Electrostatic potentials direct reaction field

The basic reactions of thiolsulfonates have been known for sometime (Field et al., 1961, 1964), but more recently, they have been applied to the study of protein interactions by site-directed modification of native cysteines or through modification of cysteines introduced at particular points in proteins by mutagenesis. Such studies have yielded insights into the structure and binding site characteristics of proteins (Kirley, 1989). Pascual et al. (1998) used AEAETS to probe the acetylcholine receptor from the extracellular side of the membrane in order to investigate the molecular accessibility and electrostatic potential within the open and closed channel. 

In chapter 2, Profs. Contreras, Perez and Aizman present the density functional (DF) theory in the framework of the reaction field (RF) approach to solvent effects. In spite of the fact that the electrostatic potentials for cations and anions display quite a different functional dependence with the radial variable, they show that it is possible in both cases to build up an unified procedure consistent with the Bom model of ion solvation. The proposed procedure avoids the introduction of arbitrary ionic radii in the calculation of insertion energy. Especially interesting is the introduction of local indices in the solvation energy expression, the effect of the polarizable medium is directly expressed in terms of the natural reactivity indices of DF theory. The paper provides the theoretical basis for the treatment of chemical reactivity in solution. 

The form of this equation makes explicit the fact that intermolecular forces do depend upon their vibrational states as well as on their electronic states. Due to the antisymmetrization of the global electronic wave function, Vaia2(R R12) contains Coulomb exchange terms and a direct term formed by the Coulomb multipole interactions and the infinite order perturbation electrostatic effects embodied in the reaction field potential [21, 22],... 

Tel. 412-621-2050, fax 412-621-3563, e-mail info gaussian.com Gaussian 92. Ab initio molecular orbital calculations (Hartree-Fock, Direct HF, Moller-Plesset, Cl, Reaction Field Theory, electrostatic potential-derived charges, vibrational frequencies, etc.). Input and output of molecular structures in formats of many other molecular modeling systems. Browse for archival storage of computed results. VAX, Cray, DEC-RISC (Ultrix), Fujitsu (UXP/M), Kubota, IBM RS/6000, Multiflow, Silicon Graphics, Sun, and other versions. Gaussian 90 for Convex, FPS-500, Fujitsu (MSP), IBM (VM, MVS), HP-700, and NEC SX/3 systems. 

The solute charge distribution can be represented by atom centered point charges or as multipole expansions. Of course, if the solute is treated quantum mechanically the charge distribution can be obtained directly from its wave function. Depending on the solvation model, the electrostatic potential derived from the wave function is fitted to atomic charges or multipoles that are then used to construct the solvent reaction field. 

The rate of an electrochemical ion transfer reaction depends on the electrode potential or on the charge density at the electrode surface. Therefore, simulations in the presence of an external electrostatic field E were performed [230], The direction of the field was chosen such that the ion is attracted toward the surface. The resulting solvent free energy curves are shown in Fig. 35 they have been adjusted by subtracting the constant force in the center of the lamina that is due to incomplete screening of the external field. Clearly, the solvent barrier decreases with increasing electric field. 

The membrane is permeable for the ionic species K and the solvent, i.e. water molecules. When an uncharged membrane is placed between two solutions containing two different activities and of species K, then a phase transfer of charge carriers occurs. The direction of this transfer depends on the gradient of the electrochemical potential. This results in a charging of the phase boundary and creation of an electric field. The initially favoured ion transfer will be slowed down and in the end a further net transfer will be stopped because of electrostatic repulsion forces, and the forth and back transfer of ions will cancel. In electrochemical equilibrium both reactions have the same rate, and the potential difference is constant. Assuming that (i) no temperature or pressure gradient exists across the membrane, (ii) the solvent in both solutions is the same, e.g. water and (iii) no diffusion potential within the membrane occurs, then the electrochemical potentials in the two phases are equal in case of electrochemical equilibrium ... 

The fonnation of quinone anions in bacterial reaction centers produces distinctive optical absorption shifts of the bacteiiopheophytin and bacteiiochlorophyll chromophores . The absence of a direct molecular contact between the quinones and these chromophores " suggests that the electrochromic shifts may be due to an electrostatic interaction between the optical transition dipoles of the chromophores and the electric field associated with the quinone anions. In this case, the electrochromism induced by the quinone anions potentially provides an opportunity to examine the propagation of electric fields, and hence the local dielectric, within the reaction center protein. 

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