Electrostatic Energy Contributions Research Articles

Electrostatic interactions in sperm whale myoglobin. Site specificity, roles in structural elements, and external electrostatic potential distributions.

The electrostatic free energy contribution to the stability of sperm whale ferrimyoglobin was evaluated according to the static accessibility modified Tanford-Kirkwood model. The electrostatic free energy contribution of each distinct structural element was divided into one term arising from interactions between it and other elements (interelemental) and another from interactions within the particular element itself (intraelemental). At pH 7 the majority of the terms were found to be stabilizing. The interelemental terms are the dominant ones for most structural elements. The small interelemental terms of the C and D helices are compensated by large intraelemental interactions which stabilize these short helices. Perturbations in pH can be accommodated by the structural elements through a redistribution of stabilizing and destabilizing interactions. The electrostatic potentials calculated at the surface of the protein indicate that the internal compensation of local potentials achieved during folding results in a generally neutral protein-solvent interface save for two distinct areas of nonzero potential. The accessibility of each charged atom to solvent was analyzed in terms of the surface area lost to charged, polar and nonpolar atoms separately. The net solvent accessibility lost parallels closely that lost to nonpolar atoms alone, indicating a specific role for nonpolar atoms in defining dielectric shielding of charged atoms, aside from their participation in the well-known hydrophobic interactions.

MgAl2O4ion positions calculated for the F43m space group

The positional parameters of the spinel MgAl2O4 were calculated using as a basis the space group F43m. The Al3+ and O2- ion positions were varied, and the lattice energy was found for each configuration. The minimum lattice energy, summing electrostatic and repulsive energy contributions, was found to be -0.248495*10-15 J/unit cell. This minimum energy occurred for ionic positional parameters satisfying Fd3m symmetry requirements. From the variations of energy with position, as well as other experimental considerations, it still seems appropriate to consider the space group for MgAl2O4 to be F43m. The specific values for the positional parameters yielding the minimum energy thus correspond to the intersection of energy surfaces of the two groups in question.

Interatomic interaction potential dependence on the relative motion energy of atoms

Abstract A theory of interatomic interaction potentials for the nonstatic case is given. It is shown that in the framework of Thomas-Fermi-Dirac approximation interatomic potential may be separated into three parts: electrostatic interaction, kinetic and exchange energy contribution. The second and the third of these depend on the Fermi-spheres overlap in the momentum space which is absent when the relative speed of atoms is high enough. The electrostatic part of the potential does not depend on this speed and can be calculated as in the static case. The general formulae for the kinetic and exchange parts of interaction potential are derived. Some applications to the channeling theory and nuclear stopping power theory are discussed.

Change of free energy in heteromolecular nucleation: Electrostatic energy contribution

The presence of foreign particles in vapor is known to promote the formation of clusters and phase transition at lower supersaturation. Our primary object here is to assess the contribution of the electrostatic energy to the total free energy change owing to the presence of a charged or uncharged foreign particle (atom or molecule) during the formation of heteromolecular clusters. Unlike the Thomson’s ’’electrothermodynamic’’ theory, we introduce the contribution of the higher electric moment (dipole) and polarization of the foreign particle, in addition to its monopole charge contribution. The present theory predicts: (1) there is no strong charge sign preference in the promotion of nucleation, (2) the size (’’radius’’) differences of ions or neutral foreign particles largely determine differences in the free energy change and thus in the promotion of nucleation, and (3) the differences in the dipole moments and polarizabilities of the charged or uncharged foreign particles are also important for such distinctions, particularly in the case of the foreign particles of nearly the same size. In the limiting case of zero polarizability and zero dipole moment associated with an ion, our present theory recovers exactly the same functional dependency on the charge, ionic radius, and the cluster size as in the electrothermodynamic theory of Thomson. .

Protein-protein interactions: nature of the electrostatic stabilization of deoxyhemoglobin tetramer formation.

The summed electrostatic free energy contributions to deoxyhemoglobin A0 tetramer formation were computed at a series of pH and ionic strength values as the difference between the computed values for the tetramer and for the sum of the four individual chains. The electrostatic stabilization of each monomer is similar and close to that for myoglobin. At ionic strength 0.10 M the electrostatic contribution to the stability of the tetramer is approximately 35 kcal/mol at pH 6.0 and 18 kcal/mol at pH 9.6. The specific contribution to the stabilization of the tetramer, (sigma delta G"i,el)tet, is obtained by difference and shows a broad plateau above 7 kcal/mol over the range from pH 6.0 to 8.0, which is nearly obliterated by pH 9.6. By examination of the contributions of individual sites under the above summation, it is found that sites in the alpha chains are responsible for virtually the entire stabilizing effects in tetramer formation. The major differences on tetramer formation are sensed at eight sites. The stabilization provided by four of these sites results simply from changes in solvent exposure of sites in the given monomers as the tetramer is assembled. They are offset in part by changes at three sites that sense the greatest destabilization and that are responsible for the near cancellation of effects among the beta-chain sites. The general implications for the stabilization of molecular assemblies are considered.

Internal energy of AB2X4 crystalline compounds. Electrostatic energy contribution for compounds with closed shell cations

Abstract Madelung energy calculations are performed for two different lattices in which some of AB2X4 compounds (A = M+2, B = M−3, X = O−2, S−2, Se−2, Te−2) crystalline. A gross evaluation of the covalent bond energy contribution in the same crystals is made and some considerations are draw from the comparison between the two above quoted energy contributions.

Theoretical conformational analysis of 2-phosphopropionic acid, 2-phospho,3-olo-propionic acid and d-2,3-diphosphoglycerate

Partition energy method (PEM) has been used to calculate the molecular conformation of three organic phosphates of biological interest, namely the 2-phosphopropionic acid, the 2-phospho-3-olo-propionic acid and the d-2,3 diphosphoglycerate (DPG). The analysis shows that the results for the symmetrically independent conformations of these phosphates indicate a strong dependence upon the electrostatic contributions. This contribution is critical in the case of DPG because of the presence of two phosphate groups. Considering only van der Waals non-bonded and torsional energy contributions, seven conformations are allowed. Including the electrostatic energy contributions an extended conformation is preferred. The derived data are discussed in relation to experimental results.

Conformational studies of quaternary ammonium ions. Part III. Conformational analyses of substituted piperidinium ions by 1H nuclear magnetic resonance spectroscopy and evaluation of the contribution of electrostatic interaction energy in controlling conformation

Conformations of piperidinium and NN-dimethylpiperidinium ions having a hydroxy- or acetoxy-group at C-4 or C-3, and of the corresponding free amines in solution have been studied by 1H n.m.r. spectroscopy. The population of the form with an axial hydroxy- or acetoxy-group of these ammonium ions is larger than that of the corresponding form in the free amines, without exception. The effect of 1,3-diaxial interaction energy and electrostatic interaction energy is discussed and an approximate value for the contribution of the latter in the conformation control of some ammonium ions is estimated.

Domain model for anomalously fast diffusion

Electrostatic calculations are performed in idealized β-Al 2O 3, containing domains with displaced sodium ions. The average additional energy per sodium ion is calculated for several sizes and shapes of the domains. The movement of the domain walls is considered to be essential in the process of anomalously fast conduction. The electrostatic energy contribution to the activation energy for wall movement is calculated. Some rules are formulated which might permit the selection of possible super ionic conductors by electrostatic calculations in ionic compounds.

Application of Pseudopotentials to the Theory of Self-Diffusion in Metals

A pseudopotential theory for the energetics of vacancy migration is formulated by a total-energy comparison of two vacancy-lattice configurations. Band-structure and electrostatic energy contributions are found to be significant. Energy of motion is determined as a function of diffusing atom and vacancy positions for the three common metal geometries. When combined with a prior theory for formation energy, activation energies for self-diffusion are obtained. Reasonable agreement with experiment is noted for the close-packed geometries using phenomenological pseudopotentials obtained from previous studies. For example, the observed anisotropy in hcp diffusion is accurately described for magnesium. The neglect of lattice relaxation in our simple model becomes a more significant effect in the case of the bcc alkali metals. Here the calculated formation energies are considerably larger than experiment, leading to overestimation of the activation energies for the self-diffusion of vacancies. When our approach is used to test various pseudopotential form factors, the model potentials which best fit the phonon dispersion curves produce the most reasonable values of activation energy.

The effect of the polar moiety of lipids on bilayer conductance induced by uncouplers of oxidative phosphorylation

Bilayer membranes were formed from decane, cholesterol, and three lipids isolated fromStaphylococcus aureus: positively charged lysyl phosphatidylglycerol (LysPG), negatively charged phosphatidylglycerol (PG), and neutral diglucosyldiglyceride (DiGluDiGly). The uncouplers of oxidative phosphorylation, 2,4-dinitrophenol (DNP) and 3-t-butyl,5-chloro,2'-chloro,4'-nitrosalicylanilide (S 13), increased the electrical conductance of all three differently charged bilayers. S 13 was found to be the most effective reagent of the known uncouplers in increasing conductance of the bilayers. The conductance induced by uncouplers was investigated as a function of pH and uncoupler concentration. The pH of maximum conductance for each uncoupling agent was dependent on both the uncoupler and the lipid; it was lower for each uncoupler in LysPG and higher in PG compared to DiGluDiGly bilayers. At a pH below the optimum for LysPG, the conductance of the positively charged membrane was 500 times and of the neutral one 10 times higher than that of the negatively charged bilayer at equal uncoupler concentration and pH. Above the pH optimum for DiGluDiGly, the conductance was approximately equal for the positive and neutral membranes, but was lower in PG bilayers. Conductance depended linearly on uncoupler concentration. The bilayer conductance induced by S 13 was entirely due to increased proton permeability in all three lipids. The findings are consistent with the role of uncouplers as "carriers" for protons across the hydrocarbon interior of lipid membranes. The differences in conductance of differently charged lipid bilayers at equal uncoupler concentration, as well as the change of pH optimum of conductance with lipid charge, can be explained in terms of an electrostatic energy contribution of the fixed lipid charges to the distribution of the uncoupler anion between the aqueous and the membrane phases.