Mechanism of coupling periodate-oxidized nucleosides to proteins

This paper describes a mechanism of coupling periodate-oxidized nucleosides to proteins. Each of the dialdehyde groups of a periodate-oxidized nucleoside is shown to couple to lysine residues on different protein molecules through Schiff bases, thereby cross-linking different protein molecules, forming a polymer. This is in contrast to the previous model in which nucleosides were suggested to couple to proteins through a morpholine structure. The cross-linked structure of the nucleoside-antigen, significantly different when compared to the native protein, may affect the specificity and the efficiency of antibody production.

Synthesis and Flammability of Bromine-Containing Polybutadiene

COOH-terminated polybutadiene has been converted into COCl-terminated polybutadiene which was further chemically cross-linked with bisphenol-A and tetrabromobisphenol-A. The flammability characteristics of bisphenol-A and tetrabromobisphenol-A resins have been examined using thermogravimetric, ignition temperature and mass burning rate data. Mass burning rate results support that tetrabromobisphenol-A cross-linked polybutadiene is flame retardant compared to the corresponding non brominated compositions.

Thermal polymerization of acrylamide in the solid state

The role of imperfections in thermal polymerization of acrylamide in the solid state was studied. The polymer yield and the degree of polymerization are highly dependent on the particle size and on the pressure to which the monomer is subjected prior to polymerization reaction. There is an enhancement in the rate of polymerization in air unlike in the case of radiation-induced polymerization. Thermal polymerization of acrylamide in pelletized form results in the formation of water-soluble linear polymer and water-insoluble cross-linked product with the evolution of ammonia. The activation energy (E) values obtained in the present investigation reveal that basically there are two processes taking place, one with E = 34–36 kcal/mole, corresponding to the initiation process, and the other with E = 19 ± 3 kcal/more for the propagation process.

Borax Mediated Layer-by-Layer Self-Assembly of Neutral Poly(vinyl alcohol) and Chitosan

We report a multilayer film of poly(vinyl alcohol) (PVA)-borate complex and chitosan by using a layer-by-layer approach. PVA is an uncharged polymer, but hydroxyl functional groups of PVA can be crosslinked by using borax as a cross-linking agent. As a result electrostatic charges and intra- and interchain cross-links are introduced in the PVA chain and provide physically cross-linked networks. The PVA-borate was then deposited on a flat Substrate as well as on colloidal particles with chitosan as an oppositely charged polyelectrolyte. Quartz crystal microbalance. scanning electron microscopy, and atomic force microscopy were used to follow the growth of thin film oil flat substrate. Analogous experiments were performed on melamine formaldehyde colloidal particles (3-3.5 mu m) to quantify the process for the preparation of hollow rnicrocapsules. Removal of the core in 0.1 N HCI results in hollow microcapsules. Characterization of microcapsules by transmission electron microscopy revealed formation of stable microcapsules. Further, self-assembly of PVA-borate/chitosan was loaded with the anticancer drug doxorubicin, and release rates were determined at different pH Values to highlight the drug delivery potential of this system.

Thiol-Disulfide Interchange Mediated Reversible Dendritic Megamer Formation and Dissociation

We report the formation of dynamic, reversible cross-linked dendritic megamers and their dissociation to monomeric dendrimers, through a thiol-disulfide interchange reaction. For this study, poly(alkyl aryl ether) dendrimers up to three-generations presenting thiol functionalities, were prepared. The series from zero to three generations of dendrimers were installed with 3, 6, 12, and 24 thiol functionalities at their peripheries. Upon synthesis, cross-linking of the dendrimer was accomplished through disulfide bond formation. The cross-linking of dendrimers was monitored through optical density changes at 420 nm. Dense cross-linking led to visible precipitation of dendritic megamers and the morphologies of the megamers were characterized by transmission electron microscopy. The disulfide cross-links between megamer monomers could be dissociated readily upon reduction of disulfide bond by dithiothreitol reagent. Preliminary studies show that dendritic megamers encapsulate C-60 and the efficiency of encapsulation increased with increasing generation of dendritic megamer.

Functional characterization of the precursor and spliced forms of RecA protein of Mycobacterium tuberculosis

The recA locus of pathogenic mycobacteria differs from that of nonpathogenic species because it contains large intervening sequences nested in the RecA homology region that are excised by an unusual protein-splicing reaction. In vivo assays indicated that Mycobacterium tuberculosis recA partially complemented Escherichia coli recA mutants for recombination and mutagenesis. Further, splicing of the 85 kDa precursor to 38 kDa MtRecA protein was necessary for the display of its activity, in vivo. To gain insights into the molecular basis for partial and lack of complementation by MtRecA and 85 kDa proteins, respectively, we purified both of them to homogeneity. MtRecA protein, but not the 85 kDa form, bound stoichiometrically to single-stranded DNA in the presence of ATP. MtRecA protein was cross-linked to 8-azidoadenosine 5'-triphosphate with reduced efficiency, and kinetic analysis of ATPase activity suggested that it is due to decreased affinity for ATP. In contrast, the 85 kDa form was unable to bind ATP, in the presence or absence of ssDNA and, consequently, was entirely devoid of ATPase activity. Molecular modeling studies suggested that the decreased affinity of MtRecA protein for ATP and the reduced efficiency of its hydrolysis might be due to the widening of the cleft which alters the hydrogen bonds and the contact area between the enzyme and the substrate and changes in the disposition of the amino acid residues around the magnesium ion and the gamma-phosphate. The formation of joint molecules promoted by MtRecA protein was stimulated by SSB when the former was added first. The probability of an association between the lack and partial levels of biological activity of RecA protein(s) to that of illegitimate recombination in pathogenic mycobacteria is considered.

Synthesis, structure and magnetic behavior of a new three-dimensional Manganese phosphite-oxalate: [C2N2H10][Mn-2(II)(OH2)(2)(HPO3)(2)(C2O4)]

A novel manganese phosphite-oxalate, [C2N2H10][Mn-2(II)(OH2)(2)(HPO3)(2)(C2O4)] has been hydothermally synthesized and its structure determined by single-crystal X-ray diffraction. The structure consists of neutral manganese phosphite layers, [Mn(HPO3)](infinity), formed by MnO6 octahedra and HPO3 units, cross-linked by the oxalate moieties. The organic cations occupy the middle of the 8-membered one dimensional channels. Magnetic studies indicate weak antiferromagnetic interactions between the Mn2+ ions. (C) 2009 Elsevier Inc. All rights reserved.

A novel fibre-coated sorbent for rapid removal of heavy metals from wastewater. Sorption of Zinc(II) from dilute aqueous solutions in the presence of high concentrations of common salts

The commercial acrylic fibre "Cashmilon" was partially hydrolyzed to convert a fraction of its nitrile (-CN) groups to carboxylic acid (-COOH) groups and then coated with polyethylenimine (PEI) resin and cross-linked with glutaraldehyde to produce a novel gel-coated fibrous sorbent with multiple functionalities of cationic, anionic and chelating types, and significantly faster sorption kinetics than bead-form sorbents. The sorption properties of the fibrous sorbent were measured using Zn(II) in aqueous solution as the sorbate to determine the effects of pH and the presence of common ions in the solution on the sorption capacity. The rate of sorption on the gel-coated fibre was measured in comparison with that on Amberlite IRA-68 weak-base resin beads, to demonstrate the marked difference between fibre and bead-form sorbents in their kinetic behaviour.

A novel fibre-coated sorbent for rapid removal of heavy metals from wastewater. Sorption of Zinc(II) from dilute aqueous solutions in the presence of high concentrations of common salts

The commercial acrylic fibre "Cashmilon" was partially hydrolyzed to convert a fraction of its nitrile (-CN) groups to carboxylic acid (-COOH) groups and then coated with polyethylenimine (PEI) resin and cross-linked with glutaraldehyde to produce a novel gel-coated fibrous sorbent with multiple functionalities of cationic, anionic and chelating types, and significantly faster sorption kinetics than bead-form sorbents. The sorption properties of the fibrous sorbent were measured using Zn(II) in aqueous solution as the sorbate to determine the effects of pH and the presence of common ions in the solution on the sorption capacity. The rate of sorption on the gel-coated fibre was measured in comparison with that on Amberlite IRA-68 weak-base resin beads, to demonstrate the marked difference between fibre and bead-form sorbents in their kinetic behaviour.

Ring-Opening-Induced Toughening of a Low-Permittivity Polymer-Metal Interface

Integrating low dielectric permittivity (low-k) polymers to metals is an exacting fundamental challenge because poor bonding between low-polarizability moieties and metals precludes good interfacial adhesion. Conventional adhesion-enhancing methods such as using intermediary layers are unsuitable for engineering polymer/metal interfaces for many applications because of the collateral increase in dielectric permittivity. Here, we demonstrate a completely new approach without surface treatments or intermediary layers to obtain an excellent interfacial fracture toughness of > 13 J/m(2) in a model system comprising copper. and a cross-linked polycarbosilane with k similar to 2.7 obtained by curing a cyclolinear polycarbosilane in air.Our results suggest that interfacial oxygen catalyzed molecularring-opening and anchoring of the opened ring moieties of the polymer to copper is the main toughening mechanism. This novel approach of realizing adherent low-k polymer/metal structures without intermediary layers by activating metal-anchoring polymer moieties at the interface could be adapted for applications such as device wiring and packaging, and laminates and composites.

Molecular Basis for the Role of Staphylococcus aureus Penicillin Binding Protein 4 in Antimicrobial Resistance

Penicillin binding proteins (PBPs) are membrane-associated proteins that catalyze the final step of murein biosynthesis. These proteins function as either transpeptidases or carboxypeptidases and in a few cases demonstrate transglycosylase activity. Both transpeptidase and carboxypeptidase activities of PBPs occur at the D-Ala-D-Ala terminus of a murein precursor containing a disaccharide pentapeptide comprising N-acetyl-glucosamine and N-acetyl-muramic acid-L-Ala-D-Glu-L-Lys-D-Ala-D-Ala. beta-Lactam antibiotics inhibit these enzymes by competing with the pentapeptide precursor for binding to the active site of the enzyme. Here we describe the crystal structure, biochemical characteristics, and expression profile of PBP4, a low-molecular-mass PBP from Staphylococcus aureus strain COL. The crystal structures of PBP4-antibiotic complexes reported here were determined by molecular replacement, using the atomic coordinates deposited by the New York Structural Genomics Consortium. While the pbp4 gene is not essential for the viability of S. aureus, the knockout phenotype of this gene is characterized by a marked reduction in cross-linked muropeptide and increased vancomycin resistance. Unlike other PBPs, we note that expression of PBP4 was not substantially altered under different experimental conditions, nor did it change across representative hospital- or community-associated strains of S. aureus that were examined. In vitro data on purified recombinant S. aureus PBP4 suggest that it is a beta-lactamase and is not trapped as an acyl intermediate with beta-lactam antibiotics. Put together, the expression analysis and biochemical features of PBP4 provide a framework for understanding the function of this protein in S. aureus and its role in antimicrobial resistance.

PVA-SSA-HPA Mixed-Matrix-Membrane Electrolytes for DMFCs

Stabilized forms of heteropolyacids (HPAs), namely phosphomolybdic acid (PMA), phosphotungstic acid (PTA), and silicotungstic acid (STA), are incorporated into poly (vinyl alcohol) (PVA) cross-linked with sulfosuccinic acid (SSA) to form mixed-matrix membranes for application in direct methanol fuel cells (DMFCs). Bridging SSA between PVA molecules not only strengthens the network but also facilitates proton conduction in HPAs. The mixed-matrix membranes are characterized for their mechanical stability, sorption capability, ion-exchange capacity, and wetting in conjunction with their proton conductivity, methanol permeability, and DMFC performance. Methanol-release kinetics is studied ex situ by volume-localized NMR spectroscopy (employing point-resolved spectroscopy'') with the results clearly demonstrating that the incorporation of certain inorganic fillers in PVA-SSA viz., STA and PTA, retards the methanol-release kinetics under osmotic drag compared to Nafion, although PVA-SSA itself exhibits a still lower methanol permeability. The methanol crossover rate for PVA-SSA-HPA-bridged-mixed-matrix membranes decreases dramatically with increasing current density rendering higher DMFC performance in relation to a DMFC using a pristine PVA-SSA membrane. A peak power density of 150 mW/cm(2) at a load current density of 500 mA/cm(2) is achieved for the DMFC using a PVA-SSA-STA-bridged-mixed-matrix-membrane electrolyte. (C) 2010 The Electrochemical Society. [DOI: 10.1149/1.3465653] All rights reserved.

Mechanical Anisotropy in Crystalline Saccharin: Nanoindentation Studies

The nanoindentation technique has been employed to relate the mechanical properties of saccharin single crystals with their internal structure. Indentations were performed on (100) and (011) faces to assess the mechanical anisotropy. The load-displacement (P-h) curves indicate significant differences in the nature of the plastic deformation on the two faces. The P-h curves obtained on the (011) plane are smooth, reflecting homogeneous plasticity. However, displacement bursts (pop-ins) are observed in the P-h curves obtained on the (100) plane suggesting a discrete deformation mechanism. Marginal differences exist in the hardness and modulus on the two faces that may, in part, be rationalized, although one notes that saccharin has a largely three-dimensional close-packed structure. The structural origins of the fundamentally different deformation mechanisms on (100) and (011) are discussed in terms of the dimensionality of the hydrogen bonding networks. Down the (100) planes, the saccharin dimers are stacked and are stabilized by nonspecific van der Wants interactions mostly between aromatic rings. However, down the (011) planes, the molecules are stabilized by more directional and cross-linked C-H ... O hydrogen bonds. This anisotropy in crystal packing and interactions is reflected in the mechanical behavior on these faces. The displacements associated with the pop-ins were found to he integral multiples oldie molecule separation distances. Nanoindentation offers an opportunity to compare experimentally, and in a quantitative way, the various intermolecular interactions that fire present in a molecular crystal.

Unfolding of Plasmodium falciparum triosephosphate isomerase in urea and guanidinium chloride solutions. Evidence for a novel disulfide exchange reaction in a covalently crosslinked mutant

The conformational stability of Plasmodium falciparum triosephosphate isomerase (TIMWT) enzyme has been investigated in urea and guanidinium chloride (GdmCl) solutions using circular dichroism, fluorescence, and size-exclusion chromatography. The dimeric enzyme is remarkably stable in urea solutions. It retains considerable secondary, tertiary, and quaternary structure even in 8 M urea. In contrast, the unfolding transition is complete by 2.4 M GdmCl. Although the secondary as well as the tertiary interactions melt before the perturbation of the quaternary structure, these studies imply that the dissociation of the dimer into monomers ultimately leads to the collapse of the structure, suggesting that the interfacial interactions play a major role in determining multimeric protein stability. The Cm(urea)/Cm(GdmCl) ratio (where Cm is the concentration of the denaturant required at the transition midpoint) is unusually high for triosephosphate isomerase as compared to other monomeric and dimeric proteins. A disulfide cross-linked mutant protein (Y74C) engineered to form two disulfide cross-links across the interface (13-74‘) and (13‘-74) is dramatically destablized in urea. The unfolding transition is complete by 6 M urea and involves a novel mechanism of dimer dissociation through intramolecular thiol−disulfide exchange.

A crosslinked ``polymer-gel'' rechargeable lithium-ion battery electrolyte from free radical polymerization using nonionic plastic crystalline electrolyte medium

A cross-linked polymer-gel soft matter electrolyte with superior electrochemical, thermal and mechanical properties obtained from free radical polymerization of vinyl monomers in a semi-solid organic nonionic plastic crystalline electrolyte for application in rechargeable lithium-ion batteries is discussed here.