The role of electrostatics in solute-solvent interactions with the continuum (original) (raw)
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The Journal of Physical Chemistry, 1996
Electrostatic solvation free energies are calculated using a self consistent reaction field (SCRF) procedure that combines a continuum dielectric model of the solvent with both Hartree-Fock (HF) and density functional theory (DFT) for the solute. Several molecules are studied in aqueous solution. They comprise three groups: nonpolar neutral, polar neutral, and ionic. The calculated values of ∆G el are sensitive to the atomic radii used to define the solute molecular surface, particularly to the value of the hydrogen radius. However, the values of ∆G el exhibit reasonable correlation with experiment when a previously determined, physically motivated set of atomic radii were used to define the van der Waals surface of the solute. The standard deviation between theory and experiment is 2.51 kcal/mol for HF and 2.21 kcal/mol for DFT for the 14 molecules examined. The errors with HF or DFT are similar. The relative difference between the calculated values of ∆G el and experiment is largest for nonpolar neutral molecules, intermediate for polar neutral molecules, and smallest for ions. This is consistent with the expected relative importance of nonelectrostatic contributions to the free energy that are omitted in the model.
Molecular mechanics and electrostatic effects
Biophysical Chemistry, 1994
Continuum solvent models predict a quadratic charge dependence (linear response) of the free energy of a system of charged solutes. The relation between this prediction and the structure of the salvation shell around the solutes is discussed. Studies of the derivative of the free energy with respect to the charges for different reference states are shown to be a convenient way of testing the linear response assumption without resorting to the standard free energy perturbation method. We illustrate this with a system of two oppositely charged ions in aqueous solution, where nonlinearities are observed before the full charging process is completed. Since molecular mechanics (MM) simulations preserve the full nonlinearity of the problem, they are well suited to the investigation of the conditions under which linear response accurately reflects the behavior of the system. The error when using linear response theory to calculate the free energies of charging is estimated to be as large as lo-20%.
The Journal of Physical Chemistry B, 1998
The potential of mean force in aqueous solution for rotation around the two backbone dihedrals φ and ψ of the alanine dipeptide is computed in explicit water and in the continuum approximation by numerical integration of the self-energies and the generalized Born (GB) equation. The two models show good agreement. The experimentally observed increase in the gauche/trans population ratio for dichloroethane in going from the gas phase to the pure liquid is reproduced by the GB solvation model with a solvent dielectric constant of 10.5. This test case shows that the GB model gives accurate predictions also for solvents with much lower polarizability than water. For both test systems additional calculations with a finite difference Poisson equation solver yield somewhat more accurate results at a much higher computational cost than the GB solvation model.
The electric potential of a macromolecule in a solvent: A fundamental approach
Journal of Computational Physics, 1991
A general numerical method is presented to compute the electric potential for a macromolecule of arbitrary shape in a solvent with nonzero ionic strength. The model is based on a continuum description of the dielectric and screening properties of the system, which consists of a bounded internal region with discrete charges and an infinite external region. The potential obeys the Poisson equation in the internal region and the linearized Poisson Boltzmann equation in the external region, coupled through appropriate boundary conditions. It is shown how this three-dimensional problem can be presented as a pair of coupled integral equations for the potential and the normal component of the electric field at the dielectric interface. These equations can be solved by a straightforward application of boundary element techniques. The solution involves the decomposition of a matrix that depends only on the geometry of the surface and not on the positions of the charges. With this approach the number of unknowns is reduced by an order of magnitude with respect to the usual finite difference methods. Special attention is given to the numerical inaccuracies resulting from charges which are located close to the interface; an adapted formulation is given for that case. The method is tested both for a spherical geometry, for which an exact solution is available, and for a realistic problem, for which a finite difference solution and experimental verification is available. The latter concerns the shift in acid strength (pK-values) of histidines in the copper-containing protein azurin on oxidation of the copper, for various values of the ionic strength. A general method is given to triangulate a macromolecular surface. The possibility is discussed to use the method presented here for a correct treatment of long-range electrostatic interactions in simulations of solvated macromolecules, which form an essential part of correct potentials of mean force. t7,
ChemPlusChem, 1984
The conformational equi librium ap :r' sp of the title compound s was investigated by two methods : dipole moment measurement in benzene solution and IR spectroscopy of the carbonyl band in decahydronaphthalene solution. Approximately 71 % of the ap rotamers were found for 2-cyanobenzoates (II, III), and between 60 to 80% for (Z)-3-cyanopropenoates (V, VI). This result does not reveal any specia l attractive interaction between the carbonyl oxygen and cyano carbon atoms; on the other hand, it is in fair agreement with electrostatic calculations based on point charges calculated from bond moments. Any attractive interaction was not detected even on aliphatic cyano esters V/l-IX. Comparison of several model compounds and severa l kinds of approximative calculations leads us to conclude that the electrostatic model works generally beller for conformational equilibria than for ionization equilibria.
The Journal of Chemical Physics, 2003
The hybrid molecular-continuum model for polar solvation considered in this paper combines the dielectric continuum approximation for treating fast electronic ͑inertialess͒ polarization effects and a molecular dynamics ͑MD͒ simulation for the slow ͑inertial͒ polarization component, including orientational and translational solvent modes. The inertial polarization is generated by average charge distributions of solvent particles, composed of permanent and induced ͑electronic͒ components. MD simulations are performed in a manner consistent with the choice of solvent and solute charges such that all electrostatic interactions are scaled by the factor 1/ ϱ , where ϱ is the optical dielectric permittivity. This approach yields an ensemble of equilibrium solvent configurations adjusted to the electric field created by a charged or strongly polar solute. The electrostatic solvent response field is found as the solution of the Poisson equation including both solute and explicit solvent charges, with accurate account of electrostatic boundary conditions at the surfaces separating spatial regions with different dielectric permittivities. Both equilibrium and nonequilibrium solvation effects can be studied by means of this model, and their inertial and inertialess contributions are naturally separated. The methodology for computation of charge transfer reorganization energies is developed and applied to a model two-site dipolar system in the SPC water solvent. Three types of charge transfer reactions are considered. The standard linear-response approach yields high accuracy for each particular reaction, but proves to be significantly in error when reorganization energies of different reactions were compared. This result has a purely molecular origin and is absent within a conventional continuum solvent model.
An electrostatic approach for the interaction between molecules
Zeitschrift für Physikalische Chemie, 1981
In this paper we present an analysis of the electrostatic interaction between a solute molecule and its environment in non-reactive systems. The model has been applied in some calculations for the methane, ethane, methanol and hydrogen fluoride as solutes and water as a solvent. The results for the electrostatic interaction are compared with the corresponding ones obtained with the use of semi-empirical methods. The agreement is very satisfactory, and since the method developed here does not take a lot of computational time, it is quite suitable to predict stable configurations for the relative orientations of the solute and solvent molecules.
On the validity of dielectric continuum models in application to solvation in molecular solvents
The Journal of Chemical Physics, 2003
We report Monte Carlo simulations of solvation of a point dipole in dipolar-quadrupolar solvents of varying dipole moment and axial quadrupole. The simulations are carried out to test the prediction of dielectric solvation models of a monotonic increase of the absolute value of the solvation chemical potential ͉ p ͉ with the solvent dielectric constant ⑀. Dielectric constants are obtained from pure liquid simulations carried out for each solvent used in solvation simulations. A raising dependence of ͉ p ͉ on ⑀, in qualitative agreement with dielectric solvation models, is seen when the solvent dipole moment is varied at constant solvent quadrupole. An increase in the axial quadrupole at constant solvent dipole reduces the dielectric constant at the same time leading to higher ͉ p ͉ values. The simulations and dielectric models thus give the opposite dependence on the solvent quadrupole for any solvent dipole. We also show that for solvation in dipolar-quadrupolar solvents the saturation limit ͉ p ͉→const at ⑀ӷ1 predicted by linear response dielectric continuum models actually occurs in the range of nonlinear solvation.
Free energy of solvation from molecular dynamics simulations for low dielectric solvents
Journal of computational …, 2003
Using molecular dynamics simulation, we present new results for the free energy of solvation for solvents with low dielectric constants (CCl 4 , CHCl 3 , benzene). The solvation free energy is computed as the sum of three contributions originated at the cavitation of the solute by the solvent, the solute-solvent repulsion and dispersion interactions, and the electrostatic solvation of the solute. The cavitational contribution has been obtained from the Claverie-Pierotti model applied to excluded volumes obtained from distances for nearest neighbor configurations between the solute's atoms and a spherical solvent description. An electrostatic continuum model has been adapted for the computation of the electrostatic free energy of solvation, whereas the van der Waals contribution has been calculated directly from the intermolecular interactions defined by the force fields applied to the simulations. For each solvent, a large set of solute molecules containing most of the chemically interesting functionalities has been treated. The simulated solvation free energies are in very good agreement with experimental data, although a small systematical overestimation of the free energy of solvation indicates a failure of the spherical approach to the solvent molecules in the case of benzene.