Validating affinities for ion-lipid association from simulation against experiment.

Understanding biological membranes at physiological conditions requires comprehension of the interaction of lipid bilayers with sodium and potassium ions. These cations are adsorbed at palmitoyl-oleoyl-phosphatidylcholine (POPC) bilayers as indicated from previous studies. Here we compare the affinity of Na(+) and K(+) for POPC in molecular dynamics (MD) simulations with recent data from electrophoresis experiments and isothermal calorimetry (ITC) at neutral pH. NaCl and KCl were described using GROMOS or parameters matching solution activities on the basis of Kirkwood-Buff theory (KBFF), and K(+) was also described using parameters by Dang et al., all in conjunction with the Berger parameters for the lipids and the SPC water model. Apparent binding constants of GROMOS-Na(+) and KBFF-K(+) are the same within error and in good agreement with values from ITC. Although these force fields yield the same number of bound ions per number of lipids for Na(+) and K(+), they give a larger number of Na(+) ions per surface area compared to K(+), in agreement with the electrophoresis experiments, because Na(+) causes a stronger reduction in the area per lipid than K(+). The intrinsic binding constants, on the other hand, are reproduced by Dang-K(+) but overestimated by GROMOS-Na(+) and KBFF-K(+). That no ion force field reproduces the intrinsic and the apparent binding constant simultaneously arises from the fact that in MD simulations, implicitly meant to mimic neutral pH, pure PC is usually modeled with zero surface charge. In contrast, POPC at neutral conditions in experiment carries a low but significant negative surface charge and is uncharged only at acidic pH as indicated from electrophoretic mobilities. Implications for future simulation and experimental studies are discussed.