Improving the CHARMM force field for polyunsaturated fatty acid chains

CHARMM36 (C36) is the most up-to-date pairwise additive all-atom lipid force field and is able to accurately represent bilayer properties of saturated and monounsaturated lipid molecules in the natural constant particle, pressure, and temperature (NPT) ensemble. However, molecular dynamics (MD) simulations on 1-stearoyl-2-docosahexaenoyl-sn-glycerco-3-phosphocholine (SDPC) bilayers of the polyunsaturated fatty acid (PUFA) chains result in inaccuracies of the surface area per lipid (SA), deuterium order parameters (S_CD), and X-ray form factors. Therefore, in this study, high-level quantum mechanical calculations are used to improve the dihedral potential of neighboring double bonds, and the corresponding force field is referred to as C36p. The SA for SDPC at 303 K increases from 63.2 ± 0.2 (C36) to 70.8 ± 0.2 (C36p) Ų and agrees favorably with X-ray diffraction results at 297 K. The resulting S_CD are in excellent agreement with experimental values of both the sn-1 and sn-2 chains. Calculated NMR ¹³C relaxation times and X-ray form factors from MD simulations of SDPC bilayers also agree with experiments. MD simulations of 1,2-diarachidonyl- phosphatidylcholine (DAPC) bilayers are used to further validate our force field parameters on a lipid with both chains containing PUFAs. As expected, the thickness of DAPC bilayers is reduced, and the SA is increased compared to the SDPC bilayers. This update in the PUFA force field should allow for accurate MD simulations of PUFA-containing bilayers in the NPT ensemble.