MEAN MOLECULAR POTENTIALS IN A MODEL LIPID BILAYER - A MOLECULAR-DYNAMICS SIMULATION

Various mean-field potentials in a model lipid bilayer are calculated by means of molecular dynamics (MD) simulation. The bilayer assembly c onsists of 200 chain molecules. The anisotropic united atom model is e mployed for nonbonded interactions and is extended to allow bond lengt h to vary with time. The interfacial translational order is systematic ally varied and found to correlate strongly with the chain orientation al order. A new torsional potential is developed and shown to give ord er parameters in better agreement with experiment than the Padilla-Tox vaerd potential. Nonbonded interaction reduces the trans-gauche and ga uche-gauche transition barriers by 0.9-1.5 kcal/mole. The mean trans-g auche energy difference near the chain tail is close to that in liquid hydrocarbons but 0.34 kcal/mol lower than that in the highly ordered chain region. In contrast to the Marcelja model, both mean intermolecu lar dispersive and repulsive energies depend exponentially on the chai n orientational parameter and the repulsive component has a poor and i nverse correlation with the reciprocal of the chain end-to-end displac ement along the bilayer normal. Inclusion of spatial heterogeneity eff ects of the interaction energy, a treatment similar to the Gruen model [Biochim. Biophys Acta 367, 165 (1980)], does not give a better descr iption of the mean intermolecular interaction. A new and unified model for the mean intermolecular interaction energy is developed based on our present MD simulation data. Various possible chain configurations which are responsible for these results are discussed. Finally our MD results suggest that, consistent with the ''wobble in a cone'' model, a chain molecule can rotate eely within an angular range without being subjected to a strong potential force. (C) 1995 American Institute of Physics.