Chemistry Commons^™

Synthesis Of (+)-Decarestrictine L, Julie Lukesh, William A. Donaldson

Chemistry Faculty Research and Publications

A synthesis of (+)-decarestrictine L 1, a cholesterol biosynthesis inhibitory metabolite isolated from Penicillium simplicissimum, is described. Beginning from tri-O-acetyl- -glucal, alkylation with trimethylaluminum introduced the axial methyl group at C-2 in a stereoselective fashion. Chain extension at the C-6 carbon was accomplished by generation of the primary tosylate, followed by displacement with cyanide anion. The synthesis of (+)-1 was completed in 13 steps and 6.3% overall yield.

Nanocomposites Based On Poly (Ε-Caprolactone) (Pcl)/Clay Hybrid: Polystyrene, High Impact Polystyrene, Abs, Polypropylene And Polyethylene, Xiaoxia Zheng, Charles A. Wilkie

Chemistry Faculty Research and Publications

Nanocomposites of polystyrene, high impact polystyrene, acrylonitrile–butadiene–styrene terploymer, polypropylene and polyethylene have been prepared using an organically-modified clay that contains polycaprolactone—PCL-modified clay. Depending upon the mode of preparation of the PCL-modified clay, all three types of nanocomposites, immiscible, intercalated and exfoliated, may be produced. The materials have been characterized by X-ray diffraction, transmission electron microscopy, cone calorimetry, thermogravimetric analysis, and the evaluation of mechanical properties.

Molecular Recognition Of No/No+ Via Multicenter (Charge-Transfer) Binding To Bridged Diarene Donors. Effect Of Structure On The Optical Transitions And Complexation Thermodynamics, Sergiy V. Rosokha, Sergey V. Lindeman, Rajendra Rathore, Jay K. Kochi

Chemistry Faculty Research and Publications

Bridged diarenes form very strong [1:1] complexes with nitrosonium/nitric oxide in which the NO moiety is optimally sandwiched in the cleft between a pair of cofacial aromatic rings which act as a molecular “Venus flytrap”. The spectral features of these associates are generally similar to those for [1:1] and [2:1] nitrosonium complexes with mononuclear alkyl-substituted benzenes, and they are appropriately described within the LCAO molecular-orbital methodology and the Mulliken (charge-transfer) formulation of donor/acceptor electronic transitions. The thermodynamics study indicates that the efficient binding is determined by (i) the close matching of the donor/acceptor redox potentials and (ii) the ability of …

An Xps Investigation Of Thermal Degradation And Charring On Poly(Vinyl Chloride)–Clay Nanocomposites, Jianxin Du, Dongyan Wang, Charles A. Wilkie, Jiangqi Wang

Chemistry Faculty Research and Publications

More information concerning the thermal degradation and charring of nanocomposites of poly(vinyl chloride), dioctyl phthalate and clay has been obtained by the use of X-ray photoelectron spectroscopy and the acquisition of the carbon (C1s), chlorine (Cl2p), and oxygen (O1s) spectra. In the cases of polystyrene–clay and poly(methyl methacrylate)–clay nanocomposites, it has been shown that the clay migrates to the surface as the temperature is raised and the polymer degrades, thereby confirming the barrier properties as a mechanism by which these materials function. For PVC–clay nanocomposites the surface at high temperatures is dominated by carbon, and not the oxygen of the …

A Stibonium-Modified Clay And Its Polystyrene Nanocomposite, Dongyan Wang, Charles A. Wilkie

Chemistry Faculty Research and Publications

Triphenylhexadecylstibonium trifluoromethylsulfonate has been prepared and ion-exchanged with sodium montmorillonite to obtain a new organically-modified clay. The clay has higher thermal stability than an ammonium clay; only a portion of the alkyl chain is lost during degradation and all of the antimony is retained. This clay has been used to prepare a polystyrene nanocomposite in which the clay is not uniformly distributed throughout the polymer. Nonetheless the polymer does insert into the clay layers and the d-spacing of the clay expands from 2.0 to 3.0 nm. The enhanced thermal stability of this system may mean that it could be …

In Situ Reactive Blending To Prepare Polystyrene-Clay And Polypropylene-Clay Nanocomposites, Dongyan Wang, Charles A. Wilkie

Chemistry Faculty Research and Publications

Nanocomposites of polystyrene and polypropylene with organically-modified clay may be prepared by melt blending in a Brabender mixer the clay and the polymer. The presence of maleic anhydride increases the likelihood of nanocomposite formation for polystyrene but is less important for polypropylene. The materials that result are immiscible materials, in that the clay is not uniformly distributed throughout the polymer matrix, but there is polymer inserted between the clay layers. The results from cone calorimetry suggest that nanocomposite formation has occurred, since there is a significant reduction in the peak heat release rate.

Synergy Between Conventional Phosphorus Fire Retardants And Organically-Modified Clays Can Lead To Fire Retardancy Of Styrenics, Grace Chigwada, Charles A. Wilkie

Chemistry Faculty Research and Publications

Polystyrene–clay nanocomposites combined with phosphorous-containing fire retardants have been prepared and used to explore the thermal stability and mechanical properties of the polymer formed. The amounts of fire retardants and clay used were varied to study the effect of each on thermal stability and mechanical properties of the polymer. The samples were prepared by bulk polymerization and analyzed by X-ray diffraction, thermogravimetric analysis, cone calorimetry, Fourier Transform infrared spectroscopy and the evaluation of mechanical properties. The thermal stability of the polymers is enhanced by the presence of the phosphorus-containing fire retardants.

Iron-Mediated Preparation Of Vinylcyclopropanes. Scope, Mechanism, And Applications, Young K. Yun, Kamil Godula, Yeyu Cao, William A. Donaldson

Chemistry Faculty Research and Publications

The addition of stabilized carbon nucleophiles to tricarbonyl(1-methoxycarbonylpentadienyl)iron(1+) cation (1a) proceeds via attack at C2 on the face of the ligand opposite the Fe(CO)₃ group to generate tricarbonyl(pentenediyl)iron complexes 2. Oxidation of complexes 2 affords vinylcyclopropanecarboxylates in good yield. In general, the relative stereochemistry about the cyclopropane ring reflects reductive elimination with retention of configuration. In cases where the C2 substituent is bulky (i.e., 2b) the major cyclopropane product 9b represents ring closure with inversion at C3. A mechanism involving π−σ−π rearrangement of the initially oxidized (pentenediyl)iron species is proposed to account for these results. …

Preparation And Flammability Properties Of Polyethylene-Clay Nanocomposites, Jinguo Zhang, Charles A. Wilkie

Chemistry Faculty Research and Publications

Polyethylene (PE)–clay nanocomposites have been prepared using melt blending in a Brabrender mixer. X-ray diffraction and transmission electron microscopy were used to characterize the nano-structure of these composites while the thermal stability was evaluated from thermogravimetric analysis and the flammability parameters using cone calorimetry. It is found that the PE–clay nanocomposites have a mixed immiscible-intercalated structure and there is better intercalation when maleic anhydride is combined with the polymer and clay to be melt blended. The reduction in peak heat release rate is 30–40%.

Flame Retardancy Of Polystyrene Nanocomposites Based On An Oligomeric Organically-Modified Clay Containing Phosphate, Xiaoxia Zheng, Charles A. Wilkie

Chemistry Faculty Research and Publications

Novel modified clays, which may enable the formation of flame retarded polystyrene nanocomposites by melt or solution blending, have been prepared using an ammonium salt which contains an oligomeric material consisting of vinylbenzyl chloride, styrene and vinyl phosphate reacting with dimethylhexadecylamine. These nanocomposites have been characterized by X-ray diffraction, transmission electron microscopy, thermogravimetric analysis, cone calorimetry and the evaluation of mechanical properties. Melt blending is an effective, economical way to produce intercalated nanocomposites with greatly reduced peak heat release rate and a decreased total heat release; the polymer does not all undergo thermal degradation.

Exfoliated Poly(Methyl Methacrylate) And Polystyrene/ Nanocomposites Occur When The Clay Cation Contains A Vinyl Monomer, Shengpei Su, Charles A. Wilkie

Chemistry Faculty Research and Publications

The bulk polymerization of methyl methacrylate and styrene in the presence of an organically modified clay containing a vinyl group that can be involved in the polymerization produces exfoliated nanocomposites. These nanocomposites have been characterized by X-ray diffraction, transmission electron microscopy, thermogravimetric analysis, mechanical properties, and cone calorimetry. The onset temperature of thermal degradation increases with the mechanical properties. The peak heat release rate is significantly reduced for nanocomposites containing 3 or 5% clay. © 2003 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 41: 1124–1135, 2003

Kinetic And Structural Characterization Of Manganese(Ii)-Loaded Methionyl Aminopeptidases, Ventris M. D'Souza, Sabina I. Swierczek, Nathaniel J. Cosper, Lu Meng, Shane Ruebush, Alicja J. Copik, Robert A. Scott, Richard C. Holz

Chemistry Faculty Research and Publications

Manganese(II) activation of the methionyl aminopeptidases from Escherichia coli (EcMetAP-I) and the hyperthermophilic archaeon Pyrococcus furiosus (PfMetAP-II) was investigated. Maximum catalytic activity for both enzymes was obtained with 1 equiv of Mn(II), and the dissociation constants (K_d) for the first metal binding site were found to be 6 ± 0.5 and 1 ± 0.5 μM for EcMetAP-I and PfMetAP-II, respectively. These K_d values were verified by isothermal titration calorimetry (ITC) and found to be 3.0 ± 0.2 and 1.4 ± 0.2 μM for EcMetAP-I and PfMetAP-II, respectively. The …

Doming Modes And Dynamics Of Model Heme Compounds, Dennis D. Klug, Marek Z. Zgierski, John S. Tse, Zhenxian Liu, James R. Kincaid, Kazimierz Czarnecki, Russell J. Hemley

Chemistry Faculty Research and Publications

Synchrotron far-IR spectroscopy and density-functional calculations are used to characterize the low-frequency dynamics of model heme FeCO compounds. The “doming” vibrational mode in which the iron atom moves out of the porphyrin plane while the periphery of this ring moves in the opposite direction determines the reactivity of oxygen with this type of molecule in biological systems. Calculations of frequencies and absorption intensities and the measured pressure dependence of vibrational modes in the model compounds are used to identify the doming and related normal modes.

The Aminopeptidase From Aeromonas Proteolytica: Structure And Mechanism Of Co-Catalytic Metal Centers Involved In Peptide Hydrolysis, Richard C. Holz

Chemistry Faculty Research and Publications

Enzymes containing multi-metal active sites are central to numerous biological processes and, consequently, characterization of their structure and function is a problem of outstanding importance. One of the least-explored groups of enzymes is the hydrolases that contain dinuclear metal centers. These enzymes play key roles in carcinogenesis, tissue repair, and protein degradation processes. In addition, some of these enzymes can catalyze the hydrolysis of phosphorus(V) compounds found in nerve gases and agricultural neurotoxins. The determination of detailed reaction mechanisms for these enzymes is required for the design of highly potent, specific inhibitors that can function as potential pharmaceuticals. Hydrolytic enzymes …

Enantioselective Synthesis Of The C11–C17 Segment Of Soraphen A1Α Via Organoiron Methodology, Yeyu Cao, Ahmad F Eweas, William A. Donaldson

Chemistry Faculty Research and Publications

The C11–C17 segment of the antifungal agent soraphen A_1α was prepared from glyceraldehyde acetonide in nine steps. The C12 stereocenter is derived from glyceraldehyde, while the C17 stereocenter as introduced by 1,6-asymmetric control via the coordinated Fe(CO)₃.

The C11–C17 segment of the antifungal agent soraphen A1α, with required inverted stereochemistry at C17, was prepared. The C12 stereocenter is derived from glyceraldehyde, while the C17 stereocenter is introduced by 1,6-asymmetric induction via a coordinated Fe(CO)₃.

Crown Ether-Modified Clays And Their Polystyrene Nanocomposites, Hongyang Jao, Jin Zhu, Alexander B. Morgan, Charles A. Wilkie

Chemistry Faculty Research and Publications

Crown ether-modified clays were obtained by the combination of sodium and potassium clays with crown ethers and cryptands. Polystyrene nanocomposites were prepared by bulk polymerization in the presence of these clays. The structures of nanocomposites were characterized by X-ray diffraction and transmission electron microscopy. Their thermal stability and flame retardancy were measured by thermogravimetric analysis and cone calorimetry, respectively. Nanocomposites can be formed only from the potassium clays; apparently the sodium clays are not sufficiently organophilic to enable nanocomposite formation. The onset temperature of the degradation is higher for the nanocomposites compared to virgin polystyrene, and the peak heat release …

The 1.20 Å Resolution Crystal Structure Of The Aminopeptidase From Aeromonas Proteolytica Complexed With Tris: A Tale Of Buffer Inhibition, William Desmarais, David L. Bienvenue, Krzysztof P. Bzymek, Richard C. Holz, Gregory A. Petsko, Dagmar Ringe

Chemistry Faculty Research and Publications

The aminopeptidase from Aeromonas proteolytica (AAP) is a bridged bimetallic enzyme that removes the N-terminal amino acid from a peptide chain. To fully understand the metal roles in the reaction pathway of AAP we have solved the 1.20 Å resolution crystal structure of native AAP (PDB ID = 1LOK). The high-quality electron density maps showed a single Tris molecule chelated to the active site Zn²⁺, alternate side chain conformations for some side chains, a sodium ion that mediates a crystal contact, a surface thiocyanate ion, and several potential hydrogen atoms. In addition, the high precision of the atomic …

Synthetic Studies Directed Toward The Phorboxazoles: Preparation Of The C3–C15 Bisoxane Segment And Two Stereoisomers, Patrick Bernard Greer, William A. Donaldson

Chemistry Faculty Research and Publications

A synthetic approach to the C3–C15 segment of the cytotoxic marine metabolite phorboxazoles is described. This segment consists of a methylene linked bisoxane structure. The first pyran ring was constructed by a Lewis acid catalyzed diene–aldehyde cyclocondensation. The β-C-glucoside substitution pattern of this ring was established by a stereoselective allylation. Ozonolysis of vinyl group and enantioselective allylation of the racemic aldehyde generated two separable homoallylic alcohols (−)-22 and (+)-23. The Mosher's esters of each alcohol were determined to be >90% de. Reaction of (−)-22 with acryloyl chloride, followed by ring closing metathesis gave the …

Synthesis Of Cyclopropanes Via Organoiron Methodology: Stereoselective Preparation Of Cis-2-(2’-Carboxycyclopropyl)Glycine, Nathaniel J Wallock, William A. Donaldson

Chemistry Faculty Research and Publications

A stereoselective route to cis-2-(2′-carboxycyclopropyl)glycine has been developed. exo-Nucleophilic addition to the (bicyclo[5.1.0]octadienyl)iron(1+) cation establishes the relative stereochemistry at the cyclopropane ring and the α-stereocenter. Subsequent removal of the metal and cleavage of the cyclic diene gave the protected target 10, which upon hydrolysis gave 1.

A stereoselective route to cis-2-(2′-carboxycyclopropyl)glycine has been developed. exo-Nucleophilic addition to the (bicyclo[5.1.0]octadienyl)iron(1+) cation establishes the relative stereochemistry at the cyclopropane ring and the α-stereocenter.

Electrochemistry And Spectroscopy Of Sulfate And Thiosulfate Complexes Of Iron Porphyrins, Philip Walter Crawford, Michael D. Ryan

Chemistry Faculty Research and Publications

The electrochemical and spectroscopic properties of the complex formed by the addition of thiosulfate to ferric porphyrins were examined. The NMR spectrum of the thiosulfate–ferric porphyrin complex was consistent with a high-spin ferric complex, while the EPR spectrum at liquid nitrogen temperatures indicated that the complex under these conditions was low-spin. Such behavior has been previously observed for other ferric porphyrin complexes. The visible spectra were characterized by a shift in the Soret band to higher energies, with smaller changes in the longer wavelength region. The complex was reasonably stable in DMF, but slowly reduced over several hours to Fe …

Molecular Structures Of The Metastable Charge-Transfer Complexes Of Benzene (And Toluene) With Bromine As The Pre-Reactive Intermediates In Electrophilic Aromatic Bromination, Alexandr V. Vasilyev, Sergey V. Lindeman, Jay K. Kochi

Chemistry Faculty Research and Publications

Successful crystallization and X-ray crystallographic analyses of the highly metastable (1∶1) complexes of bromine with benzene and toluene establish the unique (localized) structure B that differs in notable ways from the long-accepted (delocalized) structure A. Furthermore, we demonstrate the (highly structured) charge-transfer complexes [C₆H₆,Br₂] and [CH₃C₆H₅,Br₂] to be the pre-reactive intermediates that are converted (via an overall Br⁺ transfer) to the Wheland intermediates in electrophilic aromatic bromination. The role of the dative ion pairs [C₆H₆˙⁺ Br₂˙ …

Overexpression And Divalent Metal Binding Properties Of The Methionyl Aminopeptidase From Pyrococcus Furiosus, Lu Meng, Shane Ruebush, Ventris M. D'Souza, Alicja J. Copik, Susumu Tsunasawa, Richard C. Holz

Chemistry Faculty Research and Publications

The gene encoding for the methionyl aminopeptidase from the hyperthermophilic archaeon Pyrococcus furiosus (PfMetAP-II; EC 3.4.11.18) has been inserted into a pET 27b(+) vector and overexpressed in Escherichia coli. The new expression system resulted in a 5-fold increase in purified enzyme obtained from a 5 L fermentor growth. The as-purified PfMetAP-II enzyme, to which no exogenous metal ions or EDTA was added, was found to have 1.2 equiv of zinc and 0.1 equiv of iron present by ICP-AES analysis. This enzyme had a specific activity of 5 units/mg, a 60-fold decrease from the fully loaded Fe(II) …

Silicon-Methoxide-Modified Clays And Their Polystyrene Nanocomposites, Jin Zhu, Paul Start, Kenneth A. Mauritz, Charles A. Wilkie

Chemistry Faculty Research and Publications

Silicon-methoxide-containing modified clays were obtained through cationic exchange between the sodium clay and the ammoniumcation of [3-(trimethoxysilyl) propyl]octadecyldimethylammoniumchloride (Si18). The nanocomposites were prepared through bulk polymerization of styrene in which the Si18 clay was dispersed. The silicon-methoxide offers the possibility of reaction between the methoxide and a clay hydroxyl group to link together the cation and the clay. The nanocomposites were characterized by X-ray diffraction, transmission electron microscopy, and atomic force microscopy. Their thermal stability and flame retardancy were measured by thermogravimetric analysis (TGA) and cone calorimetry. Linkage between the silicon and the clay apparently occurs in the clay but …

Fire Retardant Halogen-Antimony-Clay Synergism In Polypropylene Layered Silicate Nanocomposites, Marco Zanetti, Giovanni Camino, Domenico Canavese, Alexander B. Morgan, Frank K. Lamelas, Charles A. Wilkie

Chemistry Faculty Research and Publications

Nanocomposites of polypropylene-graft-maleic anhydride with organically modified clays have been prepared and characterized by X-ray diffraction and transmission electron microscopy. Their combustion behavior has been evaluated using oxygen consumption cone calorimetry. Synergy is observed between the nanocomposite formed and conventional vapor phase fire retardants, such as the combination of decabromodiphenyloxide and antimony oxide. The presence of bromine and antimony does not affect the heat release rate curves of the virgin polymer.

A Comparison Of Various Methods For The Preparation Of Polystyrene And Poly(Methyl Methacrylate) Nanocomposites, Dongyan Wang, Jin Zhu, Qiang Yao, Charles A. Wilkie

Chemistry Faculty Research and Publications

Polymer−clay nanocomposites of styrene and methyl methacrylate have been prepared by bulk, solution, suspension, and emulsion polymerization as well as by melt blending. Two different organic modifications of montmorillonite have been used:  one contains a styryl monomer on the ammonium ion while the other has no double bond. The organic modification as well as the mode of preparation determines if the material will be exfoliated or intercalated. Exfoliation is more likely to occur if the ammonium ion contains a double bond which can participate in the polymerization reaction, but the mere presence of this double bond is not sufficient to …

Preparation Of Pvc- Clay Nanocomposites By Solution Blending, Dongyan Wang, Charles A. Wilkie

Chemistry Faculty Research and Publications

PVC-clay nanocomposites have been prepared by solution blending of the polymer with both sodium montmorillonite and an organically modified clay. The nanocomposites have been characterized by X-ray diffraction and transmission electron microscopy; in all cases the materials are best described as mixed immiscible—intercalated nanocomposites, Thermogravimetric analysis shows that, in some cases, the onset temperature of the degradation is enhanced, and the fraction of nonvolatile residue is always decreased. This decrease in the amount of char indicates a significant change in the degradation pathway of the polymer, caused by the presence of the clay. For some preparations, the tensile strength and …

Three-Dimensional Structure Of The Complexin/Snare Complex, Xiaocheng Chen, Diana R. Tomchick, Evgenii L. Kovrigin, Demet Araç, Mischa Machius, Thomas C. Südhof, Josep Rizo

Chemistry Faculty Research and Publications

During neurotransmitter release, the neuronal SNARE proteins synaptobrevin/VAMP, syntaxin, and SNAP-25 form a four-helix bundle, the SNARE complex, that pulls the synaptic vesicle and plasma membranes together possibly causing membrane fusion. Complexin binds tightly to the SNARE complex and is essential for efficient Ca²⁺-evoked neurotransmitter release. A combined X-ray and TROSY-based NMR study now reveals the atomic structure of the complexin/SNARE complex. Complexin binds in an antiparallel α-helical conformation to the groove between the synaptobrevin and syntaxin helices. This interaction stabilizes the interface between these two helices, which bears the repulsive forces between the apposed membranes. These results …

Hydrolysis Of Thionopeptides By The Aminopeptidase From Aeromonas Proteolytica:  Insight Into Substrate Binding, David L. Bienvenue, Danuta M. Gilner, Richard C. Holz

Chemistry Faculty Research and Publications

A series of l-leucine aniline analogues were synthesized that contained either a carbonyl or thiocarbonyl as a part of the amide bond. Additionally, the para-position on the phenyl ring of several substrates was altered with various electron-withdrawing or donating groups. The kinetic constants K_m and k_cat were determined for the hydrolysis of each of these compounds in the presence of the aminopeptidase from Aeromonas proteolytica (AAP) containing either Zn(II) or Cd(II). The dizinc(II) form of AAP ([ZnZn(AAP)]) was able to cleave both carbonyl and thiocarbonyl containing peptide substrates with similar efficiency. However, the dicadmium(II) form of AAP …

Steric Hindrance As A Mechanistic Probe For Olefin Reactivity:  Variability Of The Hydrogenic Canopy Over The Isomeric Adamantylideneadamantane/Sesquihomoadamantene Pair (A Combined Experimental And Theoretical Study), Rajendra Rathore, Sergey V. Lindeman, C.-J. Zhu, T. Mori, R. Schleyer, Jay K. Kochi

Chemistry Faculty Research and Publications

Access to each CC face of adamantylideneadamantane (AA) and sesquihomoadamantene (SA) is hindered by the hydrogenic canopy consisting of four β-hydrogens; otherwise, these olefins have quite normal environments. X-ray crystallography and density functional (DFT) calculations show a 0.5 Å larger annular opening in the protective cover of AA than that in SA. This contributes to the remarkable differences in reactivity toward various reagents, not only by limiting access to the olefin site in SA but also by inhibiting reactions which force these hydrogens closer together. Thus, AA is subject to typical olefin-addition reactions with bromine, …

Novel Bis-Arene (Sandwich) Complexes With No+ Acceptor. Isolation, X-Ray Crystallography And Electronic Structure, Sergiy V. Rosokha, Sergey V. Lindeman, Jay K. Kochi

Chemistry Faculty Research and Publications

The unusual charge-transfer complexes of various arene donors (ArH) with the nitrosonium cation (NO⁺) resulting from bimolecular [1 ∶ 1] associations can be extended at suitably high ArH concentrations to termolecular processes leading to the analogous [2 ∶ 1] complexes. Spectral analyses of the intense color changes accompanying the arene interaction with NO⁺ provide optimum conditions for the isolation of pure crystalline ternary complexes. Single crystal X-ray crystallographic determinations establish the unique sandwich structure consisting of the NO moiety interposed (parallel) between a pair of cofacial arene donors—reminiscent of the well-known transition metal sandwich complexes with aromatic …