Electrostatic potential fields

In order to describe the effects of the double layer on the particle motion, the Poisson equation is used. The Poisson equation relates the electrostatic potential field to the charge density in the double layer, and this gives rise to the concepts of zeta-potential and surface of shear. Using extensions of the double-layer theory, Debye and Huckel, Smoluchowski,... 

Breneman, C., and M. Martinov. 1996. The Use of the Electrostatic Potential Fields in QSAR and QSPR. In Molecular Electrostatic Potentials Concepts and Applications, edited by J. S. Murray and K. D. Sen. Elsevier, Amsterdam. 

The origin of stereofacial selectivity in electrophilic additions to methylene-cyclohexanes (2) and 5-methylene-l,3-dioxane (3) has been elucidated experimentally (Table 2) and theoretically. Ab initio calculations suggest that two electronic factors contribute to the experimentally observed axial stereoselectivity for polarizable electrophiles (in epoxidation and diimide reduction) the spatial anisotropy of the HOMO (common to both molecules) and the anisotropy in the electrostatic potential field (in the case of methylenedioxane). The anisotropy of the HOMO arises from the important topological difference between the contributions made to the HOMO by the periplanar p C-H a-bonds and opposing p C—O or C—C cr-bonds. In contrast, catalytic reduction proceeds with equatorial face selectivity for both the cyclohexane and the dioxane systems and appears to be governed largely by steric effects. ... 

More recently, with the significant increases in computer power even on desktop PCs, methods for directly matching 3-D features of molecules have become more prevalent. Features here generally refer to various types of molecular fields, some such as electron density ( steric ) and electrostatic-potential fields are derived from fundamental physics (30,31) while others such as lipophilic potential fields (32) are constructed in an ad hoc manner. Molecular fields are typically represented as continuous functions. Discrete fields have also been used (33) albeit somewhat less frequently except in the case of the many CoMFA-based studies (34). 

An alternative approach (30) treats the steric and electrostatic potential fields directly. The steric field is generally given by an expression similar to that in Eq. 2.70,... 

Wild, D. J., Willett, P. (1996) Similarity searching in files of three-dimensional chemical structures. Alignment of molecular electrostatic potential fields with a genetic algorithm. / Chem Inf Comput Sci 36, 159-167. 

Carruthers, JA, and Wigley, K.J., The estimation of electrostatic potentials, fields and energies in a rectangular metal tank containing charged fuel, J. Inst. Petroleum, 48, No. 462, 180-195, 1963. 

Wild, D.J. and Willet, P. Similarity Searching in Files ofThree-Dimensional Chemical Structures. Alignment of Molecular Electrostatic Potential Fields with a Genetic Algorithm./. Chem. Inf. Comput Sci., 1996,36, 159-167. 

Surface complexation models (SCM s) provide a rational interpretation of the physical and chemical processes of adsorption and are able to simulate adsorption in complex geochemical systems. Chemical reactions at the solid-solution interface are treated as surface complexation reactions analogous to the formation of complexes in solution. Each reaction is defined in terms of a mass action equation and an equilibrium constant. The activities of adsorbing ions are modified by a coulombic term to account for the energy required to penetrate the electrostatic-potential field extending away from the surface. Detailed information on surface complexation theory and the models that have been developed, can be found in (Stumm et al., 1976 ... 

To obtain the second equality we have integrated by parts using Eq. (1.30). According to (1.224) the electrostatic potential field is seen to arise from two charge densities the regular p(r) and an additional contribution associated with the dipole density pp(r ) = —Vr P(r). We will refer to p(r) as the external charge density. This reflects a picture of a dielectric solvent with added ionic charges. 

A. 15.5 Three of the following. (1) A central reference ion of a specific charge can be represented as a point charge. (2) This central ion is surrounded by a cloud of smeared-out charge contributed by the participation of all of the other ions in solution. (3) The electrostatic potential field in the solution can be described by an equation that combines and linearizes the Poisson and Boltzmann equations. (4) No ion — ion interactions except the electrostatic interaction given by a l/z dependence are to be considered (i.e., dispersion forces and ion - dipole forces are to be excluded). (5) The solvent simply provides a dielectric medium, and the ion — solvent interactions are to be ignored, so that the bulk permittivity of the solvent can be used. 

Breneman C, Martinov M. The use of the electrostatic potential field in QSAR and QSPR. In Murray JS, Sen KD, eds. Molecular Electrostatic Potentials Concepts and Applications. Amsterdam Elsevier, 1996 143-179. 

Electrons entering into any material are affected by the electrostatic potential field V and, as a result, change their phase. For electrons running in the z direction, the phase change is described by... 

Comparison of the proposed mimetic with the peptide has taken on many levels of sophistication. Molecular overlays of low energy conformations of the mimetic and relevant conformations of the peptide have, in some cases, been performed based solely on visual inspection of the two structures. Sometimes there are many possible ways in which geometric correspondence between important recognition elements on the peptide and mimetic can be obtained. Computational methods that attempt to limit researcher prejudice in the overlap process by searching through the numerous possibilities have also been reported.i-55 159 Comparison of the relative electrostatic potential fields, the geometric location of specific recognition elements, and the overall molecular shape and steric volume have all been considered in these types of analyses. 

Falconi, M. Rotilio, G. Desideri, A. Modeling the three-dimensional structure and electrostatic potential field of the two Cu, Zn superoxide dismutase variants from Xenopus laevis. Proteins 1991,10, 149-155. 

In order to understand this Interaction, we have calculated the electrostatic potential field of plastocyanin. For our calculations, we chose the Del-Phi program of Klapper et al. (5) because It allows us to vary parameters such as dielectric constant, solvent Ionic strength and Stern layer thickness. In addition, we were able to vary oxidation state of PC and the pH. 

Fig. 2. The effect of oxidation state on the electrostatic potential field surrounding spinach PC. The parameters used to calculate the electric field were protein/solvent dielectric constants = 2/80 solvent ionic strength = 150 mM Stern layer = 2 A. Isopotential lines are shown for kT/e of 4.0, 2.0, 1.0, 0.5, 0.25, -0.25, -0.5, -1.0, -2.0, -4.0. a) Oxidized PC b) Reduced PC c) Oxidized - reduced PC (Ionic strength = 30 mM).

Fig. 3. The effect of solvent Ionic strength, protein dielectric constant and pH of the medium on the electrostatic potential field for oxidized spinach PC. a) Ionic strength = 15 mM b)Protein dielectric constant = 80 c) pH = 5.0...

The effect of pH on the electrostatic potential field of PC Is Important, because It experiences a pH of 15 upon Illumination of the thylakolds. Some of the carboxylic acid residues on PC should be protonated under these conditions, since pH 5 Is close to their pK s. Protonation of these residues will change the magnitude of the electrostatic potential, and hence the Interaction of PC with Its reaction partners. We made a very simple approximation to the low pH case by decreasing the charge on all carboxylic residues on PC by 50 X. It can be seen (Fig. 3c) that the potential field around Tyr 83 Is decreased while that at His 87 Is Increased. These results agree with known effects of pH on the Interaction of PC with cyt f (6). 

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