The Structure of a Fluid Lipid Bilayer

 

The picture represents fully resolved images, and is an accurate representation of the true thermal motion of the molecules. This thermal motion is a fundamental and important feature of fluid bilayers that plays a critical role in peptide-bilayer interactions. Although the calculation was based at low hydration, 5-6 water molecules per lipid demonstrates that the overall structure changes in only subtle ways as the water content is increased.

 

 

 

Several features of the fluid lipid structure are important:

 

The great amount of thermal disorder is revealed by the widths of the probability densities.

The combined thermal thickness of the interfacial regions (defined by the distribution of the waters of hydration) is about equal to the 3 nm thickness of the hydrophobic core.

The thermal thickness of a single interface (1.5 nm) can easily accommodate unfolded and folded polypeptide chains, as illustrated schematically by the end-on representation of an a-helix with diameter ~ 1 nm (typical of helices in membrane proteins). In this light, the common cartoons of bilayers that assign a diminutive thickness to the bilayer headgroup/interface region can be seen as misleading.

The interfaces are chemically highly heterogeneous; they are rich in possibilities for non-covalent interactions with peptides. Because the interfaces are the sites of first contact, they are especially important in the folding and insertion of non-constitutive membrane proteins such as toxins. But, they are also important for the folding and stability of constitutive membrane proteins because significant portions of their mass contact the interfaces.

Polarity Profile of the Fluid lipid bilayer: Besides being chemically heterogeneous, the interfaces are, not surprisingly, regions in which dramatic changes in polarity occur over small distances.This profile represents the average density of atomic partial charges, both + and - , calculated by weighting the absolute values of the charge densities by the number density and group volume at each position across the bilayer. The very steep gradient of polarity in the interfaces is consistent with calculated interfacial electrostatic free energy profiles of charged membranes recently reviewed by Murray et al.