Crystalline titanium dichloride—an active catalyst in ethylene polymerization. I. Catalyst activation

Crystalline titanium dichloride, in the absence of organometallic cocatalyst, is a very poor catalyst for the polymerization of ethylene. It is transformed into a very active catalyst through mechanical activation (ball-milling). This catalyst is active in the absence not only of organometallic cocatalysts, but also metals and compounds (such as aluminium and AlCl₃) capable of forming organometallic compounds in situ (i.e., with ethylene, before polymerization starts). Ball-milling causes not only the expected increase in surface area but also disproportionation of Ti⁺⁺ to Ti⁺⁺⁺ and metallic titanium, as well as a crystal phase change to a structure not previously identified with those of TiCl₂ or TiCl₃. Catalyst activity (polymerization rate) is shown to be proportional to surface area and a direct function of Ti⁺⁺ content of the catalyst; an empirical equation relates catalyst activity to surface area and to Ti⁺⁺ lost through disproportionation. Titanium trichloride was found to be inactive in the absence of organometallic cocatalyst, even after ball-milling. The difference in structure of the catalytically active species in the conventional Ziegler (organometallic cocatalyst) and in the titanium dichloride catalyst are discussed. The mechanism of polymerization is compared with that of the supported (CrO₃ on SiO₂/Al₂O₃ and MoO₃ on Al₂O₃) catalyst systems.     

Redox buffering of hypochlorous acid by thiocyanate in physiologic fluids

The major antimicrobial products of neutrophilic myeloperoxidase (MPO) in physiologic fluids are hypochlorous acid (HOCl) and hypothiocyanite (OSCN-), and the former is generally believed to be the killing agent. However, we have determined that HOCl oxidizes SCN- in a facile nonenzymic reaction. The observed kinetics and computational models substantiate the hypothesis that SCN- serves to moderate the potential autotoxicity of HOCl by restricting its lifetime in physiologic fluids. Furthermore, the oxidizing equivalents of HOCl are preserved in OSCN-, a more discriminate biocide that is not lethal to mammalian cells.     

Fluorescence properties of recombinant tropomyosin containing tryptophan, 5-hydroxytryptophan and 7-azatryptophan

Tropomyosin mutants containing either tryptophan (122W), 5-hydroxytryptophan (5OH122W) or 7-azatryptophan (7N122W) have been expressed in Escherichia coli and their fluorescence properties studied. The fluorescent amino acids were located at position 122 of the tropomyosin primary sequence, corresponding to a solvent-exposed position c of the coiled-coil heptapeptide repeat. The emission spectrum of the probe in each mutant is blue-shifted slightly with respect to that of the probe in water. The fluorescence anisotropy decays are single exponential, with a time constant of 2-3 ns while the fluorescence lifetimes of the probes incorporated into the proteins, in water, are nonexponential. Because tryptophan in water has an intrinsic nonexponential fluorescence decay, it is not surprising that the fluorescence decay of 122W is well described by a triple exponential. The fluorescence decays in water of the nonnatural amino acids 5-hydroxytryptophan and 7-azatryptophan (when emission is collected from the entire band) are single exponential. Incorporation into tropomyosin induces triple-exponential fluorescence decay in 5-hydroxytryptophan and double-exponential fluorescence decay in 7-azatryptophan. The range of lifetimes observed for 5-hydroxyindole and 5-hydroxytryptophan at high pH and in the nonaqueous solvents were used as a base with which to interpret the lifetimes observed for the 5OH122W and indicate that the chromophore exists in several solvent environments in both its protonated and unprotonated forms in 5OH122W.     

Laryngeal function in wind instrumentalists: The woodwinds

Utilizing fiberoptic laryngoscopy, we describe the dynamics of laryngeal action in woodwind instrumentalists. This laryngeal function included patterns of vocal fold motion and changes in laryngeal postures which vary with the type of instrument, muscial style of playing, and individual differences of performers.     

Adaptive polynomial preconditioning for hermitian indefinite linear systems

This paper explores the use of polynomial preconditioned CG methods for hermitian indefinite linear systems,Ax=b. Polynomial preconditioning is attractive for several reasons. First, it is well-suited to vector and/or parallel architectures. It is also easy to employ, requiring only matrix-vector multiplication and vector addition. To obtain an optimum polynomial preconditioner we solve a minimax approximation problem. The preconditioning polynomial,C(), is optimum in that it minimizes a bound on the condition number of the preconditioned matrix,C(A)A. We also characterize the behavior of this minimax polynomial, which makes possible a thorough understanding of the associated CG methods. This characterization is also essential to the development of an adaptive procedure for dynamically determining the optimum polynomial preconditioner. Finally, we demonstrate the effectiveness of polynomial preconditioning in a variety of numerical experiments on a Cray X-MP/48. Our results suggest that high degree (20–50) polynomials are usually best.This research was supported in part by the Applied Mathematical Sciences subprogram of the Office of Energy Research, U.S. Dept. of Energy, by Lawrence Livermore National Laboratory under contract W-7405-ENG-48.This research was supported in part by the Dept. of Energy and the National Science Foundation under grant DMS 8704169.This research was supported in part by U.S. Dept. of Energy grant DEFG02-87ER25026 and by National Science Foundation grant DMS 8703226.     

Iodinated polyesters as a versatile platform for radiopaque biomaterials

The design of polymeric biomaterials with long‐lasting X‐ray contrast could advance safe and effective implants and contrast agents. Herein, a new set of wholly aliphatic, iodinated polyesters are synthesized and evaluated as high‐contrast biomaterials and nanoparticle contrast agents for general computed tomography imaging. A single iodinated monomer is used to synthesize a variety of aliphatic polyesters with tunable thermal and mechanical properties. These iodinated polyesters are end‐functionalized with a photocurable methacrylate group, which allows easy processability. The resulting materials exhibit no cytotoxicity and are radiopaque, containing over 40% iodine by weight after processing. The polymers can be formulated into lipid–polymer hybrid nanoparticles using a modified nanoprecipitation method. Initial studies indicate that these nanoparticles show good continual contrast over 60 minutes with no uptake into the kidneys. The work presented here illustrates a novel platform for iodinated polyesters that exhibit high radiopacity and processability, low cost, and no cytotoxicity. © 2017 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2017 , 55, 2171–2177     

Insights into the binding of GABA to the insect RDL receptor from atomistic simulations: a comparison of models

The resistance to dieldrin (RDL) receptor is an insect pentameric ligand-gated ion channel (pLGIC). It is activated by the neurotransmitter γ-aminobutyric acid (GABA) binding to its extracellular domain; hence elucidating the atomistic details of this interaction is important for understanding how the RDL receptor functions. As no high resolution structures are currently available, we built homology models of the extracellular domain of the RDL receptor using different templates, including the widely used acetylcholine binding protein and two pLGICs, the Erwinia Chrysanthemi ligand-gated ion channel (ELIC) and the more recently resolved GluCl. We then docked GABA into the selected three dimensional structures, which we used as starting points for classical molecular dynamics simulations. This allowed us to analyze in detail the behavior of GABA in the binding sites, including the hydrogen bond and cation-π interaction networks it formed, the conformers it visited and the possible role of water molecules in mediating the interactions; we also estimated the binding free energies. The models were all stable and showed common features, including interactions consistent with experimental data and similar to other pLGICs; differences could be attributed to the quality of the models, which increases with increasing sequence identity, and the use of a pLGIC template. We supplemented the molecular dynamics information with metadynamics, a rare event method, by exploring the free energy landscape of GABA binding to the RDL receptor. Overall, we show that the GluCl template provided the best models. GABA forming direct salt-bridges with Arg211 and Glu204, and cation-π interactions with an aromatic cage including Tyr109, Phe206 and Tyr254, represents a favorable binding arrangement, and the interaction with Glu204 can also be mediated by a water molecule.     

Bioorganic Chemistry of Hypothiocyanite

Abstract

Hypothiocyanite (OSCN^?) plays an important role in the human host defense system as a nonimmunological antimicrobial agent. Although many conjugate reactions of proteins have been attributed to OSCN^?, there is little precedence for such reactions in small-molecule chemistry. We will discuss the derivative species that are in equilibrium with OSCN^?, including hypothiocyanous acid (HOSCN), thiocyanogen [(SCN) ₂ ], and trithiocyanate [(SCN) ₃ ^?], the first organic derivatives of this mixture to be fully characterized, and we will describe a new method of synthesizing hypothiocyanite.     

The mechanical efficiency of natural materials

The materials of nature, for example cellulose, lignin, keratin, chitin, collagen and hydroxyapatite, and the structures made from them, for example bamboo, wood, antler and bone, have a remarkable range of mechanical properties. These can be compared by presenting them as material property charts, well known for the materials of engineering. Material indices (significant combinations of properties) can be plotted on to the charts, identifying materials with extreme values of an index, suggesting that they have evolved to carry particular modes of loading, or to sustain large tensile or flexural deformations, without failure. This paper describes a major revision and update of a set of property charts for natural material published some 8 years ago by Ashby et al. with examples of their use to study mechanical efficiency in nature.