Influence of rotation and isotope effects on the dynamics of the N((2)D)+H(2) reactive system and of its deuterated variants

Integral cross sections and thermal rate constants have been calculated for the N((2)D)+H(2) reaction and its isotopic variants N((2)D)+D(2) and the two-channel N((2)D)+HD by means of quasiclassical trajectory and statistical quantum-mechanical model methods on the latest ab initio potential-energy surface [T.-S. Ho et al., J. Chem. Phys. 119, 3063 (2003)]. The effect of rotational excitation of the diatom on the dynamics of these reactions has been investigated and interesting discrepancies between the classical and statistical model calculations have been found. Whereas a net effect of reagent rotation on reactivity is always observed in the classical calculations, only a very slight effect is observed in the case of the asymmetric N((2)D)+HD reaction for the statistical quantum-mechanical method. The thermal rate constants calculated on this Potential-Energy Surface using quasiclassical trajectory and statistical model methods are in good agreement with the experimental determinations, although the latter are somewhat larger. A reevaluation of the collinear barrier of the potential surface used in the present study seems timely. Further theoretical and experimental studies are needed for a full understanding of the dynamics of the title reaction.     

A thiazole-containing tripodal ligand: synthesis, characterization, and interactions with metal ions and matrix metalloproteinases

A new tripodal ligand, tris[2-(((2-thiazolyl)methylidene)amino)ethyl]amine (Tatren), has been synthesized and characterized by NMR, IR, and UV-visible absorbance spectroscopy and elemental analysis. Tatren forms stable complexes with transition metal ions (Zn(2+), 1; Mn(2+), 2; Co(2+), 3) and the alkaline earth metal ions (Ca(2+), 4; Mg(2+), 5). Single-crystal X-ray structures of 1, 2, and 5 revealed six-coordinate chelate complexes with formula [M(Tatren)](ClO(4))(2) in which the metal centers are coordinated by three thiazolyl N atoms and three acyclic imine N atoms. Crystals of 1, 2, and 5 are monoclinic, P2(1)/c space group. Crystals of 4 are triclinic, P space group. The Ca(2+) complex is eight-coordinate with all N atoms of Tatren and one water molecule coordinated to the metal ion. Spectrophotometric titrations show that formation constants for the chelates of metal ions are >1 in methanol. Free Tatren inhibits the catalytic domain of matrix metalloproteinase-13 (MMP-13, collagenase-3) with K(i) = 3.5 +/- 0.6 microM. Molecular mechanics-based docking calculations suggest that one leg of Tatren coordinates to the catalytic Zn(2+) in MMPs-2, -9, and -13 with significant hydrogen bonding to backbone amide groups. High-level DFT calculations suggest that, in the absence of nonbonded interactions between Tatren and the enzyme, the most stable first coordination sphere of the catalytic Zn(2+) is achieved with three imidazolyl groups from His residues and two imine N atoms from one leg of Tatren. While complexes (1-3) do not inhibit MMP-13 to a significant extent, 4 does (K(i) = 30 +/- 10 microM). Hence, this study shows that tripodal chelating ligands of this class and their Ca(2+) complexes have potential as active-site inhibitors for MMPs.     

Intramolecular charge transfer in pyrromethene laser dyes: photophysical behaviour of PM650.

Absorption and fluorescence (steady-state and time-correlated) techniques are used to study the photophysical characteristics of the pyrromethene 650 (PM650) dye. The presence of the cyano group at the 8 position considerably shifts the absorption and fluorescence bands to lower energies with respect to other related pyrromethene dyes; this is attributed to the strong electron-acceptor character of the cyano group, as is theoretically confirmed by quantum mechanical methods. The fluorescence properties of PM650 are intensively solvent-dependent. The fluorescence band is shifted to lower energies in polar/protic solutions, and the evolution of the corresponding wavelength with the solvent is analysed by a multicomponent linear regression. The fluorescence quantum yield and the lifetime strongly decrease in polar/protic solvents, which can be ascribed to an extra nonradiative deactivation, via an intramolecular charge-transfer state (ICT state), favoured in polar media.     

Oxygen/nitrogen ordering in lanthanum new phase (La3Si8N11O4)

High-resolution ¹⁷O NMR spectra have been collected for crystalline samples of lanthanum new phase, La₃Si₈N₁₁O₄. In conjunction with previously published ²⁹Si and ¹⁵N spectra obtained for this phase, and in the light of the high-quality crystal structure data reported recently, a more detailed interpretation of the NMR spectra is presented than was possible in previous studies. The non-bridging oxygens in the structure are responsible for the single sharp peak seen in the ¹⁷O spectrum at 188 ppm; the remaining oxygens, occupying bridging sites shared with nitrogen, show up only weakly on the ¹⁷O spectrum as a broad diffuse band centered around zero ppm. The peak at −57.3 ppm on the ²⁹Si spectrum is believed to correspond to an overlap of [SiN₄] and [SiON₃] environments, with the −68.2 ppm peak corresponding to an [SiO₂N₂] environment.     

Aqueous dispersions of single-wall and multiwall carbon nanotubes with designed amphiphilic polycations

Poor solubility of single-walled and multiwalled carbon nanotubes (NTs) in water and organic solvents presents a considerable challenge for their purification and applications. Macromolecules can be convenient solubilizing agents for NTs and a structural element of composite materials for them. Several block copolymers with different chemical functionalities of the side groups were tested for the preparation of aqueous NT dispersions. Poly(N-cetyl-4-vinylpyridinium bromide-co-N-ethyl-4-vinylpyridinium bromide-co-4-vinylpyridine) was found to form exceptionally stable NT dispersions. It is suggested that the efficiency of macromolecular dispersion agents for NT solubilization correlates with the topological and electronic similarity of polymer-NT and NT-NT interactions in the nanotube bundles. Raman spectroscopy and atomic force and transmission electron microcopies data indicate that the polycations are wrapped around NTs forming a uniform coating 1.0-1.5 nm thick. The ability to wind around the NT originates in the hydrophobic attraction of the polymer backbone to the graphene surface and topological matching. Tetraalkylammonium functional groups in the side chains of the macromolecule create a cloud of positive charge around NTs, which makes them hydrophilic. The prepared dispersions could facilitate the processing of the nanotubes into composites with high nanotube loading for electronic materials and sensing. Positive charge on their surface is particularly important for biological and biomedical applications because it strengthens interactions with negatively charged cell membranes. A high degree of spontaneous bundle separation afforded by the polymer coating can also be beneficial for NT sorting.     

Studies on some experimental parameters of radiometric flow injection analysis (RFIA) using137Cs and60Co

A simple radiometric flow injection analyzer with NaI(Tl) detector was used for the study of some experimental parameters of¹³⁷Cs and⁶⁰Co determination. A sample transfer system was developed employing both merging zone and sample injection principle.     

Towards rod-shaped molecules based on the twelve-vertex ferratricarbollides

The compounds with a single and double -CH₂C₆H₄CH₂- spacer, [CpFeC₃B₈H₁₀-NH-CH₂C₆H₄CH₂-NH-C₃B₈H₁₀FeCp] and [CpFeC₃B₈H₁₀-N-(CH₂C₆H₄CH₂)₂-N-C₃B₈H₁₀FeCp], represent the first example of designed shaping by extremely stable cyclopentadienyl-ferratricarbollide (CpFeTCB) cages into rigid molecular constructions approaching linear arrangement.     

Preparation of hollow beads of poly(styrene-co-divinyl benzene)

Two methods are described for the production of hollow beads by copolymerization of styrene and divinyl benzene. Characterization of the beads is described and growth mechanisms proposed.     

Using wavelets to analyze AFM images of thin films: surface micelles and supported lipid bilayers

This paper presents micro- and nanoanalysis of thin films based on images obtained by atomic force microscopy (AFM). The analysis exploits the discrete wavelet transform and the resulting wavelet spectrum to study surface features. It is demonstrated that the wavelet technique can characterize micro- and nanosurface features and distinguish between similar surface structures. The use of a feature extraction method is shown. The method involves the separation of certain frequency content from the original AFM images and analyzing the data independently to gain quantitative information about the images. By using the feature extraction method, soft surfaces in water are analyzed and nanofeatures are measured. The packing of surface micelles of sodium dodecyl sulfate on a self-assembled monolayer is analyzed. The characteristics of pore formation, due to penetration of the antibacterial peptide protegrin, into a solid-supported lipid bilayer are quantified. The sizes of the pores are obtained, and it is observed that the line tension of the pores reduces the fluctuations of the lipid bilayer.     

An Improved HPLC Post-Column Methodology for the Identification of Free Radical Scavenging Phytochemicals in Complex Mixtures

The importance of natural antioxidants principally relates to their health-promoting properties. The discovery of new sources of established compounds or more potent compounds is a costly exercise and any technique capable of aiding this procedure would be highly significant. An approach combining chromatographic separation, component analysis and post-column identification of free radical scavenging has been reported. However, its effectiveness is dependent upon various factors and the number of samples analyzed without operator intervention is restricted. A more applicable technique using a 10% to 100% methanol with 2% acetic acid mobile phase and a citric acid-sodium citrate buffered methanolic 1,1-diphenyl-2-picrylhydrazyl free radical reagent is presented, whereby free radical scavengers can be detected within crude extracts of variable polarity without special considerations. As an illustration, acetone, 70% (aq.) methanol and acetonitrile-soluble extracts were prepared from sage (Salvia officinalis L.) for extraction of polar and non-polar radical scavengers and analyzed. Radical scavenging components were identified as carnosic, caffeic and rosmarinic acids and luteolin-7-O-glycoside. Others radical scavengers were tentatively identified as benzoic and hydroxycinnamic acid derivatives, flavonoids and diterpenoids. Through the application of this technique, carnosic acid and rosmarinic acid were identified as the principal free radical scavengers.