryl substituents.

Synthesis and supramolecular assembly of the bifunctional borinic acid 1,2-fcB(OH)](2)

Treatment of the chloro-substituted diboradiferrocene derivative 1 with Me3SiOMe and subsequent hydrolysis resulted in formation of the novel organometallic bis(borinic acid) derivative 3. The assembly of 3 into supramolecular structures via hydrogen bonding and reversible covalent boron-oxygen bond formation was explored. Upon crystallization from acetone or THF one-dimensional chains form in which molecules of 3 alternately serve as hydrogen bond donors and acceptors. The additional OH hydrogens that are not involved in hydrogen bonding within the polymeric chains undergo hydrogen bonding to the solvent molecules. Removal of the solvent was achieved at moderate temperature under high vacuum. While the polymeric chains remain intact, in the absence of the solvent as a hydrogen bond acceptor, short contacts to the Cp rings of neighboring polymer strands lead to a network-like structure. At higher temperatures, further dehydration occurs with formation of B-O-B linkages as confirmed by MALDI-TOF mass spectrometry. Oligomers with up to 15 repeating units (30 ferrocenes) were detected.

Synergistic Lewis base and anion-binding catalysis for the enantioselective vinylogous addition of deconjugated butenolides to allenoates

An enantioselective vinylogous umpolung addition of deconjugated butenolides to allenoates has been developed for the first time with the help of synergistic combination of an achiral phosphine and a chiral squaramide, and represents the first example of a catalytic enantioselective C gamma-C gamma bond formation between two different carbonyl partners.

Structural basis for the redox sensitivity of the Mycobacterium tuberculosis SigK-RskA sigma-anti-sigma complex

The host-pathogen interactions in Mycobacterium tuberculosis infection are significantly influenced by redox stimuli and alterations in the levels of secreted antigens. The extracyto-plasmic function (ECF) sigma factor sigma(K) governs the transcription of the serodominant antigens MPT70 and MPT83. The cellular levels of sigma(K) are regulated by the membrane-associated anti-sigma(K) (RskA) that localizes sigma(K) in an inactive complex. The crystal structure of M. tuberculosis sigma(K) in complex with the cytosolic domain of RskA (RskAcyto) revealed a disulfide bridge in the -35 promoter-interaction region of sigma(K). Biochemical experiments reveal that the redox potential of the disulfide-forming cysteines in sigma(K) is consistent with its role as a sensor. The disulfide bond in sigma(K) influences the stability of the sigma(K)-RskA(cyto) complex but does not interfere with sigma(K)-promoter DNA interactions. It is noted that these disulfide-forming cysteines are conserved across homologues, suggesting that this could be a general mechanism for redox-sensitive transcription regulation.

INDUCTION OF FOLDED STRUCTURES IN DESIGNED PEPTIDES USING CONFORMATIONALLY CONSTRAINED RESIDUES

Folding into compact globular structures, with well-defined modules of secondary structure, appears to be a characteristic of long polypeptide chains, with a specific patterning of coded amino acid residues along the length of sequence. Cooperative hydrogen bond driven secondary structure formation and solvent forces, which contribute favorably to the entropy of folding, by promoting compaction of the polymeric chain, have long been discussed as major determinants of the folding process. First principles design approaches, which use non-coded amino acids, employ an alternative structure directing strategy, by using amino acid residues which exhibit a strong conformational bias for specific regions of the Ramachandran map. This overview of ongoing studies in the authors' laboratory, attempts to explore the use of conformationally restricted amino acid residues in the design of peptides with well-defined secondary structures. Short peptides composed of 20 genetically coded amino acids usually exist in solution as an ensemble of equilibrating conformations. Apolar peptide sequences, which are readily soluble in organic solvents like chloroform and methanol, facilitate formation of structures which are predominately driven by intramolecular hydrogen bond formation. The choice of sequences containing residues with a limited range of conformational choices strongly favors formation of local turn structures, stabilized by short range intramolecular hydrogen bonds. Two residue beta-turns can nucleate either helical or hairpin folding, depending on the precise conformation of the turn segment Restriction of the conformational space available to amino acid residues is easily achieved by introduction of an additional alkyl group at the C alpha carbon atom or by side chain backbone cyclization, as in proline. Studies of synthetic sequences incorporating two prototype residues alpha-aminoisobutyric acid (Aib) and D-proline (DPro) illustrate the utility of the strategy in construction of helices and hairpins. Extensions to the design of conformationally switchable sequences and structurally defined hybrid peptides containing backbone homologated residues are also surveyed.

Pressure induced manifold enhancement of Li-kinetics in FCC fullerene

The reduction of the diffusion energy barrier for Li in electrodes is one of the required criteria to achieve better performances in Li ion batteries. Using density functional theory based calculations, we report a pressure induced manifold enhancement of Li-kinetics in bulk FCC fullerene. Scanning of the potential energy surface reveals a diffusion path with a low energy barrier of 0.62 eV, which reduces further under the application of hydrostatic pressure. The pressure induced reduction in the diffusion barrier continues till a uniform volume strain of 17.7% is reached. Further enhancement of strain increases the barrier due to the repulsion caused by C-C bond formation between two neighbouring fullerenes. The decrease in the barrier is attributed to the combined effect of charge transfer triggered by the enhanced interaction of Li with the fullerene as well as the change in profile of the local potential, which becomes more attractive for Li. The lowering of the barrier leads to an enhancement of two orders of magnitude in Li diffusivity at room temperature making pressurized bulk fullerene a promising artificial solid electrolyte interface (SEI) for a faster rechargeable battery.

Metallacyclocumulenes: A Theoretical Perspective on the Structure, Bonding, and Reactivity

CONSPECTUS: Transition metals help to stabilize highly strained organic fragments. Metallacycles, especially unsaturated ones, provide much variety in this area. We had a sustained interest in understanding new C-C bond formation reactions affected by binuclear transition metal fragments Cp2M. One such study led to the exploration of the bimetallic C-C cleavage and coupled complexes, where the acetylide ligands bridge two metal atoms. The underlying M-C interaction in these complexes inspired the synthesis of a five-membered cyclocumulene complex, which opened a new phase in organometallic chemistry. The metallacyclocumulene produces a variety of C-C cleavage and coupled products including a radialene complex. Group 4 metallocenes have thus unlocked a fascinating chemistry by stabilizing strained unsaturated C4 organic fragments in the form of five-membered metallacyclocumulenes, metallacyclopentynes, and metallacycloallenes. Over the years, we have carried out a comprehensive theoretical study to understand the unusual stability and reactivity of these metallacycles. The unique (M-C-beta) interaction of the internal carbon atoms with the metal atom is the reason for unusual stability of the metallacycles. We have also shown that there is a definite dependence of the C-C coupling and cleavage reactions on the metal of metallacyclocumulenes. It demonstrates unexpected reaction pathways for these reactions. Based on this understanding, we have predicted and unraveled the stabilization factors of an unusual four-membered metallacycloallene complex. Indeed, our prediction about a four-membered heterometallacycle has led to an interesting bonding situation, which is experimentally realized. This type of M-C bonding is intriguing from a fundamental perspective and has great relevance in synthesizing unusual structures with interesting properties. In this Account, we first give a short prologue of what led to the present study and describe the salient features of the structure and bonding of the metallacyclocumulenes. The unusual reaction pathway of this metallacycle is explored next. Similar features of the metallacyclopentynes and metallacycloallenes are briefly mentioned. Then, we discuss the exploitation of the unique M-C bonding to design some exotic molecules such as a four-membered metallacycloallene complex. Our efforts to build a conceptual framework to understand these metallacycles and to exploit their chemistry continue.

A bio-attuned ratiometric hydrogen sulfate ion selective receptor in aqueous solvent: structural proof of the H-bonded adduct

A new cell permeable quinazoline based receptor (1) selectively senses HSO4- ions of nanomolar region in 0.1 M HEPES buffer (ethanol-water: 1/5, v/v) at biological pH over other competitive ions through the proton transfer followed by hydrogen bond formation and subsequent anion coordination to yield the LHSO4]-LH+center dot 3H(2)O (2) ensemble, which has been crystallographically characterised to ensure the structure property relationship. This non-cytotoxic HSO4- ion selective biomarker has great potential to recognize the intercellular distribution of HSO4- ions in HeLa cells under fluorescence microscope.

Properties of the chalcogenide-carbon nano tubes and graphene composite materials

Composite can deliver more than the individual elemental property of the material. Specifically chalcogenide- multi walled carbon nano tubes and chalcogenide- bilayer graphene composite materials could be interesting for the investigation, which have been less covered by the investigators. We describe micro structural properties of Se55Te25Ge20, Se55Te25Ge20 + 0.025% multi walled carbon nano tubes and Se55Te25Ge20 + 0.025% bilayer graphene materials. This gives realization of the alloying constituents inclusion/or diffusion inside the multi walled carbon nano tubes and bilayer graphene under the homogeneous parent alloy configuration. Raman spectroscopy, X-ray photoelectron spectroscopy, UV/Visible spectroscopy and Fourier transmission infrared spectroscopy have also been carried out under the discussion. A considerable core energy levels peak shifts have been noticed for the composite materials by the X-ray photoelectron spectroscopy. The optical energy band gaps are measured to be varied in between 1.2 and 1.3 eV. In comparison to parent (Se55Te25Ge20) alloy a higher infrared transmission has been observed for the composite materials. Subsequently, variation in physical properties has been explained on the basis of bond formation in solids. (C) 2014 Elsevier B. V. All rights reserved.

Negative hyperconjugation and red-, blue- or zero-shift in X-Z center dot center dot center dot Y complexes

A generalized explanation is provided for the existence of the red-and blue-shifting nature of X-Z bonds (Z = H, halogens, chalcogens, pnicogens, etc.) in X-Z center dot center dot center dot Y complexes based on computational studies on a selected set of weakly bonded complexes and analysis of existing literature data. The additional electrons and orbitals available on Z in comparison to H make for dramatic differences between the H-bond and the rest of the Z-bonds. The nature of the X-group and its influence on the X-Z bond length in the parent X-Z molecule largely controls the change in the X-Z bond length on X-Z center dot center dot center dot Y bond formation; the Y-group usually influences only the magnitude of the effects controlled by X. The major factors which control the X-Z bond length change are: (a) negative hyperconjugative donation of electron density from X-group to X-Z sigma* antibonding molecular orbital (ABMO) in the parent X-Z, (b) induced negative hyperconjugation from the lone pair of electrons on Z to the antibonding orbitals of the X-group, and (c) charge transfer (CT) from the Y-group to the X-Z sigma* orbital. The exchange repulsion from the Y-group that shifts partial electron density at the X-Z sigma* ABMO back to X leads to blue-shifting and the CT from the Y-group to the sigma* ABMO of X-Z leads to red-shifting. The balance between these two opposing forces decides red-, zero- or blue-shifting. A continuum of behaviour of X-Z bond length variation is inevitable in X-Z center dot center dot center dot Y complexes.

Organic-inorganic hybrid PtCo nanoparticle with high electrocatalytic activity and durability for oxygen reduction

In Pt-transition metal (TM) alloy catalysts, the electron transfer from the TM to Pt is retarded owing to the inevitable oxidation of the TM surface by oxygen. In addition, acidic electrolytes such as those employed in fuel cells accelerate the dissolution of the surface TM oxide, which leads to catalyst degradation. Herein, we propose a novel synthesis strategy that selectively modifies the electronic structure of surface Co atoms with N-containing polymers, resulting in highly active and durable PtCo nanoparticle catalysts useful for the oxygen reduction reaction (ORR). The polymer, which is functionalized on carbon black, selectively interacts with the Co precursor, resulting in Co-N bond formation on the PtCo nanoparticle surface. Electron transfer from Co to Pt in the PtCo nanoparticles modified by the polymer is enhanced by the increase in the difference in electronegativity between Pt and Co compared with that in bare PtCo nanoparticles with the TM surface oxides. In addition, the dissolution of Co and Pt is prevented by the selective passivation of surface Co atoms and the decrease in the O-binding energy of surface Pt atoms. As a result, the catalytic activity and durability of PtCo nanoparticles for the ORR are significantly improved by the electronic ensemble effects. The proposed organic/inorganic hybrid concept will provide new insights into the tuning of nanomaterials consisting of heterogeneous metallic elements for various electrochemical and chemical applications.

Surface-Bound Selectin-Ligand Binding Is Regulated By Carrier Diffusion

Two-dimensional (2D) kinetics of receptor-ligand interactions governs cell adhesion in many biological processes. While the dissociation kinetics of receptor-ligand bond is extensively investigated, the association kinetics has much less been quantified. Recently receptor-ligand interactions between two surfaces were investigated using a thermal fluctuation assay upon biomembrane force probe technique (Chen et al. in Biophys J 94:694-701, 2008). The regulating factors on association kinetics, however, are not well characterized. Here we developed an alternative thermal fluctuation assay using optical trap technique, which enables to visualize consecutive binding-unbinding transition and to quantify the impact of microbead diffusion on receptor-ligand binding. Three selectin constructs (sLs, sPs, and PLE) and their ligand P-selectin glycoprotein ligand 1 were used to conduct the measurements. It was indicated that bond formation was reduced by enhancing the diffusivity of selectin-coupled carrier, suggesting that carrier diffusion is crucial to determine receptor-ligand binding. It was also found that 2D forward rate predicted upon first-order kinetics was in the order of sPs > sLs > PLE and bond formation was history-dependent. These results further the understandings in regulating association kinetics of surface-bound receptor-ligand interactions.

Engineering thermostable fungal cellobiohydrolases

Meeting the world's growing energy demands while protecting our fragile environment is a challenging issue. Second generation biofuels are liquid fuels like long-chain alcohols produced from lignocellulosic biomass. To reduce the cost of biofuel production, we engineered fungal family 6 cellobiohydrolases (Cel6A) for enhanced thermostability using random mutagenesis and recombination of beneficial mutations. During long-time hydrolysis, engineered thermostable cellulases hydrolyze more sugars than wild-type Cel6A as single enzymes and binary mixtures at their respective optimum temperatures. Engineered thermostable cellulases exhibit synergy in binary mixtures similar to wild-type cellulases, demonstrating the utility of engineering individual cellulases to produce novel thermostable mixtures. Crystal structures of the engineered thermostable cellulases indicate that the stabilization comes from improved hydrophobic interactions and restricted loop conformations by proline substitutions. At high temperature, free cysteines contribute to irreversible thermal inactivation in engineered thermostable Cel6A and wild-type Cel6A. The mechanism of thermal inactivation in this cellulase family is consistent with disulfide bond degradation and thiol-disulfide exchange. Enhancing the thermostability of Cel6A also increases tolerance to pretreatment chemicals, demonstrated by the strong correlation between thermostability and tolerance to 1-ethyl-3-methylimidazolium acetate. Several semi-rational protein engineering approaches – on the basis of consensus sequence analysis, proline stabilization, FoldX energy calculation, and high B-factors – were evaluated to further enhance the thermostability of Cel6A.

Beta-Hydroxyimino Phosphorus Derivatives. An Efficient Tool in Organic Synthesis

[EN] The purpose of this review article is to illustrate synthetic aspects of functionalized phosphorus derivatives containing an oximo moiety at the beta-position. First section will be focused on the synthesis of phosphine oxides, phosphonates or phosphonium salts containing an oxime group. The synthesis of these derivatives comprises the carbon–phosphorus single bond construction by reaction of haloximes with phosphorus derivatives, nucleophilic addition of phosphorus reagents to carbonyl compounds, or nucleophilic addition of phosphorus reagents to nitro olefins. This section will also concentrate on the most practical routes for the synthesis of the target compounds, through carbon–nitrogen double bond formation, which are as follows: condensation processes of carbonyl compounds and hydroxylamine derivatives or addition of hydroxylamines to allenes or alkynes. The preparative use of beta-oximo phosphorus derivatives as synthetic intermediates will be discussed in a second section, comprising olefination reaction, oxidation of oximes to nitrile oxides by reaction at the C-N double bond of the oxime moiety, oxidation of these substrates to nitrosoalkenes, reduction to the corresponding hydroxylamines and some reactions at the hydroxyl group of the hydroxyimino moiety.