Molecular dynamics simulations of ice/water and lipid interfaces

Molecular dynamics (MD) studies of bulk SPC/E ice and free energy MD studies of Cl- and Na+ in an ice/water interface system are reported. The equation of state for ice 1h at 1 atmosphere using the SPC/E model of water is calculated, and an upper limit of stability for the ice structure at 260 K is established. Molecular dynamics simulations of Cl- and Na+ ions were performed to calculate ionic solvation free energies in bulk SPC/E water and ice 1h at several different temperatures, and at the basal ice 1h/water interface. For the interface we calculate the free energy of "transfer" of the ions across the ice/water interface.; Results for the ions in bulk water in the isothermal, isobaric (NPT) ensemble at 298 K and 1 atm are found to be in good agreement with experimental and comparable simulation results. Simulations performed in the canonical (NVT) ensemble are shown to give equivalent solvation free energies, and this ensemble is used for the interfacial simulations. Solvation free energies of Cl- and Na+ ions in ice at 150 K are found to be ∼30 and ∼20 kcal mol-1, respectively, less favorable than for water at room temperature. Near the melting point of the model the solvation of the ions in water are the same (within statistical error) as that measured at room temperature, and in the ice are equivalent and ∼10 kcal mol-1 less favorable than the liquid. The free energy of transfer for each ion across ice/water interface is calculated and is in good agreement with the bulk observations for the Cl- ion. However, for the model of Na+ the long-range electrostatic contribution to the free energy was more negative in the ice than the liquid, in contrast to the results observed in the bulk calculations.; Construction and molecular dynamics simulations of a solvated dimyristoylphhosphatidylcholine (DMPC) lipid bilayer containing an antifreeze protein (AFP) type I at the lipid/water interface at several temperatures were performed. Structural characteristics calculated form a base of knowledge for comparison in future work. The systems are shown to be stable and adequate in describing the characteristics of an experimental phospholipid bilayer system and therefore are suitable for use in full-scale simulation studies of AFP's in membrane systems. Possible areas for concentrating future research efforts have been highlighted; namely the acyl chain order and the polar headgroups, both of which have been implicated in experimental studies of the stabilization of model membranes by AFP's.