Molecular dynamics simulations of amphiphilic molecules

The molecular dynamics (MD) simulation method is a technique which falls under the umbrella of computational chemistry. During the last twenty years, this method has experienced explosive growth due to the significant advances that have been accomplished in computational architecture. We have employed this technique to probe the behavior of Langmuir monolayers as well as lipid bilayers. In our molecular dynamics simulations all atoms in the system under consideration are treated classically. Successive configurations of the system are generated by integrating Newton's laws of motion, resulting in a trajectory, which provides the positions and velocities of the particles as a function of time. Interactions between atoms consist of intermolecular (bonded) and intermolecular (non-bonded) interactions. Intermolecular interactions consist of bond stretching, bond angle, and dihedral interactions between covalently attached atoms. Intermolecular interactions, within a certain cut-off radius, occur between neighboring atoms and consist of van der Waals and electrostatic interactions. By performing MD simulations we are able to obtain conformational and thermodynamic information about the system under investigation. For example when studying systems with long alkyl chains, it is highly desirable to determine the tilt of the chains as well as the packing order (deuterium order parameter).; In the case of Langmuir monolayers, we examined the behavior of two different fatty acids, namely eicosanoic acid and 18-methyleicosanoic acid (18-MEA). The aim of this work was to gain an understanding the fundamental properties of 18-MEA by performing molecular dynamics simulations of Langmuir monolayers of this compound. While this unusual anteiso fatty acid has been found in minor amounts in various biological materials, its presence in mammalian hair fibers as the major covalently bound fatty acid has inspired the research described herein. It is believed to form a monolayer at the surface of human and animal fibers and is covalently attached to underlying structural proteins by a thioester bond. We have completed a significant amount of work for eicosanoic acid (arachidic acid), which is the parent form of 18-MEA. In past studies of eicosanoic acid, it was reported that a phase transition occurs when performing an MD simulation in order to obtain an isothermal phase diagram. Our studies demonstrate that this transition is actually an artifact of the method employed in the simulation. Further, it has been found that molecular dynamic simulation studies of Langmuir monolayers of these compounds has provided some insight into the structure/function relationship of the methyl group position on the behavior of these fatty acids in thin films. (Abstract shortened by UMI.)...