Home About us Contact
|
Home About us Contact | ||
|
|
||
Trimer
Kinds of Trimer Terms modified by Trimer Selected AbstractsLinear Trimer of Diruthenium Linked by Butadiyn-Diyl Units: A Unique Electronic Wire,ANGEWANDTE CHEMIE, Issue 5 2010Jie-Wen Ying Dr. Die starke elektronische Delokalisierung in der Titelverbindung (Ru,rot, C,grün, N,blau, O,violett), die durch voltammetrische, spektroskopische und magnetische Daten belegt wird, macht sie zu einem molekularen Draht. [source] The Controlled Oxidative Coupling of Ge94- Zintl Anions to a Linear Trimer [Ge9=Ge9=Ge9]6- .CHEMINFORM, Issue 30 2005Li Yong Abstract For Abstract see ChemInform Abstract in Full Text. [source] [Ge9=Ge9=Ge9]6- : A Linear Trimer of 27 Germanium Atoms.CHEMINFORM, Issue 47 2002Angel Ugrinov Abstract For Abstract see ChemInform Abstract in Full Text. [source] ChemInform Abstract: Properties of Photoexcited States of C180, a Triangle Trimer of C60CHEMINFORM, Issue 42 2001Mamoru Fujitsuka Abstract ChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 100 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a "Full Text" option. The original article is trackable via the "References" option. [source] Identification of a Highly Efficient Alkylated Pincer Thioimido,Palladium(II) Complex as the Active Catalyst in Negishi CouplingCHEMISTRY - A EUROPEAN JOURNAL, Issue 17 2009Jing Liu Dr. Abstract PdIIate complex: A novel alkylated pincer thioimido,Pd complex generated from a catalyst precursor and basic organometallic reagents (RM) was observed by in situ IR, 1H,NMR, and 13C,NMR spectroscopies for the first time and proved to be the active catalyst in stoichiometric and catalytic reactions of aryl iodides with RM (see scheme). The catalyst, as an electron-rich PdII species, promoted the Negishi coupling of aryl iodides and alkylzinc reagents with high efficiency, even at low temperatures (0 or ,20,°C). The induction period of Negishi coupling catalyzed by pincer thioamide,palladium complex 1 was investigated. A heterogeneous mechanism was excluded by kinetic studies and comparison with Negishi coupling reactions promoted by Pd(OAc)2/Bu4NBr (a palladium,nanoparticle system). Tetramer 2 was isolated from the reaction of 1 and organozinc reagents. Dissociation of complex 2 by PPh3 was achieved, and the structure of resultant complex 8 was confirmed by X-ray diffraction analysis. A novel alkylated pincer thioimido,PdII complex, 7, generated from catalyst precursor 1 and basic organometallic reagents (RM), was observed by in situ IR, 1H,NMR, and 13C,NMR spectroscopy for the first time. The reaction of 7 with methyl 2-iodobenzoate afforded 74,% of the cross-coupled product, methyl 2-methylbenzoate, together with 60,% of PdII complex 2. Furthermore, the catalyst, as an electron-rich PdII species, efficiently promoted the Negishi coupling of aryl iodides and alkylzinc reagents without an induction period, even at low temperatures (0,°C or ,20,°C). To evaluate the influence of the catalyst structure upon the induction period, complex 9 was prepared, in which the nBu groups of 1 were displaced by more bulky 1,3,5-trimethylphenyl groups. Trimer 10 was isolated from the reaction of complex 9 and basic organometallic reagents such as CyZnCl or CyMgCl (Cy: cyclohexyl); this is consistent with the result obtained with complex 1. The rate in the induction period of the model reaction catalyzed by 9 was faster than that with 1. Plausible catalytic cycles for the reaction, based upon the experimental results, are discussed. [source] The Heteropolytungstate Core {BW13O46}11, Derived as Monomer, Dimer, and TrimerCHEMISTRY - A EUROPEAN JOURNAL, Issue 25 2007Nathalie Leclerc-Laronze Dr. Abstract A study of the borotungstate system has led to the characterization of new, original compounds based on the unconventional Keggin derivative [H3BW13O46]8, ion (denoted as 1). [H3BW14O48]6, (2) and the dimer [H6B2W26O90]12, (3) crystallize as mixed cesium/ammonium salts and have been characterized by single-crystal X-ray diffraction analysis. Anion 2 reveals an unusual arrangement, consisting of an outer {W3O9} core grafted onto the monovacant [BW11O39]9, Keggin moiety and exhibits an unprecedented distorted square-pyramidal arrangement for a cis -{WO2} core. Elemental analysis, supported by bond distance analysis, is consistent with the presence of three protons distributed over the terminal oxygens of the outer {W3O7} capping fragment. The [H6B2W26O90]12, ion (3) is formally derived from the direct condensation of two [H3BW13O46]8, subunits. The cisoid arrangement of the two [BW11O39]9, subunits, coupled with the antiparallel arrangement of the two quasi-linear OW,,,OWOH2 chains within the central {W4O12} connecting group, breaks any symmetry, thereby resulting in a chiral compound. Polarography and pH-metric titrations reveal the formation of the monomeric precursor [H3BW13O46]8, (anion 1) under stoichiometric conditions. 183W NMR analysis of 2 and 3 in solution gives complex spectra, consistent with the presence of equilibria between several species. In the frame of this study, we also report on a structural re-investigation of the [H6B3W39O132]15, ion (4) based on reliable results obtained in the solid state by means of single-crystal X-ray diffraction analysis, and in solution by means of 1D and 2D COSY 183W NMR. X-ray diffraction analysis revealed the presence of three attached aquo ligands on the central {W6O15} connecting core, generating three OW,,,OWOH2 quasi-linear chains, which are responsible for the chirality of the trimeric assembly. This structural arrangement accounts for the 39-line 183W solution spectrum. The 2D COSY spectrum permits reliable assignments of the six strongly shielded resonances (around ,250 and ,400,ppm) to the six central W atoms, as well as additional assignments. The origin of such strong shielding for these particular W atoms is discussed on the basis of previously published results. Infrared data for compounds 1, 3, and 4 are also presented. [source] The application of perfluorooctanoate to investigate trimerization of the human immunodeficiency virus-1 gp41 ectodomain by electrophoresisELECTROPHORESIS, Issue 15 2008Chi-Hui Lin Abstract The transmembrane glycoprotein gp41 of human immunodeficiency virus has been proposed to form trimer-of-hairpin during virus-cell membrane fusion. To investigate its oligomerization propensity under soluble and membrane-mimic conditions, sodium salt of perfluorooctanoate (PFO) was applied. A recombinant gp41 ectodomain devoid of disulfide linkage was overexpressed in Escherichia coli and characterized by MS and circular dichroism spectropolarimetry in PFO solution in comparison to SDS. The helical content of this ectodomain in PFO is higher than that in SDS. Notably, PFO employed in PAGE clearly conduced to the formation of trimer under the optimized condition as visualized in the gel. In addition, the proteins expressed from the two mutants in the heptad repeat (HR) domains of gp41, I62P, and N126K, were also examined by the PFO-PAGE analysis for functional ramification of molecular organization. Remarkably, the I62P mutation completely abolished the gp41 trimer formation, whereas the N126K mutation resulted in a more stable trimer. The data suggested that PFO-PAGE analysis is appropriate for evaluating the effect of mutations on the trimerization of gp41 and other fusion proteins which may be implicated in the alteration of their fusogenicity. [source] Human haptoglobin structure and function , a molecular modelling studyFEBS JOURNAL, Issue 22 2008F. Polticelli Hemoglobin is the most prominent protein in blood, transporting O2 and facilitating reactive oxygen and nitrogen species detoxification. Hemoglobin metabolism leads to the release of extra-erythrocytic hemoglobin, with potentially severe consequences for health. Extra-erythrocytic hemoglobin is complexed to haptoglobin for clearance by tissue macrophages. The human gene for haptoglobin consists of three structural alleles: Hp1F, Hp1S and Hp2. The products of the Hp1F and Hp1S alleles differ by only one amino acid, whereas the Hp2 allele is the result of a fusion of the Hp1F and Hp1S alleles, is present only in humans and gives rise to a longer ,-chain. Haptoglobin consists of a dimer of ,,-chains covalently linked by a disulphide bond between the Cys15 residue of each ,-chain. However, the presence of the Hp1 and Hp2 alleles in humans gives rise to HPT1-1 dimers (covalently linked by Cys15 residues), HPT1-2 hetero-oligomers and HPT2-2 oligomers. In fact, the HPT2 variant displays two free Cys residues (Cys15 and Cys74) whose participation in intermolecular disulphide bonds gives rise to higher-order covalent multimers. Here, the complete modelling of both haptoglobin variants, together with their basic quaternary structure arrangements (i.e. HPT1 dimer and HPT2 trimer), is reported. The structural details of the models, which represent the first complete view of the molecular details of human haptoglobin variants, are discussed in relation to the known haptoglobin function(s). [source] C-terminal truncated cannabinoid receptor 1 coexpressed with G protein trimer in Sf9 cells exists in a precoupled state and shows constitutive activityFEBS JOURNAL, Issue 23 2007Chandramouli Reddy Chillakuri We have investigated the existence of a precoupled form of the distal C-terminal truncated cannabinoid receptor 1 (CB1-417) and heterotrimeric G proteins in a heterologous insect cell expression system. CB1-417 showed higher production levels than the full-length receptor. The production levels obtained in our expression system were double the values reported in the literature. We also observed that at least the distal C-terminus of the receptor was not involved in receptor dimerization, as was predicted in the literature. Using fluorescence resonance energy transfer, we found that CB1-417 and G,i1,1,2 proteins were colocalized in the cells. GTP,S binding assays with the Sf9 cell membranes containing CB1-417 and the G protein trimer showed that the receptor could constitutively activate the G,i1 protein in the absence of agonists. A CB1-specific antagonist (SR 141716A) inhibited this constitutive activity of the truncated receptor. We found that the CB1-417/G,i1,1,2 complex could be solubilized from Sf9 cell membranes and coimmunoprecipitated. In this study, we have proven that the receptor and G proteins can be coexpressed in higher yields using Sf9 cells, and that the protein complex is stable in detergent solution. Thus, our system can be used to produce sufficient quantities of the protein complex to start structural studies. [source] Disulfide bond formation through Cys186 facilitates functionally relevant dimerization of trimeric hyaluronan-binding protein 1 (HABP1)/p32/gC1qRFEBS JOURNAL, Issue 1 2002Babal Kant Jha Hyaluronan-binding protein 1 (HABP1), a ubiquitous multifunctional protein, interacts with hyaluronan, globular head of complement component 1q (gC1q), and clustered mannose and has been shown to be involved in cell signalling. In vitro, this recombinant protein isolated from human fibroblast exists in different oligomeric forms, as is evident from the results of various independent techniques in near-physiological conditions. As shown by size-exclusion chromatography under various conditions and glutaraldehyde cross-linking, HABP1 exists as a noncovalently associated trimer in equilibrium with a small fraction of a covalently linked dimer of trimers, i.e. a hexamer. The formation of a covalently-linked hexamer of HABP1 through Cys186 as a dimer of trimers is achieved by thiol group oxidation, which can be blocked by modification of Cys186. The gradual structural transition caused by cysteine-mediated disulfide linkage is evident as the fluorescence intensity increases with increasing Hg2+ concentration until all the HABP1 trimer is converted into hexamer. In order to understand the functional implication of these transitions, we examined the affinity of the hexamer for different ligands. The hexamer shows enhanced affinity for hyaluronan, gC1q, and mannosylated BSA compared with the trimeric form. Our data, analyzed with reference to the HABP1/p32 crystal structure, suggest that the oligomerization state and the compactness of its structure are factors that regulate its function. [source] Probing the role of oligomerization in the high thermal stability of Pyrococcus furiosus ornithine carbamoyltransferase by site-specific mutantsFEBS JOURNAL, Issue 14 2001Bernard Clantin The Pyrococcus furiosus ornithine carbamoyltransferase (OTCase) is extremely heat stable and maintains 50% of its catalytic activity after 60 min at 100 °C. The enzyme has an unusual quaternary structure when compared to anabolic OTCases from mesophilic organisms. It is built up of four trimers arranged in a tetrahedral manner, while other anabolic enzymes are single trimers. Residues Trp21, Glu25, Met29 and Trp33 are located in the main interfaces that occur between the catalytic trimers within the dodecamer. They participate in either hydrophobic clusters or ionic interactions. In order to elucidate the role played by the oligomerization in the enzyme stability at very high temperatures, we performed mutagenesis studies of these residues. All the variants show similar catalytic activities and kinetic properties when compared to the wild-type enzyme, allowing the interpretation of the mutations solely on heat stability and quaternary structure. The W21A variant has only a slight decrease in its stability, and is a dodecamer. The variants E25Q, M29A, W33A, W21A/W33A and E25Q/W33A show that altering more drastically the interfaces results in a proportional decrease in heat stability, correlated with a gradual dissociation of dodecamers into trimers. Finally, the E25Q/M29A/W33A variant shows a very large decrease in heat stability and is a trimer. These results suggest that extreme thermal stabilization of this OTCase is achieved in part through oligomerization. [source] Inhibitory activity of brown algal phlorotannins against hyaluronidaseINTERNATIONAL JOURNAL OF FOOD SCIENCE & TECHNOLOGY, Issue 6 2002Toshiyuki Shibata The inhibitory effect of brown algal phlorotannins on hyaluronidase was evaluated by an in vitro assay. Crude phlorotannins from the brown algae Eisenia bicyclis and Ecklonia kurome had a stronger inhibitory effect than well-known inhibitors such as catechins and sodium cromoglycate. IC50 values of the following six phlorotannins: phloroglucinol, an unknown tetramer, eckol (a trimer), phlorofucofuroeckol A (a pentamer), dieckol and 8,8,-bieckol (hexamers), were 280, 650, >800, 140, 120 and 40 ,M, respectively. The IC50 of catechin, epigallocatechin gallate and sodium cromoglycate was 620, 190 and 270 ,M, respectively. 8,8,-Bieckol, the strongest HAase inhibitor in this study, acted as a competitive inhibitor with an inhibition constant (Ki) of 35 ,M. Acetylation of the phlorotannins markedly decreased their inhibitory potency. [source] Theoretical study of the oxidative polymerization of aniline with peroxydisulfate: Tetramer formationINTERNATIONAL JOURNAL OF QUANTUM CHEMISTRY, Issue 2 2008-Marjanovi, Gordana Abstract Semi-empirical quantum chemical study of the oxidative polymerization of aniline with ammonium peroxydisulfate, in aqueous solutions without added acid, has been based on the MNDO-PM3 computations of thermodynamic, redox, and acid,base properties of reactive species and the intermediates, combined with the MM2 molecular mechanics force-field method and conductor-like screening model of solvation. The main reaction routes of aniline tetramerization are proposed. The regioselectivity of the formation of aniline tetramers by redox and electrophilic aromatic substitution reactions is analyzed. It was proved that the linear NC4 coupled tetra-aniline is formed as a dominant product by three different pathways: comproportionation redox reaction between N -phenyl-1,4-benzoquinonediimine and 4-aminodiphenylamine, the one-electron oxidation of aniline with its half-oxidized NC4 coupled trimer, and the electrophilic aromatic substitution reaction of aniline with fully oxidized NC4 coupled trianiline nitrenium cation. The electrophilic aromatic substitution reaction of the NC4 coupled aniline trimer with aniline nitrenium cation, as well as the oxidation of aniline with half-oxidized branched trimer, lead to the branched aniline tetramers. The competing character of different tetramerization routes is highlighted. The oxidative intramolecular cyclization of branched oligoanilines and polyaniline, leading to the generation of substituted phenazine units, has been predicted to accompany the classical routes of the polymerization of aniline. Various molecular (branched vs. linear) oligomeric structures produced at different level of acidity during the course of polymerization and their impact on the formation of supramolecular structures of conducting polyaniline (nanorods and nanotubes vs. granular morphology), are discussed. © 2007 Wiley Periodicals, Inc. Int J Quantum Chem, 2008 [source] The Planck,Benzinger thermal work function in the condensation of water vaporINTERNATIONAL JOURNAL OF QUANTUM CHEMISTRY, Issue 15 2006Paul W. Chun Abstract Based on the Planck,Benzinger thermal work function using Chun's method, the innate temperature-invariant enthalpy at 0 K, ,H0(T0), for the condensation of water vapor as well as the dimer, trimer, tetramer, and pentamer form in the vapor phase, was determined to be 0.447 kcal mol,1 for vapor, 1.127 for the dimer, 0.555 for the trimer, 0.236 for the tetramer, and 0.079 kcal mol,1 for the pentamer using ,G(T) data reported by Kell et al. in 1968 and Kell and McLaurin in 1969. These results suggest that the predominant dimeric form is the most stable of these n -mers. Using Nemethy and Scheraga's 1962 data for the Helmholtz free energy of liquid water, the value of ,H0(T0) was determined to be 1.21 kcal mol,1. This is very close to the value for the energy of the hydrogen bond EH of 1.32 kcal mol,1 reported by Nemethy and Scheraga, using statistical thermodynamics. It seems clear that very little energy is required for interconversion between the hypothetical supercooled water vapor and glassy water at 0 K. A hypothetical supercooled water vapor at 0 K is apparently almost as highly associated as glassy water at that temperature, suggesting a dynamic equilibrium between vapor and liquid. This water vapor condensation is highly similar in its thermodynamic behavior to that of sequence-specific pairwise (dipeptide) hydrophobic interaction, except that the negative Gibbs free energy change minimum at ,Ts,, the thermal setpoint for vapor condensation, where T,S = 0, occurs at a considerably lower temperature, 270 K (below 0°C) compared with ,350 K. The temperature of condensation ,Tcond, at which ,G(T) = 0, where water vapor begins to condense, was found to be 383 K. In the case of a sequence-specific pairwise hydrophobic interaction, the melting temperature, ,Tm,, where ,G(Tm) = 0 was found to be 460 K. Only between two temperature limits, ,Th, = 99 K and ,Tcond, = 383 K, where ,G(Tcond) = 0, is the net chemical driving force favorable for polymorphism of glassy water and hypothetical supercooled water vapor. Analysis of the water vapor condensation process based on the Planck,Benzinger thermal work function confirms that a thermodynamic molecular switch occurs at 10 K, wherein a change of sign in [,Cp(T)]cond leads to a true negative minimum in the Gibbs free energy of vapor condensation, and hence a maximum in the related equilibrium constant, Kcond. © 2006 Wiley Periodicals, Inc. Int J Quantum Chem, 2006 [source] Isolation and Characterization of a Porin-Like Outer Membrane Protein from Xanthomonas campestris pv. campestrisIUBMB LIFE, Issue 1 2002Lingyun Wang Abstract Xanthomonas campestris pv. campestris, a plant-associated pathogenic bacterium, is the causal agent of foliar spots and blights in crucifers. The major outer membrane protein, Omp37, of 37 kDa, has been identified, purified to homogeneity, and its characterization has also been carried out. Native Omp37 behaved as a trimer, as revealed by gel filtration and SDS-PAGE. FTIR measurements revealed a high ,-structure content. The pore-forming ability of the purified Omp37 was studied by the liposome swelling assay. Omp37, to our knowledge, is the first porin that has been isolated from Xanthomonas . This study clearly demonstrates that Omp37 is related to the family of trimeric bacterial porins. [source] Polymerization at the gas/solution interface: Preparation of polymer microstructures with gas bubbles as templatesJOURNAL OF APPLIED POLYMER SCIENCE, Issue 4 2007Maciej Mazur Abstract We report on the chemical polymerization of 2-methoxyaniline at the interface between an aqueous solution and air. The polymer is formed in the interfacial region, whereas the soluble trimer is yielded in the bulk of the polymerization solution. The preferential polymerization of 2-methoxyaniline is discussed in terms of monomer and oligomer accumulation at the interface, which influences the reactivity of these species and allows further polymerization. The phenomenon of polymer growth is employed to selectively deposit polymeric material onto glass slides decorated with gas microbubbles. Because of preferential polymerization at the bubble/solution interface, hemispherical features are produced on the surface of glass. When some polymeric material is mechanically removed, microrings or microholes are obtained. The anomalous polymerization of 2-methoxyaniline is compared to that of 2-methylaniline. This monomer polymerizes uniformly within the entire volume of the reaction mixture; thus, no preferential polymer formation at the gas/solution interface is observed. As a result, deposition on microbubble-decorated glass slides produces polymeric films containing a number of microholes. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci 2007 [source] An ab initio study of intermolecular interactions of nitromethane dimer and nitromethane trimerJOURNAL OF COMPUTATIONAL CHEMISTRY, Issue 3 2003Jinshan Li Abstract Different geometries of nitromethane dimer and nitromethane trimer have been fully optimized employing the density functional theory B3LYP method and the 6-31++G** basis set. Three-body interaction energy has been obtained with the ab initio supermolecular approach at the levels of MP2/6-31++G**//B3LYP/6-31++G** and MP2/aug-cc-pVDZ//B3LYP/6-31++G**. The internal rotation of methyl group induced by intermolecular interaction has been observed theoretically. For the optimized structures of nitromethane dimer, the strength of CH,ON H-bond ranges from ,9.0 to ,12.4 kJ mol,1 at the MP2/aug-cc-pVDZ//B3LYP/6-31++G** level, and the B3LYP method underestimates the interaction strength compared with the MP2 method, while MP2/6-31++G**//B3LYP/6-31++G** calculated ,EC is within 2.5 kJ mol,1 of the corresponding value at the MP4(SDTQ)/6-31G**//B3LYP/6-31++G** level. The analytic atom,atom intermolecular potential has been successfully regressed by using the MP2/6-31++G**//B3LYP/6-31++G** calculated interaction energies of nitromethane dimer. For the optimized structures of nitromethane trimer the three-body interaction energies occupy small percentage of corresponding total binding energies, but become important for the compressed nitromethane explosive. In addition, it has been discovered that the three-body interaction energy in the cyclic nitromethane trimer is more and more negative as intermolecular distances decrease from 2.2 to 1.7 Ĺ. © 2003 Wiley Periodicals, Inc. J Comput Chem 24: 345,352, 2003 [source] Ab initio and DFT studies on van der Waals trimers: The OCS · (CO2)2 complexesJOURNAL OF COMPUTATIONAL CHEMISTRY, Issue 4 2002H. Valdés Abstract Ab initio calculations [MP2, MP4SDTQ, and QCISD(T)] using different basis sets [6-31G(d,p), cc-pVXZ (X = D, T, Q), and aug-cc-pVDZ] and density functional theory [B3LYP/6-31G(d,p)] calculations were carried out to study the OCS · (CO2)2 van der Waals trimer. The DFT has proved inappropriate to the study of this type of systems where the dispersion forces are expected to play a relevant role. Three minima isomers (two noncyclic and one cyclic) were located and characterized. The most stable isomer exhibits a noncyclic barrel-like structure whose bond lengths, angles, rotational constants, and dipole moment agree quite well with the corresponding experimental values of the only structure observed in recent microwave spectroscopic studies. The energetic proximity of the three isomers, with stabilization energies of 1442, 1371, and 1307 cm,1, respectively, at the CBS-MP2/cc-pVXZ (X = D, T, Q) level, strongly suggests that the two unobserved structures should also be detected as in the case of the (CO2)3 trimer where both noncyclic and cyclic isomers have been reported to exist. The many-body symmetry-adapted perturbation theory is employed to analyze the nature of the interactions leading to the formation of the different structures. The three-body contributions are small and stabilizing for the two most stable structures and almost negligible for the cyclic isomer. © 2002 Wiley Periodicals, Inc. J Comput Chem 23: 444,455, 2002; DOI 10.1002/jcc.10041 [source] Negative and positive ion matrix-assisted laser desorption/ionization time-of-flight mass spectrometry and positive ion nano-electrospray ionization quadrupole ion trap mass spectrometry of peptidoglycan fragments isolated from various Bacillus speciesJOURNAL OF MASS SPECTROMETRY (INCORP BIOLOGICAL MASS SPECTROMETRY), Issue 2 2001Gerold Bacher Abstract A general approach for the detailed characterization of sodium borohydride-reduced peptidoglycan fragments (syn. muropeptides), produced by muramidase digestion of the purified sacculus isolated from Bacillus subtilis (vegetative cell form of the wild type and a dacA mutant) and Bacillus megaterium (endospore form), is outlined based on UV matrix-assisted laser desorption/ionization time-of-flight (MALDI-TOF) and nano-electrospray ionization (nESI) quadrupole ion trap (QIT) mass spectrometry (MS). After enzymatic digestion and reduction of the resulting muropeptides, the complex glycopeptide mixture was separated and fractionated by reversed-phase high-performance liquid chromatography. Prior to mass spectrometric analysis, the muropeptide samples were subjected to a desalting step and an aliquot was taken for amino acid analysis. Initial molecular mass determination of these peptidoglycan fragments (ranging from monomeric to tetrameric muropeptides) was performed by positive and negative ion MALDI-MS using the thin-layer technique with the matrix ,-cyano-4-hydroxycinnamic acid. The results demonstrated that for the fast molecular mass determination of large sample numbers in the 0.8,10 pmol range and with a mass accuracy of ±0.07%, negative ion MALDI-MS in the linear TOF mode is the method of choice. After this kind of muropeptide screening often a detailed primary structural analysis is required owing to ambiguous data. Structural data could be obtained from peptidoglycan monomers by post-source decay (PSD) fragment ion analysis, but not from dimers or higher oligomers and not with the necessary sensitivity. Multistage collision-induced dissociation (CID) experiments performed on an nESI-QIT instrument were found to be the superior method for structural characterization of not only monomeric but also of dimeric and trimeric muropeptides. Up to MS4 experiments were sometimes necessary to obtain unambiguous structural information. Three examples are presented: (a) CID MSn (n = 2,4) of a peptidoglycan monomer (disaccharide-tripeptide) isolated from B. subtilis (wild type, vegetative cell form), (b) CID MSn (n = 2,4) of a peptidoglycan dimer (bis-disaccharide-tetrapentapeptide) obtained from a B. subtilis mutant (vegetative cell form) and (c) CID MS2 of a peptidoglycan trimer (a linear hexasaccharide with two peptide side chains) isolated from the spore cortex of B. megaterium. All MSn experiments were performed on singly charged precursor ions and the MS2 spectra were dominated by fragments derived from interglycosidic bond cleavages. MS3 and MS4 spectra exhibited mainly peptide moiety fragment ions. In case of the bis-disaccharide-tetrapentapeptide, the peptide branching point could be determined based on MS3 and MS4 spectra. The results demonstrate the utility of nESI-QIT-MS towards the facile determination of the glycan sequence, the peptide linkage and the peptide sequence and branching of purified muropeptides (monomeric up to trimeric forms). The wealth of structural information generated by nESI-QIT-MSn is unsurpassed by any other individual technique. Copyright © 2001 John Wiley & Sons, Ltd. [source] Channel properties of template assembled alamethicin tetramers,JOURNAL OF PEPTIDE SCIENCE, Issue 11-12 2003Dr Hervé Duclohier Abstract The multiple conductance levels displayed by the antibiotic alamethicin in planar lipid bilayers is explained by a dynamic ,barrel-stave' model, the conducting pore resulting from the aggregation of up to ten helical amphipathic helical monomers. However, the precise assignment of an oligomerization state to a particular single-channel conductance substate is far from being experimentally clear. In addition, it could be useful to tailor a given channel geometry to selectively allow the permeation of solutes with different molecular sizes, whilst retaining a high voltage-dependence. To control the aggregation state of the channel, the TASP (template assembled synthetic proteins) strategy was applied to synthesize structurally defined oligomers, i.e. dimer, trimer, tetramer. The modulation of conductance properties of three alamethicin tetramers with the length and flexibility of the linkers of the ,open' or linear template is described. It is shown that the introduction of an alanine between the contiguous lysines to which are tethered C -terminally modified alamethicin helical monomers stabilizes the open channel states, whereas the alanine substitution by Pro-Gly, a reverse beta-turn promoting motif, increases voltage-dependence and leads to single-channel conductance values more in line with the expected ones from a tetrameric bundle. Copyright © 2003 European Peptide Society and John Wiley & Sons, Ltd. [source] Cationic, water-soluble, fluorene-containing poly(arylene ethynylene)s: Effects of water solubility on aggregation, photoluminescence efficiency, and amplified fluorescence quenching in aqueous solutionsJOURNAL OF POLYMER SCIENCE (IN TWO SECTIONS), Issue 19 2006Yan-Qin Huang Abstract Three novel fluorene-containing poly(arylene ethynylene)s with amino-functionalized side groups were synthesized through the Sonogashira reaction. They were poly{9,9-bis[6,-(N,N -diethylamino)hexyl]-2,7-fluorenylene ethynylene}- alt - co -{2,5-bis[3,-(N,N -diethylamino)-1,-oxapropyl]-1,4-phenylene} (P1), poly{9,9-bis[6,-(N,N -diethylamino)hexyl]-2,7-fluorenylene ethynylene} (P2), and poly({9,9-bis[6,-(N,N -diethylamino)hexyl]-2,7-fluorenylene ethynylene}- alt - co -(1,4-phenylene)) (P3). Through the postquaternization treatment of P1,P3 with methyl iodide, we obtained their cationic water-soluble conjugated polyelectrolytes (WSCPs): P1,,P3,. The water solubility was gradually improved from P3, to P1, with increasing contents of hydrophilic side chains. After examining the ultraviolet,visible absorption and photoluminescence (PL) spectra, fluorescence lifetimes, and dynamic light scattering data, we propose that with the reduction of the water solubility from P1, to P3,, they exhibited a gradually increased degree of aggregation in H2O. The PL quantum yields of P1,,P3, in H2O displayed a decreasing tendency consistent with the increased degree of aggregation, suggesting that the pronounced degree of aggregation was an important reason for the low PL quantum yields of WSCPs in H2O. Two structurally analogous water-soluble trimers of P2, and P3,, model compounds 2,7-bis(9,,9,-bis{6,-[(N,N -diethyl)- N -methylammonium] hexyl}-2,-fluorenylethynyl)-9,9-bis{6,-[(N,N -diethyl)- N -methylammonium]hexyl}fluorene hexaiodide and 1,4-bis(9,,9,-bis{6,-[(N,N -diethyl)- N -methylammonium]hexyl}-2,-fluorenylethynyl)benzene tetraiodide, were synthesized. The amplified fluorescence quenching of these WSCPs by Fe(CN)64, in H2O was studied by comparison with a corresponding analogous trimer. The effects of aggregation on the fluorescence quenching may be two-edged in these cases. © 2006 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 44: 5778,5794, 2006 [source] One-pot synthesis of star-shaped aliphatic polyesters with hyperbranched cores and their characterization with size exclusion chromatographyJOURNAL OF POLYMER SCIENCE (IN TWO SECTIONS), Issue 13 2006Tadeusz Biela Star-shaped aliphatic polyesters with hyperbranched cores were prepared by a two-step synthesis with the one-pot, arms-first core method. First, the polymerization of ,-caprolactone (CL) or L,L -lactide (LA) was initiated with aluminum isopropoxide trimer. Then, the resulting poly(CL),OAl< or poly(LA),OAl< living chains were employed as initiators for 5,5,-bis(oxepan-2-one) or 1,6-dioxaspiro[4,4]nonane-2,7-dione) polymerization. A sequence of chain growth and branching reactions led to the formation of starlike macromolecules. The progress of the polymerization was followed with size exclusion chromatography, and the products of the model reaction were also analyzed with matrix-assisted laser desorption/ionization time-of-flight mass spectrometry. [source] Characterization of ring-opening polymerization of genipin and pH-dependent cross-linking reactions between chitosan and genipinJOURNAL OF POLYMER SCIENCE (IN TWO SECTIONS), Issue 10 2005Fwu-Long Mi Abstract In this study, a novel chitosan-based polymeric network was synthesized by crosslinking with a naturally occurring crosslinking agent,genipin. The results showed that the crosslinking reactions were pH-dependent. Under basic conditions, genipin underwent a ring-opening polymerization prior to crosslinking with chitosan. The crosslink bridges consisted of polymerized genipin macromers or oligomers (7 , 88 monomer units). This ring-opening polymerization of genipin was initiated by extracting proton from the hydroxyl groups at C-1 of deoxyloganin aglycone, followed by opening the dihydropyran ring to conduct an aldol condensation. At neutral and acidic conditions, genipin reacted with primary amino groups on chitosan to form heterocyclic amines. The heterocyclic amines were further associated to form crosslinked networks with short chains of dimmer, trimer, and tetramer bridges. An accompanied reaction of nucleophilic substitution of the ester group on genipin by the primary amine group on chitosan would occur in the presence of an acid catalysis. The extent in which chitosan gels crosslinked with genipin was significantly dependent on the crosslinking pH values: 39.9 ± 3.8% at pH 5.0, 96.0 ± 1.9% at pH 7.4, 45.4 ± 1.8% at pH 9.0, and 1.4 ± 1.0% at pH 13.6 (n = 5, p < 0.05). Owing to the different crosslinking extents and different chain lengths of crosslink bridges, the genipin-crosslinked chitosan gels showed significant difference in their swelling capability and their resistance against enzymatic hydrolysis, depending on the pH conditions for crosslinking. These results indicated a direct relationship between the mode of crosslinking reaction, and the swelling and enzymatic hydrolysis properties of the genipin-crosslinked chitosan gels. The ring-opening polymerization of genipin and the pH-dependent crosslinking reactions may provide a novel way for the preparation and exploitation of chitosan-based gels for biomedical applications. © 2005 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 43: 1985,2000, 2005 [source] Identification of oleuropein oligomers in olive pulp and pomaceJOURNAL OF THE SCIENCE OF FOOD AND AGRICULTURE, Issue 10 2006Susana M Cardoso Abstract Analysis of a purified fraction from acetone extracts of olive pulp and pomace by electrospray ionisation tandem mass spectrometry (ESI-MSn) showed the presence of oleuropein oligomers, whose occurrence has not been reported previously in the literature. The main ionic species (m/z 1613) in the ESI-MS spectrum was an oleuropein trimer containing three linkages through the hydroxytyrosol backbone. In both samples, oleuropein dimers (m/z 1075), trimers comprising two hydroxytyrosol linkages (m/z 1615), tetramers (m/z 2153) and pentamers (m/z 2691) were also detected by MS. The occurrence of oleuropein oligomers was also observed by nuclear magnetic resonance (NMR). 13C, 13C distortionless enhancement by polarisation transfer DEPT 90, 13C DEPT 135, gHSQC (heteronuclear single quantum coherence) and gHMBC (heteronuclear multiple bond coherence) spectra showed all carbon and proton resonances of oleuropein with the exception of the low-mobility and asymmetric signals of the aromatic rings. Since mature olives were used in this study, it is possible that the disappearance of oleuropein that has been described to occur with the olive fruit maturation, could be associated with the formation of phenolic oligomers together with lower-molecular-weight compounds resulting from its degradation. Copyright © 2006 Society of Chemical Industry [source] Lipase-Catalyzed Ring-Opening Polymerization of Molecularly Pure Cyclic Oligomers for Use in Synthesis and Chemical Recycling of Aliphatic PolyestersMACROMOLECULAR BIOSCIENCE, Issue 6 2008Asato Kondo Abstract The lipase-catalyzed ROP of molecularly pure cyclic oligomers with a definite degree of oligomerization is analyzed with respect to the molecular weights of the resulting polymers and certain kinetic parameters of the enzymatic reaction. Cyclic BA dimers, trimers, and tetramers polymerize faster than the equivalent monomer; however, the latter produces PBA of significantly higher molecular weight. The reason is that the ring opening of the cyclic monomer is slow, leading to a lower initiator concentration than that produced by the cyclic BA dimer and trimer. Similarly, the cyclic BS dimer produces PBS of higher molecular weight than that obtained from the cyclic BS trimer. [source] Ternary complex formation between AmtB, GlnZ and the nitrogenase regulatory enzyme DraG reveals a novel facet of nitrogen regulation in bacteriaMOLECULAR MICROBIOLOGY, Issue 6 2007Luciano F. Huergo Summary Ammonium movement across biological membranes is facilitated by a class of ubiquitous channel proteins from the Amt/Rh family. Amt proteins have also been implicated in cellular responses to ammonium availability in many organisms. Ammonium sensing by Amt in bacteria is mediated by complex formation with cytosolic proteins of the PII family. In this study we have characterized in vitro complex formation between the AmtB and PII proteins (GlnB and GlnZ) from the diazotrophic plant-associative bacterium Azospirillum brasilense. AmtB,PII complex formation only occurred in the presence of adenine nucleotides and was sensitive to 2-oxoglutarate when Mg2+ and ATP were present, but not when ATP was substituted by ADP. We have also shown in vitro complex formation between GlnZ and the nitrogenase regulatory enzyme DraG, which was stimulated by ADP. The stoichiometry of this complex was 1:1 (DraG monomer : GlnZ trimer). We have previously reported that in vivo high levels of extracellular ammonium cause DraG to be sequestered to the cell membrane in an AmtB and GlnZ-dependent manner. We now report the reconstitution of a ternary complex involving AmtB, GlnZ and DraG in vitro. Sequestration of a regulatory protein by the membrane-bound AmtB,PII complex defines a new regulatory role for Amt proteins in Prokaryotes. [source] Thermal quenching of luminescence and isovalent trap model for rare-earth-ion-doped AlNPHYSICA STATUS SOLIDI (B) BASIC SOLID STATE PHYSICS, Issue 6 2007H. J. Lozykowski Abstract Investigations of the luminescent properties of Pr-, Eu-, Tb- and Tm-implanted AlN thin films at temperature in the range 9,830 K are reported. The temperature studies of photoluminescence and cathodoluminescence spectra revealed unexpectedly weak thermal quenching for all investigated rare earth (RE) ions. The maximum CL emission is observed from Eu (red) at 485 K, Tb (green) at 590 K and Tm (blue) at 530 K, respectively. For Tb- and Tm-doped AlN samples, temperature-dependent crossrelaxation processes were observed. Photoluminescence excitation spectra, obtained under UV excitation in the spectral range 200,400 nm, exhibit several bands. It is proposed that the RE ions exist in semiconductors as isolated ions (singlet), nearest-neighbor (nn) ion pairs (dimer), and three ions (trimer). The Koster,Slater and simple spherical potential-well models for RE-structured isovalent (RESI) hole trap are proposed. The exciton binding energies of RESI traps are calculated and compared with experimental thermal-quenching energies. The energy-transfer processes between the AlN host and the 4f-shell systems are emphasized as the main mechanisms for thermal-quenching processes rather than nonradiative decay of 4f transitions. (© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] Hubbard model in the ferromagnetic state.PHYSICA STATUS SOLIDI (B) BASIC SOLID STATE PHYSICS, Issue 2 2005Dimer, trimer approach Abstract We consider the Hubbard model for the sc lattice which consists of interacting, parallel lying dimer (trimer) planes. Each plane is a collection of equidistant, interacting, parallel lying dimers (trimers). We apply the exact solution of the dimer (trimer) problem after introducing the intrinsic symmetry breaking leading to the ferromagnetic ordering. We present preliminary, test results concerning the phase diagram of the system. In both approximations (dimer and trimer approach) there is a threshold for the ferromagnetic ordering at the Coulomb repulsion U = Ucr. The trimer approach gives a better saturation for the Curie temperature TC vs. U in comparison to the dimer approach. (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] Isolation and Characterisation of Procyanidins from Rumex obtusifoliusPHYTOCHEMICAL ANALYSIS, Issue 3 2007Paul Spencer Abstract An acetone:water (7:3) extract obtained from the leaves of Rumex obtusifolius was fractionated into procyanidin oligomer and polymer fractions using a linear gradient and a simple step method on Sephadex LH-20. The chemical characteristics of the procyanidin fractions were studied by 13C-NMR spectroscopy, acid-catalysed degradation in the presence of benzyl mercaptan, matrix-assisted laser desorption/ionisation time-of-flight (MALDI-TOF) MS and electrospray ionisation (ESI) MS. The 13C-NMR showed that the polymer fraction consisted predominantly of procyanidin polymers, some with galloyl groups attached. The thiolysis reaction products indicated a mean degree of polymerisation (DP) of 4.3 for the step method, and a range of 2.3,8.2 mean DP for the gradient fractionation, with epicatechin as the most abundant flavan-3-ol extension unit, while the terminal units consisted of equal proportions of catechin, epicatechin and epicatechin gallate. Singly charged ions observed in MALDI-TOF/MS showed a range of oligomeric procyanidins and their polygalloyl derivatives. These species (in the range DP 2,7) were also observed by ESI/MS but the spectra were more complex due to overlapping multiply charged ions. Isolation of oligomers from the Sephadex LH-20 fraction by chromatography on polyamide and C18 yielded B1, B2, B3 and B7 dimers, an A-type trimer and a B2 3,3,- O -digallate. Copyright © 2007 John Wiley & Sons, Ltd. [source] Qualitative analysis and HPLC isolation and identification of procyanidins from vicia fabaPHYTOCHEMICAL ANALYSIS, Issue 2 2004Rachid Merghem Abstract The soluble proanthocyanidins of the coloured seed coats of Vicia faba L. were isolated and separated by solvent partition. The chemical characteristics of the proanthocyanidins were elucidated by total oxidation and partial degradation in the presence of phloroglucinol followed by HPLC analysis. The native extract of proanthocyanidins contained (+)-gallocatechin, (,)-epigallocatechin, (+)-catechin and (,)-epicatechin units. Oligomeric procyanidins were puri,ed by chromatography on Sephadex LH-20 and the accessible compounds were isolated by RP-HPLC using a Licrospher Li 100 Column. The structures of the puri,ed oligomeric procyanidins were elucidated using a procedure involving TLC, UV spectroscopy, ESI-MS and HPLC analysis of the products from the phloroglucinol reaction. The major condensed tannins of Vicia faba comprise six compounds identi,ed as two A-type procyanidin dimers, the procyanidin dimers B1, B2 and B3, and a procyanidin trimer. Copyright © 2004 John Wiley & Sons, Ltd. [source] | ||||||||||||||||||||||||||||||||||||||||