Molecular dynamics simulations and 2H NMR study of a lipid bilayer
Molecular Dynamics (MD) simulations were performed on a two-component lipid bilayer system composed of a mixture of neutral galactosylceramide (GalCer) and charged dipalmitoylphosphatidylglycerol (DPPG) lipid molecules in a liquid crystalline phase in order to help understand the nature of the short-range interactions. Two lipid bilayers with the GalCer:DPPG ratios of 9.04:1 (system-1) and 3:1 (system-2) have been prepared and simulated. System-1 represents a 'collapsed' lipid bilayer state, with a narrow water space between the bilayers and system-2 represents an 'expanded' state with a wide (100 A?) water space between the bilayers. The simulations were validated by comparing with the experimental data for several important aspects of the bilayer structure and dynamics, such as the deuterium order parameters of the lipid chains, the area per lipid, the bilayer thickness, the thickness of the chain region and the diffusion constants of lipids and water. The interaction of water with the GalCer and DPPG oxygen atoms resulted in a strong water ordering via a spherical hydration shell and the formation of hydrogen bonds (H-bonds). The formation of intermolecular H-bonds was observed between hydroxyl groups from the opposing GalCer sugar units, giving the energy of adhesion in the range from -1.0 to -3.4 erg/cm2. We suggest that this value is the contribution of the H-bond component to the net adhesion energy between GalCer bilayers.