Figure 3.

Simulations enforcing interbilayer flips of lipid tails are used to measure the energy barrier $\Delta {\overline{E}}_{\alpha }$ for interbilayer flips. (a) From two adhering bilayers (head beads are blue/green, tail beads are omitted for clarity), a single lipid is selected (several red and one yellow tail beads), and a slowly moving external harmonic force F applied to one of its tail beads (yellow), until the tail has flipped to the other bilayer, so that the lipid has assumed a splayed configuration with one tail inserted in each bilayer as shown in panel c. (b) The energy landscape E_α for the bead is sketched as a function of the displacement z of the yellow bead. It has a high barrier in the center corresponding to the repulsive headgroups and increases to the sides reflecting displacement of the headgroup into the hydrophobic region.