RESUMO
The dynamical properties of amphiphilics in Newton black films, as well as those of the water confined between the two charged hydrophilic surfaces, have been calculated via a series of molecular dynamic calculations in several films with different water contents. A charged semiflexible amphiphilic model and the TIP5P model of water are used in our simulations [Z. Gamba, J. Chem. Phys. 129, 164901 (2008)]. We calculate the diffusion coefficients, reorientational dynamics, and the atomic density profile of water molecules as a function of the number of water molecules per amphiphilic (n(w)). We also analyze the reorientational motion of the amphiphilics and determine a strong correlation between the dynamics of water molecules and the translational and reorientational dynamics of the amphiphilics, as well as a correlation between the reorientational dynamics of the amphiphilics belonging to the upper and lower halves of the studied thin films.
Assuntos
Água/química , Algoritmos , Ânions/química , Difusão , Modelos Químicos , Simulação de Dinâmica Molecular , Movimento (Física) , Sódio/química , Dodecilsulfato de Sódio/química , Eletricidade Estática , Propriedades de Superfície , Tensoativos/química , Temperatura , Fatores de TempoRESUMO
We propose a very simple but "realistic" model of amphiphilic bilayers, simple enough to be able to include a large number of molecules in the sample but nevertheless detailed enough to include molecular charge distributions, flexible amphiphilic molecules, and a reliable model of water. All these parameters are essential in a nanoscopic scale study of intermolecular and long range electrostatic interactions. We also propose a novel, simple, and more accurate macroscopic electrostatic field for model bilayers. This model goes beyond the total dipole moment of the sample, which on a time average is zero for this type of symmetrical samples; i.e., it includes higher order moments of this macroscopic electric field. We show that by representing it with a superposition of Gaussians, it can be analytically integrated, and therefore its calculation is easily implemented in a molecular dynamics simulation (even in simulations of nonsymmetrical bi- or multilayers). In this paper we test our model by molecular dynamics simulations of Newton black films.
Assuntos
Eletricidade Estática , Água/química , Modelos Moleculares , Reprodutibilidade dos Testes , Fatores de TempoRESUMO
In order to study the electrostatic properties of a single biological membrane (not an stack of bilayers), we propose a very simple and effective external potential that simulates the interaction of the bilayer with the surrounding water and that takes into account the microscopic pair distribution functions of water. The electrostatic interactions are calculated using Ewald sums but, for the macroscopic electrostatic field, we use an approximation recently tested in simulations of Newton black films that essentially consists in a coarsed fit (perpendicular to the bilayer plane) of the molecular charge distributions with Gaussian distributions. The method of effective macroscopic and external potentials is extremely simple to implement in numerical simulations, and the spatial and temporal charge inhomogeneities are then roughly taken into account. As examples of their use, several molecular dynamics simulations of simple models of a single biological membrane, of neutral or charged polar amphiphilics, with or without water (using the TIP5P intermolecular potential for water) are included. The numerical simulations are performed using a simplified amphiphilic model which allows the inclusion of a large number of molecules in these simulations, but nevertheless taking into account molecular charge distributions, flexible amphiphilic molecules, and a reliable model of water. All these parameters are essential in a nanoscopic scale study of intermolecular and long range electrostatic interactions. This amphiphilic model was previously used by us to simulate a Newton black film, and, in this paper, we extend our investigation to bilayers of the biological membrane type.