Structure of Fluid Lipid Bilayers

 

The structure fluid lipid bilayers is determined by means of X-ray and neutron diffraction methods. A molecular understanding of the interaction of peptides and proteins with lipid bilayers requires experimental knowledge of the structure of the membrane bilayer, the transbilayer location of bound peptides, the structures the peptides adopt, and the changes that occur in the bilayer structure as a result of partitioning. Because cellular membranes must be in a fluid state for normal cell function, it is the structure of fluid (L alpha-phase) bilayers that is relevant to understanding the interactions of peptides in molecular detail.

Unfortunately, the high thermal disorder of fluid bilayers precludes atomic-resolution three-dimensional crystallographic images. Useful structural information can nevertheless be obtained by diffraction methods because multilamellar bilayers (liquid crystals) obtained from phospholipids by dispersal in water or by deposition on surfaces are highly periodic along the bilayer normal. This one-dimensional crystallinity allows the distribution of matter along the bilayer normal to be determined from combined x-ray and neutron diffraction measurements.

The 'structure' of a fluid bilayer is consequently defined as the time-averaged spatial distributions of the structural groups of the lipid (carbonyls, phosphates, etc.) projected onto an axis normal to the bilayer plane. Such distributions are equivalent to probability densities from which the probability of finding a particular structural group at a specific location can be determined.