The main structural element of the biological membrane is the lipid bilayer. Lipid molecules, when brought into contact with water, spontaneously organize themselves into a bilayer leaflet: The polar lipid headgroups remain in the aqueous environment while the fatty acid tails form the inner hydrophobic core. The lipid bilayer is thus a “sandwich”-like structure with the polar group as the “bread” and the fatty acyl chain as the “butter.” The structure of the lipid bilayer and the interaction of the lipid molecules with their environment, such as metal ions, peptides, and proteins, are the themes presented here. Using solid-state nuclear magnetic resonance (NMR) techniques, a quantitative analysis of the molecular ordering and dynamics of a lipid bilayer has become possible with a segment-to-segment resolution. Lipid bilayers—and also intact biological membranes—are not rigid but can be classified in physical terms as smectic liquid crystals. The lipids within each bilayer undergo rapid translational and rotational motion. The packing of the hydrocarbon chains is best described in terms of statistical order profiles. In contrast, well-defined conformations are observed for the glycerol backbone and to some extent also for the polar head groups. Both the order profile and the orientation of the polar groups can vary considerably depending on the external conditions and constitute regulatory elements for the function of the biological membrane.
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