Extension of the Slipids Force Field to Polyunsaturated Lipids

The all-atomic force
field Slipids (Stockholm Lipids) for lipid
bilayers simulations has been extended to polyunsaturated lipids.
Following the strategy adopted in the development of previous versions
of the Slipids force field, the parametrization was essentially based
on high-level ab initio calculations. Atomic charges
and torsion angles related to polyunsaturated lipid tails were parametrized
using structures of dienes molecules. The new parameters of the force
field were validated in simulations of bilayers composed of seven
polyunsaturated lipids. An overall good agreement was found with available
experimental data on the areas per lipids, volumetric properties of
bilayers, deuterium order parameters, and scattering form factors.
Furthermore, simulations of bilayers consisting of highly polyunsaturated
lipids and cholesterol molecules have been carried out. The majority
of cholesterol molecules were found in a position parallel to bilayer
normal with the hydroxyl group directed to the membrane surface, while
a small fraction of cholesterol was found in the bilayer center parallel
to the membrane plane. Furthermore, a tendency of cholesterol molecules
to form chain-like clusters in polyunsaturated bilayers was qualitatively
observed.