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.