Comparing Simulations of Lipid Bilayers to Scattering Data: The GROMOS 43A1-S3 Force Field

Simulations of DOPC at T = 303 K were performed using the united atom force field 43A1-S3 at six fixed projected areas, A_P = 62, 64, 66, 68, 70, and 72 Ų, as well as a tensionless simulation that produced an average A_NPT = 65.8 Ų. After a small undulation correction for the system size consisting of 288 lipids, results were compared to experimental data. The best, and excellent, fit to neutron scattering data occurs at an interpolated A_N = 66.6 Ų and the best, but not as good, fit to the more extensive X-ray scattering data occurs at A_X = 68.7 Ų. The distance ΔD_B–H between the Gibbs dividing surface for water and the peak in the electron density profile agrees with scattering experiments. The calculated area compressibility K_A = 277 ± 10 mN/m is in excellent agreement with the micromechanical experiment. The volume per lipid V_L is smaller than volume experiments which suggests a workaround that raises all the areas by about 1.5%. Although A_X ≠ A_N ≠ A_NPT, this force field obtains acceptable agreement with experiment for A_L = 67.5 Ų (68.5 Ų in the workaround), which we suggest is a better DOPC result from 43A1-S3 simulations than its value from the tensionless NPT simulation. However, nonsimulation modeling obtains better simultaneous fits to both kinds of scattering data, which suggests that the force fields can still be improved.