Molecular Dynamics Simulations and 2H NMR Study of the
GalCer/DPPG Lipid Bilayer
Tatiana Zaraiskaya 1 and Kenneth R. Jeffrey 2*
1 Department of Physics, University of Guelph
2 Department of Physics, University Guelph
* To whom correspondence should be addressed. E-mail: krj{at}physics.uoguelph.ca.
Submitted on October 14, 2004
Revised on December 9, 2004
Accepted on 28 February 2005
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Abstract |
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Molecular Dynamics (MD) simulations were performed on a two-component lipid bilayer system in the liquid crystalline phase at constant pressure and constant temperature. The lipid bilayers were composed of a mixture of neutral galactosylceramide (GalCer) and charged dipalmitoylphosphatidylglycerol (DPPG) lipid molecules. Two lipid bilayer systems were prepared with GalCer:DPPG ratio 9:1 (system 1) and 3:1 (system 2). System 1 represents a collapsed state lipid bilayer, with a narrow water space between the bilayers and system 2 represents an expanded state with a fluid space of a ~100 Å. The number of lipid molecules used in each simulation was 1024 and the length of the production run simulation was 10 ns. The simulations were validated by comparing the simulation results for several important aspects of the bilayer structure and dynamics with experimental data. Deuterium order parameters obtained from 2H NMR experiments for DPPG chains are in a very good agreement with those obtained from MD simulations. The surface area per GalCer lipid molecule was estimated to be 60.8±1.1 Å2. From the simulated electron density profiles the bilayer thickness defined as the distance between the phosphorus peaks across the bilayer was calculated to be 42.1 Å and the thickness of the hydrocarbon chain region was found to be 32.8 Å. Both simulation systems revealed a tendency for cooperative bilayer undulations as expected in the liquid crystalline phase. The interaction of water with the GalCer and DPPG oxygen atoms results in a strong water ordering in a spherical hydration shell and the formation of hydrogen bonds (H-bonds). Each GalCer lipid molecule makes 8.6±0.1 H-bonds with the surrounding water, while each DPPG lipid molecule makes 8.3±0.1 H-bonds. The number of water molecules per GalCer or DPPG in the hydration shell was estimated to be 10-11 based on theradial distribution functions. The formation of the intermolecular hydrogen bonds was observed between hydroxyl groups from the opposing GalCer sugar headgroups, giving an energy of adhesion in the range between -1.0 and -3.4 erg/cm2. We suggest that this value is the contribution of the hydrogen bond component to the net adhesion energy between GalCer bilayers in the liquid crystalline phase.
Key Words:
deuterium NMR, dipalmitoylphosphatidylglycero, galactosylceramide, interaction energy, lipid bilayer, molecular dynamics simulation
