Chemistry Commons^™

A Tga/Ftir And Mass Spectral Study On The Thermal Degradation Of Bisphenol A Polycarbonate, Bok Nam Jang, Charles A. Wilkie

Chemistry Faculty Research and Publications

The thermal degradation of polycarbonate under nitrogen was studied using TGA/FTIR, GC/MS and LC/MS as a function of mass loss. The gases evolved during degradation were inspected by in situ FTIR and then the evolved products were collected and analysed using FTIR, GC–MS and LC–MS. The structures of the evolved products are assigned on the basis of FTIR and GC/MS results. The main thermal degradation pathways follow chain scission of the isopropylidene linkage, and hydrolysis/alcoholysis and rearrangement of carbonate linkages. In the case of chain scission, it was proposed that methyl scission of isopropylidene occurs first, according to the bond …

Function Of The Signal Peptide And N- And C-Terminal Propeptides In The Leucine Aminopeptidase From Aeromonas Proteolytica, Krzysztof P. Bzymek, Ventris M. D'Souza, Guanjing Chen, Heidi Campbell, Alice Mitchell, Richard C. Holz

Chemistry Faculty Research and Publications

The leucine aminopeptidase from Aeromonas proteolytica (also known as Vibrio proteolyticus) (AAP) is a metalloenzyme with broad substrate specificity. The open reading frame (ORF) for AAP encodes a 54 kDa enzyme, however, the extracellular enzyme has a molecular weight of 43 kDa. This form of AAP is further processed to a mature, thermostable 32 kDa form but the exact nature of this process is unknown. Over-expression of different forms of AAP in Escherichia coli (with AAP's native leader sequence, with and without the N- and/or C-terminal propeptides, and as fusion protein) has allowed a model for the processing of …

Halide Recognition Through Diagnostic “Anion–Π” Interactions: Molecular Complexes Of CL−, Br−, And I− With Olefinic And Aromatic Π Receptors, Yaroslav S. Rosokha, Sergey V. Lindeman, Sergiy V. Rosokha, Jay K. Kochi

Chemistry Faculty Research and Publications

Intense colorations and new charge-transfer absorption bands are observed upon addition of a halide (Cl⁻, Br⁻, I⁻) to neutral organic π acceptors with electron-deficient olefinic and aromatic centers. These phenomena results from noncovalent anion–π interactions (shown schematically), which were confirmed by X-ray crystallography.

The Catalytic Role Of Glutamate 151 In The Leucine Aminopeptidase From Aeromonas Proteolytica, Krzysztof P. Bzymek, Richard C. Holz

Chemistry Faculty Research and Publications

Glutamate 151 has been proposed to act as the general acid/base during the peptide hydrolysis reaction catalyzed by the co-catalytic metallohydrolase from Aeromonas proteolytica (AAP). However, to date, no direct evidence has been reported for the role of Glu-151 during catalytic turnover by AAP. In order to elucidate the catalytic role of Glu-151, altered AAP enzymes have been prepared in which Glu-151 has been substituted with a glutamine, an alanine, and an aspartate. The Michaelis constant (K_m) does not change upon substitution to aspartate or glutamine, but the rate of the reaction changes drastically in the following …

Characterization Of Epoxy Resin (Su-8) Film Using Thickness-Shear Mode (Tsm) Resonator Under Various Conditions, Jeanne Hossenlopp, Lizhong Jiang, Richard Cernosek, Fabien Josse

Chemistry Faculty Research and Publications

Characterization of an epoxy resin film, commonly known as SU-8, is presented using thickness shear mode (TSM) quartz resonator. The impedance-admittance characteristics of the equivalent circuit models of the unperturbed and coated resonators are analyzed to extract the storage modulus and loss modulus (G' and G''). Those parameters are needed to establish SU-8 film as an effective wave-guiding layer in the implementation of guided shear-horizontal surface acoustic wave (SH-SAW) sensor platforms. Both cured and uncured polymer films are studied at the fundamental and third harmonic frequencies of the TSM resonators. The storage modulus (G') and loss …

Polystyrene Magadiite Nanocomposites, Dongyan Wang, David D. Jiang, Jaclyn Pabst, Zhidong Han, Jianqi Wang, Charles A. Wilkie

Chemistry Faculty Research and Publications

An organically modified magadiite has been prepared and used to make a mixed intercalated-exfoliated polystyrene nanocomposite by bulk polymerization. This system gives excellent improvement in mechanical properties, but the thermogravimetric analysis curves do not show any change in the onset of the degradation and the degradation pathway is not changed from that for virgin polystyrene, unlike the situation for an aluminosilicate clay, monlmorillonite. By cone calorimetry, the peak heat release rate is not changed, again unlike the results with the aluminosilicate. This suggests that not all clays exhibit the same behavior in nanocomposite formation.

Kinetic Analysis Of The Thermal Degradation Of Polystyrene-Montmorillonite Nanocomposite, Serge Bourbigot, Jeffery W. Gilman Sr., Charles A. Wilkie

Chemistry Faculty Research and Publications

Nanocomposites exhibit a combination of unique properties, such as increased heat distortion temperature, reduced permeability, reduced flammability and improved mechanical properties. In this work, a polystyrene (PS) clay nanocomposite was prepared via bulk polymerization using a novel organically modified montmorillonite (MMT). The organic-modifier is the N,N-dimethyl-n-hexadecyl-(4-vinylbenzyl) ammonium chloride (VB16). The thermal stability of PS–VB16 compared to pure PS is examined in pyrolytic and thermo-oxidative conditions. It is then studied using a kinetic analysis. It is shown that the stability of PS is significantly increased in the presence of clay. The thermal behavior of PS and …

Arenediazonium Salts As Versatile (Meso-Ionic) Tectons. Charge-Transfer Intercalates And Clathrates In Unusual Crystalline Networks Self-Assembled With Neutral Aromatic Π-Donors, Sergey V. Lindeman, Jay K. Kochi

Chemistry Faculty Research and Publications

Synopsis

Arenediazonium salts as tectonic building elements for crystal engineering possess a unique array of binding properties, including (a) strong acceptor affinity toward electronegative or neutral σ-donors; (b) enhanced C−H acidity for extensive formation of C−H···X bonds; and (c) π-acceptor properties of the electron-deficient arene substituent for charge-transfer complex formation with appropriate aromatic π-donors. Shown: Donor/acceptor intercalate in the layered structure of 3,5(NO₂)C₆H₃N₂⁺ BF₄^- with naphthalene.

Abstract

The surprisingly uniform crystal packings of various arenediazonium salts (ArN₂⁺X^-) largely consist of two-dimensional ionic layers from …

Study On The Thermal Stability Of Polystyryl Surfactants And Its Modified Clay Nanocomposites, Shengpei Su, David D. Jiang, Charles A. Wilkie

Chemistry Faculty Research and Publications

Five oligomeric styrene surfactants, N,N,N-trimethylpolystyrylammonium, N,N-dimethyl-N-benzylpolystyrylammonium, N,N-dimethyl-N-hexadecylpolystyrylammonium, 1,2-dimethyl-3-polystyrylimidazolium, and triphenylpolystyrylphosphonium chlorides were synthesized and used to prepare organically modified clays. Both styrene and methyl methacrylate nanocomposites were prepared by melt blending and the type of nanocomposite was evaluated by X-ray diffraction and transmission electron microscopy. The thermal stability of the organically modified clays and the nanocomposites were studied by thermogravimetric analysis; these systems do give clays which have good thermal stability and may be useful for melt blending with polymers that must be processed at …

Preparation Of Nanocomposites From Styrene And Modified Graphite Oxides, Fawn Marie Uhl, Charles A. Wilkie

Chemistry Faculty Research and Publications

Graphite oxide was prepared and modified with several ammonium salts and these modified graphite oxides were used to prepare nanocomposites with polystyrene by in situ polymerization of styrene monomer and by melt blending with polystyrene. Nanocomposites were characterized by X-ray diffraction, cone calorimetry, thermogravimetric analysis and the evaluation of mechanical properties. Nanocomposites are formed by in situ polymerization but not by melt blending; the graphite oxide undergoes thermal degradation at the temperature of melt blending so nanocomposite formation would be unlikely. Mechanical properties of the melt blended nanocomposites are improved relative to the virgin polystyrene while those prepared by in …

Synthesis And Reactivity Of Tricarbonyl(1-Methoxycarbonyl-5-Phenylpentadienyl)Iron(1+) Cation, Subhabrata Chaudhury, William A. Donaldson, Dennis W. Bennett, Daniel T. Haworth, Tasneem Siddiquee, Jennifer M. Kloss

Chemistry Faculty Research and Publications

Tricarbonyl(1-methoxycarbonyl-5-phenylpentadienyl)iron(1+) hexafluorophosphate (7) was prepared in two steps from tricarbonyl(methyl 6-oxo-2,4-hexadienoate)iron. While addition of carbon and heteroatom nucleophiles to 7 generally occurs at the phenyl-substituted dienyl carbon to afford (2,4-dienoate)iron products, the addition of phthalimide proceeded at C2 to afford a (pentenediyl)iron product (18). Complex 18 was structurally characterized by X-ray diffraction analysis.

The reaction of the title cation with carbon and heteroatom nucleophiles was examined. In general, the products arise from nucleophilic attack at C5 to give E,E- or E,Z-dienoate iron complexes. Addition of phthalimide anion proceeds at C2 …

Polybutadiene Modified Clay And Its Nanocomposites, Shengpei Su, David D. Jiang, Charles A. Wilkie

Chemistry Faculty Research and Publications

A butadiene-modified clay was prepared by ionic exchange between sodium montmorillonite and a butadiene surfactant; the butadiene surfactant was obtained from the reaction of vinylbenzyl chloride grafted polybutadiene with a tertiary amine. Nanocomposites of polystyrene, high impact polystyrene, acrylonitrile–butadiene–styrene terpolymer, poly(methyl methacrylate), polypropylene and polyethylene were prepared by melt blending this modified clay with the virgin polymers. The nanocomposites were characterized by X-ray diffraction, transmission electron microscopy, thermogravimetric analysis, cone calorimetry and the evaluation of mechanical properties. A morphological study of PBD-modified clay–polymer nanocomposites shows that all the composites are immiscible micro-composites. The consistency of the result from XRD and …

Fighting Antibiotic Resistance: The Dape-Encoded N-Succinyl-L, L-Diaminopimelic Acid Desuccinylase From Haemophilus Influenzae In A Dinuclear Metallohydrolase, Nathaniel J. Cosper, David L. Bienvenue, Jacob E. Shokes, Danuta M. Gilner, Takashi Tsukamoto, Robert A. Scott, Richard C. Holz

Chemistry Faculty Research and Publications

No abstract provided.

Reactivity Of (Bicyclo[5.1.0]Octadienyl)Iron(1+) Cations: Application To The Synthesis Of Cis-2-(2’-Carboxycyclopropyl)Glycines, Nathaniel J. Wallock, William A. Donaldson

Chemistry Faculty Research and Publications

The addition of carbon and heteroatom nucleophiles to (bicyclo[5.1.0]octadienyl)Fe(CO)₂L⁺ cations 5 or 8 (L = CO, PPh₃) generally proceeds via attack at the dienyl terminus on the face of the ligand opposite to iron to generate 6-substituted (bicyclo[5.1.0]octa-2,4-diene)iron complexes (11 or 13). In certain cases, these products are unstable with respect to elimination of a proton and the nucleophilic substituent to afford (cyclooctatetraene)Fe(CO)₂L (4 or 7). Decomplexation of 13f, arising from addition of phthalimide to 8, gave N-(bicyclo[5.1.0]octa-3,5-dien-2-yl)phthalimide (19). Oxidative cleavage of 19 (RuCl …

Poly(Methyl Methacrylate), Polypropylene And Polyethylene Nanocomposite Formation By Melt Blending Using Novel Polymerically-Modified Clays, Shengpei Su, David D. Jiang, Charles A. Wilkie

Chemistry Faculty Research and Publications

Two new organically-modified clays that contain an oligomeric styrene or methacrylate have been prepared and used to produce nanocomposites of poly(methyl methacryate), polypropylene and polyethylene. Intercalated nanocomposites and, in some cases, exfoliated or mixed intercalated/exfoliated nanocomposites of all of these polymers have been produced by melt blending in a Brabender mixer. The use of the styrene-containing clay permits the direct blending of the clay with polypropylene, without the usual need for maleation, to produce the nanocomposites. The systems have all been characterized by X-ray diffraction, transmission electron microscopy, thermogravimetric analysis, cone calorimetry and the measurement of mechanical properties. These novel …

Systems-Based Design Of Bi-Ligand Inhibitors Of Oxidoreductases: Filling The Chemical Proteomic Toolbox, Daniel S. Sem, Bonnie Bertolaet, Brian Baker, Edcon Chang, Aurora Demertrina Costache, Stephen Coutts, Qing Dong, Mark Hansen, Victor Hong, Xuemei Huang, Richard M. Jack, Richard Kho, Henk Lang, Chen-Ting Ma, David Meininger, Maurizio Pellacchia, Fabrice Pierre, Hugo Villar, Lin Yu

Chemistry Faculty Research and Publications

Genomics-driven growth in the number of enzymes of unknown function has created a need for better strategies to characterize them. Since enzyme inhibitors have traditionally served this purpose, we present here an efficient systems-based inhibitor design strategy, enabled by bioinformatic and NMR structural developments. First, we parse the oxidoreductase gene family into structural subfamilies termed pharmacofamilies, which share pharmacophore features in their cofactor binding sites. Then we identify a ligand for this site and use NMR-based binding site mapping (NMR SOLVE) to determine where to extend a combinatorial library, such that diversity elements are directed into the adjacent substrate site. …

The Thermal Degradation Of Nanocomposites That Contain An Oligomeric Ammonium Cation On The Clay, Shengpei Su, Charles A. Wilkie

Chemistry Faculty Research and Publications

The thermal degradation of polystyrene, high-impact polystyrene, ABS terpolymer, poly(methyl methacrylate), polypropylene and polyethylene nanocomposites has been studied using thermogravimetric analysis coupled to Fourier transform infrared spectroscopy, TGA/FT-IR. The nanocomposites that have been studied include immiscible, intercalated and exfoliated systems and the evolved gases do not depend upon the type of nanocomposite and are qualitatively similar to those of the virgin polymer. In the case of the styrenics, the presence of clay promotes the production of oligomer, rather than monomer. It is suggested that this change in evolved products may offer an explanation for why some polymers give large reduction …

Novel Polymerically-Modified Clays Permit The Preparation Of Intercalated And Exfoliated Nanocomposites Of Styrene And Its Copolymers By Melt Blending, Shengpei Su, David D. Jiang, Charles A. Wilkie

Chemistry Faculty Research and Publications

Two new organically-modified clays have been made and used to produce nanocomposites of polystyrene, high impact polystyrene and acrylonitrile–butadiene–styrene terploymer. At a minimum, intercalated nanocomposites of all of these polymers have been produced by melt blending in a Brabender mixer and, in some cases, exfoliated nanocomposites have been obtained. The systems have all been characterized by X-ray diffraction, transmission electron microscopy, thermogravimetric analysis, cone calorimetry and the measurement of mechanical properties. These novel new clays open new opportunities for melt blending of polymers with clays to obtain nanocomposites with important properties.

A Carbocation Substituted Clay And Its Styrene Nanocomposite, Jinguo Zhang, Charles A. Wilkie

Chemistry Faculty Research and Publications

A substituted tropylium ion can be ion-exchanged onto montmorillonite to give a novel organically-modified clay. One can prepare a polystyrene nanocomposite of this clay by emulsion, but not bulk, polymerization. This is the first example of a clay that contains a carbocation and its use to prepare a polymer-clay nanocomposite. Both the clay and its nanocomposites exhibit outstanding thermal stability. Characterization by X-ray diffraction, transmission electron microscopy, cone calorimetry, thermogravimetric analysis and the evaluation of mechanical properties shows that a mixed intercalated-exfoliated nanocomposite is obtained.

Far-Infrared Spectroelectrochemistry: A Study Of Linear Molybdenum/Iron/Sulfur Clusters, Michael D. Ryan, Lin Li

Chemistry Faculty Research and Publications

The far-infrared spectroelectrochemistry of linear M/Fe/S (M=Mo, W) complexes was investigated in methylene chloride and dichloroethane. With CsI as spectral windows, bands above 200 cm⁻¹ can be observed in methylene chloride, except for a weak methylene chloride band at 450 cm⁻¹. Substitution of dichloroethane for methylene chloride, solvents of nearly identical electrochemical properties, allows one to observe solute bands in the 450-cm⁻¹ region. The far-infrared spectroelectrochemistry of [MoFe₂S₄Cl₄]²⁻ and its tungsten analogue was investigated. The disappearance of the oxidation bands and the appearance of bands due to the reduced …

Additional Xps Studies On The Degradation Of Poly(Methyl Methacryalte) And Polystyrene Nanocomposites, Jianxin Du, Jianqi Wang, Shengpei Su, Charles A. Wilkie

Chemistry Faculty Research and Publications

XPS studies have been undertaken on exfoliated nanocomposites of polystyrene and poly(methyl methacrylate). One can clearly see that carbon is lost and that oxygen, silicon and aluminum accumulate at the surface of the degrading polymer. The concentration of aluminum at the surface is very low at the beginning of the experiment but makes a large jump at the same temperature at which carbon is lost and oxygen begins to accumulate at the surface. It appears that the ratio of silicon to aluminum changes as the polymer is lost. A brief discussion is given to explain the origin of oxygen at …

Thermal Properties Of Thermoplastics, Charles A. Wilkie, Michael A. Mckinney

Chemistry Faculty Research and Publications

No abstract provided.

Polybutadiene Modified Clay And Its Polystyrene Nanocomposites, Shengpei Su, David D. Jiang, Charles A. Wilkie

Chemistry Faculty Research and Publications

Vinylbenzyl chloride was allowed to react with a low-molecular-weight oligomer of butadiene in order to give a material that could then be used to alkylate an amine to form an ammonium salt. This ammonium salt was ion-exchanged onto a clay to give a new organically modified clay, which was used to prepare polystyrene-clay nanocomposites by solution and bulk polymerization and by melt blending. The nanocomposites were characterized by using X-ray diffraction, transmission electron microscopy, thermogravimetric analysis, cone calorimetry, and the evaluation of mechanical properties. These systems show good nanodispersion and improvement in thermal and mechanical properties. J. Vinyl Addit. Technol. …

Structural Effects Of Carbon Monoxide Coordination To Carbon Centers. Π And Σ Bindings In Aliphatic Acyl Versus Aromatic Aroyl Cations, M. G. Davlieva, Sergey V. Lindeman, Ivan S. Neretin, Jay K. Kochi

Chemistry Faculty Research and Publications

The binding of carbon monoxide to carbon centers has been examined with two series of aromatic and aliphatic oxocarbonium ions that are successfully isolated as crystalline and highly reactive (hygroscopic) aroylium and acylium salts with poorly coordinating counteranions. X-Ray crystallographic analyses at −150 °C afford precise structural parameters for the characteristic linear carbonyl bond (r_CO) and the bond to the carbon centers (r_Cα). The correlations of these structural parameters evaluated for alkyl (Me, Et and i-Pr) and aryl (p-Me, 2,4,6-trimethyl, p-MeO and p-fluorophenyl) oxocarbonium ions with the corresponding carbonyl …

Investigation Of Nanodispersion In Polystyrene-Montmorillonite Nanocomposites By Solid State Nmr, Serge Bourbigot, David L. Vanderhart, Jeffery W. Gilman Sr., Walid Awad, Rick D. Davis, Alexander B. Morgan, Charles A. Wilkie

Chemistry Faculty Research and Publications

Nanocomposites result from combinations of materials with vastly different properties in the nanometer scale. These materials exhibit many unique properties such as improved thermal stability, reduced flammability, and improved mechanical properties. Many of the properties associated with polymer–clay nanocomposites are a function of the extent of exfoliation of the individual clay sheets or the quality of the nanodispersion. This work demonstrates that solid-state NMR can be used to characterize, quantitatively, the nanodispersion of variously modified montmorillonite (MMT) clays in polystyrene (PS) matrices. The direct influence of the paramagnetic Fe3, embedded in the aluminosilicate layers of MMT, on polymer protons within …

The Role Of The Dielectric Barrier In Narrow Biological Channels: A Novel Composite Approach To Modeling Single-Channel Currents, Artem B. Mamonov, Rob D. Coalson, Abraham Nitzan, Maria G. Kurnikova

Chemistry Faculty Research and Publications

A composite continuum theory for calculating ion current through a protein channel of known structure is proposed, which incorporates information about the channel dynamics. The approach is utilized to predict current through the Gramicidin A ion channel, a narrow pore in which the applicability of conventional continuum theories is questionable. The proposed approach utilizes a modified version of Poisson-Nernst-Planck (PNP) theory, termed Potential-of-Mean-Force-Poisson-Nernst-Planck theory (PMFPNP), to compute ion currents. As in standard PNP, ion permeation is modeled as a continuum drift-diffusion process in a self-consistent electrostatic potential. In PMFPNP, however, information about the dynamic relaxation of the protein and the …

Synthesis Of Tin Oxide Nanocrystalline Phases Via Use Of Tin (Ii) Halide Precursors, Hongmei Deng, Frank K. Lamelas, Jeanne Hossenlopp

Chemistry Faculty Research and Publications

Nanocrystalline tin oxides are synthesized via precipitation from heated solutions as well as from a novel above-solution vapor deposition route that occurs at low temperatures and atmospheric pressure. Crystalline phases are characterized via powder X-ray diffraction. Samples precipitated from reactions of SnCl2 are found to exist primarily as mixtures of tetragonal SnO and tetragonal SnO2 or tetragonal SnO2 and tin(II) oxyhydroxide (Sn6O4- (OH)4), depending on reaction conditions. A mixed tin(II)/tin(IV) sample is shown to produce a rarely observed form of the intermediate oxide Sn3O4 upon annealing in air at 600 °C. SnBr2 exclusively forms tetragonal SnO2 via precipitation. Variation in …

Kinetic And Spectroscopic Characterization Of The H178a Methionyl Aminopeptidase From Escherichia Coli, Alicja J. Copik, Sabina I. Swierczek, W. Todd Lowther, Ventris M. D'Souza, Brian W. Matthews, Richard C. Holz

Chemistry Faculty Research and Publications

To gain insight into the role of the strictly conserved histidine residue, H178, in the reaction mechanism of the methionyl aminopeptidase from Escherichia coli (EcMetAP-I), the H178A mutant enzyme was prepared. Metal-reconstituted H178A binds only one equivalent of Co(II) or Fe(II) tightly with affinities that are identical to the WT enzyme based on kinetic and isothermal titration calorimetry (ITC) data. Electronic absorption spectra of Co(II)-loaded H178A EcMetAP-I indicate that the active site divalent metal ion is pentacoordinate, identical to the WT enzyme. These data indicate that the metal binding site has not been affected by altering H178. …

Regio- And Stereoselective Ruthenium Catalyzed Hydrovinylation Of 1,3-Dienes: Application To The Generation Of A 20s-Steroidal Sidechain, Zhengjie He, Chae S. Yi, William A. Donaldson

Chemistry Faculty Research and Publications

The addition of ethylene to 1,3-dienes and 1-vinylcycloalkenes, catalyzed by two ruthenium complexes, proceeds in a regioselective fashion to afford 3-methyl-1,4-dienes as products. For a steroidal-based 1-vinylcycloalkene, the addition is stereospecific, giving a product with a 20(S) configuration.

Co-Catalytic Metallopeptidases As Pharmaceutical Targets, Richard C. Holz, Krzysztof P. Bzymek, Sabina I. Swierczek

Chemistry Faculty Research and Publications

Understanding the reaction mechanism of co-catalytic metallopeptidases provides a starting point for the design and synthesis of new molecules that can be screened as potential pharmaceuticals. Many of the enzymes that contain co-catalytic metallo-active sites play important roles in cellular processes such as tissue repair, protein maturation, hormone level regulation, cell-cycle control and protein degradation. Therefore, these enzymes play central roles in several disease states including cancer, HIV, stroke, diabetes, bacterial infections, neurological processes, schizophrenia, seizure disorders, and amyotrophic lateral sclerosis. The mechanism of AAP, an aminopeptidase from Aeromonas proteolytica, is one of the best-characterized examples of a metallopeptidase …