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Effect of the barometric phase transition of a DMPA bilayer on the lipid/water interface. An atomistic description by molecular dynamics simulation.

Departmento Química Física y TermodinAmica Aplicada, Ed. Marie Curie, Campus de Rabanales, Universidad de Córdoba, 14014 Córdoba, Spain.
The Journal of Physical Chemistry B (Impact Factor: 3.61). 01/2008; 111(49):13726-33. DOI: 10.1021/jp075948v
Source: PubMed

ABSTRACT Understanding the structure and dynamics of phospholipid bilayers is of fundamental relevance in biophysics, biochemistry, and chemical physics. Lipid Langmuir monolayers are used as a model of lipid bilayers, because they are much more easily studied experimentally, although some authors question the validity of this model. With the aim of throwing light on this debate, we used molecular dynamics simulations to obtain an atomistic description of a membrane of dimyristoylphosphatidic acid under different surface pressures. Our results show that at low surface pressure the interdigitation between opposite lipids (that is, back-to-back interactions) controls the system structure. In this setting and due to the absence of this effect in the Langmuir monolayers, the behavior between these two systems differs considerably. However, when the surface pressure increases the lipid interdigitation diminishes and so monolayer and bilayer behavior converges. In this work, four computer simulations were carried out, subjecting the phospholipids to lateral pressures ranging from 0.17 to 40 mN/m. The phospholipids were studied in their charged state because this approach is closer to the experimental situation. Special attention was paid to validating our simulation results by comparison with available experimental data, therebeing in general excellent agreement between experimental and simulation data. In addition, the properties of the lipid/solution interface associated with the lipid barometric phase transition were studied.

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    ABSTRACT: 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.
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