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PubMed1999

Effect of added copolymer on the critical properties of polymer mixtures.

Monte Carlo simulation and finite-size analysis are used to determine the critical properties of a binary homopolymer blend and of a polymer blend that has been compatibilized with a random copolymer. Determination of nu, the critical exponent for the correlation length, for the mixture of two homopolymers shows that the binary blend exhibits properties that are consistent with Ising behavior (nu=0.63). Similar results for the compatibilized blend show that the added copolymer acts very much like an impurity; lowering the mixing transition temperature and increasing the value of nu to a value that is in qualitative agreement with Fisher renormalization (nu=0.69). These results are important as copolymers are often added to polymer blends to act as compatibilizers. These results, therefore, show that the analysis of the phase behavior of these mixtures must be analyzed with the understanding that its critical exponents differ from those of a binary polymer mixture.

Journal Article↗
PubMed2002

Elastic properties of polymer interfaces: aggregation of pure diblock, mixed diblock, and triblock copolymers.

Block copolymers adsorbing to an interface between two immiscible homopolymers modify the elastic constants of this interface. Within self-consistent field calculations for Gaussian chains, we determine how the bending constants vary in dependence on the block copolymer concentration and architecture. Four phenomena are discussed. (i) When a tricritical or isotropic Lifshitz critical point is approached in a ternary mixture by varying the concentration of diblock copolymers or changing temperature, the elastic constants vanish. We determine the corresponding power laws, and show that the de Gennes-Taupin criterium for the stability of lamellar phases against undulations and the Ginzburg-Landau criterium for bulk fluctuations yield identical predictions for the validity of the mean-field approximation. (ii) Addition of a small amount of diblock copolymers modifies the bending rigidity. If the diblock copolymers are comparable in length to the homopolymers, adsorption of the diblocks reduces the bending rigidity. If the diblocks are much longer, they increase the bending rigidity. Only for an extreme ratio of chain lengths (>100), the predictions for polymers tethered to an infinitely thin, impenetrable sheet become accurate. (iii) Mixtures of short and long symmetric diblock copolymers are studied, as well as mixtures of two asymmetric diblocks, which are obtained by exchanging the long and short ends. The saddle-splay modulus is found to be the same in both mixtures, while the bending rigidity is significantly smaller in the latter case. (iv) The role of the block copolymer architecture is studied by comparing the effect of triblock copolymers with the effect of diblocks with the same overall length and composition. We propose that triblock copolymers are a very efficient way to control the spontaneous curvature of an interface.

Journal Article↗
PubMed2000

Improving polymeric microemulsions with block copolymer polydispersity.

Recent experiments have demonstrated that block copolymers are capable of stabilizing immiscible homopolymer blends producing bicontinuous microemulsion. The stability of these polymeric alloys requires the copolymer to form flexible, nonattractive monolayers along the homopolymer interfaces. We predict that copolymer polydispersity can substantially and simultaneously improve the monolayers in both of these respects. Furthermore, polydispersity should provide similar improvements in systems, such as colloidal suspensions and polymer/clay composites, that utilize polymer brushes to suppress attractive interactions.

Journal Article↗
PubMed2002

Polyproline II helical structure in protein unfolded states: lysine peptides revisited.

The left-handed polyproline II (PPII) helix gives rise to a circular dichroism spectrum that is remarkably similar to that of unfolded proteins. This similarity has been used as the basis for the hypothesis that unfolded proteins possess considerable PPII helical content. It has long been known that homopolymers of lysine adopt the PPII helical conformation at neutral pH, presumably a result of electrostatic repulsion between side chains. It is shown here that a seven-residue lysine peptide also adopts the PPII conformation. In contrast with homopolymers of lysine, this short peptide is shown to retain PPII helical character under conditions in which side-chain charges are heavily screened or even neutralized. The most plausible explanation for these observations is that the peptide backbone favors the PPII conformation to maximize favorable interactions with solvent. These data are evidence that unfolded proteins do indeed possess PPII content, indicating that the ensemble of unfolded states is significantly smaller than is commonly assumed.

Circular Dichroism↗
PubMed1996

Lipopolysaccharides of Escherichia coli K12 strains that express cloned genes for the Ogawa and Inaba antigens of Vibrio cholerae O1: identification of O-antigenic factors.

Structural and serological studies were performed with the lipopolysaccharide (LPS) expressed by Escherichia coli K12 strains No. 30 and No. 64, into which cosmid clones derived from Vibrio cholerae O1 NIH 41 (Ogawa) and NIH 35A3 (Inaba) had been introduced, respectively. The two recombinant strains, No. 30 (Ogawa) and No. 64 (Inaba), produced LPS that included, in common, the O-polysaccharide chain composed of an alpha (1-->2)-linked N-(3-deoxy-L-glycero-tetronyl)-D-perosamine (4-amino-4,6-dideoxy-D-manno-pyranose) homopolymer attached to the core oligosaccharide of the LPS of E. coli K12. Structural analysis revealed the presence of N-(3-deoxy-L-glycero-tetronyl)-2-O-methyl-D-perosamine at the non-reducing terminus of the O-polysaccharide chain of LPS from No. 30 (Ogawa) but not from No. 64 (Inaba). Serological analysis revealed that No. 30 (Ogawa) and No. 64 (Inaba) LPS were found to share the group antigen factor A of V. cholerae O1. They were distinguished by presence of the Ogawa antigen factor B [co-existing with relatively small amounts of the Inaba antigen factor (c)] in the former LPS and the Inaba antigen factor C in the latter LPS. It appears, therefore, that No. 30 (Ogawa) and No. 64 (Inaba) have O-antigenic structures that are fully consistent with the AB(c) structure for the Ogawa and the AC structure for the Inaba O-forms of V. cholerae O1, respectively. Thus, the present study clearly confirmed our previous finding that the Ogawa antigenic factor B is substantially related to the 2-O-methyl group at the non-reducing terminus of the alpha (1-->2)-linked N-(3-deoxy-L-glycero-tetronyl)-D-perosamine homopolymer that forms the O-polysaccharide chain of LPS of V. cholerae O1 (Ogawa).

Antigens, Bacterial↗
PubMed2006

Alpha-haemoglobin stabilising protein is a quantitative trait gene that modifies the phenotype of beta-thalassaemia.

It has been suggested that altered levels or function of alpha-haemoglobin stabilising protein (AHSP), an erythroid-specific protein that binds specifically to free alpha-(haemo)globin, might account for some of the clinical variability in beta-thalassaemia. To assess the variation of AHSP expression, mRNA levels in circulating reticulocytes of 103 healthy individuals were measured by quantitative reverse transcription-polymerase chain reaction. AHSP expression varied up to threefold, and did not correlate with age or sex. A systematic survey of the AHSP locus identified eight sequence variants, of which six were common. Four common variants, including the longer homopolymer (T18) in the putative promoter, are strongly associated with AHSP expression. Reporter assays in K562 cells showed that the activity of the shorter (T15) reporter was relatively lower than that of the T18 reporter. In a study of nine anaemic patients who were heterozygous for beta-thalassaemia and also heterozygous for the triplicated alpha-globin gene (alpha alpha alpha/alpha alpha), frequency of the shorter homopolymer was higher than expected. AHSP expression is variable, with cis control accounting for some of its variance. In some families, the subtle altered levels in AHSP related to the AHSP genotype appears to be a relevant contributory factor in the haematological phenotype.

Adult↗
PubMed1977

The hyphal wall of Mucor mucedo. 2. Hexosamine-containing polymers.

Nitrous acid, which specifically depolymerises polymers containing hexosamines with a primary amino group, was used to analyse the hexosamine-containing polymers in the hyphal wall of Mucor mucedo. N-Acetylglucosamine was found to occur in three polymeric fractions. One fraction which was solubilised by HNO2 treatment contained-N-acetylglucosamine interspersed with glucosamine; no homopolymer of glucosamine (chitosan) was detected. Another fraction became HNO2-soluble after treatment with pronase or alkali; this points to the occurrence of a heteropolymer containing N-acetylglucosamine and glucosamine in which some of the glucosamine residues are linked to peptides via their amino groups. The residue remaaining after pronase and HNO* treatment appeared to consist of a homopolymer of N-acetylglucosamine (chitin).

Acetylglucosamine↗
PubMed1994

The structure of the O-specific chain of Legionella pneumophila serogroup 1 lipopolysaccharide.

The O-polysaccharide chain of Legionella pneumophila Philadelphia strain 1 (serogroup 1) lipopolysaccharide was investigated by means of 1H- and 13C-NMR spectroscopy, and chemical analysis. It was found to consist of an alpha-(2-->4) interlinked homopolymer of a 5-acetamidino-7-acetamido-8-O-acetyl-3,5,7,9-tetradeoxy-nonulos onic acid possessing most likely the D-glycero-L-galacto configuration, representing the first example of an acidic homopolymer of a higher sugar of this class. The ladder-like banding pattern exhibiting small distances between individual bands in the SDS/PAGE is compatible with a monosaccharide repeating unit.

Carbohydrate Conformation↗
PubMed1995

A potential soft tissue filling material: chloramphenicol loaded poly(D,L-lactide) sponges.

Poly(D,L-lactide) (PDLLA) homopolymers were produced by the ring opening polymerization of a D,L-lactide dimer by using stannous chloride as the catalyst. Chloramphenicol loaded PDLLA sponges were pre- pared by a solvent evaporation procedure by using the PDLLA homopolymers with three different molecular weights (i.e., 11,000, 20,000 and 35,000 daltons). Chloramphenicol loading was changed by using three different solvents (i.e., acetone, ethyl acetate, and acetonitrile) and by changing the initial polymer concentration and its molecular weight and the initial concentration of the drug. Higher degradation rates of the chloramphenicol loaded PDLLA sponges in alkaline pH 9.0 and at 37 degrees C were observed. Chloramphenicol release rates were also high at these conditions. It was concluded that chloramphenicol release was both degradation and diffusion controlled.

Acetonitriles↗
PubMedDate not supplied

Regulation of carbohydrate metabolism during Giardia encystment.

Giardia intestinalis trophozoites encyst when they are exposed to bile. During encystment, events related to the inducible synthesis of a novel N-acetyl-D-galactosamine (GalNAc) homopolymer, occur. Within the first 6 h of encystment, mRNA for glucosamine 6-P isomerase (GPI), the first inducible enzyme unique to this pathway appears, oxygen uptake rates double from non-encysting levels, and metronidazole (MTZ) inhibits oxygen uptake. Within 12 h, GPI and its activity are detectable and OU decreases 50% from non-encysting levels; glucose's stimulation and MTZ's inhibition of oxygen uptake cease. In contrast, aspartate uptake remained constant throughout the 40 h monitored. Two genes, gpi 1 and 2 encode for GPI, but only gpi1 is expressed during encystment. Glucosamine 6-P (GlcN6P), the synthetic product of GPI, activates UDP-N-acetylglucosamine (UDP-GlcNAc) pyrophosphorylase, a downstream enzyme, 3 to 5-fold in the direction of UDP-GlcNAc synthesis. UDP-GlcNAc is epimerized to UDP-GalNAc and UDP-GalNAc is polymerized by "cyst wall synthase" (beta 1 --> 3 GalNAc transferase) into a highly insoluble beta 1,3-linked homopolymer. This GalNAc polysaccharide, the major component of cyst wall filaments, forms, in conjunction with polypeptides, the outer cyst wall of Giardia.

Acetylgalactosamine↗
PubMed2005

Directed assembly of block copolymer blends into nonregular device-oriented structures.

Self-assembly is an effective strategy for the creation of periodic structures at the nanoscale. However, because microelectronic devices use free-form design principles, the insertion point of self-assembling materials into existing nanomanufacturing processes is unclear. We directed ternary blends of diblock copolymers and homopolymers that naturally form periodic arrays to assemble into nonregular device-oriented structures on chemically nanopatterned substrates. Redistribution of homopolymer facilitates the defect-free assembly in locations where the domain dimensions deviate substantially from those formed in the bulk. The ability to pattern nonregular structures using self-assembling materials creates new opportunities for nanoscale manufacturing.

Journal Article↗
PubMed1999

Production of poly(3-hydroxybutyric acid-co-4-hydroxybutyric acid) and poly(4-hydroxybutyric acid) without subsequent degradation by Hydrogenophaga pseudoflava.

A Hydrogenophaga pseudoflava strain was able to synthesize poly(3-hydroxybutyric acid-co-4-hydroxybutyric acid) [P(3HB-co-4HB)] having a high level of 4-hydroxybutyric acid monomer unit (4HB) from gamma-butyrolactone. In a two-step process in which the first step involved production of cells containing a minimum amount of poly(3-hydroxybutyric acid) [P(3HB)] and the second step involved polyester accumulation from the lactone, approximately 5 to 10 mol% of the 3-hydroxybutyric acid (3HB) derived from the first-step culture was unavoidably reincorporated into the polymer in the second cultivation step. Reincorporation of the 3HB units produced from degradation of the first-step residual P(3HB) was confirmed by high-resolution 13C nuclear magnetic resonance spectroscopy. In order to synthesize 3HB-free poly(4-hydroxybutyric acid) [P(4HB)] homopolymer, a three-stage cultivation technique was developed by adding a nitrogen addition step, which completely removed the residual P(3HB). The resulting polymer was free of 3HB. However, when the strain was grown on gamma-butyrolactone as the sole carbon source in a synthesis medium, a copolyester of P(3HB-co-4HB) containing 45 mol% 3HB was produced. One-step cultivation on gamma-butyrolactone required a rather long induction time (3 to 4 days). On the basis of the results of an enzymatic study performed with crude extracts, we suggest that the inability of cells to produce 3HB in the multistep culture was due to a low level of 4-hydroxybutyric acid (4HBA) dehydrogenase activity, which resulted in a low level of acetyl coenzyme A. Thus, 3HB formation from gamma-butyrolactone is driven by a high level of 4HBA dehydrogenase activity induced by long exposure to gamma-butyrolactone, as is the case for a one-step culture. In addition, intracellular degradation kinetics studies showed that P(3HB) in cells was completely degraded within 30 h of cultivation after being transferred to a carbon-free mineral medium containing additional ammonium sulfate, while P(3HB-co-4HB) containing 5 mol% 3HB and 95 mol% 4HB was totally inert in interactions with the intracellular depolymerases. Intracellular inertness could be a useful factor for efficient synthesis of the P(4HB) homopolymer and of 4HB-rich P(3HB-co-4HB) by the strain used in this study.

3-Hydroxybutyric Acid↗
PubMed1989

Fine structure of A and M antigens from Brucella biovars.

Brucella A and M epitopes were found on single O-polysaccharide chains of all biotype strains of this species. Lipopolysaccharides from the type and reference strains of five of the six Brucella species, B. abortus, B. melitensis, B. suis, B. canis, and B. neotomae, were extracted and purified. Analysis by sodium dodecyl sulfate-polyacrylamide gel electrophoresis, in conjunction with silver staining and immunoblotting developed by monoclonal antibodies, showed bands characteristic of A, M, or mixed A and M antigens. The A antigen previously described as an exclusively alpha 1,2-linked homopolymer of 4,6-dideoxy-4-formamido-D-mannopyranose was shown by 1H and 13C nuclear magnetic resonance spectroscopy to possess a fine structure consistent with the low-frequency occurrence of alpha 1, 3-linked 4,6-dideoxy-4-formamido-D-mannopyranose residues. This feature was previously attributed only to the M antigen, which is also a homopolymer of the same sugar. B. melitensis biotype 3 and B. suis biotype 4 lipopolysaccharides showed characteristics of mixed A and M antigens. Immunoabsorption of these O polysaccharides on a column of immobilized A-antigen-specific monoclonal antibody enriched polymer chains with A-antigen characteristics but did not eliminate M epitopes. Composite A- and M-antigen characteristics resulted from O polysaccharides in which the frequency of alpha 1,3 linkages, and hence, M-antigen characteristics, varied. All biotypes assigned as A+ M- expressed one or two alpha 1,3-linked residues per polysaccharide O chain. M antigens (M+ A-) also possessed a unique M epitope as well as a tetrasaccharide determinant common to A-antigen structures. B. canis and B. abortus 45/20, both rough strains, expressed low-molecular-weight A antigen.

Antibodies, Monoclonal↗
PubMed2001

Interactions of surfactant proteins A and D with Saccharomyces cerevisiae and Aspergillus fumigatus.

Surfactant proteins A (SP-A) and D (SP-D) are members of the collectin family of calcium-dependent lectins and are important pulmonary host defense molecules. Human SP-A and SP-D and rat SP-D bind to Aspergillus fumigatus conidia, but the ligand remains unidentified. To identify a fungal ligand for SP-A and/or SP-D, we examined the interactions of the proteins with Saccharomyces cerevisiae. SP-D but not SP-A bound yeast cells, and EDTA inhibited the binding. SP-D also aggregated yeast cells and isolated yeast cell walls. Treating yeast cells to remove cell wall mannoprotein did not reduce SP-D binding, and SP-D failed to aggregate chitin. However, SP-D aggregated yeast glucan before and after treatment with a beta(1-->3)-glucanase, suggesting a specific interaction between the collectin and beta(1-->6)-glucan. In support of this idea, SP-D-induced yeast aggregation was strongly inhibited by pustulan [a beta(1-->6)-linked glucose homopolymer] but was not inhibited by laminarin [a beta(1-->3)-linked glucose homopolymer]. Additionally, pustulan but not laminarin strongly inhibited SP-D binding to A. fumigatus. The pustulan concentration for 50% inhibition of SP-D binding to A. fumigatus is 1.0 +/- 0.3 microM glucose equivalents. Finally, SP-D showed reduced binding to the beta(1-->6)-glucan-deficient kre6 yeast mutant. Taken together, these observations demonstrate that beta(1-->6)-glucan is an important fungal ligand for SP-D and that glycosidic bond patterns alone can determine if an extended carbohydrate polymer is recognized by SP-D.

Aspergillus fumigatus↗
PubMed1984

Heterogeneity of the polyribocytidylic acid tract in aphthovirus: biochemical and biological studies of viruses carrying polyribocytidylic acid tracts of different lengths.

In this paper we report a study of a sample of foot-and-mouth disease virus carrying two polyribocytidylic acid [poly(C)] tracts of different lengths. By plaque purification in tissue culture, we isolated two populations of particles, one carrying the long poly(C) tract and the other carrying only the short homopolymer. The fingerprints of both viruses were indistinguishable from each other and from that of the virus present in the original sample, suggesting that the main difference between the two types of particles is limited to the poly(C) tracts of their genomic RNAs, to the flanking sequences of the poly(C) tract, or to both. In addition, some biological properties of these viruses are reported, such as stability upon serial passages in different cell lines, plaque size, and pathogenicity for cattle. The results indicate that the size of the poly(C) tract is not directly related to the virulence of these viruses. However, the size of the homopolymer could play a role in determining their efficiency of replication, and it appears that the particles with the short poly(C) tract might have some replicative advantage over those carrying the long one.

Animals↗
PubMed1985

Neuraminidase associated with coliphage E that specifically depolymerizes the Escherichia coli K1 capsular polysaccharide.

Plaque morphology indicated that the five Escherichia coli K1-specific bacteriophages (A to E) described by Gross et al. (R. J. Gross, T. Cheasty, and B. Rowe, J. Clin. Microbiol. 6:548-550, 1977) encode K1 depolymerase activity that is present in both the bound and free forms. The free form of the enzyme from bacteriophage E was purified 238-fold to apparent homogeneity and in a high yield from ammonium sulfate precipitates of cell lysates by a combination of CsCl density gradient ultracentrifugation, gel filtration, and anion-exchange chromatography. The enzyme complex had an apparent molecular weight of 208,000, as judged from its behavior on sodium dodecyl sulfate-polyacrylamide gel electrophoresis, and was dissociated by sodium dodecyl sulfate at 100 degrees C to yield two polypeptides with apparent molecular weights of 74,000 and 38,500. Optimum hydrolytic activity was observed at pH 5.5, and activity was strongly inhibited by Ca2+; the Km was 7.41 X 10(-3) M. Rapid hydrolysis of both the O-acetylated and non-O-acetylated forms of the K1 antigen, an alpha 2----8-linked homopolymer of N-acetylneuraminic acid, and of the meningococcus B antigen was observed. Limited hydrolysis of the E. coli K92 antigen, an N-acetylneuraminic acid homopolymer containing alternating alpha 2----8 and alpha 2----9 linkages, occurred, but the enzyme failed to release alpha 2----3-, alpha 2----6-, or alpha 2----9-linked sialic residues from a variety of other substrates.

Antigens, Bacterial↗
PubMed1996

The RGG box motif of the herpes simplex virus ICP27 protein mediates an RNA-binding activity and determines in vivo methylation.

ICP27 is an essential herpes simplex virus type 1 nuclear regulatory protein that is required for efficient viral gene expression. Although the mechanism by which ICP27 regulates genes is unknown, a variety of evidence suggests that it functions posttranscriptionally, and recent studies indicate that it is an RNA-binding protein. Previously, we noted that a short arginine- and glycine-rich sequence in ICP27 (residues 138 to 152) is similar to an RGG box motif, a putative RNA-binding determinant found in a number of cellular proteins (W. Mears, V. Lam, and S. Rice, J. Virol. 69:935-947, 1995). In the present study, we have further investigated ICP27's association with RNA and examined the role of the RGG box in RNA binding. We find that ICP27 binds efficiently to RNA homopolymers composed of poly(G) and weakly to poly(U) RNA homopolymers. Poly(G) binding activity maps to the N-terminal 189 residues of ICP27 and requires the RGG box sequence. Using a northwestern blotting assay, we demonstrate that the RGG box alone (residues 140 to 152) can mediate RNA binding when attached to a heterologous protein. As many cellular RGG box proteins are methylated on arginine residues, we also investigated the in vivo methylation status of ICP27. Our results demonstrate that ICP27 is methylated in herpes simplex virus-infected cells. Methylation is dependent on the presence of the RGG box, suggesting that one or more arginine residues in the RGG box sequence are modified. These data demonstrate that ICP27 displays the characteristics of an RGG box-type RNA-binding protein.

Amino Acid Sequence↗
PubMed2002

Characterization of RNA determinants recognized by the arginine- and proline-rich region of Us11, a herpes simplex virus type 1-encoded double-stranded RNA binding protein that prevents PKR activation.

The herpes simplex virus Us11 gene product inhibits activation of the cellular PKR kinase and associates with a limited number of unrelated viral and cellular RNA molecules via a carboxyl-terminal 68-amino-acid segment rich in arginine and proline. To characterize the determinants underlying the recognition of an RNA target by Us11, we employed an in vitro selection technique to isolate RNA ligands that bind Us11 with high affinity from a population of molecules containing an internal randomized segment. Binding of Us11 to these RNA ligands is specific and appears to occur preferentially on conformational isoforms that possess a higher-order structure. While the addition of unlabeled poly(I. C) reduced binding of Us11 to a selected radiolabeled RNA, single-stranded homopolymers were not effective competitors. Us11 directly associates with poly(I. C), and inclusion of an unlabeled selected RNA in the reaction reduces poly(I. C) binding, while single-stranded RNA homopolymers have no effect. Finally, Us11 binds to defined, double-stranded RNA (dsRNA) molecules that exhibit greater sequence complexity. Binding to these dsRNA perfect duplexes displays a striking dependence on length, as 39-bp or shorter duplexes do not bind efficiently. Furthermore, this interaction is specific for dsRNA as opposed to dsDNA, implying that the Us11 RNA binding domain can distinguish nucleic acid duplexes containing 2' hydroxyl groups from those that do not. These results establish that Us11 is a dsRNA binding protein. The arginine- and proline-rich Us11 RNA binding domain is unrelated to known dsRNA binding elements and thus constitutes a unique recognition motif that interacts with dsRNA. The ability of Us11 to bind dsRNA may be important for inhibiting activation of the cellular PKR kinase in response to dsRNA.

Arginine↗