Bottom: Molecular dynamics image* of a somewhat more realistic lipid bilayer membrane. The polar moieties of the lipid headgroup are shown in color and the acyl chain carbons are shown in black. Notice that the interface between the polar and nonpolar parts of the memrbane is indiscriminate, and occupies a significant fraction of the membrane thickness. This molecular dynamics image was based on the experimentally determined structural information shown in the two images below.
*H Heller, M Schaefer, & K Schulten, "Molecular dynamics simulation of a bilayer of 200 lipids in the gel and in the liquid-crystal phases" J. Phys. Chem. 97:8343-60, 1993.
The red line is the calculated density of atomic partial charge. It is an approximate measurement of the polarity gradient across the membrane. This charge profile clearly demonstrates the broad interfacial polarity gradient.
*Wiener MC, White SH. Structure of a fluid dioleoylphosphatidylcholine bilayer determined by joint refinement of x-ray and neutron diffraction data. III. Complete structure. Biophys J 1992, 61(2):437-47.
Many of the important interactions in membrane protein folding take place in this interface.
