Structural characterization of single-walled carbon nanotube bundles by experiment and molecular simulation

A procedure, combining molecular simulation, Raman spectroscopy, and standard nitrogen adsorption, is developed for structural characterization of single-walled carbon nanotube (SWNT) samples. Grand canonical Monte Carlo simulations of nitrogen adsorption are performed on the external and internal adsorption sites of homogeneous arrays of SWNTs of diameters previously determined by Raman spectroscopy of the sample. The results show the importance of the peripheral grooves of a nanotube bundle at low relative pressure and the insensitivity of nanotube diameter toward adsorption on the external surface of the bundle at higher pressures. Simulations also reveal that samples containing thin nanotubes have less internal adsorption capacity that saturates at lower pressure than those comprising large diameter nanotubes. The fraction of open-ended nanotubes in a sample can be estimated by scaling the simulated internal adsorption inside nanotubes to obtain a near perfect fit between simulated and experimental isotherms. This procedure allows extrapolation of adsorption properties to conditions in which all nanotubes in the sample are open-ended.     

Studies on the inactivation of bovine liver enoyl-CoA hydratase by (methylenecyclopropyl)formyl-CoA: elucidation of the inactivation mechanism and identification of cysteine-114 as the entrapped nucleophile

The inhibitory properties of (methylenecyclopropyl)formyl-CoA (MCPF-CoA), a metabolite derived from a natural amino acid, (methylenecyclopropyl)glycine, against bovine liver enoyl-CoA hydratase (ECH) were characterized. We have previously demonstrated that MCPF-CoA specifically targets ECHs, which catalyze the reversible hydration of alpha,beta-unsaturated enoyl-CoA substrates to the corresponding beta-hydroxyacyl-CoA products. Here, we synthesized (R)- and (S)-diastereomers of MCPF-CoA to examine the stereoselectivity of this inactivation. Both compounds were shown to be competent inhibitors for bovine liver ECH with nearly identical second-order inactivation rate constants (k(inact)/K(I)) and partition ratios (k(cat)/k(inact)), indicating that the inactivation is nonstereospecific with respect to ring cleavage. The inhibitor, upon incubation with bovine liver ECH, labels a tryptic peptide, ALGGGXEL, near the active site of the protein, where X is the amino acid that is covalently modified. Cloning and sequence analysis of bovine liver ECH gene revealed the identity of the amino acid residue entrapped by MCPF-CoA as Cys-114 (mature sequence numbering). On the basis of gHMQC (gradient heteronuclear multiple quantum coherence) analysis with [3-(13)C]-labeled MCPF-CoA, the ring cleavage is most likely induced by the nucleophilic attack at the terminal carbon of the exomethylene group (C(2)'). We propose a plausible inactivation mechanism that involves relief of ring strain and is consistent with examples found in the literature. In addition, these studies provide important clues for future design of more efficient and selective inhibitors to control and/or regulate fatty acid metabolism.     

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Summary An efficient and economical GC method for rapid determination of FAMEs in rapeseed-mustard is described. The seeds were transmethylated with acetylchloride, using microwave heating and separation achieved on a 3 m column packed with a mixture of 2% SP-2300 and 3% SP-2310 on Chromosorb ‘W’. The method is compared with the conventional heating method and extended efficiently for half-seed analysis. The fatty acid composition of the FAMEs mixtures prepared by both methods was similar with highly significant correlation coefficients (P<0.001).     

Theoretical and experimental investigation of morphology and temperature effects on adsorption of organic vapors in single-walled carbon nanotubes

Hexane adsorption on single-walled carbon nanotube (SWNT) bundles is studied by both simulation and experimentally using a previously developed computer-aided methodology, which employed a smaller physisorbed probe molecule, nitrogen, to explore the porosity of nanotube samples. Configurational-bias grand canonical Monte Carlo simulation of hexane adsorption on localized sites of the bundles is carried out to predict adsorption on their external surface and in their internal sites. These localized isotherms are then combined into a global isotherm for a given sample by using knowledge of its tube-diameter distribution and structural parameters, such as the fraction of open-ended nanotubes and the external surface area of bundles in samples, which have been independently determined from the standard nitrogen adsorption isotherm. The near-perfect replication of experimental isotherms demonstrates the validity of our method for structural characterization of SWNT samples. The effect of temperature on adsorption is also studied and the simulation results are extrapolated to predict the limiting hexane adsorption capacity of the samples. The similarity between the hexane adsorption isotherms and those of other organic molecules demonstrates that the adsorption mechanisms explored here are not specific to hexane, and that the proposed methodology can be potentially applicable to other sorbates with equal success.     

Spectrophotometric Determination of Rhenium(IV) with Thiocyanate,TX-100 and N,N′-Diphenylbenzamidine

A spectrophotometric method to determine rhenium(IV) at trace level is based on the extraction of Re(IV)-SCN^? complex in sulphuric acid media with N,N′-diphenylbenzamidine(DPBA) in presence of a non-ionic surfactant triton X-100 (TX-100) in chloroform. The complex shows maximum absorbance at 435 nm with amolar absorptivity value of 4.24 × 10⁴ L mol^?1 cm^?1 at an acidity range 3.5-6.5 M H₂SO₄. The method followed Beer's Law for the system Re(IV)-SCN^?(TX-100)-DPBA upto 4.0 μg Re(IV) mL^?1. The detection limit of the method is 5 ppb. None of the tested foreign ions, except molybdenum(VI), interfere with the determination of rhenium. The interference due to molybdenum could effectively be removed by prior precipitation with oxine. The effect of various analytical parameters on the extraction of the metal are discussed.     

Cyclization of synthetic cis-1,4-polyisoprene

Cyclization of synthetic cis-1,4-polyisoprene was carried out by using TiCl₄ and p-toluenesulfonic acid catalyst. The average cyclicity and structure of the polymers cyclized to a different degree was studied by infrared and high-temperature NMR spectroscopy. The methods of synthesizing an average mono-, bi-, tri-, or polycyclic polymer that could be distinguished by high-temperature NMR spectroscopy and followed by gel-permeation chromatography are discussed.     

IR absorption in p-type pb0.97Sn0.03Se

IR absorption in p-type melt grown Pb_0.97Sn_0.03Se crystals is reported. The results for the room temperature absorption coefficient (α) in the wavelength region 2–15 μm are analysed. The indirect absorption edge is found to be at 0.26 eV for this ternary alloy. In the longwavelength region α is found to be proportional to λ², in agreement with the classical free carrier absorption expression. The conductivity effective mass of holes is found to be 0.067 m₀ at 300°K.     

Molecular complexity from aromatics: synthesis of highly functionalized spiro δ-lactones

A novel, general, and efficient method has been developed for the synthesis of highly functionalized spiro δ-lactones from aromatic precursors. Our methodology involves a tandem oxidative dearomatization-rearrangement of tertiary furyl carbinols and ring-closing metathesis as key features. This method allows an access to the spirolactones attached to carbocycles of various sizes.     

A practical method,NaOCl-mediated,to prepare thiabendazoles via intramolecular amination reaction

A series of new chlorinated thiabendazoles (6a–m) have been synthesized from readily available anilines and 4-cyanothiazole in moderate to good yields. All synthesized compounds were fully characterized using ¹H NMR, ¹³C NMR, IR, and mass spectrometry. Additionally, the structure of the compound (6f) was confirmed by single-crystal X-ray diffraction. In addition, synthesis of 2-substituted benzimidazoles and 2-phenyl benzothiazole was investigated using our optimized conditions and the outcome is presented herein.