Global homeomorphisms and covering projections on metric spaces

For a large class of metric spaces with nice local structure, which includes Banach–Finsler manifolds and geodesic spaces of curvature bounded above, we give sufficient conditions for a local homeomorphism to be a covering projection. We first obtain a general condition in terms of a path continuation property. As a consequence, we deduce several conditions in terms of path- liftings involving a generalized derivative, and in particular we obtain an extension of Hadamard global inversion theorem in this context. Next we prove that, in the case of quasi-isometric mappings, some of these sufficient conditions are also necessary. Finally, we give an application to the existence of global implicit functions. O. Gutú and J. A. Jaramillo were supported in part by D.G.E.S. (Spain) Grant BFM2003-06420.     

Insights into the binding properties of a cuprous ion embedded in the tren cap of a calix[6]arene and supramolecular trapping of an intermediate

Coordination of Cu(I) to a tren unit that is covalently linked to a calix[6]arene has been explored. The resulting complex revealed itself very stable in solution under an inert atmosphere, but extremely sensitive to O2 in solution as well as in the solid state. Therefore, its binding properties towards non-redox ligands have been studied in detail. The electron-rich metal center displays moderate affinity for nitrilo ligands compared to the calix[6]tris-pyridine ligand. Indeed, the binding enthalpy with acetonitrile is only -30 kJ mol(-1), whereas it is -72 kJ mol(-1) with the tris-pyridine system. In contrast, CO binding is relatively strong due to important pi-back donation from the metal center, as evidenced by the CO stretch, which was found to be less energetic (2075 cm(-1)) than that measured for ligands based on aromatic donors such as imidazole or pyridine. The conformational and dynamic properties of this calix-system have also been studied in detail. With an empty cavity or with the very small CO guest-ligand, the calix-core undergoes partial self-inclusion leading to dissymmetrical conformations. In contrast, nitrilo ligands act as "shoe-trees" that maintain the calix-core in a C(3v) symmetrical cone conformation. Very interestingly, the variable T study relative to the ligand exchange process highlighted a two-step dissociative pathway, where Cu-N bond cleavage/formation is differentiated from the nitrilo guest expulsion/inclusion from/into the calixarene cavity.     

Innovative methodologies for the N-protection of N-alkylimidazole groups: application to the first synthesis of a water-soluble calix[6]arene presenting three ammonium substituents at the large rim and three neutral N-donors at the small rim

The first synthesis of a cationic amphiphile calixarene ligand, which bears three neutral imidazole donors on one side of the hydrophobic cone and three quaternary ammonium substituents on the other side, is reported. The synthetic strategy relies on two key steps: (i) the "small rim-directed" selective ipso-nitration at the large rim and (ii) a protection-deprotection sequence of the N-alkylimidazole groups, for which two equally efficient novel methods (coordination to Zn(II) or to a cyanoborane) are presented.     

Controlling Internal Pore Sizes in Bicontinuous Polymeric Nanospheres

Complex polymeric nanospheres were formed in water from comb‐like amphiphilic block copolymers. Their internal morphology was determined by three‐dimensional cryo‐electron tomographic analysis. Varying the polymer molecular weight (MW) and the hydrophilic block weight content allowed for fine control over the internal structure. Construction of a partial phase diagram allowed us to determine the criteria for the formation of bicontinuous polymer nanosphere (BPN), namely for copolymers with MW of up to 17 kDa and hydrophilic weight fractions of ≤0.25; and varying the organic solvent to water ratio used in their preparation allowed for control over nanosphere diameters from 70 to 460 nm. Significantly, altering the block copolymer hydrophilic–hydrophobic balance enabled control of the internal pore diameter of the BPNs from 10 to 19 nm.     

Total Synthesis of Anti-MRSA Active Diorcinols and Analogues

Diorcinols and related prenylated diaryl ethers were reported to exhibit activity against methicillin-resistant clinical isolates of Staphylococcus aureus (MRSA). Within these lines, we report the first total synthesis of diorcinol D, I, J, the proposed structure of verticilatin and recently isolated antibacterial diaryl ether by using an efficient and highly divergent synthetic strategy. These total syntheses furnish the diaryl ethers in only five to seven steps employing a Pd-catalyzed diaryl ether coupling as the key step. The total synthesis led to the structural revision of the natural product verticilatin, which has been isolated from a plant pathogenic fungus. Furthermore, these structures were tested in order to determine their antibacterial activities against different MRSA strains as well as further Gram-positive and -negative bacteria.     

Comparison of water isotope-ratio determinations using two cavity ring-down instruments and classical mass spectrometry in continuous ice-core analysis

We present a detailed comparison between subsequent versions of commercially available wavelength-scanned cavity ring-down water isotope analysers (L2120-i and L2130-i, Picarro Inc.). The analysers are used in parallel in a continuous mode by adaption of a low-volume flash evaporation module. Application of the analysers to ice-core analysis is assessed by comparison between continuous water isotope measurements of a glacial ice-core from Severnaya Zemlya with discrete isotope-ratio mass spectrometry measurements performed on parallel samples from the same ice-core. The great advances between instrument versions, particularly in the measurement of δ²H, allow the continuous technique to achieve the same high level of accuracy and precision obtained using traditional isotope spectrometry techniques in a fraction of the experiment time. However, when applied to continuous ice-core measurements, increased integration times result in a compromise of the achievable depth resolution of the ice-core records.     

Calorimetric study on coordination of tridentate imidazolyl calix[6]arene ligands to zinc ion in organic solvents

Complexation of three kinds of tris(imidazolyl)calix[6]arenes containing alternate p-substituents (Calix-tBu, R(1) = R(2) = tert-butyl; Calix-NH(2), R(1) = tert-butyl, R(2) = NH(2); Calix-NO(2), R(1) = tert-butyl, R(2) = NO(2)) with Zn(ClO(4))(2)(H(2)O)(6) in acetonitrile, methanol, and THF was investigated via isothermal titration calorimetry (ITC). For the coordination of these calixarene ligands to Zn(II) in acetonitrile, typical one-phase exothermic titration curves were obtained, indicating the formation of 1:1 ligand-Zn(II) complexes accompanied by large conformational changes of the ligands. In contrast, the complexation in methanol was endothermic and dominated by favorable entropy changes. The entropy gains were achieved by extensive desolvation from both Zn(II) and the ligands. ITC measurements suggest a 2:1 ligand-Zn(II) complex formation in THF in the presence of excess ligands (Calix-NH(2) and Calix-NO(2)). The 2:1 complexes were converted to 1:1 complexes upon further addition of Zn(ClO(4))(2)(H(2)O)(6). The results indicate the important role of a coordinating solvent (acetonitrile) for direct formation of the 1:1 complexes under the conditions of excess ligand. Complexation of a ditopic ligand (Calix-Tri) with three triazole moieties on the wider rim was also studied via ITC. The first coordination of the imidazole moieties to Zn(II) was an exothermic process. This was followed by the entropically favorable coordination of the triazole moieties to the divalent cation. We have also investigated exchange of the fourth ligand (H(2)O) of the Zn(II) complex of Calix-NH(2) with butylamine, heptylamine, acetonitrile, and acetamide in a noncompetitive solvent, THF. The ΔH(0) tended to decrease upon increasing the electron-pair-donating ability of the guest ligand, whereas it was also affected by an entropic term due to restricted rotation of the guest ligand inside the calixarene cavity.     

Ionic concentration effects on reverse micelle size and stability: implications for the synthesis of nanoparticles

We present a systematic investigation and analysis of the structure and stability of reverse micelle systems with the addition of NH(4)OH, ZrOCl(2), and Al(NO(3))(3) salts. We demonstrate that the reverse micelle size decreases with increasing salt additions until one reaches a critical concentration, which characterizes the onset of system destabilization. The concept of an electrical double layer, as it applies to reverse micelles, is considered for explaining features of destabilization, including the initial decrease in reverse micelle size, the destabilization concentration, and the effect of cation valence. We propose that the reduction in size prior to instability is caused by compression of the reverse micelle electrical double layers, as higher concentrations of salts are present. The reduced thickness of the electrical double layers allows the decaying potentials to move into closer proximity to each other before generating enough repulsion to balance the forces for reverse micelle formation and form a new equilibrium average reverse micelle size. The point of reverse micelle instability has been related to the formation of a two-phase system as a result of the inability to further compress the salt co-ions in the core of the reverse micelles, which would cause an excessive repulsive force between the overlapping potentials. We have extracted a critical potential of -89 nV between the two overlapping potentials for the AOT/water/isooctane (ω(0) = 10) systems studied. All these effects have important implications for the preparation of nanopowders by reverse micelle synthesis. If the reverse micelles are unstable before the precipitates are formed, then the advantage of reverse micelle synthesis is immediately lost.     

A generic platform for the addressable functionalisation of electrode surfaces through self-induced "electroclick"

A novel and general strategy for the immobilisation of functional objects onto electrodes is described. The concept is based on the addition of two pendant ethynyl groups onto a bis(pyridyl)amine derivative, which acts as a molecular platform. This platform is pre-functionalised with an N(3)-tagged object of interest by Huisgen cycloaddition to one of the ethynyl groups in biphasic conditions. Hence, when complexed by Cu(II) , this molecular-object holder can be immobilised, by a "self-induced electroclick", through the second ethynyl group onto N(3)-alkanethiol self-assembled monolayers on a gold electrode. Two different functional groups, a redox innocent ((CH(2))(3)-Ph) and an electrochemical probe (ferrocene), were immobilised by following this strategy. The in situ electrochemical grafting showed, for both systems, that the kinetics of immobilisation is fast. The voltammetric characterisation of the surface-tagged functionalised copper complexes indicated that a good surface coverage was achieved and that a moderately fast electron-transfer reaction occurs. Remarkably, in the case of the redox-active ferrocenyl-immobilised system, the electrochemical response highlighted the involvement of the copper ion of the platform in the kinetics of the electron transfer to the ferrocene moiety. This platform is a promising candidate for applications in surface addressing in areas as diverse as biology and materials.     

Back-to-back Schottky diodes: the generalization of the diode theory in analysis and extraction of electrical parameters of nanodevices

We report on the analysis of nonlinear current-voltage characteristics exhibited by a set of blocking metal/SnO(2)/metal. Schottky barrier heights in both interfaces were independently extracted and their dependence on the metal work function was analyzed. The disorder-induced interface states effectively pinned the Fermi level at the SnO(2) surface, leading to the observed Schottky barriers. The model is useful for any two-terminal device which cannot be described by a conventional diode configuration.