Controlled assembly of carbon nanotubes by designed amphiphilic Peptide helices

Carbon nanotubes have properties potentially useful in diverse electrical and mechanical nanoscale devices and for making strong, light materials. However, carbon nanotubes are difficult to solubilize and organize into architectures necessary for many applications. In the present paper, we describe an amphiphilic alpha-helical peptide specifically designed not only to coat and solubilize carbon nanotubes, but also to control the assembly of the peptide-coated nanotubes into macromolecular structures through peptide-peptide interactions between adjacent peptide-wrapped nanotubes. The data presented herein show that the peptide folds into an amphiphilic alpha-helix in the presence of carbon nanotubes and disperses them in aqueous solution by noncovalent interactions with the nanotube surface. Electron microscopy and polarized Raman studies reveal that the peptide-coated nanotubes assemble into fibers with the nanotubes aligned along the fiber axis. Most importantly, the size and morphology of the fibers can be controlled by manipulating solution conditions that affect peptide-peptide interactions.     

The Pr2Se3-PrSe2 system: Studies of the phase relationships and the modulated crystal structure of PrSe1.85

Thermochemical investigations have been carried out to elucidate the phase relationships in the system Pr₂Se₃-PrSe₂ and to construct the p_Se-T-x diagram. The result of the tensimetric and thermogravimetric studies showed the thermal decomposition of PrSe₂ to be a step-wise process due to the formation of a sequence of discrete intermediate phases with the compositions PrSe_1.9, PrSe_1.85, PrSe_1.8, and Pr₂Se₃. PrSe_1.85 and PrSe_1.8 have been previously considered as selenium-deficient non-stoichiometric phases. Conclusions derived from the thermodynamic study were verified by X-ray diffraction and electron microscopy studies on single crystals of PrSe_1.85 which were obtained from reactions of praseodymium and selenium in stoichiometric amounts in a KCl flux at 1070 K. A two-dimensional modulation could be observed for PrSe_1.85 in X-ray and high-resolution transmission electron microscopy. The structure of PrSe_1.85 was solved and refined in superspace group P4/n()(β-α)00 with lattice parameters of a=4.137(1) Å, c=8.398(2) Å of the basic unit cell and α=β=0.293(1). The origin of the modulation can be attributed to a site occupancy wave and a charge density wave in its planar selenium layer. The experimentally determined magnetic moments indicate Pr³⁺ for the polyselenides PrSe₂, PrSe_1.9 and PrSe_1.85.     

Comproportionation of cationic and anionic tungsten complexes having an N-heterocyclic carbene ligand to give the isolable 17-electron tungsten radical CpW(CO)2(IMes)(•)

A series consisting of a tungsten anion, radical, and cation, supported by the N-heterocyclic carbene 1,3-bis(2,4,6-trimethylphenyl)imidazol-2-ylidene (IMes) and spanning formal oxidation states W(0), W(I), and W(II), has been synthesized, isolated, and characterized. Reaction of the hydride CpW(CO)(2)(IMes)H with KH and 18-crown-6 gives the tungsten anion [CpW(CO)(2)(IMes)](-)[K(18-crown-6)](+). Electrochemical oxidation of [CpW(CO)(2)(IMes)](-) in MeCN (0.2 M (n)Bu(4)N(+)PF(6)(-)) is fully reversible (E(1/2) = -1.65 V vs Cp(2)Fe(+?/0)) at all scan rates, indicating that CpW(CO)(2)(IMes)(?) is a persistent radical. Hydride transfer from CpW(CO)(2)(IMes)H to Ph(3)C(+)PF(6)(-) in MeCN affords [cis-CpW(CO)(2)(IMes)(MeCN)](+)PF(6)(-). Comproportionation of [CpW(CO)(2)(IMes)](-) with [CpW(CO)(2)(IMes)(MeCN)](+) gives the 17-electron tungsten radical CpW(CO)(2)(IMes)(?). This complex shows paramagnetically shifted resonances in the (1)H NMR spectrum and has been characterized by IR spectroscopy, low-temperature EPR spectroscopy, and X-ray diffraction. CpW(CO)(2)(IMes)(?) is stable with respect to disproportionation and dimerization. NMR studies of degenerate electron transfer between CpW(CO)(2)(IMes)(?) and [CpW(CO)(2)(IMes)](-) are reported. DFT calculations were carried out on CpW(CO)(2)(IMes)H, as well as on related complexes bearing NHC ligands with N,N' substituents Me (CpW(CO)(2)(IMe)H) or H (CpW(CO)(2)(IH)H) to compare to the experimentally studied IMes complexes with mesityl substituents. These calculations reveal that W-H homolytic bond dissociation energies (BDEs) decrease with increasing steric bulk of the NHC ligand, from 67 to 64 to 63 kcal mol(-1) for CpW(CO)(2)(IH)H, CpW(CO)(2)(IMe)H, and CpW(CO)(2)(IMes)H, respectively. The calculated spin density at W for CpW(CO)(2)(IMes)(?) is 0.63. The W radicals CpW(CO)(2)(IMe)(?) and CpW(CO)(2)(IH)(?) are calculated to form weak W-W bonds. The weakly bonded complexes [CpW(CO)(2)(IMe)](2) and [CpW(CO)(2)(IH)](2) are predicted to have W-W BDEs of 6 and 18 kcal mol(-1), respectively, and to dissociate readily to the W-centered radicals CpW(CO)(2)(IMe)(?) and CpW(CO)(2)(IH)(?).     

Study on the reactions of 4-amino-5-nitro-6-phenylethynylpyrimidines with amines and thiols

Reactions of 4-amino-5-nitro-6-phenylethynylpyrimidines with amines and thiols have been investigated. Pyridine catalyzes rearrangement of the title compounds into 6-phenyl-7-oxo-7H-pyrrolo[3,2-d]pyrimidine-5-oxides. Primary and secondary amines and thiols take part in a regio- and stereoselective addition reaction to the triple bond of 5-nitro-6-phenylethynylpyrimidines to form the corresponding syn-addition (in the case of secondary amines) or anti-addition (in the case of primary amines or thiols) products.     

Effect of electron-donating and electron-withdrawing groups on peptide/single-walled carbon nanotube interactions

Nano-1, a designed peptide, has been demonstrated to efficiently disperse individual single-walled carbon nanotubes (SWNTs) by folding into an amphiphilic alpha-helix wherein the phenylalanine (Phe) residues on the hydrophobic face of the helix interact via pi-stacking with the aromatic surface of the SWNT. In this study, the ability of electron-donating (hydroxyl) and electron-withdrawing (nitro) groups on the phenyl ring of Phe to affect the interactions between the peptide and SWNTs is examined by substituting the Phe residues in the nano-1 sequence with tyrosine and p-nitro-phenylalanine, respectively. Atomic force microscopy measurements and optical absorption spectroscopy revealed that the ability to disperse individual SWNTs increases with increasing electron density of the aromatic residue on the hydrophobic face of the amphiphilic helical peptides. Scanning tunneling spectroscopy (STS) and Raman analyses were used to examine the effect of noncovalent protein functionalization on the electronic properties of SWNTs. Small shifts in the Raman G band peak for the peptide/SWNT composites, as well as weak features that appear near the Fermi energy (Ef) in the STS dI/dV spectra of the peptide-coated SWNTs, are suggestive of a weak charge-transfer interaction between the peptides and the SWNTs.     

On the nature of bonds between carbon atoms

The combined effect of conjugation and van der Waals' interaction has been calculated for carbon sp²-sp² single bonds of different lengths and for different molecular conformations. The bond shortening effect of conjugation is found to be comparable to that of hybridization. The attractive van der Waals' interaction, on the other hand, is always found to be small. It seems thus most improbable that this interaction should be the cause of the planarity of butadiene.     

Bioassay-directed chemical analysis utilizing LC–MS: a tool for identifying estrogenic compounds in water samples?

A bioassay-directed chemical analysis (BDCA) scheme has been developed which combines a yeast screen for estrogenic activity with LC–MS detection after liquid–liquid extraction and fractionation by size exclusion chromatography. Focusing on sewage-treatment plant (STP) effluents, the approach aims at characterizing the substances responsible for estrogenic effects in aquatic systems. Initial results show a strong response of STP effluent extracts in the yeast screen. Estrone, bisphenol A, and nonylphenol have been identified as substances being partly responsible for observed estrogenic activity. However, confirmation experiments with synthetic samples revealed that the estrogenic effect potentials of the samples could not be completely assigned to specific compounds. Further improvement of the limits of detection of the analytical scheme is needed to enable identification and quantification of potent estrogenic compounds at low concentrations.     

Finite-Volume Discretisations for Flow in Fractured Porous Media

Over the last decades, finite-volume discretisations for flow in porous media have been extended to handle situations where fractures dominate the flow. Successful discretisations have been based on the discrete fracture-matrix models to yield mass conservative methods capable of explicitly incorporating the impact of fractures and their geometry. When combined with a hybrid-dimensional formulation, two central concerns are the restrictions arising from small cell sizes at fracture intersections and the coupling between fractures and matrix. Focusing on these aspects, we demonstrate how finite-volume methods can be efficiently extended to handle fractures, providing generalisations of previous work. We address the finite-volume methods applying a general hierarchical formulation, facilitating implementation with extensive code reuse and providing a natural framework for coupling of different subdomains. Furthermore, we demonstrate how a Schur complement technique may be used to obtain a robust and versatile method for fracture intersection cell elimination. We investigate the accuracy of the proposed elimination method through a series of numerical simulations in 3D and 2D. The simulations, performed on fractured domains containing permeability heterogeneity and anisotropy, also demonstrate the flexibility of the hierarchical framework.