Molecular simulation of dioleoylphosphatidylcholine lipid bilayers at differing levels of hydration

The structure and dynamics of the lipid and water components of dioleoylpho sphatidylcholine bilayers at various levels of hydration were studied using molecular dynamics (MID) simulations. Equilibration of these systems proce eded by use of a hybrid MID and configurational-bias Monte Carlo technique using one atmosphere of pressure normal to the membrane and a set point for the lateral area derived from experimental Bragg spacings, combined with e xperimentally derived specific volumes for each of the system components. M embrane surface tensions were observed to be of the order of tens of dyn/cm . The transbilayer molecular fragment peak positions at low hydration were found to agree with experimental neutron and x-ray scattering profiles and previously published simulations. For hydration levels of 5.4, 11.4, and 16 waters/lipid, molecular fragment distributions and order parameters for th e headgroup, lipid chains, and water were quantified. Spin-lattice relaxati on rates and lateral self-diffusion coefficients of water agreed well with results from experimental nuclear magnetic resonance studies. Relaxation ra tes of the choline segments and chemical shift anisotropies for the phospha te and carbonyls were computed. Headgroup orientation, as measured by the P -N vector, showed enhanced aligment with the membrane surface at low hydrat ion. The sign of the membrane dipole potential reversed at low hydration, w ith the membrane interior negative relative to the interlamellar region. Ca lculation of the number of water molecules in the headgroup hydration shell , as a function of hydration level, supports the hypothesis that the break point in the curve of Bragg spacing versus hydration level near 12 waters/l ipid, observed experimentally by Hristova and White (1998. Biophys. J. 74:2 419-2433), marks the completion of the first hydration shell.