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At least 649 records · Page 36Linked to original sources
PubMed1992

On the mapping of electrostatic properties from the multipole description of the charge density.

A method is presented to calculate the electrostatic potential, the electric field and the electric-field gradient in a crystal from the atomic multipole expansion of the experimental charge density, as described by the Hansen-Coppens formalism [Hansen & Coppens (1978), Acta Cryst. A34, 909-921]. The electrostatic properties are expressed in terms of the positions and the charge-density parameters of the individual atoms. Contributions due to the procrystal charge density and the deformation charge density are compared. The method is illustrated by the calculation of the electrostatic potential maps of fully deuterated benzene and of iron(II) tetraphenylporphyrin.

Chemical Phenomena↗
PubMed1999

Molecular fragment electric moments derived from the fit of the experimental electrostatic potential. Application to the water molecule.

The electrostatic potential is a multicenter property that can be expressed as a sum of the contributions of electric moments located at each atomic site of a molecule. Independently of the model used to generate the electrostatic potential around the system, these atomic moments can be accurately obtained by the fit of this physical property outside the van der Waals envelop. However, the larger the system, the greater the number of parameters. In this study a way is proposed to reduce the number of centers in the representation of the electrostatic potential which becomes a sum of fragment contributions rather than atomic ones. A sample of six water molecules in different crystal environments was chosen to discuss the derived values of the electric moments referred to the molecular center of mass.

Journal Article↗
PubMed1999

Experimental charge density and electrostatic potential in nicotinamide.

The accurate crystal structure of nicotinamide, 3-pyridinecarboxamide, was determined from X-ray and neutron diffraction experiments: C(6)H(6)N(2)O, M(r) = 122.13, monoclinic, P2(1)/c, Z = 4. The electron distribution at 150 K was determined by the maximum entropy method and the electrostatic potential in the crystal was calculated by Fourier convolution of the electron distribution. The electrostatic properties of the nicotinamide molecule depend on the molecular conformation. The asymmetric electrostatic potential field observed above and below the pyridine-ring plane is related to the rotation of the carboxamide group with respect to the pyridine plane. The positive potential peak at the C4 atom of the pyridine ring extends to the C=O-group side of the plane. The asymmetry of the potential on the C4 atom is consistent with the stereospecificity of hydride transfer in NAD(+)/NADH oxidoreduction.

Journal Article↗
PubMed2004

Impact of a Poisson-Boltzmann electrostatic restraint on protein structures refined at medium resolution.

The Poisson-Boltzmann formulism has been developed as a restraint for electrostatic interactions during the crystallographic refinement of macromolecules. It accounts implicitly for the effects of solvent and mobile ions, which are usually not included as restraints in the refinement of experimental structures. The electrostatic restraint has been implemented by combining software for numerically solving the three-dimensional Poisson-Boltzmann equation with a package for stereochemically restrained refinement. Its application to medium-resolution protein structures leads to a reduced free R factor, overfitting and to improved interactions in salt bridges and between polar or charged amino acids and the solvent. In contrast, Coulombic and screened Coulombic treatments did not lead to significant gains. The work leads to a modest improvement in refinement methods, confirmation that the Poisson-Boltzmann formulism is more consistent with experimental structure than the Coulombic approach, and to a reduction in the discrepancy between experimental and electrostatically optimized atomic models.

Crystallography↗
PubMed1997

The effects of metal binding on a nucleobase: the experimental charge density and electrostatic potential in 1H(+)-adeniniumtrichlorozinc(II) at 123 K and its relationship to that in adenine hydrochloride hemihydrate.

The charge-density distribution in 1H(+)-adeniniumtrichlorozinc(II) has been determined from X-ray diffraction data collected to sin theta/lambda = 1.32 A(-1) at 123 K. The electrostatic potential, isolated from the crystal lattice, and the deformation density in the nucleobase have been calculated following multipole refinements based on the rigid pseudoatom model of Stewart. These and the molecular dimensions have been compared with results from the charge-density study of adenine hydrochloride hemihydrate by Cunane & Taylor [Acta Cryst. (1993). B49, 524-530] to determine the effects of metal binding on the nucleobase. The main conclusions are that while the bond lengths and angles in the pyrimidine ring are similar, those in the imidazole ring are significantly perturbed on complexation; lone-pair electron density at N7 is observed in both structures and lies significantly off the plane of the nucleobases; the positive electrostatic potential of the complexed base extends much further from the molecule than in the uncomplexed one and the regions of negative potential at N3 and N7 are depleted in the complexed base. The observed enhancement of positive electrostatic potential in the nucleobase on binding to zinc is presented in support of a model for the mechanism of reversible unwinding of DNA in the presence of zinc ions.

Journal Article↗
PubMed1998

A microfabricated electrostatic haptic display for persons with visual impairments.

An electrostatic haptic display with three 7 x 7 electrode arrays of three different sizes was fabricated on a 4-in wafer using lithographic microfabrication techniques. The display utilizes electrostatic stimulation to generate a tactile sensation of texture on a scanning finger. The tactile sensation appeared to be a result of increased friction and vibration due to the electrostatic forces between the finger skin and the electrodes. Various spatial tactile patterns (lines, circles, squares, and triangles, etc.) can be presented on the display. Experiments of threshold, line separation, and pattern recognition were performed on subjects with visual impairments to study the spatial resolution and information transmission on arrays of variant electrode size and spacing. Two columns with two-column spacing can be resolved with 80% accuracy on the small array, for a spatial resolution of 5.8 mm in terms of edge-to-edge electrode distance. The overall percentages of correct recognition for the patterns were 68.3, 72.1, and 71.3% on the small, medium, and large arrays, respectively. While subject is an important factor for both threshold and pattern recognition, electrode size was statistically significant for threshold only. Frequency and duty cycle of the stimulation waveform did not show statistical significance.

Adult↗
PubMed2004

Precompensated excitation waveforms to suppress harmonic generation in MEMS electrostatic transducers.

Microelectromechanical systems (MEMS) electrostatic-based transducers inherently produce harmonics as the electrostatic force generated in the transmit mode is approximately proportional to the square of the applied voltage signal. This characteristic precludes them from being effectively used for harmonic imaging (either with or without the addition of microbubble-based contrast agents). The harmonic signal that is nonlinearly generated by tissue (or contrast agent) cannot be distinguished from the inherent transmitted harmonic signal. We investigated two precompensation methods to cancel this inherent harmonic generation in electrostatic transducers. A combination of finite element analysis (FEA) and experimental results are presented. The first approach relies on a calculation, or measurement, of the transducer's linear transfer function, which is valid for small signal levels. Using this transfer function and a measurement of the undesired harmonic signal, a predistorted transmit signal was calculated to cancel the harmonic inherently generated by the transducer. Due to the lack of perfect linearity, the approach does hot work completely in a single iteration. However, with subsequent iterations, the problem becomes more linear and converges toward a very satisfactory result (a 18.6 dB harmonic reduction was achieved in FEA simulations and a 20.7 dB reduction was measured in a prototype experiment). The second approach tested involves defining a desired function [including a direct current (DC) offset], then taking the square root of this function to determine the shape of the required input function. A 5.5 dB reduction of transmitted harmonic was obtained in both FEA simulation and experimental prototypes test.

Journal Article↗
PubMedDate not supplied

Electrostatic tactile display with thin film slider and its application to tactile telepresentation systems.

A new electrostatic tactile display is proposed to realize compact tactile display devices that can be incorporated with virtual reality systems. The tactile display of this study consists of a thin conductive film slider with stator electrodes that excite electrostatic forces. Users of the device experience tactile texture sensations by moving the slider with their fingers. The display operates by applying two-phase cyclic voltage patterns to the electrodes. The display is incorporated into a tactile telepresentation system to realize explorations of remote surface textures with real-time tactile feedback. In the system, a PVDF tactile sensor and a DSP controller automatically generate voltage patterns to present surface texture sensations through the tactile display. A sensor, in synchronization with finger motion on the tactile display, scans a texture sample and outputs information about the sample surface. The information is processed by a DSP and fed back to the tactile display in real time. The tactile telepresentation system was evaluated in texture discrimination tests and demonstrated a 79 percent correct answer ratio. A transparent electrostatic tactile display is also reported in which the tactile display is combined with an LCD to realize a visual-tactile integrated display system.

Computer Peripherals↗
PubMed2005

The evolutionary origins and catalytic importance of conserved electrostatic networks within TIM-barrel proteins.

Conservation of function is the basic tenet of protein evolution. Conservation of key electrostatic properties is a frequently employed mechanism that leads to conserved function. In a previous report, we identified several conserved electrostatic properties in four protein families and one functionally diverse enzyme superfamily. In this report, we demonstrate the evolutionary and catalytic importance of electrostatic networks in three ubiquitous metabolic enzymes: triosephosphate isomerase, enolase, and transaldolase. Evolutionary importance is demonstrated using phylogenetic motifs (sequence fragments that parallel the overall familial phylogeny). Phylogenetic motifs frequently correspond to both catalytic residues and conserved interactions that fine-tune catalytic residue pKa values. Further, in the case of triosephosphate isomerase, quantitative differences in the catalytic Glu169 pKa values parallel subfamily differentiation. Finally, phylogenetic motifs are shown to structurally cluster around the active sites of eight different TIM-barrel families. Depending upon the mechanistic requisites of each reaction catalyzed, interruptions to the canonical fold may or may not be identified as phylogenetic motifs.

Amino Acid Sequence↗
PubMed2001

Elongation of the BH8 beta-hairpin peptide: Electrostatic interactions in beta-hairpin formation and stability.

An elongated version of the de novo designed beta-hairpin peptide, BH8, has allowed us to gain insight into the role of electrostatic interactions in beta-hairpin stability. A Lys-Glu electrostatic pair has been introduced by adding a residue at the beginning and at the end of the N-terminal and C-terminal strands, respectively, of the beta-hairpin structure, in both orientations. The two resulting peptides and controls having Ala residues at these positions and different combinations of Ala with Lys, or Glu residues, have been analyzed by nuclear magnetic resonance (NMR), under different pH and ionic strength conditions. All of the NMR parameters, in particular the conformational shift analysis of Calpha protons and the coupling constants, (3)J(HNalpha), correlate well and the population estimates are in reasonable agreement among the different methods used. In the most structured peptides, we find an extension of the beta-hairpin structure comprising the two extra residues. Analysis of the pH and salt dependence shows that ionic pairs contribute to beta-hairpin stability. The interaction is electrostatic in nature and can be screened by salt. There is also an important salt-independent contribution of negatively charged groups to the stability of this family of beta-hairpin peptides.

Amino Acid Sequence↗
PubMed1988

Electrostatic potential of macromolecules measured by pKa shift of a fluorophore. 1. The 3' terminus of 16S RNA.

We have investigated the use of the pH-sensitive fluorescein label as a probe for electrostatic potential in macromolecules. The practicality of this technique is demonstrated by its application to the 16S RNA molecule. The dependence of the electrostatic potential upon ionic conditions and upon the presence of ribosomal proteins and the state of the RNA was studied. The combination of electrostatic and anisotropy data emphasizes the rôle of the 30S ribosomal proteins, rather than of the renaturation of the 16S RNA or the presence of the 50S subunit, in shaping the environment of the 3' terminus of the 16S RNA in the active ribosome.

Electrochemistry↗
PubMed1992

Inhibition of cytochrome-P450 reductase by polyols has an electrostatic nature.

The present study was undertaken to examine the nature of the inhibitory action of glycerol on the liver microsomal monooxygenase system. In agreement with earlier observations, glycerol inhibited benzphetamine N-demethylation by liver microsomes of the phenobarbital-treated rabbit. The presence of glycerol in the medium did not affect binding of the substrate to cytochrome P450. Another polyol, ethylene glycol, was equally efficient in inhibiting benzphetamine N-demethylation. Both also inhibited reduction of rabbit cytochrome P450 LM2, cytochrome c and potassium ferricyanide by NADPH-cytochrome-P450 reductase in microsomes. Recently, we showed that the stimulation of electron transfer by increased ionic strength is due to neutralization of electrostatic interaction between NADPH-cytochrome-P450 reductase and its charged redox partners [Voznesensky, A. I. & Schenkman, J. B. (1992) J. Biol. Chem. 267, 14669-14676]. Polyols have an opposite effect to that of salt on ionic properties of a solution. They decrease the dielectric constant, thereby promoting electrostatic interactions between proteins. Addition of polyols decreased the conductivity of the medium. When rates of electron transfer to charged acceptors, cytochrome P450, cytochrome c and potassium ferricyanide, at various salt and polyol concentrations, relative to activities in 200 mM sodium phosphate, were plotted as a function of the conductivity the data for each acceptor fit on the same line. In contrast, neither alteration of ionic strength nor polyol addition affected the rate of electron transfer from NADPH-cytochrome-P450 reductase to an uncharged acceptor 1,4-benzoquinone. The data obtained is consistent with our earlier suggestion that charge repulsion limits redox interactions between rabbit cytochrome P450 LM2 and its reductase at low ionic strength, and suggest that the observed action of polyols is the result of enhancement of electrostatic interactions that inhibits electron transfer between NADPH-cytochrome-P450 reductase and its charged redox partners. In congruence with the hypothesis, the Km of rabbit cytochrome P450 LM2 for NADPH-cytochrome-P450 reductase was increased almost one order of magnitude by elevating the glycerol content from 5% to 25% (by vol.) without a change in Vmax.

Animals↗
PubMed1994

The role of electrostatic charge in the membrane insertion of colicin A. Calculation and mutation.

The bacterial toxin colicin A binds spontaneously to the surfaces of negatively charged membranes. The surface-bound toxin must subsequently, however, become an acidic 'molten globule' before it can fully insert into the lipid bilayer. Clearly, electrostatic interactions must play a significant role in both events. The electrostatic field around the toxin in solution was calculated using the finite-difference Poisson-Boltzmann method of the Delphi programme and the known X-ray structure. A large positively charged surface was identified which could be involved in the binding of colicin to negatively charged membranes. The applicability of the result was tested by also calculating the fields around modelled structures of the closely related colicins B and N. Surprisingly, colicin N showed a similar charge distribution in spite of its isoelectric point of pI 10.20 (colicin A has pI 5.44). One reason for this is the strong conservation of certain negative charges in all colicins. There is a single highly conserved aspartate residue (Asp78) on the positively charged face which provides a small but discrete region of negative charge. This residue, Asp78, was replaced by asparagine in the mutant D78N. D78N binds faster to negatively charged vesicles but inserts only half as fast as the wild-type protein into the membrane core. This indicates that, first, the initial membrane binding has a significant electrostatic component and, second, that the isolated charge on Asp78 plays a role in the formation of the insertion intermediate.

Amino Acid Sequence↗
PubMed1997

Electrostatic effects of smooth muscle calponin on actin assembly.

The contribution of electrostatic interactions to the effects of chicken gizzard calponin on the kinetics of actin polymerization and the bundling of F-actin were characterized by a combination of fluorescence, light-scattering, co-sedimentation, and electron-microscopic methods. Stoichiometric amounts of calponin accelerate actin polymerization in low-ionic-strength solutions, but this effect is diminished at [KCI] = 150 mM. At low ionic strengths, micromolar concentrations of calponin induce the formation of large bundles of actin filaments, and lower concentrations of calponin quench the fluorescence of pyrene-labeled F-actin. The latter effect is related to binding of calponin to F-actin rather than to bundling of the filaments. The concentration of calponin required to bundle a fixed concentration of actin filaments increases with increasing ionic strength, as the average diameter of the bundles decreases. Millimolar concentrations of ATP, GTP or ITP are equally efficient at dispersing actin bundles to single filaments or smaller aggregates, even though a significant fraction of calponin remains bound to F-actin. Our findings show that the binding of calponin to actin is determined at least in part by electrostatic interactions, and that the polycationic nature of calponin is primarily responsible for the formation of F-actin bundles via its ability to reduce the electrostatic repulsion between the negatively charged actin filaments.

Actins↗
PubMed2004

Identification of domains on the extrinsic 23 kDa protein possibly involved in electrostatic interaction with the extrinsic 33 kDa protein in spinach photosystem II.

To elucidate the domains on the extrinsic 23 kDa protein involved in electrostatic interaction with the extrinsic 33 kDa protein in spinach photosystem II, we modified amino or carboxyl groups of the 23 kDa protein to uncharged methyl ester groups with N-succinimidyl propionate or glycine methyl ester in the presence of a water-soluble carbodiimide, respectively. The N-succinimidyl propionate-modified 23 kDa protein did not bind to the 33 kDa protein associated with PSII membranes, whereas the glycine methyl ester-modified 23 kDa protein completely bound. This indicates that positive charges on the 23 kDa protein are important for electrostatic interaction with the 33 kDa protein associated with the PSII membranes. Mapping of the N-succinimidyl propionate-modified sites of the 23 kDa protein was performed using Staphylococcus V8 protease digestion of the modified protein followed by determination of the mass of the resultant peptide fragments with MALDI-TOF MS. The results showed that six domains (Lys11-Lys14, Lys27-Lys38, Lys40, Lys90-Lys96, Lys143-Lys152, Lys166-Lys174) were modified with N-succinimidyl propionate. In these domains, Lys11, Lys13, Lys33, Lys38, Lys143, Lys166, Lys170 and Lys174 were wholly conserved in the 23 kDa protein from 12 species of higher plants. These positively charged lysyl residues on the 23 kDa protein may be involved in electrostatic interactions with the negatively charged carboxyl groups on the 33 kDa protein, the latter has been suggested to be important for the 23 kDa binding [Bricker, T.M. & Frankel, L.K. (2003) Biochemistry42, 2056-2061].

Amino Acid Sequence↗
PubMedDate not supplied

Electrostatic fields in ionography.

There is renewed interest in diagnostic radiology in electrostatic methods of imaging, such as xeroradiography and ionography. This is due to the fact that edge contrast can be achieved, aiding in the visualization of soft tissue tumors. In analyzing the image forming properties of these system, we chose to solve the electrostatic problems by the method of images. We present methods and recursion formulas for calculating electrostatic fields due to (a) any charge distribution on a slab of dielectric (the solution involves a single infinite series) or (b) the same as the above with the introduction of an additional ground plane parallel to the dielectric surface (the solution now involves double infinite series). Analysis of these fields suggests new methods of controlling edge contrast and development configurations where the field which penetrates through the foil is used to produce the final image rather than the field above the charged surface.

Electric Conductivity↗
PubMed1983

Influence of salts on electrostatic interactions between poliovirus and membrane filters.

Neither solutions of salts nor solutions of detergents or of an alcohol at pH 4 are capable of eluting poliovirus adsorbed to membrane filters. However, solutions containing both a salt, such as magnesium chloride or sodium chloride, and a detergent or alcohol at pH 4 were capable of eluting adsorbed virus. The ability of ions to promote elution of virus at low pH in the presence of detergent or alcohol was dependent on the size of the ions and the ionic strength of the medium. These results suggest that both electrostatic and hydrophobic interactions are important in maintaining virus adsorption to membrane filters. Hydrophobic interactions can be disrupted by detergents or alcohols. It appears that electrostatic interactions can be disrupted by raising the pH of a solution or by adding certain salts. Disruption of either electrostatic or hydrophobic interactions alone does not permit efficient elution of the adsorbed virus at low pHs. However, when both interactions are disrupted, most of the poliovirus adsorbed to membrane filters is eluted, even at pH 4.

Adsorption↗