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Electrostatic Repulsion (electrostatic + repulsion)
Selected AbstractsX-Ray Structure Analyses of syn/anti -Conformers of N -Furfuroyl-, N -Benzoyl-, and N -Picolinoyl-Substituted (2R)-Bornane-10,2-sultam DerivativesHELVETICA CHIMICA ACTA, Issue 8 2008Karolina Koszewska Abstract The synthesis and the X-ray structure of the three new N -(arylcarbonyl)-substituted derivatives 2a,2c of (2R)-bornane-10,2-sultam are presented and discussed. Direct comparison of the solid-state analyses shows that the dipole-directed SO2/CO anti- /syn- conformations may be very sensitive to weak electronic/electrostatic repulsions of the heteroatom lone pairs. The optimum interactions are reached when the lone pair of the , -positioned heteroatom is oriented in the O(3)C(11)N(1) plane. Such rare syn -conformations may be observed with at least up to 1.8,kcal/mol higher energy as compared to their ground states. Additionally, these anti/syn- conformations are also very sensitive to external influences such as, for example, the crystal-packing forces. [source] Real Time Electrochemical Monitoring of DNA/PNA Dissociation by Melting Curve AnalysisELECTROANALYSIS, Issue 14 2009Xiaoteng Luo Abstract An immobilization-free electrochemical method is reported for real-time monitoring of the DNA hybrid dissociation between a ferrocene labeled peptide nucleic acid (PNA) and a fully-complementary or single-base-mismatched DNA. This method takes advantages of electrostatic charge characteristics and interactions among the neutrally charged PNA, the negatively charged DNA and the negatively charged electrode surface made of indium tin oxide (ITO). When a ferrocene labeled PNA (Fc-PNA) sequence is hybridized to a complementary DNA strand, electrostatic repulsion between the negatively charged PNA/DNA hybrid and the negative ITO surface retards the diffusion of the electroactive Fc to the electrode, resulting in a much reduced electrochemical signal. On the other hand, when the Fc-PNA is dissociated from the hybrid at elevated temperatures, the neutrally charged Fc-PNA easily diffuses to the electrode with an enhanced electrochemical signal. Therefore, an electrochemical melting curve of the Fc-PNA/DNA hybrid can be obtained by measuring the Fc signal with the increasing temperature. This strategy allows monitoring of the dissociation of the DNA hybrid in real time, which might lead to a simple detection method for single nucleotide polymorphism (SNP) analysis. [source] Multi-walled carbon nanotube composites with polyacrylate prepared for open-tubular capillary electrochromatographyELECTROPHORESIS, Issue 19 2010Jian-Lian Chen Abstract A new phase containing immobilized carbon nanotubes (CNTs) was synthesized by in situ polymerization of acid-treated multi-walled CNTs using butylmethacrylate (BMA) as the monomer and ethylene dimethacrylate as the crosslinker on a silanized capillary, forming a porous-layered open-tubular column for CEC. Incorporation of CNT nanomaterials into a polymer matrix could increase the phase ratio and take advantage of the easy preparation of an OT-CEC column. The completed BMA-CNT column was characterized by SEM, ATR-IR, and EOF measurements, varying the pH and the added volume organic modifier. In the multi-walled CNTs structure, carboxylate groups were the major ionizable ligands on the phase surface exerting the EOF having electroosmotic mobility, 4.0×104,cm2,V,1,S,1, in the phosphate buffer at pH 2.8 and RSD values (n=5), 3.2, 4.1, and 4.3%, for three replicate capillaries at pH 7.6. Application of the BMA-CNT column in CEC separations of various samples, including nucleobases, nucleosides, flavonoids, and phenolic acids, proved satisfactory upon optimization of the running buffers. Their optima were found in the borate buffers at pH 9.0/50,mM, pH 9.5/10,mM/50% v/v ACN, and pH 9.5/30,mM/10% v/v methanol, respectively. The separations could also be used to assess the relative contributions of electrophoresis and chromatography to the CEC mechanism by calculating the corresponding velocity and retention factors. Discussions about interactions between the probe solutes and the bonded phase included the ,,, interactions, electrostatic repulsion, and hydrogen bonding. Furthermore, a reversed-phase mode was discovered to be involved in the chromatographic retention. [source] Decreasing effective nanofluidic filter size by modulating electrical double layers: Separation enhancement in microfabricated nanofluidic filtersELECTROPHORESIS, Issue 23 2008Hansen Bow Abstract Conventional methods for separating biomolecules are based on steric interactions between the biomolecules and randomly oriented gel fibers. The recently developed artificial molecular sieves also rely on steric interactions for separation. In this work, we present an experimental investigation of a method that can be used in these sieves to increase separation selectivity and resolution. This method exploits the electrostatic repulsion between the charged molecules and the charged nanofluidic structure. Although this method has been mentioned in the previous work, it has not been examined in detail. We characterize this method by comparing the selectivity with that achieved in devices with different dimensions. The results of this study are relevant to the optimization of chip-based gel-free biomolecule separation and analysis. [source] 1,8-Bis(dialkylamino)-4,5-dinitronaphthalenes and 4,5-Bis(dimethylamino)naphthalene-1,8-dicarbaldehyde as "Push,Pull" Proton Sponges: When and Why Formyl Groups Become Stronger ,-Electron Acceptors than Nitro GroupsEUROPEAN JOURNAL OF ORGANIC CHEMISTRY, Issue 8 2009Valery A. Ozeryanskii Abstract Single-crystal X-ray studies of four representatives of "push,pull" proton sponges, namely 1,8-bis(dimethylamino)-, 1,8-bis(diethylamino)-, 1,8-bis(dipropylamino)-4,5-dinitronaphthalenes and 4,5-bis(dimethylamino)naphthalene-1,8-dicarbaldehyde have been performed at low and ambient temperatures. The most interesting and unexpected result is that the formyl groups in the peri -dialdehyde display stronger ,-acceptor effects than the nitro groups. This phenomenon is ascribed to smaller steric demands of the CHO groups, their lower electrostatic repulsion, and specific packing forces. The naphthalene cores of all but one of the molecules are markedly twisted (21,26°) while that of the diethylamino derivative is not (<5°), providing different and somewhat unpredictable ways of resonance stabilization and steric relaxation. The through-conjugation in the above compounds is also discussed for gas and solution phases on the basis of theoretical calculations, UV/Vis and 1H NMR spectra.(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2009) [source] Non-Covalent Interactions of Organic Halogen Compounds with Aromatic Systems , Analyses of Crystal Structure DataEUROPEAN JOURNAL OF ORGANIC CHEMISTRY, Issue 6 2005Dariusz Swierczynski Abstract The Cambridge Structural Database showed in mid 2002 about 20.000 structures containing halogen atoms and aryl rests with distances d between the aryl center and the halogen atom, which would allow both hydrogen bonds with the aromatic hydrogens and/or van der Waalsinteractions with the ,-cloud. The latter are characterized by short distances d and by small angles , between the vector along the aryl centroid,halogen line and the perpendicular vector originating in the aryl center (the plane normal). The cases with d < 3.0 Å for F; and d < 3.5 Å for Cl, Br or I (outliers neglected), and , , 10 ± 5°, indicating predominating van der Waals forces, amount to several hundreds. The majority of fragments exhibit larger d and , values, in line with avoidance of electrostatic repulsion between the negative partial charges of the halogens and the ,-cloud center, and with an increasing electrostatic attraction with the aromatic hydrogen atoms. The corresponding hydrogen bonds are characterized by longer distances d and by angle values of , > 60° (about 40 % of the fragments), with only very few cases close to linear hydrogen bonds (then with , = 90°). Compounds containing metal,halogen bonds were omitted in view of possible strong orientational interferences. The structures were screened with four different halogen binding modes, all of them containing the halogen attached to a carbon atom, but with different hybrizations at the connecting carbon centers. The results show only small differences in the distance distributions, with a slight preference for smaller , values for sp2 frameworks. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2005) [source] The role of residues R97 and Y331 in modulating the pH optimum of an insect ,-glycosidase of family 1FEBS JOURNAL, Issue 24 2003Sandro R. Marana The activity of the digestive ,-glycosidase from Spodoptera frugiperda (Sf,gly50, pH optimum 6.2) depends on E399 (pKa = 4.9; catalytic nucleophile) and E187 (pKa = 7.5; catalytic proton donor). Homology modelling of the Sf,gly50 active site confirms that R97 and Y331 form hydrogen bonds with E399. Site-directed mutagenesis showed that the substitution of R97 by methionine or lysine increased the E399 pKa by 0.6 or 0.8 units, respectively, shifting the pH optima of these mutants to 6.5. The substitution of Y331 by phenylalanine increased the pKa of E399 and E187 by 0.7 and 1.6 units, respectively, and displaced the pH optimum to 7.0. From the observed ,pKa it was calculated that R97 and Y331 contribute 3.4 and 4.0 kJ·mol,1, respectively, to stabilization of the charged E399, thus enabling it to be the catalytic nucleophile. The substitution of E187 by D decreased the pKa of residue 187 by 0.5 units and shifted the pH optimum to 5.8, suggesting that an electrostatic repulsion between the deprotonated E399 and E187 may increase the pKa of E187, which then becomes the catalytic proton donor. In short the data showed that a network of noncovalent interactions among R97, Y331, E399 and E187 controls the Sf,gly50 pH optimum. As those residues are conserved among the family 1 ,-glycosidases, it is proposed here that similar interactions modulate the pH optimum of all family 1 ,-glycosidases. [source] B,Z DNA Transition Triggered by a Cationic Comb-Type CopolymerADVANCED FUNCTIONAL MATERIALS, Issue 22 2009Naohiko Shimada Abstract The conformational transition from right-handed B,DNA to left-handed Z,DNA,the B,Z transition,has received increased attention recently because of its potential roles in biological systems and its applicability to bionanotechnology. Though the B,Z transition of poly(dG,dC),·,poly(dG,dC) is inducible under high salt concentration conditions (over 4,M NaCl) or by addition of multivalent cations, such as hexaamminecobalt(III), no cationic polymer were known to induce the transition. In this study, it is shown by circular dichroism and UV spectroscopy that the cationic comb-type copolymer, poly(L -lysine)- graft -dextran, but not poly(L -lysine) homopolymer or a basic peptide, induces the B,Z transition of poly(dG,dC),·,poly(dG,dC). At a cationic amino group concentration of 10,4,M the copolymer stabilizes Z,DNA. The transition pathway from the B to the Z form is different to that observed previously. We speculate that the cationic backbone of the copolymer, which reduces electrostatic repulsion among DNA phosphate groups, and the hydrophilic dextran chains, which reduce activity of water, cooperate to induce the B,Z transition. The copolymer specifically modified the micro-environment around DNA molecules to induce Z,DNA formation through stable and spontaneous inter-polyelectrolyte complex formation. [source] The two-Yukawa model and its applications: the cases of charged proteins and copolymer micellar solutionsJOURNAL OF APPLIED CRYSTALLOGRAPHY, Issue 2007Sow-Hsin Chen Charged and uncharged colloidal systems are known from experiment to display an extremely rich phase behavior, which is ultimately determined by the effective pair potential between particles in solution. As a confirmation, the recent striking observation of an equilibrium cluster phase in charged globular protein solutions [Stradner, Sedgwick, Cardinaux, Poon, Egelhaaf & Schurtenberger (2004). Nature, 432, 492,495] has been interpreted as the effect of competing short-range attractive and long-range repulsive interactions. The `two-Yukawa (2Y) fluid' model assumes an interparticle potential consisting of a hard core plus an attractive and a repulsive Yukawa tail. We show that this rather simple model can indeed explain satisfactorily the structural properties of diverse colloidal materials, measured in small-angle neutron scattering (SANS) experiments, including the cases of equilibrium cluster formation and soft-core repulsion. We apply this model to the analysis of SANS data from horse-heart cytochrome c protein solutions (whose effective potential can be modeled as a hard-sphere part plus a short-range attraction and a weaker screened electrostatic repulsion) and micellar solutions of a triblock copolymer (whose effective potential can be modeled as a hard-sphere part plus a repulsive shoulder and a short-range attraction). The accuracy of the 2Y model predictions is successfully tested against Monte Carlo simulations in both cases. [source] Role of Reduced Ionic Strength and Low pH in Gelation of Chicken Breast Muscle ProteinJOURNAL OF FOOD SCIENCE, Issue 1 2005S. Ke And ABSTRACT: Elastic gels with a high moisture content of 88% were prepared at an acidic pH and low ionic strength. The relationship among pH, ionic strength, water-holding capacity, and fold score of gels was investigated. A decrease of pH from 4.1 to 3.7 or below increased gel elasticity and significantly decreased water loss under pressure (P < 0.05). In the presence of sodium chloride, gels made at pH 3.5 to 3.7 had decreased elasticity and increased water loss under pressure. Prior freezing increased the water loss of gels under pressure. Gels made with phosphoric acid and hydrochloric acid lost less water under pressure than those made with citric acid. The percentage loss of water from cylindrical gels was inversely related to the height of the cylinders, suggesting that surface effects were involved. These results suggest that net positive charges on the protein molecules at low pH produced electrostatic repulsion, which was a major driving force for water uptake in the low-salt gels. [source] Physicochemical Properties of Frozen Red Hake (Urophycis chuss) Mince as Affected by Cryoprotective IngredientsJOURNAL OF FOOD SCIENCE, Issue 7 2000P.Z. Lian ABSTRACT: Effects of alginate, iota-carrageenan, soy protein concentrate (SPC), sodium tripolyphosphate (STPP), and sorbitol on physicochemical and sensory properties of red hake mince stored at ,20 °C for 17 wk were investigated. The addition of 0.4% alginate, 4% sorbitol, and 0.3% STPP kept the mince from hardening and improved its dispersibility during mixing. This is believed to be a result of reduced protein denaturation as evidenced by higher levels of water, salt, SDS-soluble proteins, and higher free sulfhydryl contents than the untreated control mince, which hardened with compact texture and exhibited considerable syneresis. Alginate appears to be responsible for preventing muscle fiber interaction through electrostatic repulsion and chelating Ca2+, thus improving dispersibility. Sorbitol and STPP with or without SPC and iota-carrageenan did not improve the dispersibility. [source] The effective hard particle model provides a simple, robust, and broadly applicable description of nonideal behavior in concentrated solutions of bovine serum albumin and other nonassociating proteinsJOURNAL OF PHARMACEUTICAL SCIENCES, Issue 12 2007Allen P. Minton Abstract Published data on the concentration dependence of osmotic pressure of solutions of bovine serum albumin in 0.15 M NaCl at concentrations up to greater than 400 g/L are shown to be described to within experimental uncertainty by a simple one-parameter model in which protein molecules are represented by effective hard spherical particles. The volume of the effective hard particle reflects both steric and electrostatic repulsion and thus varies with pH and ionic strength. The pH dependence of the effective volume is shown to agree well with that previously obtained from analysis of the concentration dependence of sedimentation equilibrium and static light scattering. © 2007 Wiley-Liss, Inc. and the American Pharmacists Association J Pharm Sci 96: 3466,3469, 2007 [source] Magnetic field effects on the copolymerization of water-soluble and ionic monomersJOURNAL OF POLYMER SCIENCE (IN TWO SECTIONS), Issue 2 2009Ignacio Rintoul Abstract The effect of magnetic field (MF) on the radical copolymerization of a series of water-soluble and ionic monomers is presented including acrylamide (AM), acrylic acid (AA), its ionized form acrylate (A,), and diallyldimethylammonium chloride (DADMAC). The following combinations have been studied: AM/AA, AM/A,, AM/DADMAC, and AA/DADMAC. In addition to the MF, strong electrostatic interactions are present for the majority of monomer combinations and conditions. Although the monomer consumption rate (Rp) increased up to 65% applying a MF of 0.1 Tesla, the composition of the resulting copolymers was not affected under such conditions. Despite this increase of Rp by MF, the electrostatic repulsion between ionic monomers and charged growing radicals dominates Rp and governs the copolymer composition with and without MF. The order of the experimentally obtained reactivity ratios reflects the extent of electrostatic interaction: rAM/AA (1.41) < r (3.10) < rAA/DADMAC (4.25) < rAM/DADMAC (6.95) and rAA/AM (2.20) > rDADMAC/AA (0.25) > r (0.17) > rDADMAC/AM (0.03). Overall, weak MF offers to reduce the production time without modifying the product composition. © 2008 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 47: 373,383, 2009 [source] Long-term stability of an ambient self-curable latex based on colloidal dispersions in water of two reactive polymersJOURNAL OF POLYMER SCIENCE (IN TWO SECTIONS), Issue 12 2005Xiaonong Chen Abstract An ambient self-curable latex (ASCL) was prepared via the blending of colloidal dispersions in water of a chloromethylstyrene-functionalized copolymer and a tertiary-amine-functionalized copolymer. Upon casting and drying under ambient conditions, the ASCL could generate crosslinked continuous polymer films. The crosslinking occurred via the Menschutkin reaction (quaternization) between the two types of functional groups. Because this reaction was reversible at high temperatures, the films could be decrosslinked and hence were self-curable. The prepared ASCL exhibited excellent colloidal and chemical stability during long-term storage: no significant changes in the colloidal properties, such as the particle size, electrophoretic mobility, and crosslinking reactivity, were observed after 48 months of storage. The electrophoretic measurements indicated that the electrostatic repulsion between the negatively charged particles of the ASCL was responsible for the excellent stability. © 2005 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 43: 2598,2605, 2005 [source] Surface-enhanced Raman scattering study of the red dye laccaic acidJOURNAL OF RAMAN SPECTROSCOPY, Issue 10 2007M. V. Cañamares Abstract FT-Raman and surface-enhanced Raman scattering (SERS) spectroscopy were applied to the study of lac dye, a highly fluorescent anthraquinone red dye. The SERS spectra were obtained at different pH values, on Ag nanoparticles prepared by chemical reduction with citrate and hydroxylamine, and at several excitation wavelengths, in order to find the best experimental conditions for the detection of the lac dye. The lower detection limit was achieved using nanoparticles prepared by reduction with hydroxylamine, excitation at 514.5 nm, and slightly acidic pH conditions, thus exploiting a combination of factors including lower electrostatic repulsion between dye and nanoparticles and resonance Raman enhancement. A comparison between the adsorption of laccaic acid (LA) and carminic acid (CA), another anthraquinone red dye, was also done, based on the SERS spectra of both dyes. Copyright © 2007 John Wiley & Sons, Ltd. [source] Influence of Impurities on Dispersion Properties of Bayer AluminaJOURNAL OF THE AMERICAN CERAMIC SOCIETY, Issue 12 2004J. Kiennemann Among the different impurities contained in Bayer alumina (Ca, Fe, Na, Si, Mg), calcium was found to greatly influence the dispersion of alumina particles in water. Up to 90% of calcium is dissolved at acidic pH, whereas calcium remains on the alumina surface in the basic pH range and screens the negative Al,O, charges. The presence of calcium, through reducing repulsive interactions between particles, has a negative effect on the dispersion of alumina. The adsorption of the Na+ salt of poly(acrylic acid) (PAA-Na) is strongly influenced by Ca2+/PAA-Na interactions in suspension with an increase of the maximum adsorbed amount of PAA-Na in the presence of calcium. The amount of PAA-Na needed to reach a high electrostatic repulsion and a minimum of viscosity is 2 times higher in the presence of 400 pm calcium than for a low calcium content (,80 ppm). Finally, with an appropriate amount of PAA-Na, a similar state of dispersion can be reached with or without the presence of calcium. [source] Electrostatic Stabilization of Ultrafine Titania in EthanolJOURNAL OF THE AMERICAN CERAMIC SOCIETY, Issue 3 2002Johnny Widegren The colloidal stability of a suspension of very fine titania (TiO2) powder in ethanol has been investigated. The electrokinetic behavior and stability ratios were determined as a function of operational pH and salt concentration. We show that the stability is strongly dependent on ionic strength; the TiO2suspensions will aggregate relatively rapidly at ionic strengths >10,3M, while suspensions exhibiting long-term stability can be made at ,10,4M, providing that the zeta potential of the TiO2particles is sufficiently high, i.e., |,|= 40,50 mV. Theoretical estimates of the stability based on electrostatic repulsion show a relatively good correlation with the experimental results, in particular in the acidic operational pH (pH*) range. [source] Conformational Effects on Photophysical Characteristics of C5,C5,-linked Dihydrothymine Dimers in Solution,PHOTOCHEMISTRY & PHOTOBIOLOGY, Issue 6 2000T. Ito ABSTRACT Photophysical characteristics of N-substituted C5,C5,-linked dihydrothymine dimers (1a,b[meso], meso compounds of [5R,5,S]-bi-5,6-dihydrothymines; 1a,b[rac], racemic compounds of [5R,5,R]-bi-5,6-dihydrothymines and [5S,5,S]-bi-5,6-dihydrothymines) in aqueous solution with varying contents of less-polar aprotic solvent such as tetrahydrofuran or dioxane have been investigated by UV-absorption, and steady-state and time-resolved fluorescence spectroscopies. Among the C5,C5,-linked dimers, (5R,5,S)-bi-5,6-dihydro-1-methylthymine (1a[meso]) showed a redshifted weak UV-absorption band at 270,350 nm and excimer fluorescence emission at ,max= 370 nm with a quantum yield (,F) of ,0.1 in phosphate buffer (pH < 10) at 293 K. Racemic compound of 5,6-dihydro-1-methylthymine dimer (1a[rac]), meso and racemic compounds of 5,6-dihydro-1,3-dimethylthymine dimers (1b[meso] and 1b[rac]) in phosphate buffer were nonfluorescent under similar conditions. The UV-absorption and fluorescence spectral characteristics of 1a[meso] in aqueous solution were interpreted in terms of intramolecular stacking interactions between the dihydropyrimidine chromophores leading to a preferential "closed-shell" conformation both in the ground state and the excited singlet state. In basic solutions at pH > pKa= 11.7, the fluorescence quantum yield of 1a[meso] decreased due to a dominant "open-shell" conformation resulting from the electrostatic repulsion between the deprotonated dihydrothymine chromophores of 1a[meso] in a dianion form. [source] Theoretical studies of the spin Hamiltonian parameters and the local structure for the orthorhombic V4+ in CaYAlO4PHYSICA STATUS SOLIDI (B) BASIC SOLID STATE PHYSICS, Issue 6 2008Chuan-Ji Fu Abstract The spin Hamiltonian parameters (g -factors and the hyperfine structure constants) and the local structure for the orthorhombic V4+ in CaYAlO4 are theoretically studied from the perturbation formulas of these parameters for a 3d1 ion in orthorhombically distorted octahedra. Based on the present studies, this center is explicitly assigned to V4+ occupying the Al3+ site, associated with one next-nearest-neighbor Mg2+ replacing the host Al3+ (i.e. MgAl) in the ab -plane due to the charge compensation. Since the effective charge of the compensator is negative (,e), the ligand O2, intervening in the impurity V4+ and the MgAl would undergo a small displacement (,0.08 Å) towards the central ion because of the electrostatic repulsion. The local structure of this center can be described as elongation of the oxygen octahedron along the c- axis and the slight orthorhombic distortion in the perpendicular direction. The theoretical spin Hamiltonian parameters and the optical transitions based on the above local structure show reasonable agreement with the experimental data. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] Effect of acids on in situ polyaniline film formationPOLYMER INTERNATIONAL, Issue 8 2004Prof MM Ayad Abstract The chemical oxidation of aniline with ammonium persulfate (APS) to form polyaniline (PANI) films has been studied in different aqueous acid media such as sulfuric, nitric, phosphoric and acetic acids. A comparison was made between the yields of PANI film deposition during the polymerization from these media with the corresponding one obtained previously from aqueous HCl solution. The degradation of the formed PANI films at the beginning of polymerization obtained at the higher concentrations of HCl is absent when the other acids under consideration were used. The effect of acid concentration on the yield and growth rate of the PANI film was studied. The variation in the yield and growth rate of the polymer films at different acid concentrations was explained on the basis of the electrostatic repulsion and the screening effects. Copyright © 2004 Society of Chemical Industry [source] The solution structure of the methylated form of the N-terminal 16-kDa domain of Escherichia coli Ada proteinPROTEIN SCIENCE, Issue 3 2006Hiroto Takinowaki N-Ada16k, the N-terminal 16-kDa domain of the Ada protein; meC38 N-Ada16k, the Cys38 methylated form of N-Ada16k; MTase, methyltransferase; HTH, helix-turn-helix; NMR, nuclear magnetic resonance; MALDI-TOF MS, matrix assisted laser desorption/ionization time of flight mass spectrometry; MNU, methylnitrosourea Abstract The N-terminal 16-kDa domain of Escherichia coli Ada protein (N-Ada16k) repairs DNA methyl phosphotriester lesions by an irreversible methyl transfer to its cysteine residue. Upon the methylation, the sequence-specific DNA binding affinity for the promoter region of the alkylation resistance genes is enhanced by 103 -fold. Then, it acts as a transcriptional regulator for the methylation damage. In this paper, we identified the methyl acceptor residue of N-Ada16k and determined the solution structure of the methylated form of N-Ada16k by using NMR and mass spectrometry. The results of a 13C-filtered 1H- 13C HMBC experiment and MALDI-TOF MS and MS/MS experiments clearly showed that the methyl acceptor residue is Cys38. The solution structure revealed that it has two distinct subdomains connected by a flexible linker loop: the methyltransferase (MTase) subdomain with the zinc,thiolate center, and the helical subdomain with a helix-turn-helix motif. Interestingly, there is no potential hydrogen bond donor around Cys38, whereas the other three cysteine residues coordinated to a zinc ion have potential donors. Hence, Cys38 could retain its inherent nucleophilicity and react with a methyl phosphotriester. Furthermore, the structure comparison shows that there is no indication of a remarkable conformational change occurring upon the methylation. This implies that the electrostatic repulsion between the negatively charged DNA and the zinc,thiolate center may avoid the contact between the MTase subdomain and the DNA in the nonmethylated form. Thus, after the Cys38 methylation, the MTase subdomain can bind the cognate DNA because the negative charge of the zinc,thiolate center is reduced. [source] Rheology control by modulating hydrophobic and inclusive associations of side-groups in poly (acrylic acid)ASIA-PACIFIC JOURNAL OF CHEMICAL ENGINEERING, Issue 5 2009Jie Wang Abstract In this article we demonstrated that the viscosities of modified poly (acrylic acid) (PAA) solutions were tunable by modulating the hydrophobic and inclusion association between alkyl groups and ,-cyclodextrin (,-CD) groups grafted to PAA. The viscosity can be controlled by changing the host,guest molar ratio, alkyl chain length, polymer concentration, salt concentration, pH value, temperature, or addition of native ,-CD. A viscosity maximum for inclusive polymer networks constructed by mixing hydrophobically modified PAA (HMPAA) and ,-CD,modified PAA (,-CDPAA) appeared at the alkyl : ,-CD molar ratio of 1:1, which implies the inclusion association between HM and ,-CD grafts is binary. Longer side chain length or higher polymer concentration led to higher viscosity for aqueous HMPAA solution with only hydrophobic association or its mixture with ,-CDPAA with inclusion association. Monotonically increasing the ionic strength or pH value resulted in a viscosity maximum due to the competition between electrostatic repulsion and hydrophobic or inclusive association. The hydrophobic interactions of alkyl groups could be masked by native ,-CD, and the networks of HMPAA and ,-CDPAA mixture deconstructed upon addition of native ,-CD molecules. The storage modulus and loss modulus of hydrophobic HMPAA and inclusive HMPAA + ,-CDPAA solutions obey time,temperature superposition. The horizontal and vertical temperature shift factors obeyed a simple-exponential Arrhenius relationship, where the activation energies for hydrophobic association system were found to be 3.4 and , 12.1 kJ/mol, and for inclusive association system 53.9 and , 2.9 kJ/mol, respectively. Copyright © 2009 Curtin University of Technology and John Wiley & Sons, Ltd. [source] Effects of excluded volume and polydispersity on solution properties of lentinan in 0.1 M NaOH solution,BIOPOLYMERS, Issue 4 2005Xufeng Zhang Abstract Seven lentinan fractions of various weight-average molecular weights (Mw), ranging from 1.45 × 105 to 1.13 × 106 g mol,1 were investigated by static light scattering and viscometry in 0.1M NaOH solution at 25°C. The intrinsic viscosity [,] , Mw and radius of gyration ,s2, , Mw relationships for lentinan in 0.1M NaOH solution were found to be represented by [,] = 5.1 × 10,3M cm3 g,1 and ,s2, = 2.3 × 10,1M nm, respectively. Focusing on the effects of the Mw polydispersity with the Schulz,Zimm distribution function, the data of Mw, ,s2,, and [,] was analyzed on the basis of the Yoshizaki,Nitta,Yamakawa theory for the unperturbed helical wormlike chain combined with the quasi-two-parameter (QTP) theory for excluded-volume effects. The persistence length, molecular weight per unit contour length, and the excluded-volume strength were determined roughly to be 6.2 nm, 980 nm,1, and 0.1, respectively. Compared with the theoretical value calculated by the Monte Carlo model, the persistence length is longer than that of the single (1 , 3)-,- D -glucan chain. The results revealed that lentinan exists as single-stranded flexible chains in 0.1M NaOH solution with a certain degree of expansion due to the electrostatic repulsion from the interaction between the OH, anions and lentinan molecules. © 2005 Wiley Periodicals, Inc. Biopolymers 78: 187,196, 2005 [source] The Stereospecific Ligand Exchange at a Pseudo-Benzylic T -4 Iridium Centre in Planar-Chiral Cycloiridium (,6 -Arene)tricarbonylchromium ComplexesCHEMISTRY - A EUROPEAN JOURNAL, Issue 41 2009Jean-Pierre Djukic Dr. Abstract The stereospecificity of ligand exchange at the IrIII centre of a cycloiridium arenetricarbonylchromium complex has been established experimentally by various analytical methods as well as by X-ray diffraction structural analysis and computational investigations. Two new cases of phenyl and methyl iridium(III) complexes have been prepared by reaction of (,)-chlorido{2-[(tricarbonyl)(,6 -phenylene-,C1,)chromium(0)]pyridine-,N}(pentamethylcyclopentadienyl)iridium(III) with PhMgBr and MeMgBr. The determining influence of electrostatic repulsion has been established by means of density functional theory at the Becke,Perdew/TZP(ZORA) level by using, among other means, energy partitioning analysis. It is also shown that the Cr(CO)3 fragment is likely to ease the ionic cleavage of the IrCl bond in chlorido cycloiridium tricarbonylchromium complexes in a way similar to that already established for the solvolysis of benzyl halide complexes, that is, through a direct interaction of the Cr0 centre with the cationic IrIII centre. [source] Colorimetric Sensing of Adenosine Based on Aptamer Binding Inducing Gold Nanoparticle AggregationCHINESE JOURNAL OF CHEMISTRY, Issue 10 2009Xueping Liu Abstract A simple and rapid colorimetric approach for the determination of adenosine has been developed via target inducing aptamer structure switching, thus leading to Au colloidal solution aggregation. In the absence of the analytes, the aptamer/gold nanoparticle (Au NP) solution remained well dispersed under a given high ionic strength condition in that the random-coil aptamer was readily wrapped on the surface of the Au NPs, which resulted in the enhancement of the repulsive force between the nanoparticles due to the high negative charge density of DNA molecules. While in the presence of adenosine, target-aptamer complexes were formed and the conformation of the aptamer was changed to a folded structure which disfavored its adsorption on the Au NP surface, thus leading to the reduction of the negative charge density on each Au NP and then the reduced degree of electrostatic repulsion between Au nanoparticles. As a result, the aggregation of the Au colloidal solution occurred. The changes of the absorption spectrum could be easily monitored by a UV-Vis spectrophotometer. A linear correlation exists between the ratio of the absorbance of the system at 522 to 700 nm (A522 nm/A700 nm) and the concentration of adenosine between 100 nmol·L,1 and 10 µmol·L,1, with a detection limit of 51.5 nmol·L,1. [source] Evaluation of factors influencing membrane performanceENVIRONMENTAL PROGRESS & SUSTAINABLE ENERGY, Issue 4 2005Weihua Peng Abstract Three commercial water treatment membranes, TFC-S (Koch membranes, San Diego, CA), ESPA1, and NTR7450 (Hydranautics, San Diego, CA), were tested under various physical and chemical conditions to investigate their fouling behaviors. It was found that TFC-S always displayed the greatest rate of flux decline, ESPA1 displayed a mild trend in flux decline, and NTR7450 presented a nearly stable flux. Multivariable regression models showed that the flux decline rates for TFC-S and ESPA1 were controlled by the initial permeate flux, whereas their initial (that is, instantaneous) foulings were controlled by the interaction between permeate drag and electrostatic repulsions. Feed bacteria concentration also contributed to the initial fouling of ESPA1 as a result of cell deposition on the membrane surface. NTR7450 showed an initial decline in flux followed by a steady flux, and its initial fouling was significantly affected by feed water total organic carbon (TOC) arising from the initial accumulation of colloidal organic particles on the surface. © 2005 American Institute of Chemical Engineers Environ Prog, 2005 [source] Role of hydrophobic effect in the salt-induced dimerization of bovine ,-lactoglobulin at pH 3,BIOPOLYMERS, Issue 12 2009Giuseppe Graziano Abstract ,-Lactoglobulin is a dimeric protein around neutral pH, but the monomer becomes the dominant species at pH 3.0 due to strong electrostatic repulsions between the positively charged molecules. It has been found that the addition of salts to water at pH 3.0 favors the dimerization of ,-lactoglobulin. In particular, the dimer is the dominant species at 1M NaCl, 1M GuHCl, and 25 mM NaClO4 [Sakurai, Oobatake, and Goto, Protein Sci 2001, 10, 2325,2335]. The effect of these salt conditions on the strength of hydrophobic interaction has been calculated by means of a simple but physically sound approach. The obtained estimates indicate that: (a) the hydrophobic interaction contribution is strengthened in 1M NaCl and 1M GuHCl with respect to pure water, but not in 25 mM NaClO4; (b) anion binding on the positively charged surface of protein molecules has to be the major factor for the salt-induced dimerization. © 2009 Wiley Periodicals, Inc. Biopolymers 91: 1182,1188, 2009. This article was originally published online as an accepted preprint. The "Published Online" date corresponds to the preprint version. You can request a copy of the preprint by emailing the Biopolymers editorial office at biopolymers@wiley.com [source] "Nanoions": Fundamental Properties and Analytical Applications of Charged NanoparticlesCHEMPHYSCHEM, Issue 15 2007Kyle J. M. Bishop Abstract Mixtures of oppositely charged nanoparticles (NPs) precipitate sharply only at the point of NP electroneutrality. This behavior,reminiscent of the threshold precipitation of inorganic ions,is specific to the nanoscale and can be attributed to the formation of like-charged NP clusters stabilized in solution by mutual electrostatic repulsions. NP titrations based on this phenomenon provide a uniquely accurate tool for measuring charges tethered onto nanoscopic objects and for studying the thermodynamics of surface reactions at the nanoscale. [source] | ||||||||||||||||||||||||||||