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Net Charge (net + charge)
Kinds of Net Charge Selected AbstractsComputational studies of the cone and 1,2,3 alternate calix[6]arene bis-crown-4 isomers: structures, NMR shifts, atomic charges, and steric compressionJOURNAL OF PHYSICAL ORGANIC CHEMISTRY, Issue 12 2009Meghan N. Lawson Abstract The cone and 1,2,3 alternate isomers of calix[6]arene bis-crown-4 were investigated computationally. Structural optimizations, energies, bond distances, and Mulliken charges were calculated by the application of the B3LYP/6-31g(d) method/basis, followed by NMR calculations via both B3LYP/6-31g(d) and HF/6-31g(d). Calculations were completed at three different levels of imposed symmetry, and two calculations investigated the chloroform solvent effects. Better NMR results were obtained from HF/6-31g(d) calculations that did not impose molecular symmetry constraints. Consideration of solvent effects improved ground state energies, but other improvements were minimal and not significant enough to justify the added computational expense of solvent calculations. Overall results are consistent with known experimental assignments and were valuable for assigning previously unknown NMR peaks. Net charges, electrostatic forces, and local dipoles , but not bond lengths , are strongly correlated to spectroscopic manifestations of steric compression. Copyright © 2009 John Wiley & Sons, Ltd. [source] The dependence of the electrophoretic mobility of small organic ions on ionic strength and complex formationELECTROPHORESIS, Issue 5 2010Stuart A. Allison Abstract The ionic strength dependence of the electrophoretic mobility of small organic anions with valencies up to ,3 is investigated in this study. Provided the anions are not too aspherical, it is argued that shape and charge distribution have little influence on mobility. To a good approximation, the electrophoretic mobility of a small particle should be equal to that of a model sphere with the same hydrodynamic radius and same net charge. For small ions, the relaxation effect (distortion of the ion atmosphere from equilibrium due to external electric and flow fields) is significant even for monovalent ions. Alternative procedures of accounting for the relaxation effect are examined. In order to account for the ionic strength dependence of a specific set of nonaromatic and aromatic anions in aqueous solution, it is necessary to include complex formation between the anion with species in the BGE. A number of possible complexes are considered. When the BGE is Tris-acetate, the most important of these involves the complex formed between anion and Tris, the principle cation in the BGE. When the BGE is sodium borate, an anion,anion (borate) complex appears to be important, at least when the organic anion is monovalent. An algorithm is developed to analyze the ionic strength dependence of the electrophoretic mobility. This algorithm is applied to two sets of organic anions from two independent research groups. [source] Numerical calculations of the pH of maximal protein stabilityFEBS JOURNAL, Issue 1 2004The effect of the sequence composition, three-dimensional structure A large number of proteins, found experimentally to have different optimum pH of maximal stability, were studied to reveal the basic principles of their preferenence for a particular pH. The pH-dependent free energy of folding was modeled numerically as a function of pH as well as the net charge of the protein. The optimum pH was determined in the numerical calculations as the pH of the minimum free energy of folding. The experimental data for the pH of maximal stability (experimental optimum pH) was reproducible (rmsd = 0.73). It was shown that the optimum pH results from two factors , amino acid composition and the organization of the titratable groups with the 3D structure. It was demonstrated that the optimum pH and isoelectric point could be quite different. In many cases, the optimum pH was found at a pH corresponding to a large net charge of the protein. At the same time, there was a tendency for proteins having acidic optimum pHs to have a base/acid ratio smaller than one and vice versa. The correlation between the optimum pH and base/acid ratio is significant if only buried groups are taken into account. It was shown that a protein that provides a favorable electrostatic environment for acids and disfavors the bases tends to have high optimum pH and vice versa. [source] The porcine trophoblastic interferon-,, secreted by a polarized epithelium, has specific structural and biochemical propertiesFEBS JOURNAL, Issue 11 2002Avrelija Cenci At the time of implantation in the maternal uterus, the trophectoderm of the pig blastocyst is the source of a massive secretion of interferon-gamma (IFN-,), together with lesser amounts of IFN-,, a unique species of type I IFN. This trophoblastic IFN-, (TrIFN-,) is an unprecedented example of IFN-, being produced spontaneously by an epithelium. We therefore studied some of its structural and biochemical properties, by comparison with pig IFN-, from other sources, either natural LeIFN-, (from adult leucocytes), or recombinant. Biologically active TrIFN-, is a dimeric molecule, of which monomers are mainly composed of a truncated polypeptide chain with two glycotypes, unlike LeIFN-, which is formed of at least two polypeptide chains and four glycotypes. TrIFN-, collected in the uterus lumen was enzymatically deglycosylated and analysed by mass spectrometry (MALDI-TOF). The data revealed that the more abundant polypeptide has a mass of 14.74 kDa, corresponding to a C-terminal cleavage of 17 residues from the expected 143-residue long mature sequence. A minor polypeptide, with a mass of 12.63 kDa, corresponds to a C-terminal truncation of 36 amino acids. MALDI-TOF analysis of tryptic peptides from the glycosylated molecule(s) identifies a single branched carbohydrate motif, with six N -acetylgalactosamines, and no sialic acid. The only glycan microheterogeneity seems to reside in the number of l -fucose residues (one to three). The lack of the C-terminal cluster of basic residues, and the presence of nonsialylated glycans, result in a very low net charge of TrIFN-, molecule. However, the 17-residue truncation does not affect the antiproliferative activity of TrIFN-, on different cells, among which is a porcine uterine epithelial cell line. It is suggested that these specific properties might confer on TrIFN-, a particular ability to invade the uterine mucosa and exert biological functions beyond the endometrial epithelium. [source] Physico-chemical requirements for cellular uptake of pAntp peptideFEBS JOURNAL, Issue 5 2001Role of lipid-binding affinity The pAntp peptide, corresponding to the third helix of the Antennapedia homeodomain, is internalized by a receptor-independent process into eucaryotic cells. The precise mechanism of entry remains unclear but the interaction between the phospholipids of plasma membrane and pAntp is probably involved in the translocation process. In order to define the role of peptide,lipid interaction in this mechanism and the physico-chemical properties that are necessary for an efficient cellular uptake, we have carried out an Ala-Scan mapping. The peptides were labeled with a fluorescent group (7-nitrobenz-2-oxo-1,3-diazol-4-yl-; NBD) and their cell association was measured by flow cytometry. Furthermore, we determined the fraction of internalized peptide by using a dithionite treatment. Comparison between cell association and cell uptake suggests that the affinity of pAntp for the plasma membrane is required for the import process. To further investigate which are the physico-chemical requirements for phospholipid-binding of pAntp, we have determined the surface partition coefficient of peptides by titrating them with phospholipid vesicles having different compositions. In addition, we estimated by circular dichroism the conformation adopted by these peptides in a membrane-mimetic environment. We show that the phospholipid binding of pAntp depends on its helical amphipathicity, especially when the negative surface charge density of phospholipid vesicles is low. The cell uptake of pAntp, related to lipid-binding affinity, requires a minimal hydrophobicity and net charge. As pAntp does not seem to translocate through an artificial phospholipid bilayer, this might indicate that it could interact with other cell surface components or enters into cells by a nonelucidated biological mechanism. [source] The influence of pH and digestion with commercial enzymes on calcium adsorption in casabeINTERNATIONAL JOURNAL OF FOOD SCIENCE & TECHNOLOGY, Issue 9 2006Petra Beatriz Navas Summary The Ca2+ binding capacity of a sample of casabe made from cassava (Manihot esculenta) was evaluated by using adsorption isotherms after a digestion process with commercial enzymes. It was found that enzymatic treatment increased the ability of vegetable material to retain calcium and to release endogenous mineral ions as a consequence of possible modifications to the carbohydrate matrix. Untreated casabe did not release mineral ions. pH also influenced the retention of Ca2+: at pH 4.5 release was the main process but adsorption increases with alkalinity up to pH 8.5. The Ca concentrations at which neither adsorption nor release occurred [Ca2+]e were as follows: 5.2 mm (pH 4.5), 3.5 mm (pH 7.1), and 0.63 mm (pH 8.5). The pH effect was explained by an increase in the density of negatively charged functional groups produced by ionization reactions at pH below the point of zero net charge (pHo) which was evaluated by using the Gouy,Chapman double layer model. Values of pHo were 6.4 for raw material and 4.1 after digestion with enzymes. In both cases, the density of positively charged sites below pHo was much higher that the density of negatively charged sites above pHo. [source] Local HSAB principle in the conjugate addition of p -substituted thiophenols to cyclohexenoneINTERNATIONAL JOURNAL OF QUANTUM CHEMISTRY, Issue 1 2005Rocío Meza Abstract In the formation of new bonds through the Michel-type addition of thiolates to a ,,,-conjugate system, it is observed that the reaction takes place faster if the thio donor compound contains an electron-rich group or the enone acceptor is highly electron deficient. In contrast, the hard,soft acid,base (HSAB) principle predicts that this reaction is favored when a soft,soft interaction between the reactants takes place. Taking into account that softness is related to a barely electronegative atom of high polarizability, we consider it of interest to analyze the effect of charge transfer of a p -substituent on the softness of sulfur in thiophenols, as well as its impact in the conjugate addition to 2-cyclohexen-1-one. Experiments-in-competition, net charge of X-groups at the p -position of the aromatic ring, the global and local softness at sulfur, and the electrophilicity, obtained by density functional theory (DFT), led to the observation that the reaction is faster for electron attractor thiophenols. The softness at sulfur increases by delocalization of charge through the aromatic ring. © 2005 Wiley Periodicals, Inc. Int J Quantum Chem, 2005 [source] DFT calculations of light-induced excited states and comparison with time-resolved crystallographic resultsINTERNATIONAL JOURNAL OF QUANTUM CHEMISTRY, Issue 5 2005Philip Coppens Abstract DFT calculations of the ground and first excited states of several transition metal complexes have been performed to complement time-resolved diffraction experiments. The results from different functionals and relativistic treatments are tested against both diffraction and spectroscopic values. Calculations of the d8,d8 complex [Pt2(pyrophosphite)4]4, quantitatively reproduce metal,metal shortening on excitation to the triplet state and support bond formation between the two metal centers, as do calculations on [Rh2(1,3-diisocyanopropane)4]2+. Results on homoleptic and heteroleptic copper(I) 2,9-dimethyl,1,10-phenanthroline (dmp) complexes, which are investigated because of their potential for solar energy capture, confirm considerable molecular deformations on excitation. The distortion calculated for the isolated complex [Cu(dmp)(dmpe)]+ (dmpe=1,2-bis(dimethylphosphino)ethane) is significantly larger than observed in the crystal, indicating the constraining effect of the crystalline environment. The change in the net charge of the Cu atom upon photo-induced metal-to-ligand charge transfer is less than 0.2 e, showing the limitations of the formal Cu(I),Cu(II) designation. Electron density difference maps show a pronounced change in electronic structure of the Cu atom on excitation. © 2004 Wiley Periodicals, Inc. Int J Quantum Chem, 2005 [source] Antibacterial peptides: basic facts and emerging conceptsJOURNAL OF INTERNAL MEDICINE, Issue 3 2003H. G. Boman Abstract., Boman HG (Microbiology and Tumor Biology Center, Karolinska Institutet, Stockholm, Sweden). Antibacterial peptides: basic facts and emerging concepts (Review). J Intern Med 2003; 254: 197,215. Antibacterial peptides are the effector molecules of innate immunity. Generally they contain 15,45 amino acid residues and the net charge is positive. The cecropin type of linear peptides without cysteine were found first in insects, whilst the defensin type with three disulphide bridges were found in rabbit granulocytes. Now a database stores more than 800 sequences of antibacterial peptides and proteins from the animal and plant kingdoms. Generally, each species has 15,40 peptides made from genes, which code for only one precursor. The dominating targets are bacterial membranes and the killing reaction must be faster than the growth rate of the bacteria. Some antibacterial peptides are clearly multifunctional and an attempt to predict this property from the hydrophobicity of all amino acid side chains are given. Gene structures and biosynthesis are known both in the fruit fly Drosophila and several mammals. Humans need two classes of defensins and the cathelicidin-derived linear peptide LL-37. Clinical cases show that deficiencies in these peptides give severe symptoms. Examples given are morbus Kostmann and atopic allergy. Several antibacterial peptides are being developed as drugs. [source] Origin of the conformation dependence of protein charge-state distributions in electrospray ionization mass spectrometryJOURNAL OF MASS SPECTROMETRY (INCORP BIOLOGICAL MASS SPECTROMETRY), Issue 1 2003Rita Grandori Abstract The influence of tertiary structure on the electrospray ionization mass spectra of proteins is a well known and broadly exploited phenomenon. However, the underlying mechanism is not well understood. This paper discusses the bases and the implications of the two current hypotheses (solvent accessibility and Coulombic repulsions), pointing out the remaining open questions. Evidence reported here supports a third hypothesis, i.e. that intramolecular interactions in folded proteins play a key role in determining the observed charge-state distributions. It is proposed that native protein structures stabilize to a large extent pre-existing charges of the opposite polarity to the net charge of the ion, preventing their neutralization during the electrospray process. Thus, the higher charge states of unfolded conformations, relative to the folded structure, would not derive from a more extensive ionization of the former, but rather from a higher content of neutralizing charges in the latter. This interpretation allows several other problematic observations to be explained, including the different shapes of the spectra of folded and unfolded proteins, the discrepancies between observed and predicted gas-phase reactivity of protein ions and the apparent inconsistency of positive- and negative-ion mode results. Copyright © 2003 John Wiley & Sons, Ltd. [source] Isolation and biological characterization of HIV-1 BG intersubtype recombinants and other genetic forms circulating in Galicia, SpainJOURNAL OF MEDICAL VIROLOGY, Issue 12 2006Lucía Pérez-Alvarez Abstract The biological characteristics of HIV-1 primary isolates of different recombinant forms (RFs) and non-B subtypes from Galicia, Spain, were investigated and the relationships between biological phenotype and evolution of infection were determined. Peripheral blood mononuclear cells (PBMCs) were obtained during the follow-up of 32 patients infected with HIV-1 non-subtype B genetic forms, characterized in partial sequences of pol (protease-reverse transcriptase) and env V3 region: 12 (37.5%) circulating RFs (CRFs), 9 (28.1%) unique RFs (URFs), and 11(34.4%) non-B subtypes. Primary isolates were obtained by coculture with donor PBMCs. Syncytium-inducing (SI) phenotype was examined in MT2 cell line and coreceptor use in GHOST and U87.CD4 cells. Fifty percent of tissue culture infective dose (TCID50) and viral phenotype based on V3 net charge and Geno2phenocoreceptor bioinformatic method were determined. Fifty-four HIV-1 primary isolates were obtained. CRF14_BG and BG URFs represented the largest group, being all SI/X4, independently of the CD4+ cell count, viral load, or the duration of infection. By contrast, 10 of 11 CRF02_AG viruses were NSI/R5. The prediction of co-receptor use was concordant with biological characterization in all NSI/R5 and in 23 of 26 SI/X4 isolates. The presence of SI/X4 or SI/X4,R5 isolates at early stages of the infection in addition to a decrease in CD4+ counts below 500 cells/µl between 2 and 6 years since diagnosis was observed in all patients infected with CRF14_BG and BG URFs. These data contrast with the usual progression in B subtype infections, in which SI/X4 viruses rarely predominate in the early years of HIV-1 infection. J. Med. Virol. 78:1520,1528, 2006. © 2006 Wiley-Liss, Inc. [source] A brief introduction to cell-penetrating peptidesJOURNAL OF MOLECULAR RECOGNITION, Issue 5 2003Pontus Lundberg Abstract Cell membranes act as protective walls to exclude most molecules that are not actively imported by living cells. This is an efficient way for a cell to prevent uncontrolled influx or efflux of solutes, which otherwise would be harmful to it. Only compounds within a narrow range of molecular size, polarity and net charge are able to diffuse effectively through cell membranes. In order to overcome this barrier for effective delivery of membrane-impermeable molecules, several chemical and physical methods have been developed. These methods, e.g. electroporation, and more recent methods as cationic lipids/liposomes, have been shown to be effective for delivering hydrophobic macromolecules. The drawbacks of these harsh methods are, primarily, the unwanted cellular effects exerted by them, and, secondly, their limitation to in vitro applications. The last decade's discovery of cell-penetrating peptides translocating themselves across cell membranes of various cell lines, along with a cargo 100-fold their own size, via a seemingly energy-independent process, opens up the possibility for efficient delivery of DNA, antisense peptide nucleic acids, oligonucleotides, proteins and small molecules into cells both in vitro and in vivo. Copyright © 2003 John Wiley & Sons, Ltd. [source] Selective enhancement of the activity of C-terminally truncated, but not intact, acetylcholinesteraseJOURNAL OF NEUROCHEMISTRY, Issue 1 2008Martina Zimmermann Abstract Acetylcholinesterase (AChE) is one of the fastest enzymes approaching the catalytic limit of enzyme activity. The enzyme is involved in the terminal breakdown of the neurotransmitter acetylcholine, but non-enzymatic roles have also been described for the entire AChE molecule and its isolated C-terminal sequences. These non-cholinergic functions have been attributed to both the developmental and degenerative situation: the major form of AChE present in these conditions is monomeric. Moreover, AChE has been shown to lose its typical characteristic of substrate inhibition in both development and degeneration. This study characterizes a form of AChE truncated after amino acid 548 (T548-AChE), whose truncation site is homologue to that of a physiological form of T-AChE detected in fetal bovine serum that has lost its C-terminal moiety supposedly due to proteolytic cleavage. Peptide sequences covered by this C-terminal sequence have been shown to be crucially involved in both developmental and degenerative mechanisms in vitro. Numerous studies have addressed the structure,function relationship of the AChE C-terminus with T548-AChE representing one of the most frequently studied forms of truncated AChE. In this study, we provide new insight into the understanding of the functional characteristics that T548-AChE acquires in solution: T548-AChE is incubated with agents of varying net charge and molecular weight. Together with kinetic studies and an analysis of different molecular forms and aggregation states of T548-AChE, we show that the enzymatic activity of T548-AChE, an enzyme verging at its catalytic limit is, nonetheless, apparently enhanced by up to 800%. We demonstrate, first, how the activity of T548-AChE can be enhanced through agents that contain highly positive charged moieties. Moreover, the un-competitive mechanism of activity enhancement most likely involves the peripheral anionic site of AChE that is reflected in delayed substrate inhibition being observed for activity enhanced T548-AChE. The data provides evidence towards a mechanistic and functional link between the form of AChE unique to both development and degeneration and a C-terminal peptide of T-AChE acting under those conditions. [source] Semi-empirical studies of substituent effects on the ionization of bicyclooctane carboxylic acids and quinuclidinesJOURNAL OF PHYSICAL ORGANIC CHEMISTRY, Issue 2 2003Helena Diez y Riega Abstract Semiempirical AM1 calculations were performed for a representative series of 4-substituted bicyclooctane carboxylic acids and quinuclidines. It was found that the Hammett constant, ,I, and the Swain and Lupton field constant, F, correlate linearly with the differences in the heat of formation of isodesmic reactions. These constants also correlate with the charges on the acid moiety of the bicyclooctane acids and their anions, and with the hydrogen net charge on the protonated quinuclidines. For all cases, the NO2 was the poorest correlated substituent. Copyright © 2003 John Wiley & Sons, Ltd. [source] Substituent effect on the [2,+,3] cycloaddition of (E)-,-Nitrostyrene with (Z)- C,N -Diarylnitrones,JOURNAL OF PHYSICAL ORGANIC CHEMISTRY, Issue 2 2002Andrzej Baranski Abstract The kinetics of the title reaction were studied in toluene at 80,°C by means of HPLC. The overall rate constant (ktotal) of the reaction of (E)-,-nitrostyrene (1) with (Z)- C -phenyl- N -arylnitrones (2a,g) was found to increase with increasing Hammett ,-constant, whereas in the case of (Z)- C -aryl- N -phenylnitrones (2g,p) and the same nitrostyrene the rate decreased. The substituent effect in these reactions is inconsistent with FMO treatments of cycloaddition rates. For both reaction series good linear relationships were obtained between log ktotal and the net charge on the carbon atom in the >CN(O), moiety of the nitrones calculated by the AM1 method. Copyright © 2001 John Wiley & Sons, Ltd. [source] Molecular Dynamics Simulations of the Orientation and Reorientational Dynamics of Water and Polypyrrole Rings as a Function of the Oxidation State of the PolymerMACROMOLECULAR THEORY AND SIMULATIONS, Issue 1 2005Jose J. López Cascales Abstract Summary: Polypyrrole is one of the most widely-studied conducting polymers due to its steady electrochemical response and good chemical stability in different solvents, including organic and inorganic ones. In this work, we provide for the first time valuable information in atomic detail concerning the steady and dynamic properties of pyrrole rings as a function of the oxidation state of the polymer. The study was carried out by Classical molecular dynamics simulation, where the system was modelled by 256 polypyrrole chains of 10 pyrrole rings each. Water was explicitly introduced in our simulations. Besides the uncharged or reduced state, two steady oxidation states of the polymer have been simulated by introducing a net charge (+1) on 85 and 256 of the polypyrrole chains. To balance the charges emerging in these oxidised states, 85 and 256 chloride ions (Cl,1) respectively, were introduced into the system. From an analysis of the simulated trajectories, the orientation and relaxation times of water and pyrrole rings were evaluated for the different oxidation states of the polymer across the polypyrrole/water interface. The calculated densities for different oxidation states describe the swelling or shrinking process during electrochemical oxidation or reduction respectively. The rotational relaxation times calculated for the polypyrrole rings decrease with increasing oxidation of the polymer, which is in a good agreement with experimental electrochemical data. Almost no variation in pyrrole ring orientation was measured for the different oxidation states of the polymer, even compared with polypyrrole bulk. As regards the water structure in the vicinity of the polypyrrole/water interface, both the orientation and orientation relaxation time were strongly affected by the presence of charges in the polymer. Thus, the water dipole was strongly orientated in the vicinity of the water/polypyrrole interface and its orientational relaxation time increased by one order of magnitude compared with bulk water, even when only one-third of the total polymer chains were oxidised. The results attained in this work were validated with experimental results, when they were available. Polypyrrole ring orientation and water orientation at the polypyrrole/water interface. (a) 256 rPPy and (b)171 rPPy,+,85 oPPy. [source] The Role of the Extracellular Matrix in Tissue Distribution of Macromolecules in Normal and Pathological Tissues: Potential Therapeutic ConsequencesMICROCIRCULATION, Issue 4 2008Helge Wiig ABSTRACT The interstitial space is a dynamic microenvironment that consists of interstitial fluid and structural molecules of the extracellular matrix, such as glycosaminoglycans (hyaluronan and proteoglycans) and collagen. Macromolecules can distribute in the interstitium only in those spaces unoccupied by structural components, a phenomenon called interstitial exclusion. The exclusion phenomenon has direct consequences for plasma volume regulation. Early studies have assigned a major role to collagen as an excluding agent that accounts for the sterical (geometrical) exclusion. More recently, it has been shown that the contribution of negatively charged glycosaminoglycans might also be significant, resulting in an additional electrostatical exclusion effect. This charge effect may be of importance for drug uptake and suggests that either the glycosaminoglycans or the net charge of macromolecular substances to be delivered may be targeted to increase the available volume and uptake of macromolecular therapeutic agents in tumor tissue. Here, we provide an overview of the structural components of the interstitium and discuss the importance the sterical and electrostatical components have on the dynamics of transcapillary fluid exchange. [source] Baryon loading and the Weibel instability in gamma-ray burstsMONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Issue 4 2006M. Fiore ABSTRACT The dynamics of two counter-streaming electron,positron,ion unmagnetized plasma shells with zero net charge is analysed in the context of magnetic field generation in gamma-ray burst internal shocks due to the Weibel instability. The effects of large thermal motion of plasma particles, arbitrary mixture of plasma species and space charge effects are taken into account. We show that, although thermal effects slow down the instability, baryon loading leads to a non-negligible growth rate even for large temperatures and different shell velocities, thus guaranteeing the robustness and the occurrence of the Weibel instability for a wide range of scenarios. [source] Structural properties of trimers and tetramers of ribonuclease APROTEIN SCIENCE, Issue 10 2001Arianna Nenci Abstract Ribonuclease A aggregates (dimers, trimers, tetramers, pentamers) can be obtained by lyophilization from 40% acetic acid solutions. Each aggregate forms two conformational isomers distinguishable by different basic net charge. The crystal structure of the two dimers has recently been determined; the structure of the higher oligomers is unknown. The results of the study of the two trimeric and tetrameric conformers can be summarized as follows: (1) RNase A trimers and tetramers form by a 3D domain-swapping mechanism. N-terminal and C-terminal types of domain swapping could coexist; (2) the secondary structures of the trimeric and tetrameric conformers do not show significant differences if compared with the secondary structure of monomeric RNase A or its two dimers; (3) a different exposure of tyrosine residues indicates that in the aggregates they have different microenvironments; (4) the two trimeric and tetrameric conformers show different susceptibility to digestion by subtilisin; (5) dimers, trimers, and tetramers of RNase A show unwinding activity on double-helical poly(dA-dT) , poly(dA-dT), that increases as a function of the size of the oligomers; (6) the less basic conformers are more stable than the more basic ones, and a low concentration in solution of trimers and tetramers favors their stability, which is definitely increased by the interaction of the aggregates with poly(dA-dT) , poly(dA-dT); (7) the products of thermal dissociation of the two trimers indicate that their structures co ld be remarkably different. The dissociation products of the two tetramers allow the proposal of two models for their putative structures. [source] New insight on ,-lactoglobulin binding sites by 1-anilinonaphthalene-8-sulfonate fluorescence decayPROTEIN SCIENCE, Issue 10 2000Maddalena Collini Abstract The fluorescence time decay parameters of the ,-lactoglobulin-1-anilinonaphthalene-8-sulfonate complex have been investigated under physical and chemical perturbations (2 < pH < 8 and added electrolyte 0 < NaCl < 0.5 M) to obtain new insight on the nature of the protein binding interactions. A double exponential decay of the bound probe lifetime has been confirmed by the presence of a longer component, 11 to 14.5 ns, and a shorter component, 2.5 to 3.5 ns. The two lifetimes are ascribed to different binding modes associated also with different exposure to the solvent; in particular, the longer component is attributed to binding inside the hydrophobic beta barrel, while a "surface" site is suggested for the shorter component. A detailed analysis of the lifetime fractional intensities correlates the binding constants with ionic strength and supports the presence of electrostatic effects at both sites. A Debye,Hückel approach, applied to extrapolate the electrostatic free energy contribution vs. pH at vanishing ionic strength, gives interesting clues on the effective charge felt by the ANS ligands in the proximity of each site. In particular, binding is found to parallel the aspartate and glutamate titrations between pH 3 and pH 4.5; the "surface" site mainly responds to the presence of these local titrating charges while the "internal" site more closely follows the overall protein net charge. [source] After-effects of near-threshold stimulation in single human motor axonsTHE JOURNAL OF PHYSIOLOGY, Issue 3 2005Hugh Bostock Subthreshold electrical stimuli can generate a long-lasting increase in axonal excitability, superficially resembling the phase of superexcitability that follows a conditioning nerve impulse. This phenomenon of ,subthreshold superexcitability' has been investigated in single motor axons in six healthy human subjects, by tracking the excitability changes produced by conditioning stimuli of different amplitudes and waveforms. Near-threshold 1 ms stimuli caused a mean decrease in threshold at 5 ms of 22.1 ± 6.0% (mean ±s.d.) if excitation occurred, or 6.9 ± 2.6% if excitation did not occur. The subthreshold superexcitability was maximal at an interval of about 5 ms, and fell to zero at 30 ms. It appeared to be made up of two components: a passive component linearly related to conditioning stimulus amplitude, and a non-linear active component. The active component appeared when conditioning stimuli exceeded 60% of threshold, and accounted for a maximal threshold decrease of 2.6 ± 1.3%. The passive component was directly proportional to stimulus charge, when conditioning stimulus duration was varied between 0.2 and 2 ms, and could be eliminated by using triphasic stimuli with zero net charge. This change in stimulus waveform had little effect on the active component of subthreshold superexcitability or on the ,suprathreshold superexcitability' that followed excitation. It is concluded that subthreshold superexcitability in human motor axons is mainly due to the passive electrotonic effects of the stimulating current, but this is supplemented by an active component (about 12% of suprathreshold superexcitability), due to a local response of voltage-dependent sodium channels. [source] Role of acetylation and charge in antimicrobial peptides based on human ,-defensin-3APMIS, Issue 7 2009EMILIOS ANDREW PAPANASTASIOU Cationic antimicrobial peptides are an evolutionarily ancient and essential element of innate immunity in higher organisms. The precise mechanism by which these peptides exert their antimicrobial activity on bacteria is not well understood. Decapeptides based on the C-terminus of human ,-defensin-3 were designed and evaluated to study the role of charge in defining the antimicrobial activity and selectivity of these peptides against Escherichia coli. Acetylated derivatives of these peptides were prepared in order to further evaluate how positively charged primary amines contribute to potency in these small antimicrobial peptides. These peptides enabled us to explore the relationship between net charge, charge distribution and antimicrobial activity. While the results indicate that net charge is a major factor in antimicrobial activity in these peptides, the actual relationship between charge and potency appears to be more complex. [source] Salt tolerant membrane adsorbers for robust impurity clearanceBIOTECHNOLOGY PROGRESS, Issue 6 2009William T. Riordan Abstract Clearance of impurities such as viruses, host cell protein (HCP), and DNA is a critical purification design consideration for manufacture of monoclonal antibody therapeutics. Anion exchange chromatography has frequently been utilized to accomplish this goal; however, anion exchange adsorbents based on the traditional quaternary amine (Q) ligand are sensitive to salt concentration, leading to reduced clearance levels of impurities at moderate salt concentrations (50,150 mM). In this report, membrane adsorbers incorporating four alternative salt tolerant anion exchange ligands were examined for impurity clearance: agmatine, tris-2-aminoethyl amine, polyhexamethylene biguanide (PHMB), and polyethyleneimine. Each of these ligands provided greater than 5 log reduction value (LRV) for viral clearance of phage ,X174 (pI , 6.7) at pH 7.5 and phage PR772 (pI , 4) at pH 4.2 in the presence of salt. Under these same conditions, the commercial Q membrane adsorber provided no clearance (zero LRV). Clearance of host-cell protein at pH 7.5 was the most challenging test case; only PHMB maintained 1.5 LRV in 150 mM salt. The salt tolerance of PHMB was attributed to its large positive net charge through the presence of multiple biguanide groups that participated in electrostatic and hydrogen bonding interactions with the impurity molecules. On the basis of the results of this study, membrane adsorbers that incorporate salt tolerant anion exchange ligands provide a robust approach to impurity clearance during the purification of monoclonal antibodies. © 2009 American Institute of Chemical Engineers Biotechnol. Prog., 2009 [source] Insulin transport across porous charged membranes: Effect of the electrostatic interactionBIOTECHNOLOGY PROGRESS, Issue 5 2009Shaoling Zhang Abstract Insulin transport phenomena across a series of porous charged membranes were studied at two pH conditions (pH 3.3 and pH 7.4) in this article. The membranes were prepared by pore-surface modification of porous poly(acrylonitrile) (PAN) membranes by grafting with weak acidic and basic functional groups. The insulin partition coefficient K between the membrane and solution was estimated from the equilibrium adsorption amount in the batch adsorption experiment. The insulin effective diffusion coefficient D inside the membrane was determined as a fitting parameter by matching the diffusion model with the experimental data of the diffusion measurement. Both K and D correlated well with the charge properties of the insulin and membrane: when the insulin and membrane carried opposite net charge, the partition coefficient showed relatively larger values, while the effective diffusion coefficient was reduced. The insulin permeability coefficient P obtained from the experimental results agreed with that estimated from the partition coefficient and effective diffusion coefficient. These results suggested that the combined effects of the solubility and diffusivity on the permeability coefficient complicated the relationship between the permeability and the charge properties of the insulin and membrane. Additionally, our calculation supported that insulin permeability was reduced by the boundary layer between the membrane and solution. © 2009 American Institute of Chemical Engineers Biotechnol. Prog., 2009 [source] A Theoretical Study on the Mechanism of C2H4 Oxidation over a Neutral V3O8 ClusterCHEMPHYSCHEM, Issue 8 2010Yan-Ping Ma Dr. Abstract Density functional theory (DFT) calculations are used to investigate the reaction mechanism of V3O8+C2H4. The reaction of V3O8 with C2H4 produces V3O7CH2+HCHO or V3O7+CH2OCH2 overall barrierlessly at room temperature, whereas formation of hydrogen-transfer products V3O7+CH3CHO is subject to a tiny overall free energy barrier (0.03 eV), although the formation of the last-named pair of products is thermodynamically more favorable than that of the first two. These DFT results are in agreement with recent experimental observations. The (Ob)2V(OtOt). (b=bridging, t=terminal) moiety containing the oxygen radical in V3O8 is the active site in the reaction with C2H4. Similarities and differences between the reactivities of (Ob)2V(OtOt). in V3O8 and the small VO3 cluster [(Ot)2VOt.] are discussed. Moreover, the effect of the support on the reactivity of the (Ob)2V(OtOt). active site is evaluated by investigating the reactivity of the cluster VX2O8, which is obtained by replacing the V atoms in the (Ob)3VOt support moieties of V3O8 with X atoms (X=P, As, Sb, Nb, Ta, Si, and Ti). Support X atoms with different electronegativities influence the oxidative reactivity of the (Ob)2V(OtOt). active site through changing the net charge of the active site. These theoretical predictions of the mechanism of V3O8+C2H4 and the effect of the support on the active site may be helpful for understanding the reactivity and selectivity of reactive O. species over condensed-phase catalysts. [source] Evaluation of Intrinsic Ionization and Complexation Constants of TiO2 and Mg-Fe Hydrotalcite-like CompoundsCHINESE JOURNAL OF CHEMISTRY, Issue 10 2006Wan-Guo Hou Abstract The intrinsic surface reaction constants, pKinta1, pKinta2, p*KintC and p*KintA, were evaluated by a modified double extrapolation (MDE) for TiO2 without structural charge and Mg-Fe hydrotalcite-like compounds (HTlc) with structural charge, respectively. The results of intrinsic surface reaction constants for TiO2 were compared with those obtained by class double extrapolation (CDE) in literature. Furthermore, the values of intrinsic surface reaction constants obtained by MDE were used to simulate the charging behaviors of the materials. The following conclusions were obtained. For TiO2 without structural charge, the pKinta1 and pKinta2 evaluated by MDE are equal to those by CDE, however the p*KintC and p*KintA evaluated by MDE are much different from those by CDE. In principle, the results of the p*KintC and p*KintA evaluated by MDE are more accurate than those by CDE. The values of intrinsic surface reaction constants obtained by MDE can excellently simulate the charging curves for TiO2 with the triple layer model (TLM). For HTlc with positive structural charge, the results of *KintC=0 and *KintA,, were obtained by MDE, which means the inert electrolyte chemical binding does not exist; the point of zero net charge (PZNC) of c -independence also exist as the same as solid without structural charge, and the pHPZNC obtained by the acid-base titration can excellently be simulated and the surface charging tendency can be simulated to a great extent using the pKinta1 and pKinta2 evaluated by MDE and the diffuse layer model (DLM). [source] Enantioselective Recognition of Aspartic Acids by Chiral Ligand Exchange PotentiometryELECTROANALYSIS, Issue 11 2004Yanxiu Zhou Abstract Enantioselective resolution is realized by combining potentiometry with ligand exchange (CE) in a new method called chiral ligand exchange potentiometry (CLEP). A chiral selector, N -carbobenzoxy- L -aspartic acid (N-CBZ-L-Asp), preferentially recognizes D -aspartic acid (D-Asp) and undergoes ligand exchange with the enantiomeric labile coordination complexes of [Cu(II)(D-Asp)2] or [Cu(II)(L-Asp)2] to form a diastereoisomeric complex [(D-Asp)Cu(II)(N-CBZ-L-Asp)] (a) or [(L-Asp)Cu(II)(N-CBZ-L-Asp)] (b). Considerable stereoselectivity occurs in the formation of these diastereoisomeric complexes, and their net charges were ,2 (a) and 0 (b), respectively, resulting in different Nernst factor (electrode slope), thus enabling chiral D-Asp to be distinguished by potentiometry without any pre- or postseparation processes. [source] Modeling the semi-empirical electrotopological index in QSPR studies for aldehydes and ketonesJOURNAL OF CHEMOMETRICS, Issue 5 2009Érica Silva Souza Abstract The semi-empirical electrotopological index, ISET, used for quantitative structure,retention relationship (QSRR) models firstly developed for alkanes and alkenes, was remodeled for organic functions such as ketones and aldehydes. The ISET values for hydrocarbons are calculated through the atomic charge values obtained from a Mulliken population analysis using the semi-empirical AM1 method and their correlation with the SETi values attributed to the different types of carbon atoms according to experimental data. For ketones and aldehydes the interactions between the molecules and the stationary phase are slowly increased relative to the hydrocarbons, due to the charge redistribution that occurs in the presence of heteroatoms. For these polar molecules the increase in the interactions was included in the calculation of the ISET values through the dipole moment of the whole molecule and also through an equivalent local dipole moment related to the net charges of the atoms of the CO and HCO functional groups. Our findings show that the best definition of an equivalent local dipole moment is clearly dependent on the specific features of the charge distribution in the polar region of the molecules (e.g. ketones and aldehydes), which allows them to be distinguished. Thus, the QSRR models for 15 aldehydes and 42 ketones obtained using the remodeled ISET were of good quality as shown by the statistical parameters. The ability of this remodeled index to include charge distribution and structural details opens a new way to study the correlations between the molecular structure and retention indices in gas chromatography. Copyright © 2009 John Wiley & Sons, Ltd. [source] | |||||||||||||||||||