Dr. Ioan Kosztin
Department of Physics and Astronomy
University of Missouri
Columbia, MO

Monday, March 30, 2009
3:00 pm (CT)
3269 Beckman Institute

Abstract

Individual atoms and lipid molecules in biologically relevant phospholipid bilayers have an extremely rich dynamics that extend on a wide range of time and length scales. Computer modeling and simulations can be used effectively both to investigate the dynamics of such complex systems and to interpret the results of experiments (e.g., inelastic neutron scattering) employed to probe them. Here we present an all-atom molecular dynamics (MD) simulation, combined with theoretical modeling, to investigate the dynamics of lipid atoms and molecules in a hydrated diyristoyl-phosphatidylcholine (DMPC) lipid bilayer. From the analysis of a 0.1 microsecond MD trajectory we find that the time evolution of the mean square displacement (MSD) of lipid atoms and molecules exhibits three well separated dynamical regions: (1) for short times (t < 10 fs) the motion is ballistic with a quadratic in time MSD; (2) for intermediate times (10 ps 30 ns) the MSD is linear in time, corresponding to ordinary Fickian diffusion. The origin of the extended anomalous, sub-diffusive region is attributed to the polymeric nature of the lipid molecules, characterized by connectivity and flexibility. We propose a memory function approach for calculating the MSD over the entire time range, from the ballistic to the Fickian diffusion regimes. The lateral self-diffusion coefficient of lipid molecules determined by employing the memory function approach is found to be in good agreement with the one calculated directly from the long time MD trajectory of the lipid bilayer. Finally, we show that the proposed memory function approach is suitable to correctly describe and interpret inelastic neutron scattering experiments on phospholipid bilayers.