United Atom Lipid Parameters for Combination with the Optimized Potentials for Liquid Simulations All-Atom Force Field.

We have developed a new united-atom set of lipid force field parameters for dipalmitoylphosphatidylcholine (DPPC) lipid bilayers that can be combined with the all-atom optimized potentials for liquid simulations (OPLS-AA) protein force field. For this, all torsions have been refitted for a nonbonded 1-4 scale factor of 0.5, which is the standard in OPLS-AA. Improved van der Waals parameters have been obtained for the acyl lipid tails by matching simulation results of bulk pentadecane against recently improved experimental measurements. The charge set has been adjusted from previous lipid force fields to allow for an identical treatment of the alkoxy ester groups. This reduces the amount of parameters required for the model. Simulation of DPPC bilayers in the tension-free NPT ensemble at 50 °C gives the correct area per lipid of 62.9 ± 0.1 Å(2), which compares well with the recently refined experimental value of 63.0 Å(2). Electron density profiles and deuterium order parameters are similarly well reproduced. The new parameters will allow for improved simulation results in microsecond scale peptide partitioning simulations, which have proved problematic with prior parametrizations.