Equilibrium or quenched: fundamental differences between lipid monolayers, supported bilayers, and membranes.

In this work, we establish fundamental differences between the structure and packing of lipids in monolayers, supported bilayers, and multilayer films. High resolution grazing incidence X-ray diffraction reveals that monolayer structure is largely retained upon deposition onto substrates with the area per molecule controlled by deposition pressure. Such structural changes are consistent with a quenched rather than equilibrated supported membrane structure. Supported bilayers formed by vesicle fusion exhibit structural similarity to bilayers deposited at 38 mN/m, whereas packing in lipid multilayers more closely resembled bilayers deposited below 30 mN/m. At the molecular level, coupling between opposing lipid acyl chains is observed for all deposition pressures with the outer leaflet templating on the inner leaflet. Leaflet coupling induces a small condensation in the area per lipid molecule and a surprising increase in acyl chain tilt. Moreover, supported lipid bilayers exhibit preferential acyl chain alignment: the system cannot be modeled with freely rotating acyl chains as in free-standing lipid monolayers. Such acyl chain alignment is consistent with orientational texture of lipid tilt directors at larger length scales. These findings clearly demonstrate that supported, gel-phase bilayer membrane structure can be controlled and maintained by deposition onto solid supports and that increasing surface pressure induces preferential alignment of the acyl chains both within and between membrane leaflets.