United-Atom Acyl Chains for CHARMM Phospholipids

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, although they convey only 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 nonbonded
parameters were fitted to reproduce experimental data and free energy
surfaces of all-atom model systems. Comparative simulations of fluid-phase
POPC bilayers were 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.