Structure and stability of DPPE planar bilayers

Biomembrane mimics in the form of supported planar bilayers allow the application of a wide range of surface and interface analytical techniques. The structure and phase-behavior of single and double bilayers of 1,2-dipalmitoylphosphoethanolamine (DPPE) were investigated by specular neutron reflectivity for their viability as biomembrane mimics. Whilst single bilayer samples were found to exhibit stable gel and fluid structures, double bilayers were found to be intrinsically unstable in the fluid phase as a planar structure. A Bragg peak was observed in the reflectivity data at just above the gel-to-fluid transition temperature, indicating the partial rearrangement of the upper bilayer into a repeat stacked structure. The lower bilayer was structurally stable. The structure and phase-behaviour of a double bilayer containing a ratio of 9 : 1 DPPE/cholesterol was also investigated to assess the stabilising effect of cholesterol on the upper bilayer. The presence of cholesterol completely destabilised the upper bilayer, causing it to detach 7 °C below the gel-to-fluid transition temperature of DPPE. It is possible that the cholesterol increases the overall conical shape of DPPE molecule by residing in the chain region of the lipid.