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Repeat Units (repeat + unit)

Distribution by Scientific Domains
Polymers and Materials Science46%
Life Sciences20%
Chemistry18%
Medical Sciences14%
Distribution within Polymers and Materials Science
Polymer Science & Technology General60%
General & Introductory Materials Science26%

Selected Abstracts

Proper expression of the O-antigen of lipopolysaccharide is essential for the virulence of Yersinia enterocolitica O:8 in experimental oral infection of rabbits

FEMS IMMUNOLOGY & MEDICAL MICROBIOLOGY, Issue 2 2003
H Najdenski
Abstract The O-antigen of lipopolysaccharide (LPS) is required for virulence in Yersinia enterocolitica serotype O:8. Here we evaluated the importance of controlling the O-antigen biosynthesis using an in vivo rabbit model of infection. Y. enterocolitica O:8 wild-type strain was compared to three mutants differing in the O-antigen phenotype: (i) the rough strain completely devoid of the O-antigen, (ii) the wzy strain that lacks the O-antigen polymerase (Wzy protein) and expresses LPS with only one repeat unit, and (iii) the wzz strain that lacks the O-antigen chain length determinant (Wzz protein) and expresses LPS without modal distribution of O-antigen chain lengths. The most attenuated strain was the wzz mutant. The wzz bacteria were cleared from the tissues by day 30, the blood parameters were least dramatic and histologically only immunomorphological findings were seen. The level of attenuation of the rough and the wzy strain bacteria was between the wild-type and the wzz strain. Wild-type bacteria were highly resistant to killing by polymorphonuclear leukocytes, the wzz strain bacteria were most sensitive and the rough and wzy strain bacteria were intermediate resistant. These results clearly demonstrated that the presence of O-antigen on the bacterial surface is not alone sufficient for full virulence, but also there is a requirement for its controlled chain length. [source]

Molar group contributions to the heat of combustion,

FIRE AND MATERIALS, Issue 3 2002
Richard N. Walters
Experimental results for the gross heat of combustion of over 140 commercial and developmental polymers and small molecules of known chemical structure were used to derive additive molar group contributions to the heat of combustion. The materials examined contained carbon, hydrogen, oxygen, nitrogen, phosphorus, sulphur, chlorine, fluorine and silicon in linear, branched and cyclic structures. Values for the molar group contributions were summed and divided by the molecular weight of the molecule or polymer repeat unit to yield the gross heat of combustion as determined by oxygen bomb calorimetry. This new method provides an accurate single step method for calculating the heat of combustion. The average relative error of the calculated gross heats of combustion is 2.8%. Published in 2002 by John Wiley & Sons, Ltd. [source]

Synthesis and Processing of Monodisperse Oligo(fluorene- co -bithiophene)s into Oriented Films by Thermal and Solvent Annealing

ADVANCED FUNCTIONAL MATERIALS, Issue 12 2009
Lichang Zeng
Abstract A series of oligo(fluorene- co -bithiophene)s, OF2Ts, have been synthesized and characterized to investigate the effects of oligomer length and pendant aliphatic structure on glassy-nematic mesomorphism. The OF2Ts comprising more than one repeat unit and their polymer analogue, PF2T, carrying 52 number-average repeat units, possess the highest occupied molecular orbital energy level at ,5.3,±,0.2,eV, but the anisotropic field-effect mobilities increase with the oligomer length. Spin coating from high-boiling chlorobenzene with and without subsequent exposure to saturated chlorobenzene vapor constitute solvent-vapor annealing and quasi-solvent annealing, respectively. Solvent-vapor annealing yields monodomain glassy-nematic films in which OF2Ts are aligned as well as with thermal annealing across a 2,cm diameter. Quasi-solvent annealing, however, amounts to kinetically trapping a lower orientational order than solvent-vapor or thermal annealing. While amenable to thermal annealing at elevated temperatures, PF2T shows no alignment at all following either strategy of solvent annealing. [source]

Analysis of intraspecies polymorphism in the ribosomal DNA cluster of the cockroach Blattella germanica

INSECT MOLECULAR BIOLOGY, Issue 2 2000
D. V. Mukha
Abstract HindIII restriction digests of the rDNA repeat unit of the German cockroach, Blattella germanica, reveal significant intraspecies sequence polymorphism. This variability is probably caused by structural differences within the nontranscribed spacer regions (NTS) of the ribosomal repeat unit. HindIII rDNA fragment polymorphisms in three cockroach strains show that individuals from different populations may have different HindIII rDNA patterns, whereas individuals within populations exhibit relatively similar rDNA patterns. We suggest that HindIII restriction fragment polymorphisms within cockroach ribosomal DNA will be a valuable tool for measuring population-level parameters within and between natural cockroach populations. [source]

Sulfonated poly(ether imide) and poly(ether sulfone) blends for direct methanol fuel cells.

JOURNAL OF APPLIED POLYMER SCIENCE, Issue 5 2008

Abstract This investigation examines characteristics of sulfonated polyether imides (SPEI) with various ion exchange capacity values (IEC) and completes previous work to enable its blends to be adopted as polyelectrolyte in direct methanol fuel cells (DMFC). Polyether imides (PEI) were sulfonated by using chlorosulfonic acid as the sulfonating agent and chloroform as the solvent. The structure of SPEI was observed by FTIR and 1H NMR. The sulfonate or sulfonic acid content of the polymers, expressed as a number per repeat unit of the polymer, was accurately determined by elemental analysis and conductometric titration. Physical properties such as solubility, intrinsic viscosities, thermal stability, and glass transition temperature (Tg) were studied for both PEI and SPEI. TGA-FTIR verified that sulfonic groups, attached to the aromatic ring in the PEI backbone, are split at 230,350°C, but the main-chain splitting temperature of SPEI is similar to that of pure polymer. The sulfonated samples exhibited good solubilities and increased glass transition temperatures (Tg values) as degree of sulfonation (DS) increased; two Tg values were detected when IEC was sufficiently high. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci, 2008 [source]

Detection of Heterobasidion annosum s. l. [(Fr.) Bref.] in Norway Spruce by Polymerase Chain Reaction

JOURNAL OF PHYTOPATHOLOGY, Issue 7 2002
G. Bahnweg
Abstract Internal transcribed spacer (ITS) sequences of the rDNA repeat unit of Heterobasidion annosum were used to design specific primers for the detection and quantification of this important forest pathogen by polymerase chain reaction (PCR). Specificity of detection was cross-checked against a variety of other fungi (saprophytes, root pathogens, mycorrhizal fungi) which may occur in the same environment. As little as 1 pg fungal DNA (equiv. to 10,40 genomes) could be detected in 200 ng spruce root DNA (from 1 mg fresh spruce root). The Heterobasidion -specific primers allowed simultaneous detection of Armillaria spp. in multiplex PCR. The method was successfully applied to increment cores of Norway spruce from the forest region Tharandter Wald (Saxonia, Germany), Oberbärenburg (East Ore Mountains, Saxonia) and Oberschleissheim (north of Munich, Bavaria). [source]

Oxidation and epoxidation of poly(1,3-cyclohexadiene)

JOURNAL OF POLYMER SCIENCE (IN TWO SECTIONS), Issue 1 2003
D. T. Williamson
Abstract Poly(1,3-cyclohexadiene) (PCHD) derivatives were synthesized via facile chemical modification reactions of the residual double bond in the repeat unit. The oxidation and degradation of PCHD was investigated to enable subsequent controlled epoxidation reactions. PCHD exhibited a 15% weight loss at 110 °C in the presence of oxygen. The oxidative degradation, demonstrated by gel permeation chromatography (GPC) and 1H NMR spectroscopy, was attributed to main-chain scission. Aldehyde and ether functional groups were introduced into the polymer during the oxidation process. PCHD was quantitatively epoxidized in the absence of deleterious oxidation with meta -chloroperoxybenzoic acid. 1H and 13C NMR spectroscopy confirmed that polymers with controlled degrees of epoxidation were reproducibly obtained. Epoxidized PCHD exhibited a glass-transition temperature at 154 °C, which was slightly higher than that of a PCHD precursor of a nearly equivalent molecular weight. Moreover, GPC indicated the absence of undesirable crosslinking or degradation, and the molecular weight distributions remained narrow. The thermooxidative stability of the fully epoxidized polymer was compared to that of the PCHD precursor, and the epoxidized PCHD exhibited an initial weight loss at 250 °C in oxygen, which was 140 °C higher than the temperature for PCHD. © 2002 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 41: 84,93, 2003 [source]

The Influence of Pendant Hydroxyl Groups on Enzymatic Degradation and Drug Delivery of Amphiphilic Poly[glycidol- block -(, -caprolactone)] Copolymers

MACROMOLECULAR BIOSCIENCE, Issue 11 2009
Jing Mao
Abstract An amphiphilic diblock copolymer PG- b -PCL with well-controlled structure and pendant hydroxyl groups along hydrophilic block was synthesized by sequential anionic ring-opening polymerization. The micellization and drug release of PG- b -PCL copolymers using pyrene as a fluorescence probe were investigated for determining the influences of copolymer composition and lipase concentration on drug loading capacity and controlled release behavior. The biodegradation of PG- b -PCL copolymers was studied with microspheres as research samples. It has been concluded that the polar hydroxyl groups along each repeat unit of hydrophilic PG block in PG- b -PCL copolymer have great influences on drug encapsulation, drug release, and enzymatic degradation of micelles and microspheres. [source]

Miscibility in Blends of Isotactic/Syndiotactic Polystyrenes at Melt or Quenched Amorphous Solid State

MACROMOLECULAR MATERIALS & ENGINEERING, Issue 11 2006
Shu Hsien Li
Abstract Summary: Miscibility in amorphous phase and behavior in a crystalline phase of blends of two semicrystalline and isomeric polymers, isotactic polystyrene (iPS) and syndiotactic polystyrene (sPS), was probed. Optical and scanning electron microscopy results indicate no discernible heterogeneity in iPS/sPS blends in either melt state or rapidly quenched amorphous super-cooled state, while the Tg behavior of the quenched amorphous blends shows an intimately mixed state of two polymer chains. The crystal forms of the blends were further analyzed to provide additional evidence of miscibility in the amorphous domain. The sPS in the iPS/sPS blends upon melt crystallization was found to predominantly exist as the more stable , -form (rather than mixed , -form and , -form in neat sPS), which also suggests evidence of miscibility in the iPS/sPS blends. The melting behavior of semicrystalline sPS in the iPS/sPS mixtures was analyzed using the Flory-Huggins approach for estimation of interactions. By measuring the equilibrium melting point of the higher-melting sPS species in the sPS/iPS blends, a small negative value, for the interaction parameter (,,,,,0.11) was found. Further, by introducing a third polymer, poly(2,6-dimethyl- p -phenylene oxide) (PPO), a ternary iPS/sPS/PPO blend system was also proven miscible, which constituted a further test for stable phase miscibility in the iPS/sPS blend. General nature of miscibility in blends composed of two crystalline isomeric polymers is discussed. Issues in dealing with blends of polymers of the same chemical repeat unit but different tacticities were addressed. X-ray diffractograms for neat sPS and iPS/sPS blends, each having been isothermally crystallized at 245,°C for 4 h. [source]

Novel Cobalt (II) Phthalocyanine-Containing Polyimides: Synthesis, Characterization, Thermal and Optical Properties

MACROMOLECULAR RAPID COMMUNICATIONS, Issue 21 2006
Eva M. Maya
Abstract Summary: A novel class of polymer has been prepared by reaction of a copolyimide containing carboxylic acid groups, and a cobalt phthalocyanine peripherally substituted with a reactive hydroxyl group. The incorporation of the phthalocyanine ring was achieved by esterification of the carboxylic acid groups of the polyimide with the free hydroxyl group of the phthalocyanine. In this work, a limited number of phthalocyanine rings have been introduced in order to avoid extensive modification of the copolyimide properties. The extent of modification (percentage of esterified carboxylic groups) was estimated to be about 18%. The resulting material showed good thermal stability and a high glass transition temperature (above 300,°C). The incorporation of the bulky phthalocyanine ring brought about a light decrease of thermal properties, relative to those of the initial polyimide. The presence of phthalocyanine was confirmed by the absorption band at 670 nm showing in the visible region for the modified polyimide. The novel copolyimide also exhibited a remarkable emission peak at 430 nm, when excited at 350 nm, corresponding to the emission of the cobalt phthalocyanine moiety. Schematic representation of the PI-CoPc molecule and the chemical structure of the repeat unit. [source]

Mitochondrial DNA differentiation between two forms of trout Salmo letnica, endemic to the Balkan Lake Ohrid, reflects their reproductive isolation

MOLECULAR ECOLOGY, Issue 12 2004
J. SELL
Abstract Mitochondrial haplotype diversity in sympatric populations of Ohrid trout, Salmo letnica was investigated by polymerase chain reaction,restriction fragment length polymorphism (PCR-RFLP) analysis of the mtDNA control region and ND1, ND3/4, ND5/6 segments. A 310 bp fragment at the 5, end, and a 340,572 bp fragment at the 3, end of the control region were sequenced from representatives of the populations studied. Based on pairwise comparison of the sequences, five new haplotypes were identified plus one identical with the brown trout Andalusian haplotype from the southern Iberian Peninsula. The combination of both RFLP and sequence data sets yielded a total of 10 composite haplotypes. A high degree of genetic subdivision between S. letnica typicus and S. letnica aestivalis populations was observed. The notion of a sympatric origin for the two morphs is discussed. Length variation of the mtDNA control region due to the presence of an 82 bp unit, tandemly repeated one to four times, in the region between the conserved sequence block-3 (CSB-3) and the gene for phenylalanine tRNA is reported. Further, we demonstrate that a single duplication of the approximately 82 bp repeat unit is a common element of the salmonid mitochondrial control region. The unique genetic structure of Ohrid trout represents a highly valuable genetic resource that deserves appropriate management and conservation. [source]

Isolation and characterization of the cell-surface polysaccharides of Porphyromonas gingivalis ATCC 53978

MOLECULAR ORAL MICROBIOLOGY, Issue 3 2000
S. I. Farquharson
The cell-surface carbohydrates of Porphyromonas gingivalis strain ATCC 53978 were isolated and partially characterized. Three separate polysaccharides were found to be present: an extracellular polysaccharide, capsular polysaccharide and lipopolysaccharide. The capsular polysaccharide, which had peculiar, gel-like viscoelastic properties, was found to be comprised of mannuronic acid, glucuronic acid, galacturonic acid, galactose, and 2-acetamido-2-deoxy- d -glucose in relative molar ratios of 0.6:0.9:0.5:0.5:1.0, respectively. The extracellular polysaccharide was found to contain mannose, rhamnose, glucose, galactose, and 2-acetamido-2-deoxy- d -glucose in relative molar ratios of 13.5:1.4:1.0:2.0:1.0, respectively. The lipopolysaccharide was found to contain an O -antigen with a regular tetrasaccharide repeat unit comprised of 4-linked ,- l -rhamnopyranosyl, 6-linked ,- d -glucopyranosyl, 3-linked ,- d -galactopyranosyl, and 4-linked 2-acetamido-2-deoxy-,- d -glucopyranosyl residues in equimolar proportions. [source]

Curing of diglycidyl ether of bisphenol-A epoxy resin using a poly(aryl ether ketone) bearing pendant carboxyl groups as macromolecular curing agent

POLYMER INTERNATIONAL, Issue 8 2009
Fuhua Liu
Abstract BACKGROUND: Reactive thermoplastics have received increasing attention in the field of epoxy resin toughening. This paper presents the first report of using a novel polyaryletherketone bearing one pendant carboxyl group per repeat unit to cure the diglycidyl ether of bisphenol-A epoxy resin (DGEBA). The curing reactions of DGEBA/PEK-L mixtures of various molar ratios and with different catalysts were investigated by means of dynamic differential scanning calorimetry and Fourier transform infrared (FTIR) spectroscopy methods. RESULTS: FTIR results for the DGEBA/PEK-L system before curing and after curing at 135 °C for different times demonstrated that the carboxyl groups of PEK-L were indeed involved in the curing reaction to form a crosslinked network, as evidenced by the marked decreased peak intensities of the carboxyl group at 1705 cm,1 and the epoxy group at 915 cm,1 as well as the newly emerged strong absorptions of ester bonds at 1721 cm,1 and hydroxyl groups at 3447 cm,1. Curing kinetic analysis showed that the value of the activation energy (Ea) was the highest at the beginning of curing, followed by a decrease with increasing conversion (,), which was attributed to the autocatalytic effect of hydroxyls generated in the curing reaction. CONCLUSION: The pendant carboxyl groups in PEK-L can react with epoxy groups of DGEBA during thermal curing, and covalently participate in the crosslinking network. PEK-L is thus expected to significantly improve the fracture toughness of DGEBA epoxy resin. Copyright © 2009 Society of Chemical Industry [source]

Thermal and mechanical properties of diglycidylether of bisphenol A/ trimethylolpropane triglycidylether epoxy blends cured with benzylpyrazinium salts

POLYMER INTERNATIONAL, Issue 5 2002
Soo-Jin Park
Abstract The effect of blend composition on thermal stability and mechanical properties of diglycidylether of bisphenol A (DGEBA)/trimethylolpropane triglycidylether (TMP) epoxy blends cured with benzylpyrazinium salts (N -benzylpyrazinium hexafluoroantimonate, BPH) as a thermal latent catalyst was investigated. The thermal stability, characterized by the initial decomposition temperature, temperature of maximum rate of weight loss, integral procedural decomposition temperature, and activation energy for decomposition, increase in DGEBA-rich compositions. This could be due to the long repeat unit and stable aromatic ring in the DGEBA. The mechanical properties are also discussed in terms of the fracture toughness (KIC), flexural and impact tests for the blend composition studied. The addition of TMP into DGEBA gives systematic improvements in fracture toughness, which results from the increase in aliphatic and flexible chain segments of TMP. © 2002 Society of Chemical Industry [source]

Recent work on entropically-driven ring-opening polymerizations: some potential applications,

POLYMERS FOR ADVANCED TECHNOLOGIES, Issue 2-3 2005
Philip Hodge
Abstract The entropically-driven ring-opening polymerization of macrocyclic monomers (>ca. 14 ring atoms per repeat unit) and/or macrocyclic oligomers is a relatively new method of polymer synthesis that exploits the well-known phenomenon of ring-chain equilibria. It attracts interest because of its novel features. For example, these ring-opening polymerizations emit no volatiles and little or no heat. This review considers the principles of entropically-driven ring-opening polymerizations, gives selected examples and discusses potential applications. The latter include micromolding, high throughput syntheses and the synthesis of supramolecular polymers. Copyright © 2005 John Wiley & Sons, Ltd. [source]

Poly[[{,4 -3-[(1H -1,2,4-triazol-1-yl)methyl]benzoato}zinc(II)] hemihydrate]: a novel two-dimensional framework formed by self-association of zinc(II) sulfate with 3-[(1H -1,2,4-triazol-1-yl)methyl]benzoic acid

ACTA CRYSTALLOGRAPHICA SECTION C, Issue 6 2010
Li Duan
The title novel two-dimensional coordination polymer, {[Zn2(C10H8N3O2)4]·H2O}n, features a {Zn2L2} bimetallic ring repeat unit {L is the 3-[(1H -1,2,4-triazol-1-yl)methyl]benzoate ligand}. Each ZnII cation of the bimetallic ring is further bonded to two other L ligands, resulting in a novel infinite two-dimensional network structure with two channels of different sizes. The crystallographically unique ZnII atom is thus six-coordinated in a distorted octahedral environment of four carboxylate O atoms and two triazole N atoms. Two of these networks interpenetrate in an orthogonal arrangement to form the full three-dimensional framework, with disordered water molecules located in the channels. [source]

Calcium and strontium thio­sulfate, CaS2O3·6H2O and SrS2O3·5H2O

ACTA CRYSTALLOGRAPHICA SECTION C, Issue 10 2004
Peter Held
In contrast to former morphological studies, the results presented here show that calcium(II) thio­sulfate hexahydrate, CaS2O3·6H2O, crystallizes centrosymmetrically in the pinacoidal class (point group ). The structure is characterized by chains, parallel to [100], of alternating S2O3 and Ca(H2O)6O2 groups sharing common O atoms. The composition of each chain link is [Ca(H2O)6(S2O3)]. The geometry is analysed and compared in detail with the structural features of monoclinic strontium(II) thio­sulfate pentahydrate, SrS2O3·5H2O, which forms layers, parallel to (100), of alternating S2O3 and Sr(H2O)4O5 groups connected via common O atoms and O,O edges. Each layer contains [Sr(H2O)3O(S2O3)], as the unique repeat unit. [source]

Hydration of polysaccharide hyaluronan observed by IR spectrometry.

BIOPOLYMERS, Issue 1 2003

Abstract This article is the first one in a series dedicated to the study of hyaluronan as observed by IR spectrometry. The goal is to determine its hydration mechanism and the structural changes this mechanism implies. Hyaluronan is a natural polysaccharide that is widely used in biomedical applications and cosmetics. Its macroscopic properties are significantly dependent on its degree of hydration. In this article we record the IR spectrum of a several micron thick dried film and deduce that four or five residual H2O molecules remain around each disaccharide repeat unit in the dried film. We then compare the spectra of sodium hyaluronan and its acid form to assign vibrational bands linked to the carboxylate group. We proceed with a qualitative analysis of the spectral changes induced by changes of temperature and hygroscopicity, two independent parameters that act by modifying the hydrogen bond network of the sample. This enables us to assign most of the vibrational bands of the hydrophilic groups and to distinguish the bands that are due to these hydrophilic groups when they are or are not hydrogen bonded. It constitutes a prerequisite for the quantitative analysis of hydration spectra that will be described in the following articles of this series. © 2002 Wiley Periodicals, Inc. Biopolymers (Biospectroscopy) 72: 10,20, 2003 [source]

Characterization of Dicarboxylic Acids for Cellulose Hydrolysis

BIOTECHNOLOGY PROGRESS, Issue 3 2001
Nathan S. Mosier
In this paper, we show that dilute maleic acid, a dicarboxylic acid, hydrolyzes cellobiose, the repeat unit of cellulose, and the microcrystalline cellulose Avicel as effectively as dilute sulfuric acid but with minimal glucose degradation. Maleic acid, superior to other carboxylic acids reported in this paper, gives higher yields of glucose that is more easily fermented as a result of lower concentrations of degradation products. These results are especially significant because maleic acid, in the form of maleic anhydride, is widely available and produced in large quantities annually. [source]

Cu[Al(ORF)4] Starting Materials and their Application in the Preparation of [Cu(Sn)]+ (n=12, 8) Complexes

CHEMISTRY - A EUROPEAN JOURNAL, Issue 27 2009
Gustavo Santiso-Quiñones Dr.
Abstract Naked copper,? A newly developed simple two-step route to weakly coordinated CuI starting materials that were used to prepare novel copper,cyclosulfur adducts, including the first M+,S12 complex (see figure, RF= C(CF3)3, C(CH3)(CF3)2, or CH(CF3)2). Reactions with the [Al{OC(CF3)3}4], counterion mimic gas-phase chemistry inside a mass spectrometer (to form [Cu(S12)]+). An effective route to stable, almost-"naked" CuI salts of weakly coordinating anions (WCAs) of the type [Al(ORF)4], has been developed. Born,Fajans,Haber cycles and theoretical calculations suggest that this methodology is useful for the generation of CuI salts regardless of the larger WCA used. The first homoleptic CuI,arene complex [Cu(1,2-F2C6H4)2]+[Al{OC(CF3)3}4], (1), the first CuI,methylenechloride complex [Cu(CH2Cl2)Al{OC(CH3)(CF3)2}4] (2), and the donor-free dimer [CuAl{OCH(CF3)2}4]2 (3) were synthesized in quantitative yields by sonicating Li[Al(ORF)4] (RF=C(CF3)3, C(CH3)(CF3)2, or CH(CF3)2), AgF, and a three-fold excess of CuI in 1,2-F2C6H4 (1) or CH2Cl2 (2, 3). Substances 1,3 are good starting materials for further CuI chemistry, and the reaction of 1,3 with the weak Lewis base cyclooctasulfur gave the first CuI,sulfur complexes of type [Cu(S12)(S8)]+[Al{OC(CF3)3}4], (4), [Cu(CH2Cl2)(S12)]+[Al{OC(CF3)3}4], (5), [A1Cu(1,5-,1,,1 -S8)CuA1] (6; A1=[Al{OC(CH3)(CF3)2}4],), and a CuI,S8 1D coordination polymer with [Cu2(S8)2A22] (7; A2=[Al{OCH(CF3)2}4],), as a monomeric repeat unit. Complexes 4 and 5 are the first example of any metal coordinated to cyclo-S12 and 4 is the first example of a complex having an element in two allotropic modifications as a ligand. [source]

Folding Control in Cyclic Peptides through N-Methylation Pattern Selection: Formation of Antiparallel ,-Sheet Dimers, Double Reverse Turns and Supramolecular Helices by 3,,, Cyclic Peptides

CHEMISTRY - A EUROPEAN JOURNAL, Issue 7 2008
Manuel Amorín Dr.
Abstract Peptide foldamers constitute a growing class of nanomaterials with potential applications in a wide variety of chemical, medical and technological fields. Here we describe the preparation and structural characteristics of a new class of cyclic peptide foldamers (3,,,-CPs) that, depending on their backbone N-methylation patterns and the medium, can either remain as flat rings that dimerize through arrays of hydrogen bonds of antiparallel ,-sheet type, or can fold into twisted double reverse turns that, in the case of double ,-turns, associate in nonpolar solvents to form helical supramolecular structures. A 3,,,-CP consists of a number of multiples of a repeat unit made up of four amino acid residues of alternating chirality: three corresponding to ,-amino acids and one to a ,-amino acid (a cis -3-aminocycloalkanecarboxylic acid). [source]

Usefulness of mec -associated direct repeat unit (dru) typing in the epidemiological analysis of highly clonal methicillin-resistant Staphylococcus aureus in Scotland

CLINICAL MICROBIOLOGY AND INFECTION, Issue 10 2008
R. V. Goering
Abstract The incidence of the epidemic methicillin-resistant Staphylococcus aureus (EMRSA) strains EMRSA-15 and EMRSA-16 in Scotland has increased dramatically, now accounting for c. 70% and c. 20% of isolates, respectively. Epidemiological tracking of these EMRSA strains is difficult, as c. 50% of EMRSA-15 and c. 35% of EMRSA-16 isolates are indistinguishable using pulsed-field gel electrophoresis (PFGE) and other typing methods. The usefulness of mec -associated direct repeat unit (dru) sequence analysis as a more sensitive approach to tracking the persistence and spread of these ,clonal' EMRSA strains in Scotland was evaluated. Analysis of 47 EMRSA-15 and 57 EMRSA-16 isolates (including two separately cultured isolates of the Harmony collection type strain) obtained from 22 hospital laboratories over an 8-year period (1997,2005) revealed 13 and 12 different dru types, respectively. Whereas some types appeared to be endemic in multiple hospitals, subtypes that may represent specific strain movement among hospitals in a given geographical region were identified in other instances. These results suggest that mec -associated dru typing may have potential for identifying and tracking specific subtypes of otherwise indistinguishable epidemic MRSA isolates such as those in Scotland. [source]

Random Copolymer Films with Molecular-Scale Compositional Heterogeneities that Interfere with Protein Adsorption

ADVANCED FUNCTIONAL MATERIALS, Issue 21 2009
Salmaan H. Baxamusa
Abstract Smooth surfaces with compositional heterogeneities at a molecular-length scale are presented with the goal of disrupting surface,protein interactions. These surfaces are synthesized by utilizing photoinitiated chemical vapor deposition (piCVD) to deposit thin films of random copolymers consisting of highly hydrophilic and highly hydrophobic comonomers. Swellability, wettability, and surface roughness could be systematically controlled by tuning the copolymer composition. The surface composition was dynamic, and the surface reconstructed based on the hydration state of the film. Proteins adsorbed to the copolymer films less readily than to either of the respective homopolymers, indicating a synergistic effect resulting from the random copolymer presenting molecular-scale compositional heterogeneity. These results provide direct evidence that protein adsorption can be disrupted by such surfaces and a simple analytical model suggests that the heterogeneities occur over areas encompassing 4,5 repeat units of the polymer. The synthetic method used to create these films can be used to coat arbitrary geometries, enabling practical utility in a number of applications. [source]

The carboxy-terminal tail region of human Cav2.1 (P/Q-type) channel is not an essential determinant for its subcellular localization in cultured neurones

GENES TO CELLS, Issue 2 2005
Qiuping Hu
A recent report on the mechanism of synaptic targeting of Cav2.2 channel suggested that this process depends upon the presence of long C-terminal tail and that protein interactions mediated by SH3-binding and PDZ-binding motifs in the tail region are important. To examine the possibility that C-terminal tail of the Cav2.1 channel and the polyglutamine stretch therein are also involved in the mechanism for channel localization, we constructed several expression plasmids for human Cav2.1 channel tagged with enhanced green fluorescent protein (EGFP) and introduced them into mouse hippocampal neuronal culture. HC construct encodes short version of Cav2.1, and HS and HL encode Cav2.1 channel with a long C-terminal tail, which contains polyglutamine tract of 13 (normal range) and 28 (SCA6 disease range) repeat units, respectively. Surprisingly, transfection with HC, HS, and HL gave essentially the same results: EGFP signal was observed in cell soma, dendrites, and the axon as well. Furthermore, mutation of the PDZ-binding motif located at the C-terminus of the long version of Cav2.1, by adding FLAG tag, did not affect the localization patterns of HS and HL as well. Therefore, the C-terminal region is not indispensable for the subcellular localization of Cav2.1 channel, nor expansion of polyglutamine length affected the localization of the channel. Thus, it is possible that the localization mechanism of Cav2.1 channel is different from that of Cav2.2, though these channels share various structural and functional characteristics. [source]

Replication fork block protein, Fob1, acts as an rDNA region specific recombinator in S. cerevisiae

GENES TO CELLS, Issue 2 2002
Katsuki Johzuka
Background: The analysis of homologous recombination in the tandemly repeating rDNA array of Saccharomyces cerevisiae should provide useful information about the stability of not only the rDNA repeat but also the abundant repeated sequences on higher eukaryotic genomes. However, the data obtained so far are not yet conclusive, due to the absence of a reliable assay for detecting products of recombination in the rDNA array. Results: We developed an assay method to detect the products of unequal sister-chromatid recombination (marker-duplication products) in yeast rDNA. This assay, together with the circular rDNA detection assay, was used for the analysis. Marker-duplication occurred throughout the rDNA cluster, preferentially between nearby repeat units. The FOB1 and RAD52 genes were required for both types of recombinant formation. FOB1 showed a gene dosage effect on not only the amounts of both recombinants, but also on the copy number of the repeat. However, unlike the RAD52 gene, the FOB1 gene was not involved in homologous recombination in a non-rDNA locus. In addition, the marker-duplication products were drastically decreased in the mre11 mutant. Conclusion: Our data demonstrate that FOB1 - and RAD52 -dependent homologous recombination cause the gain and loss of a few copies of the rDNA unit, and this must be a basic mechanism responsible for amplification and reduction of the rDNA copy number. In addition, FOB1 may also play a role in the copy number regulation of rDNA tandem repeats. [source]

Molecular characterization of the spectrum of genomic deletions in the mismatch repair genes MSH2, MLH1, MSH6, and PMS2 responsible for hereditary nonpolyposis colorectal cancer (HNPCC)

GENES, CHROMOSOMES AND CANCER, Issue 2 2005
Heleen van der Klift
A systematic search by Southern blot analysis in a cohort of 439 hereditary nonpolyposis colorectal cancer (HNPCC) families for genomic rearrangements in the main mismatch repair (MMR) genes, namely, MSH2, MLH1, MSH6, and PMS2, identified 48 genomic rearrangements causative of this inherited predisposition to colorectal cancer in 68 unrelated kindreds. Twenty-nine of the 48 rearrangements were found in MSH2, 13 in MLH1, 2 in MSH6, and 4 in PMS2. The vast majority were deletions, although one previously described large inversion, an intronic insertion, and a more complex rearrangement also were found. Twenty-four deletion breakpoints have been identified and sequenced in order to determine the underlying recombination mechanisms. Most fall within repetitive sequences, mainly Alu repeats, in agreement with the differential distribution of deletions between the MSH2 and MLH1 genes: the higher number and density of Alu repeats in MSH2 corresponded with a higher incidence of genomic rearrangement at this disease locus when compared with other MMR genes. Long interspersed nuclear element (LINE) repeats, relatively abundant in, for example, MLH1, did not seem to contribute to the genesis of the deletions, presumably because of their older evolutionary age and divergence among individual repeat units when compared with short interspersed nuclear element (SINE) repeats, including Alu repeats. Moreover, Southern blot analysis of the introns and the genomic regions flanking the MMR genes allowed us to detect 6 novel genomic rearrangements that left the coding region of the disease-causing gene intact. These rearrangements comprised 4 deletions upstream of the coding region of MSH2 (3 cases) and MSH6 (1 case), a 2-kb insertion in intron 7 of PMS2, and a small (459-bp) deletion in intron 13 of MLH1. The characterization of these genomic rearrangements underlines the importance of genomic deletions in the etiology of HNPCC and will facilitate the development of PCR-based tests for their detection in diagnostic laboratories. © 2005 Wiley-Liss, Inc. [source]

Controllable Molecular Doping and Charge Transport in Solution-Processed Polymer Semiconducting Layers

ADVANCED FUNCTIONAL MATERIALS, Issue 12 2009
Yuan Zhang
Abstract Here, controlled p-type doping of poly(2-methoxy-5-(2,-ethylhexyloxy)- p -phenylene vinylene) (MEH-PPV) deposited from solution using tetrafluoro-tetracyanoquinodimethane (F4-TCNQ) as a dopant is presented. By using a co-solvent, aggregation in solution can be prevented and doped films can be deposited. Upon doping the current,voltage characteristics of MEH-PPV-based hole-only devices are increased by several orders of magnitude and a clear Ohmic behavior is observed at low bias. Taking the density dependence of the hole mobility into account the free hole concentration due to doping can be derived. It is found that a molar doping ratio of 1 F4-TCNQ dopant per 600 repeat units of MEH-PPV leads to a free carrier density of 4,×,1022,m,3. Neglecting the density-dependent mobility would lead to an overestimation of the free hole density by an order of magnitude. The free hole densities are further confirmed by impedance measurements on Schottky diodes based on F4-TCNQ doped MEH-PPV and a silver electrode. [source]

Synthesis and Processing of Monodisperse Oligo(fluorene- co -bithiophene)s into Oriented Films by Thermal and Solvent Annealing

ADVANCED FUNCTIONAL MATERIALS, Issue 12 2009
Lichang Zeng
Abstract A series of oligo(fluorene- co -bithiophene)s, OF2Ts, have been synthesized and characterized to investigate the effects of oligomer length and pendant aliphatic structure on glassy-nematic mesomorphism. The OF2Ts comprising more than one repeat unit and their polymer analogue, PF2T, carrying 52 number-average repeat units, possess the highest occupied molecular orbital energy level at ,5.3,±,0.2,eV, but the anisotropic field-effect mobilities increase with the oligomer length. Spin coating from high-boiling chlorobenzene with and without subsequent exposure to saturated chlorobenzene vapor constitute solvent-vapor annealing and quasi-solvent annealing, respectively. Solvent-vapor annealing yields monodomain glassy-nematic films in which OF2Ts are aligned as well as with thermal annealing across a 2,cm diameter. Quasi-solvent annealing, however, amounts to kinetically trapping a lower orientational order than solvent-vapor or thermal annealing. While amenable to thermal annealing at elevated temperatures, PF2T shows no alignment at all following either strategy of solvent annealing. [source]

Cationic Conjugated Polyelectrolytes with Molecular Spacers for Efficient Fluorescence Energy Transfer to Dye-Labeled DNA,

ADVANCED FUNCTIONAL MATERIALS, Issue 2 2007
Y. Woo
Abstract Two water-soluble conjugated polyelectrolytes, poly(9,9,-bis(6- N,N,N -trimethylammoniumhexyl)fluorene- alt -1,4-(2,5-bis(6- N,N,N -trimethylammoniumhexyloxy))phenylene) tetrabromide (P1i) and poly((10,10,-bis(6- N,N,N -trimethylammoniumhexyl)-10H-spiro(anthracene-9,9,-fluorene))- alt -1,4-(2,5-bis(6- N,N,N -trimethylammoniumhexyloxy))phenylene) tetrabromide (P2i) are synthesized, characterized, and used in fluorescence resonance energy transfer (FRET) experiments with fluorescein-labeled single-stranded DNA (ssDNA-Fl). P1i and P2i have nearly identical ,-conjugated backbones, as determined by cyclic voltammetry and UV-vis spectroscopy. The main structural difference is the presence of an anthracenyl substituent, orthogonal to the main chain in each of the P2i repeat units, which increases the average interchain separation in aggregated phases. It is possible to observe emission from ssDNA-Fl via FRET upon excitation of P2i. Fluorescein is not emissive within the ssDNA-Fl/P1i electrostatic complex, suggesting Fl emission quenching through photoinduced charge transfer (PCT). We propose that the presence of the anthracenyl "molecular bumper" in P2i increases the distance between optical partners, which decreases PCT more acutely relative to FRET. [source]

Donor,Acceptor-Substituted Oligo(1,4-phenylene)s

HELVETICA CHIMICA ACTA, Issue 6 2009
Soungkyoo Kim
Abstract Oligo(para -phenylene)s (DAOPPs) 2a,2d (n=1,4) with terminal donor,acceptor substitution (D=C6H13O, A=NO2) were prepared by applying Suzuki cross-couplings for chain extension and end capping. The push,pull effect induces short-reaching polarizations of the chain consisting of conjugated but twisted benzene rings, which was studied by NMR measurements. Electron excitation from the ground-state S0 to the more planar first-excited singlet state S1 is combined with a strong intramolecular charge transfer (ICT), which is documented by the red shift of the long-wavelength absorption (charge-transfer band) for short chains (one or two repeat units, n,=,1 or 2). The opposite influence of decreasing ICT and increasing conjugation length leads to a bathochromic series (,max(n+1),,max(n)) with a fast saturation of ,max (n). The effective conjugation length nECL=4 corresponds to ,, 349,nm. These results are discussed in the context of other oligo(para -phenylene)s (OPPs). [source]