Access to L- and D-iminosugar C-glycosides from a D-gluco-derived 6-azidolactol exploiting a ring isomerization/alkylation strategy

A flexible synthetic access to six-membered L- and D-iminosugar C-glycosides is reported starting from the easily available 6-azido-6-deoxy-2,3,4-tri-O-benzyl-D-glucopyranose precursor. This methodology involves a highly diastereoselective tandem ring enlargement/alkylation and a stereocontrolled ring contraction. It allows an efficient synthesis of iminosugar C-glycosides displaying structural diversity at both C-1 and C-6.     

Synthesis of planar chiral iridacycles by cationic metal π-coordination: facial selectivity, and conformational and stereochemical consequences

Facial selectivity during the π-coordination of pseudo-tetrahedral iridacycles by neutral (Cr(CO)(3)), monocationic (Cp*Ru(+)), and biscationic (Cp*Ir(2+)) metal centers was directly influenced by the coulombic imbalance in the coordination sphere of the chelated Ir center. We also showed by using theoretical calculations that the feasibility of the related metallacycles that displayed metallocenic planar chirality was dependent to the presence of an electron-donating group, such as NMe(2), which contributed to the overall stability of the complexes. When the π-bonded moiety was the strongly electron-withdrawing Cp*Ir(2+) group, the electron donation from NMe(2) resulted in major conformational changes, with a barrier to rotation of about 17?kcal mol(-1) for this group that became spectroscopically diastereotopic (high-field (1)H?NMR spectroscopy). This peculiar property is proposed as a means to introduce a new type of constitutional chirality at the nitrogen center: planar chirality at tertiary aromatic amines.     

Magnetic Co3O4 nanowires,preparation and properties

The Co₃O₄ nanowires have been successfully synthesized via modified template method.The as-prepared products have been characterized by EDS,TEM and HRTEM analysis.The magnetic behavior of it is investigated by a magnetic property measurement system.The nanowires exhibit some novel magnetic properties,which are different from its bulk material.The temperature dependence curves of magnetization in zero-field-cooling and field-cooling exhibit two peaks of antiferromagnetic at blocking temperature of～23 K and～31 K,respectively.The field dependent M（H） curves of the Co₃O₄ nanowires at T = 5 and 300 K both exhibit PM properties.Moreover,the diameter of nanowires is hence determined according to the finite size effect as approximately 7-11 nm,in consistent with the characterizations by HRTEM.     

Measurement of Aluminum–Carbon Distances Using S‐RESPDOR NMR Experiments

It is demonstrated that reliable aluminum–carbon distances can be measured in samples with ¹³C natural abundance by NMR spectroscopy. Overcoupled resonators, with only one radio‐frequency synthesizer and one amplifier, are used to irradiate in the same pulse sequence ²⁷Al and ¹³C nuclei, which differ by only 3.6 % in Larmor frequencies. The combination of ²⁷Al saturation pulse with heteronuclear dipolar recoupling yields dipolar dephasing of the ¹³C signal, which only depends on the Al? C distance and the efficiency of the saturation pulse. Therefore, reliable distances can be obtained by rapid fitting of experimental data to an analytical expression. It is demonstrated that with natural isotopic abundance this approach allows recovery of Al? C distances of 216 pm for the covalent bond in lithium tetraalkyl aluminates, commonly used as a co‐catalyst in olefin polymerization processes, and which range from 274 to 381 pm for the three carbon atoms in aluminum lactate. The accuracy of the measured internuclear distances is carefully estimated.     

23Na double-rotation NMR of sodium nucleotides leads to the discovery of a new dCMP hendecahydrate

Obtaining definitive information concerning the coordination environment of sodium ions which balance the negative charges found in nucleotides is a challenging task. We show that high resolution 1D and 2D (23)Na NMR spectra of sodium nucleotides obtained in the solid state with the use of double-rotation (DOR) provide valuable structural information. Sensitive spin diffusion homonuclear correlation experiments are used to establish the relative proximities of various pairs of crystallographically distinct Na sites and to assign the spectral resonances. Additionally, the DOR sidebands are simulated to obtain coordination information which is complementary to that obtained using multiple-quantum magic-angle spinning NMR spectra. These experiments led us to discover a new hendecahydrate of deoxycytidine monophosphate (dCMP), the structure of which is confirmed via single-crystal X-ray diffraction. This hydrate crystallizes reproducibly when deuterated water is used exclusively in the preparation process.     

Impact of metal cations on the electrocatalytic properties of Pt/C nanoparticles at multiple phase interfaces

Proton-exchange membrane fuel cells (PEMFCs) use carbon-supported nanoparticles based on platinum and its alloys to accelerate the rate of the sluggish oxygen-reduction reaction (ORR). The most common metals alloyed to Pt include Co, Ni and Cu, and are thermodynamically unstable in the PEMFC environment. Their dissolution yields the formation and redistribution of metal cations (M(y+)) within the membrane electrode assembly (MEA). Metal cations can also contaminate the MEA when metallic bipolar plates are used as current collectors. In each case, the electrical performance of the PEMFC severely decreases, an effect that is commonly attributed to the poisoning of the sulfonic acid groups of the perfluorosulfonated membrane (PEM) and the resulting decrease of the proton transport properties. However, the impact of metal cations on the kinetics of electrochemical reactions involving adsorption/desorption and bond-breaking processes remains poorly understood. In this paper, we use model electrodes to highlight the effect of metal cations on Pt/C nanoparticles coated or not with a perfluorosulfonated ionomer for the CO electrooxidation reaction and the oxygen reduction reaction. We show that metal cations negatively impact the ORR kinetics and the mass-transport resistance of molecular oxygen. However, the specific adsorption of sulfonate groups of the Nafion? ionomer locally modifies the double layer structure and increases the tolerance to metal cations, even in the presence of sulphate ions in the electrolyte. The survey is extended by using an ultramicroelectrode with cavity and a solid state cell (SSC) specifically developed for this study.     

High toxicity and specificity of the saponin 3-GlcA-28-AraRhaxyl-medicagenate, from Medicago truncatula seeds, for Sitophilus oryzae

Background

Because of the increasingly concern of consumers and public policy about problems for environment and for public health due to chemical pesticides, the search for molecules more safe is currently of great importance. Particularly, plants are able to fight the pathogens as insects, bacteria or fungi; so that plants could represent a valuable source of new molecules.

Results

It was observed that Medicago truncatula seed flour displayed a strong toxic activity towards the adults of the rice weevil Sitophilus oryzae (Coleoptera), a major pest of stored cereals. The molecule responsible for toxicity was purified, by solvent extraction and HPLC, and identified as a saponin, namely 3-GlcA-28-AraRhaxyl-medicagenate. Saponins are detergents, and the CMC of this molecule was found to be 0.65?mg per mL. Neither the worm Caenorhabditis elegans nor the bacteria E. coli were found to be sensitive to this saponin, but growth of the yeast Saccharomyces cerevisiae was inhibited at concentrations higher than 100???g per mL. The purified molecule is toxic for the adults of the rice weevils at concentrations down to 100???g per g of food, but this does not apply to the others insects tested, including the coleopteran Tribolium castaneum and the Sf9 insect cultured cells.

Conclusions

This specificity for the weevil led us to investigate this saponin potential for pest control and to propose the hypothesis that this saponin has a specific mode of action, rather than acting via its non-specific detergent properties.     

Selective photochemistry at stereogenic metal and ligand centers of cis-[Ru(diphosphine)2(H)2]: preparative, NMR, solid state, and laser flash studies

Three ruthenium complexes Λ-[cis-Ru((R,R)-Me-BPE)(2)(H)(2)] Λ-R,R-Ru1H(2), Δ-[cis-Ru((S,S)-Me-DuPHOS)(2)(H)(2)] Δ-S,S-Ru2H(2), and Λ-[cis-Ru((R,R)-Me-DuPHOS)(2)(H)(2)] Λ-R,R-Ru2H(2) (1 = (Me-BPE)(2), 2 = (Me-DuPHOS)(2)) were characterized by multinuclear NMR and CD spectroscopy in solution and by X-ray crystallography. The chiral ligands allow the full control of stereochemistry and enable mechanistic studies not otherwise available. Oxidative addition of E-H bonds (E = H, B, Si, C) was studied by steady state and laser flash photolysis in the presence of substrates. Steady state photolysis shows formation of single products with one stereoisomer. Solid state structures and circular dichroism spectra reveal a change in configuration at ruthenium for some Δ-S,S-Ru2H(2)/Λ-R,R-Ru2H(2) photoproducts from Λ to Δ (or vice versa) while the configuration for Λ-R,R-Ru1H(2) products remains unchanged as Λ. The X-ray structure of silyl hydride photoproducts suggests a residual H(1)···Si(1) interaction for Δ-[cis-Ru((R,R)-Me-DuPHOS)(2)(Et(2)SiH)(H)] and Δ-[cis-Ru((R,R)-Me-DuPHOS)(2)(PhSiH(2))(H)] but not for their Ru(R,R-BPE)(2) analogues. Molecular structures were also determined for Λ-[cis-Ru((R,R)-Me-BPE)(2)(Bpin)(H)], Λ-[Ru((S,S)-Me-DuPHOS)(2)(η(2)-C(2)H(4))], Δ-[Ru((R,R)-Me-DuPHOS)(2)(η(2)-C(2)H(4))], and trans-[Ru((R,R)-Me-DuPHOS)(2)(C(6)F(5))(H)]. In situ laser photolysis in the presence of p-H(2) generates hyperpolarized NMR spectra because of magnetically inequivalent hydrides; these experiments and low temperature photolysis with D(2) reveal that the loss of hydride ligands is concerted. The reaction intermediates [Ru(DuPHOS)(2)] and [Ru(BPE)(2)] were detected by laser flash photolysis and have spectra consistent with approximate square-planar Ru(0) structures. The rates of their reactions with H(2), D(2), HBpin, and PhSiH(3) were measured by transient kinetics. Rate constants are significantly faster for [Ru(BPE)(2)] than for [Ru(DuPHOS)(2)] and follow the substrate order H(2) > D(2) > PhSiH(3) > HBpin.     

Structural role of counterions adsorbed on self-assembled peptide nanotubes

Among noncovalent forces, electrostatic ones are the strongest and possess a rather long-range action. For these reasons, charges and counterions play a prominent role in self-assembly processes in water and therefore in many biological systems. However, the complexity of the biological media often hinders a detailed understanding of all the electrostatic-related events. In this context, we have studied the role of charges and counterions in the self-assembly of lanreotide, a cationic octapeptide. This peptide spontaneously forms monodisperse nanotubes (NTs) above a critical concentration when solubilized in pure water. Free from any screening buffer, we assessed the interactions between the different peptide oligomers and counterions in solutions, above and below the critical assembly concentration. Our results provide explanations for the selection of a dimeric building block instead of a monomeric one. Indeed, the apparent charge of the dimers is lower than that of the monomers because of strong chemisorption. This phenomenon has two consequences: (i) the dimer-dimer interaction is less repulsive than the monomer-monomer one and (ii) the lowered charge of the dimeric building block weakens the electrostatic repulsion from the positively charged NT walls. Moreover, additional counterion condensation (physisorption) occurs on the NT wall. We furthermore show that the counterions interacting with the NTs play a structural role as they tune the NTs diameter. We demonstrate by a simple model that counterions adsorption sites located on the inner face of the NT walls are responsible for this size control.     

Spectrophotometric detection of tyrosinase activity based on boronic acid-functionalized gold nanoparticles

A spectrophotometric method for the detection of tyrosinase activity is developed by utilizing the product-triggered aggregation of boronic acid-functionalized gold nanoparticles. Based on the changes of absorbance in UV-visible spectra, the assay shows extremely high sensitivity and lowered limit of detection of 1 × 10(-10) u mL(-1).