Chemistry Commons^™

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 …

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 …

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.

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

Thermal Stability And Flame Retardancy Of Poly(Methyl Methacrylate)-Clay Nanocomposites, Jin Zhu, Paul Start, Kenneth A. Mauritz, Charles A. Wilkie

Chemistry Faculty Research and Publications

Three ammonium salts, hexadecylallyldimethyl ammonium chloride (Allyl16), hexadecylvinylbenzyldimethyl ammonium chloride (VB16) and hexadecylvinylbenzyldimethyl ammonium chloride (Bz16) were synthesized and ion exchanged onto montmorillonite. Poly(methyl methacrylate)–clay nanocomposites of all three clays were prepared by bulk polymerization and the resulting nanocomposites were characterized by X-ray diffraction and transmission electron microscopy. The clays which contain a pendant double bond are more likely to give an exfoliated material while that which contains no double bond is intercalated. The thermal stability and flame retardancy were measured by thermogravimetric analysis (TGA) and Cone Calorimetry respectively.

An Xps Study Of The Thermal Degradation Of Polystyrene-Clay Nanocomposites, Jianqi Wang, Jianxin Du, Jin Zhu, Charles A. Wilkie

Chemistry Faculty Research and Publications

X-ray photoelectron spectroscopy, XPS, has been used to examine several polystyrene-clay nanocomposites. The accumulation of oxygen, from the almuniosilicate, on the surface of the polymer was observed, along with the loss of carbon. This confirms that the barrier properties of the clay provide a mechanism by which nanocomposite formation can enhance the fire retardancy of the polymers. No difference is detected depending upon the extent of exfoliation or intercalation of the nanocomposite. #2002 Elsevier Science Ltd. All rights reserved.

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 …

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) …

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 …

An Xps Investigation Of Thermal Degradation And Charring Of Pmma Clay Nanocomposites, Jianxin Du, Jin Zhu, Charles A. Wilkie, Jiangqi Wang

Chemistry Faculty Research and Publications

Poly(methyl methacrylate)–clay nanocomposites have been studied using X-ray photoelectron spectroscopy. It is clear that as the polymer undergoes thermal degradation, the clay accumulates at the surface and the barrier properties which result from this clay accumulation have been described as the reason for the decreased heat release rate for nanocomposites. The surface composition of the clay changes as the nanocomposite is heated and the changes are affected by the organic-modification that were applied to the clay in order to prepare the nanocomposite.

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 …

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.

Three-Dimensional Structure Of The Complexin/Snare Complex, Xiaocheng Chen, Diana R. Tomchick, Evgueni Kovriguine, 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 …

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˙ …

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 …

Polystyrene/Graphite Nanocomposites: Effect On Thermal Stability, Fawn Marie Uhl, Charles A. Wilkie

Chemistry Faculty Research and Publications

Nanocomposites consisting of polymer and clay have been shown to exhibit a significant reduction in flammability and an increase in mechanical properties. This work examines the effect of thermal stability and mechanical properties of nanocomposites prepared from potassium graphite and styrene. Synthesis of nanocomposites was accomplished by using potassium graphite (KC₈) as the initiator in the polymerization of styrene. A slight increase in thermal stability is observed but mechanical properties are decreased.

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 …