Examining the contributions of lipid shape and headgroup charge on bilayer behavior.

To better understand bilayer property dependency on lipid electrostatics and headgroup size, we use atomistic molecular dynamics simulations to study negatively charged and neutral lipid membranes. We compare the negatively charged phosphatidic acid (PA), which at physiological pH and salt concentration has a negative spontaneous curvature, with the negatively charged phosphatidylglycerol (PG) and neutrally charged phosphatidylcholine (PC), both of which have zero spontaneous curvature. The PA lipids are simulated using two different sets of partial charges for the headgroup and the varied charge distribution between the two PA systems results in significantly different locations for the Na(+) ions relative to the water/membrane interface. For one PA system, the Na(+) ions are localized around the phosphate group. In the second PA system, the Na(+) ions are located near the ester carbonyl atoms, which coincides with the preferred location site for the PG Na(+) ions. We find that the Na(+) ion location has a larger effect on bilayer fluidity properties than lipid headgroup size, where the A(lipid) and acyl chain order parameter values are more similar between the PA and PG bilayers that have Na(+) ions located near the ester groups than between the two PA bilayers.