United-atom acyl chains for CHARMM phospholipids

Jérôme Hénin, Wataru Shinoda, and Michael L. Klein

J. Phys. Chem. B 112, 7008-7015, 2008.

In all-atom simulations of lipid membranes, explicit hydrogen atoms contained in the hydrocarbon region are described by a large number of degrees of freedom, while they only convey limited physical information. We propose an implicit-hydrogen model for saturated and monounsaturated acyl chains, aimed at complementing the all-atom CHARMM27 model for phospholipid headgroups. Torsional potentials and non-bonded parameters are fitted to reproduce experimental data and free energy surfaces of all-atom model systems. Comparative simulations of fluid-phase POPC bilayers are performed using the all-hydrogen force field and the present model. The hybrid model accelerates a typical bilayer simulation by about 50%, while sacrificing a minimal amount of detail with respect to the fully atomistic description. In addition, the united-atom description is energetically compatible with all-atom CHARMM models, making it suitable for simulations of complex membrane systems.