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At least 145 records · Page 8Linked to original sources
PubMed2006

Insights into the role of electrostatic forces on the behavior of dry pharmaceutical particulate systems.

PURPOSE: Under different charging conditions, particles can be either attracted or repulsed by each other, causing powder agglomeration or segregation. Such behavior can be detrimental in many processes aimed at achieving particulate mixture homogeneity. Consequently, the effects of electrostatic charges on mixing kinetics must be well understood to insure a high level of process control, product quality, and reproducibility. METHODS: In Part 1, an electrostatic charger is used to evaluate the ability of the studied particles to develop and retain an induced charge at the surface for a fixed period of time. Part 2 assesses the natural electrostatic charge developed by powders sliding across a stainless steel, plastic, or Tyvek chute. In Part 3, 2 binary systems were formed according to an experimental design under different charging conditions, and their behavior was quantified by measuring the degree of agglomeration attained. RESULTS: This work has shown that each raw material develops a different charge according to its physico-chemical properties and the type of contact surface. Electrostatic charges influence the creation of agglomerates under certain conditions. CONCLUSIONS: The presence of electrostatic charges must be accounted for in any effort to maximize mixing efficiency.

Calcium Phosphates↗
PubMed1989

Electrostatic analysis of effects of ions on the inhibition of corn root plasma membrane Mg2+-ATPase by the bivalent orthovanadate.

The microelectrophoretic mobility of corn root plasma membranes and the inhibition of the Mg+2-ATPase by vanadate were investigated under different ionic conditions. The Mg2+-ATPase was uncompetitively inhibited and a 10-fold variation of the apparent inhibition constant was observed, depending on the addition of K+ and Mg2+. The determination of the zeta potential indicated that a 5-fold decrease of the apparent inhibition constant was due to aspecific electrostatic interactions of the vanadate anion and the negative charge of the membrane. The screening and masking effects of 6 mM free Mg2+ totally abolished electrostatic interactions and allowed the direct determination of the intrinsic vanadate inhibition constant (KIi). On the other hand, a specific, non-electrostatic, effect of K+ caused a 2-fold decrease of the inhibition constant in addition to the electrostatic effect. Finally, the electrostatic analysis indicates that the Mg2+-ATPase is inhibited by the monomeric bivalent anion HVO4(2-).

Ca(2+) Mg(2+)-ATPase↗
PubMed1985

On the calculation of electrostatic interactions in proteins.

In this paper we present a classical treatment of electrostatic interactions in proteins. The protein is treated as a region of low dielectric constant with spherical charges embedded within it, surrounded by an aqueous solvent of high dielectric constant, which may contain a simple electrolyte. The complete analysis includes the effects of solvent screening, polarization forces, and self energies, which are related to solvation energies. Formulae, and sample calculations of forces and energies, are given for the special case of a spherical protein. Our analysis and model calculations point out that any consistent treatment of electrostatic interactions in proteins should account for the following. Solvent polarization is an important factor in the calculation of pairwise electrostatic interactions. Solvent polarization substantially affects both electrostatic energies and forces acting upon charges. No simple expression for the effective dielectric constant, Deff, can generally be valid, since Deff is a sensitive function of position. Solvent screening of pairwise interactions involving dipolar groups is less effective than the screening of charges. In fact for many interactions involving dipoles, solvent screening can be essentially ignored. The self energy of charges makes a large contribution to the total electrostatic energy of a protein. This must be compensated by specific interactions with other groups in the protein. Strategies for applying our analysis to proteins whose structures are known are discussed.

Electricity↗
PubMed1987

Calculations of electrostatic properties in proteins. Analysis of contributions from induced protein dipoles.

The calculation of induced dipole moments and of their contribution to electrostatic effects in proteins is implemented following the approach of Warshel. Isotropic polarizabilities are assigned to individual atoms, and the resulting deviation from pairwise interactions is treated by a self-consistent iterative procedure. We give a detailed description of how the formalism is implemented in molecular mechanics and molecular dynamics simulation procedures, and report results based on calculations performed on crystal structures of crambin, liver alcohol dehydrogenase and ribonuclease T1. We focus our analysis on evaluating the contribution of polarizability of the protein matrix to electrostatic energies, local fields, to dipole moments of peptide groups and of secondary structure elements in the polypeptide chain. Our calculations confirm that induced dipole moments in proteins provide important stabilizing contributions to electrostatic energies, and that these contributions cannot be mimicked by the usual approximations where either a continuum dielectric constant, or a distance-dependent dielectric function is used. We find that induced protein dipoles appreciably affect the magnitude and direction of local electrostatic fields in a manner that is strongly influenced by the microscopic environment in the protein. Most strongly affected are fields in charged groups that are involved in close interactions with other charged groups, while the influence on local fields of aliphatic groups is marginal. We find, moreover, that induction effects from surrounding protein atoms tend on average to increase peptide dipoles and helix macro-dipoles by about 16%, again reflecting electrostatic stabilization by the protein matrix, and show that (at least in the alpha/beta domain of alcohol dehydrogenase) the contribution of side-chains to this stabilization is significant.

Alcohol Dehydrogenase↗
PubMed2005

A new and simpler approach for the solution of the electrostatic potential differential equation. Enhanced solution for planar, cylindrical and annular geometries.

The motivation of the study performed in this project is focused on deriving a more effective, accurate, and mathematically friendly solution for the prediction of the electrostatic potential, commonly used on electrokinetic research and its related applications. In this contribution, based on the Debye-Hückel approximation, a new solution strategy for the differential equations of the electrostatic potential is proposed. In fact, a simple predictor-corrector calculation is developed to achieve more accurate predictions of electrostatic potential profiles. Furthermore, in this study the authors introduce the correction function f(AO) to the inverse Debye length, lambda. The f(AO) function improves the Debye-Hückel approximation and it is a recursive function of the electrical potential. Once the inverse Debye length, lambda, has been corrected by the f(AO) function and introduced in the simplified solution of the Poisson-Boltzmann equation (i.e., the linear approximation, due to Debye and Hückel), the electrostatic potential outcome little differs from the numerical solution of the complete (nonlinear) differential equation. This new approach embraces different geometries of interest, such as planar, cylindrical, and annular, with excellent results in all the cases and for a wide range of electrostatic potential values. This new predicting semi-analytical technique can be a useful tool on electrical field applications such as the separation of a mixture of macromolecules and the removal of contaminants in soil cleaning processes. Illustrative results are presented for the geometries identified above.

Journal Article↗
PubMed2004

Escherichia coli glutaminyl-tRNA synthetase is electrostatically optimized for binding of its cognate substrates.

Natural evolution has resulted in protein molecules displaying a wide range of binding properties that include extremes of affinity and specificity. A detailed understanding of the principles underlying protein structure-function relationships, particularly with respect to binding properties, would greatly enhance molecular engineering and ligand design studies. Here, we have analyzed the interactions of an aminoacyl-tRNA synthetase for which strong evolutionary pressure has enforced high specificity for substrate binding and catalysis. Electrostatic interactions have been identified as one efficient mechanism for enhancing binding specificity; as such, the effects of charged and polar groups were the focus of this study. The binding of glutaminyl-tRNA synthetase from Escherichia coli to several ligands, including the natural substrates, was analyzed. The electrostatic complementarity of the enzyme to its ligands was assessed using measures derived from affinity optimization theory. The results were independent of the details of the calculational parameters, including the value used for the protein dielectric constant. Glutamine and ATP, two of the natural ligands, were found to be extremely complementary to their binding sites, particularly in regions seen to make electrostatic interactions in the structure. These data suggest that the optimization of electrostatic interactions has played an important role in guiding the evolution of this enzyme. The results also show that the enzyme is able to effectively select for high affinity and specificity for the same chemical moieties both in the context of smaller substrates, and in that of a larger reactive intermediate. The regions of greatest non-complementarity between the enzyme and ligands are the portions of the ligand that make few polar contacts with the binding site, as well as the sites of chemical reaction, where overly strong electrostatic binding interactions with the substrate could hinder catalysis. The results also suggest that the negative charge on the phosphorus center of glutaminyl-adenylate plays an important role in the tight binding of this intermediate, and thus that adenylate analogs that preserve the negative charge in this region may bind substantially tighter than analogs where this group is replaced with a neutral group, such as the sulfamoyl family, which can make similar hydrogen bonds but is uncharged.

Adenosine Monophosphate↗
PubMed2005

Electrostatics of the intracellular vestibule of K+ channels.

Previous calculations using continuum electrostatic calculations showed that a fully hydrated monovalent cation is electrostatically stabilized at the center of the cavity of the KcsA potassium channel. Further analysis demonstrated that this cavity stabilization was controlled by a balance between the unfavorable reaction field due to the finite size of the cavity and the favorable electrostatic field arising from the pore helices. In the present study, continuum electrostatic calculations are used to investigate how the stability of an ion in the intracellular vestibular cavity common to known potassium channels is affected as the inner channel gate opens and the cavity becomes larger and contiguous with the intracellular solution. The X-ray structure of the calcium-activated potassium channel MthK, which was crystallized in the open state, is used to construct models of the KcsA channel in the open state. It is found that, as the channel opens, the barrier at the helix bundle crossing decreases to approximately 0 kcal/mol, but that the ion in the cavity is also significantly destabilized. The results are compared and contrasted with additional calculations performed on the KvAP (voltage-activated) and KirBac1.1 (inward rectifier) channels, as well as models of the pore domain of Shaker in the open and closed state. In conclusion, electrostatic factors give rise to energetic constraints on ion permeation that have important functional consequences on the various K+ channels, and partly explain the presence or absence of charged residues near the inner vestibular entry.

Amino Acid Sequence↗
PubMed2006

Evidence that membrane insertion of the cytosolic domain of Bcl-xL is governed by an electrostatic mechanism.

Signals from different cellular networks are integrated at the mitochondria in the regulation of apoptosis. This integration is controlled by the Bcl-2 proteins, many of which change localization from the cytosol to the mitochondrial outer membrane in this regulation. For Bcl-xL, this change in localization reflects the ability to undergo a conformational change from a solution to integral membrane conformation. To characterize this conformational change, structural and thermodynamic measurements were performed in the absence and presence of lipid vesicles with Bcl-xL. A pH-dependent model is proposed for the solution to membrane conformational change that consists of three stable conformations: a solution conformation, a conformation similar to the solution conformation but anchored to the membrane by its C-terminal transmembrane domain, and a membrane conformation that is fully associated with the membrane. This model predicts that the solution to membrane conformational change is independent of the C-terminal transmembrane domain, which is experimentally demonstrated. The conformational change is associated with changes in secondary and, especially, tertiary structure of the protein, as measured by far and near-UV circular dichroism spectroscopy, respectively. Membrane insertion was distinguished from peripheral association with the membrane by quenching of intrinsic tryptophan fluorescence by acrylamide and brominated lipids. For the cytosolic domain, the free energy of insertion (DeltaG degrees x) into lipid vesicles was determined to be -6.5 kcal mol(-1) at pH 4.9 by vesicle binding experiments. To test whether electrostatic interactions were significant to this process, the salt dependence of this conformational change was measured and analyzed in terms of Gouy-Chapman theory to estimate an electrostatic contribution of DeltaG degrees el approximately -2.5 kcal mol(-1) and a non-electrostatic contribution of DeltaG degrees nel approximately -4.0 kcal mol(-1) to the free energy of insertion, DeltaG degrees x. Calcium, which blocks ion channel activity of Bcl-xL, did not affect the solution to membrane conformational change more than predicted by these electrostatic considerations. The lipid cardiolipin, that is enriched at mitochondrial contact sites and reported to be important for the localization of Bcl-2 proteins, did not affect the solution to membrane conformational change of the cytosolic domain, suggesting that this lipid is not involved in the localization of Bcl-xL in vivo. Collectively, these data suggest the solution to membrane conformational change is controlled by an electrostatic mechanism. Given the distinct biological activities of these conformations, the possibility that this conformational change might be a regulatory checkpoint for apoptosis is discussed.

Calcium↗
PubMed2004

Tip-to-sample distance dependence of an electrostatic force in KFM measurements.

In most scanning probe methods like an atomic force microscopy, a cantilever is mechanically vibrated in order to obtain topographies. Therefore, a tip-to-sample distance periodically changes during the scanning. Since the electrostatic force, which is a long-range force, is used for potential feedback in Kelvin probe force microscopy (KFM), such mechanical vibration leads to the fluctuation of the electrostatic force between the tip and the sample. In this study, firstly, we performed two-dimensional simulations of the electric fields between surfaces of the tip and the sample and evaluated the tip-to-sample distance dependence of the electrostatic force. Secondly, we experimentally confirmed the existence of the fluctuation of the electrostatic force and the tip-to-sample distance dependence of the electrostatic force was evaluated. Both the simulations and the experiments on the tip-to-sample distance dependence showed the importance of considering the tip sidewall effect in the KFM potential determination.

Journal Article↗
PubMed2003

Association of protein-DNA recognition complexes: electrostatic and nonelectrostatic effects.

In this study the electrostatic and nonelectrostatic contributions to the binding free energy of a number of different protein-DNA recognition complexes are investigated. To determine the electrostatic effects in the protein-DNA association the Poisson-Boltzmann approach was applied. Overall the salt-dependent electrostatic free energy opposed binding in all protein-DNA complexes except one, and the salt-independent electrostatic contribution favored binding in more than half of the complexes. Further the salt-dependent electrostatic free energy increased with higher ionic concentrations and therefore complex association is stronger opposed at higher ionic concentrations. The hydrophobic effect in the protein-DNA complexes was determined from the buried accessible surface area and the surface tension. A majority of the complexes showed more polar than nonpolar buried accessible surface area. Interestingly the buried DNA-accessible surface area was preferentially hydrophilic, only in one complex a slightly more hydrophobic buried accessible surface area was observed. A quite sophisticated balance between several different free energy components seems to be responsible for determining the free energy of binding in protein-DNA systems.

Alkaline Phosphatase↗
PubMed1997

Estimating the electrostatic potential at the acetylcholine receptor agonist site using power saturation EPR.

Continuous wave EPR power saturation was used to measure electrostatic potentials at spin-labeled sites. Membrane surface potentials were estimated by power saturating the EPR spectrum of a membrane bound 14N spin-labeled amphiphile in the presence of a neutral or positively charged 15N labeled aqueous spin probe. The potentials that are measured are in good agreement with other probe measurements and with the predictions of the Gouy-Chapman-Stern theory, indicating that this is a valid approach to determine electrostatic potentials. A spin-labeled affinity probe based on maleimidobenzyltrimethylammonium was synthesized and could be derivatized to a sulfhydryl near either agonist site on the nicotinic acetylcholine receptor. The amplitudes of motion of the spin-probe on the ns time scale are significantly different when the two labeled sites are compared, and the probe is more restricted in its motion when attached to the more easily labeled site. When attached to this agonist site, power saturation EPR yields an electrostatic potential of -15 mV. Two other sulfhydryl-specific probes were used to label this site in reconstituted receptor containing membranes. These probes show less contact with the receptor and reduced electrostatic potentials, indicating that there is a strong spatial dependence to the potential at the agonist site. This work demonstrates that power saturation EPR provides a general method that can be used to estimate electrostatic potentials at any specifically spin-labeled macromolecular site.

Binding Sites↗
PubMed2002

Imaging the electrostatic potential of transmembrane channels: atomic probe microscopy of OmpF porin.

The atomic force microscope (AFM) was used to image native OmpF porin and to detect the electrostatic potential generated by the protein. To this end the OmpF porin trimers from Escherichia coli was reproducibly imaged at a lateral resolution of approximately 0.5 nm and a vertical resolution of approximately 0.1 nm at variable electrolyte concentrations of the buffer solution. At low electrolyte concentrations the charged AFM probe not only contoured structural details of the membrane protein surface but also interacted with local electrostatic potentials. Differences measured between topographs recorded at variable ionic strength allowed mapping of the electrostatic potential of OmpF porin. The potential map acquired by AFM showed qualitative agreement with continuum electrostatic calculations based on the atomic OmpF porin embedded in a lipid bilayer at the same electrolyte concentrations. Numerical simulations of the experimental conditions showed the measurements to be reproduced quantitatively when the AFM probe was included in the calculations. This method opens a novel avenue to determine the electrostatic potential of native protein surfaces at a lateral resolution better than 1 nm and a vertical resolution of approximately 0.1 nm.

Biophysical Phenomena↗
PubMed1978

Electrostatic calculations for an ion channel. II. Kinetic behavior of the gramicidin A channel.

A theoretical model of the gramicidin A channel is presented and the kinetic behavior of the model is derived and compared with previous experimental results. The major assumption of the model is that the only interaction between ions in a multiply-occupied channel is electrostatic. The electrostatic calculations indicate in a multiply-occupied channel is electrostatic. The electrostatic calculations indicate that there will be potential wells at each end of the channel and, at high concentrations, that both wells can be occupied. The kinetics are based on two reaction steps: movement of the ion from the bulk solution to the well and movement between the two wells. The kinetics for this reaction rate approach are identical to those based on the Nernst-Planck equation in the limit where the movement between the two wells is rate limiting. The experimental results for sodium and potassium are consistent with a maximum of two ions per channel. To explain the thallium results it is necessary to allow three ions per channel. It is shown that this case is compatible with the electrostatic calculations if the presence of an anion is included. The theoretical kinetics are in reasonable quantitative agreement with the following experimental measurements: single channel conductance of sodium, potassium, and thallium; bi-ionic potential and permeability ratio between sodium-potassium and potassium-thallium; the limiting conductance of potassium and thallium at high applied voltages; current-voltage curves for sodium and potassium at low (but not high) concentrations; and the inhibition of sodium conductance by thallium. The results suggest that the potential well is located close to the channel mouth and that the conductance is partially limited by the rate going from the bulk solution to the well. For thallium, this entrance rate is probably diffusion limited.

Biological Transport↗
PubMed1995

Electrostatic and hydrodynamic orientational steering effects in enzyme-substrate association.

Diffusional encounters between a dumbbell model of a cleft enzyme and a dumbbell model of an elongated ligand are simulated by Brownian dynamics. The simulations take into account electrostatic and hydrodynamic interactions between the molecules. It is shown that the primary effect of inclusion of hydrodynamic interactions into the simulation is an overall decrease in the rate constant. Hydrodynamic orientational effects are of modest size for the systems considered here. They are manifested when changes in the rate constants for diffusional encounters favored by hydrodynamic interactions are compared with those favored by electrostatic interactions as functions of the overall strength of electrostatic interactions. The electrostatic interactions modify the hydrodynamic torques by modifying the drift velocity of the substrate toward the enzyme. We conclude that simulations referring only to electrostatic interactions between an enzyme and its ligand may yield rate constants that are somewhat (e.g., 20%) too high, but provide realistic descriptions of the orientational steering effects in the enzyme-ligand encounters.

Algorithms↗
PubMed1996

Membrane binding of the colicin E1 channel: activity requires an electrostatic interaction of intermediate magnitude.

In vitro channel activity of the C-terminal colicin E1 channel polypeptide under conditions of variable electrostatic interaction with synthetic lipid membranes showed distinct maxima with respect to pH and membrane surface potential. The membrane binding energy was determined from fluorescence quenching of the intrinsic tryptophans of the channel polypeptide by liposomes containing N-trinitrophenyl-phosphatidylethanolamine. Maximum in vitro colicin channel activity correlated with an intermediate magnitude of the electrostatic interaction. For conditions associated with maximum activity (40% anionic lipid, I = 0.12 M, pH 4.0), the free energy of binding was delta G approximately -9 kcal/mol, with nonelectrostatic and electrostatic components, delta Gnel approximately -5 kcal/mol and delta Gel approximately -4 kcal/mol, and an effective binding charge of +7 at pH 4.0. Binding of the channel polypeptide to negative membranes at pH 8 is minimal, whereas initial binding at pH 4 followed by a shift to pH 8 causes only 3-10% reversal of binding, implying that it is kinetically trapped, probably by a hydrophobic interaction. It was inferred that membrane binding and insertion involves an initial electrostatic interaction responsible for concentration and binding to the membrane surface. This is followed by insertion into the bilayer driven by hydrophobic forces, which are countered in the case of excessive electrostatic binding.

Binding Sites↗
PubMed1996

Electrostatic effects on the stability of condensed DNA in the presence of divalent cations.

Cylindrical cell model Poisson-Boltzmann (P-B) calculations are used to evaluate the electrostatic contributions to the relative stability of various DNA conformations (A, B, C, Z, and single-stranded (ss) with charge spacings of 3.38 and 4.2 A) as a function of interhelix distance in a concentrated solution of divalent cations. The divalent ion concentration was set at 100 mM, to compare with our earlier reports of spectroscopic and calorimetric experiments, which demonstrate substantial disruption of B-DNA geometry. Monovalent cations neutralize the DNA phosphates in two ways, corresponding to different experimental situations: 1) There is no significant contribution to the ionic strength from the neutralizing cations, corresponding to DNA condensation from dilute solution and to osmotic stress experiments in which DNA segments are brought into close proximity to each other in the presence of a large excess of buffer. 2) The solution is uniformly concentrated in DNA, so that the neutralizing cations add significantly to those in the buffer at close DNA packing. In case 1), conformations with lower charge density (Z and ssDNA) have markedly lower electrostatic free energies than B-DNA as the DNA molecules approach closely, due largely to ionic entropy. If the divalent cations bind preferentially to single-stranded DNA or a distorted form of B-DNA, as is the case with transition metals, the base pairing and stacking free energies that stabilize the double helix against electrostatic denaturation may be overcome. Strong binding to the bases is favored by the high concentration of divalent cations at the DNA surface arising from the large negative surface potential; the surface concentration increases sharply as the interhelical distance decreases. In case 2), the concentration of neutralizing monovalent cations becomes very large and the electrostatic free energy difference between secondary structures becomes small as the interhelical spacing decreases. Such high ionic concentrations will be expected to modify the stability of DNA by changing water activity as well as by screening electrostatic interactions. This may be the root of the decreased thermal stability of DNA in the presence of high concentrations of magnesium ions.

Biophysical Phenomena↗
PubMed1998

Electrostatic contributions to the binding free energy of the lambdacI repressor to DNA.

A model based on the nonlinear Poisson-Boltzmann (NLPB) equation is used to study the electrostatic contribution to the binding free energy of the lambdacI repressor to its operator DNA. In particular, we use the Poisson-Boltzmann model to calculate the pKa shift of individual ionizable amino acids upon binding. We find that three residues on each monomer, Glu34, Glu83, and the amino terminus, have significant changes in their pKa and titrate between pH 4 and 9. This information is then used to calculate the pH dependence of the binding free energy. We find that the calculated pH dependence of binding accurately reproduces the available experimental data over a range of physiological pH values. The NLPB equation is then used to develop an overall picture of the electrostatics of the lambdacI repressor-operator interaction. We find that long-range Coulombic forces associated with the highly charged nucleic acid provide a strong driving force for the interaction of the protein with the DNA. These favorable electrostatic interactions are opposed, however, by unfavorable changes in the solvation of both the protein and the DNA upon binding. Specifically, the formation of a protein-DNA complex removes both charged and polar groups at the binding interface from solvent while it displaces salt from around the nucleic acid. As a result, the electrostatic contribution to the lambdacI repressor-operator interaction opposes binding by approximately 73 kcal/mol at physiological salt concentrations and neutral pH. A variety of entropic terms also oppose binding. The major force driving the binding process appears to be release of interfacial water from the protein and DNA surfaces upon complexation and, possibly, enhanced packing interactions between the protein and DNA in the interface. When the various nonelectrostatic terms are described with simple models that have been applied previously to other binding processes, a general picture of protein/DNA association emerges in which binding is driven by the nonpolar interactions, whereas specificity results from electrostatic interactions that weaken binding but are necessary components of any protein/DNA complex.

DNA, Viral↗
PubMed1999

Sampling field heterogeneity at the heme of c-type cytochromes by spectral hole burning spectroscopy and electrostatic calculations.

We report on a comparative investigation of the heme pocket fields of two Zn-substituted c-type cytochromes-namely yeast and horse heart cytochromes c-using a combination of hole burning Stark spectroscopy and electrostatic calculations. The spectral hole burning experiments are consistent with different pocket fields experienced at the hemes of the respective cytochromes. In the case of horse heart Zn-cytochrome c, two distinguishable electronic origins with different electrostatic properties are observed. The yeast species, on the other hand, displays a single electronic origin. Electrostatic calculations and graphics modeling using the linearized finite-difference Poisson-Boltzmann equation performed at selected time intervals on nanosecond-molecular dynamics trajectories show that the hemes of the respective cytochromes sample different potentials as they explore conformational space. The electrostatic potentials generated by the protein matrix at the heme show different patterns in both cytochromes, and we suggest that the cytochromes differ by the number of "electrostatic substates" that they can sample, thus accounting for the different spectral populations observed in the two cytochromes.

Animals↗