Electronic Transitions in Conformationally Controlled Peralkylated Hexasilanes

The photophysical properties of oligosilanes show unique conformational dependence due to σ‐electron delocalization. The excited states of the SAS, AAS, and AEA conformations of peralkylated n‐hexasilanes, in which the SiSiSiSi dihedral angles are controlled into a syn (S), anti (A), or eclipsed (E) conformation, were investigated by using UV absorption, magnetic circular dichroism (MCD), and linear dichroism spectroscopy. Simultaneous Gaussian fitting of all three spectra identified a minimal set of transitions and the wavenumbers, oscillator strengths, and MCD B terms in all three compounds. The results compare well with those obtained by using the symmetry‐adapted‐cluster configuration interaction method and almost as well with those obtained by time‐dependent density functional theory with the PBE0 functional. The conformational dependence of the transition energies and other properties of free‐chain permethylated n‐hexasilane, n‐Si₆Me₁₄, was also examined as a function of dihedral angles, and the striking effects found were attributed to avoided crossings between configurations of σσ* and σπ* character.     

Phosphatidylcholine vesicle-mediated decomposition of hydrogen peroxide

The decomposition of hydrogen peroxide (H2O2) was examined in aqueous solution (50 mM Tris-HCl buffer, pH 7.4, containing 100 mM NaCl) at 25 degrees C in pure buffer or in the presence of either vesicles or micelles formed from various phosphatidylcholines (PCs). In the absence of PCs, more than 90% of the initially added H2O2 (1.0 mM) remained intact after incubation for 120 h. The effect of the PCs on the decomposition of H2O2 was studied by using different PCs that varied in terms of number of carbon atoms in the two acyl chains n as well as in terms of the degree of unsaturation. PCs with short hydrocarbon chains (n = 4, 6-8) were dissolved in the buffer solution in the form of nonassociated monomers or as micelles in equilibrium with monomers at a fixed PC concentration of 10 mM. The presence of these short-chain PCs slightly enhanced the H2O2 decomposition rate. Micelles formed by non-lipid detergents (sodium cholate, Triton X-100, and sodium dodecylsulfate) had a similar effect. In marked contrast, PCs with long hydrocarbon chains (n > or = 10) dispersed in buffer solution as vesicles (liposomes) significantly enhanced the rate of H2O2 decomposition, with the most effective PC being 1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC) at 25 degrees C. This indicates that the packing density of the PC molecules influences the reactivity, presumably through the direct interaction of the PC assemblies with H2O2 molecules. Furthermore, in the case of vesicles formed from PCs with unsaturated acyl chains (1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine, POPC; 1,2-dioleoyl-sn-glycero-3-phosphocholine, DOPC), carbon-carbon double bond oxidation did not occur extensively under the conditions used. This indicates that the observed effect of PCs on the decomposition of H2O2 is indeed related to the assembly structure (vesicle vs micelles vs monomers) and is clearly not related to the presence of unsaturated hydrocarbon chains. Fluorescence polarization measurements of two fluorescent probes embedded either in the acyl chain region of the vesicles (DPH, 1,6-diphenyl-1,3,5-hexatriene) or on the surface of the vesicles (TMA-DPH, 1-(4-trimethylammoniumphenyl)-6-phenyl-1,3,5-hexatriene iodide) show that the presence of H2O2 leads to a decrease in the fluidity of the lipid-water surface and not to a change in the fluidity of the hydrophobic region of the vesicle bilayer. This indicates that the decomposition of H2O2 is triggered through interactions between H2O2 and the polar head group area of PC vesicles.     

Ion‐Free and Ion‐Pairing Assemblies of Anion‐Responsive π‐Electronic Systems Possessing Directly Linked Alkyl Chains

Alkyl‐substituted pyrrole‐based anion‐responsive π‐electronic systems formed supramolecular gels and liquid crystals through effective π–π stacking and van der Waals interactions. The addition of chloride as a planar cation salt afforded ion‐pairing assemblies as soft materials comprising planar receptor‐Cl^? complexes and the cation.     

Major histocompatibility complex (MHC) protein analysis by optimised two-dimensional electrophoretic methods

Human histocompatibility molecules HLA-Class I and Class II (DR, DQ, DP) were analysed using three two-dimensional protocols: nonequilibrium pH gradient electrophoresis (NEPHGE), isoelectric focusing-acidic gradient (IEF-AG) and isoelectric focusing-basic gradient (IEF-BG). The three methods differ in their carrier ampholyte combinations and electrophoretic conditions. They provide different pH gradients and therefore different electrofocusing profiles. The NEPHGE protocol was adequate for separating proteins across a broad range of pI mobilities, i.e. 4.4 pH units between the acidic and the basic end. In contrast, the IEF-AG and the IEF-BG protocols gave a separation power across a narrow pH range, 1.9 and 1.7 pH units respectively. Thus, whereas the NEPHGE protocol provides a tool for a global major histocompatibility complex (MHC) antigen profile analysis, the IEF-AG and -BG allows one to investigate subcomponents of the individual MHC chains. For example, NEPHGE analysis of the HLA Class I heavy chain revealed a single spot. However, IEF-BG revealed the presence of six equidistantly spaced spots spanning a short pH gradient with identical molecular weight. Similar improved resolution was seen for the HLA-DR, DQ, and DP molecules. The IEF acidic gradient was adequate for separating the alpha chain; the IEF basic gradient gave better resolution of the beta chains. This data provides a baseline set of conditions for both analytical and preparative MHC protein studies prior to amino acid sequencing.     

Synthesis of crown ether-terminated poly(methyl methacrylate) by radical chain transfer polymerization

Mercapto-16-crown-5 was prepared starting from tetraethyleneglycol and 3-chloro-2-chloromethyl-1-propene. Radical polymerization of methyl methacrylate was carried out in the presence of mercapto-16-crown-5 as a chain transfer agent to give crown ether-terminated poly(methyl methacrylate). The end crown group was characterized by IR and ¹H-NMR spectra. Sodium cation was selectively extracted by this crown-containing polymer. The molecular weight of the obtained polymer had influence upon the ability of extraction of sodium cation.     

Large-scale separation of metallic and semiconducting single-walled carbon nanotubes

In the applications of single-walled carbon nanotubes (SWNTs), it is extremely important to separate semiconducting and metallic SWNTs. Although several methods have been reported for the separation, only low yields have been achieved at great expense. We show a separation method involving a dispersion-centrifugation process in a tetrahydrofuran solution of amine, which makes metallic SWNTs highly concentrated to 87% in a simple way.     

Synthesis of 2,3-disubstituted benzofurans by platinum-olefin-catalyzed carboalkoxylation of o-alkynylphenyl acetals

The PtCl2-catalyzed cyclization reaction of o-alkynylphenyl acetals 1 in the presence of 1,5-cyclooctadiene produces 3-(alpha-alkoxyalkyl)benzofurans 2 in good to high yields. For example, the reaction of acetaldehyde ethyl 2-(1-octynyl)phenyl acetal (1a), acetaldehyde ethyl 2-(cyclohexylethynyl)phenyl acetal (1c), and acetaldehyde ethyl 2-(phenylethynyl)phenyl acetal (1f) in the presence of 2 mol % of platinum(II) chloride and 8 mol % of 1,5-cyclooctadiene in toluene at 30 degrees C gave the corresponding 2,3-disubstituted benzofurans 2a, 2c, and 2f in 91, 94, and 88% yields, respectively.     

Copper-catalyzed complete regio- and stereoselective cyclization of 1-aryl-3-sulfanyl-4-oxahepta-1,6-diynes triggered by alkynylation

Copper(I)-catalyzed alkynylation-cyclization of 4-oxahepta-1,6-diynes 1 with a wide variety of terminal alkynes proceeded to give (3E,4Z)-3-(phenylsulfanylmethylene)-4-(2-propynylidene)tetrahydrofuran-2-yl]benzenes 2aa-he in high yields with complete regio- and stereoselectivity.     

Unpaired electrons in the heavy-fermion superconductor CeCoIn5

Thermal conductivity and specific heat were measured in the superconducting state of the heavy-fermion material Ce(1-x)La(x)CoIn5. With increasing impurity concentration x, the suppression of T(c) is accompanied by the increase in residual electronic specific heat expected of a d-wave superconductor, but it occurs in parallel with a decrease in residual electronic thermal conductivity. This contrasting behavior reveals the presence of uncondensed electrons coexisting with nodal quasiparticles. An extreme multiband scenario is proposed, with a d-wave superconducting gap on the heavy-electron sheets of the Fermi surface and a negligible gap on the light, three-dimensional pockets.