Solid-state NMR measurements have shown that functional biological membranes are in the liquid crystalline state and that structural features of lipids in the crystalline phase are essentially carried over into the liquid crystalline state. An order parameter profile comparable for the most diverse membranes has been established. The absolute values of order parameters may, however, vary as much as a factor of two as a consequence of the large variation in lipid composition encountered in biological membranes. Membrane ordering decreases upon increasing the temperature, introducing one or several cis-double bonds into a saturated fatty acyl chain, or upon adding an amphiphilic guest molecule. In contrast, it increases up to twofold upon addition of 50% cholesterol. Transmembrane proteins barely influence the lipid order, as they perfectly match the lipid bilayer properties. Due to the action of enzymes (e.g., phospholipases) the lipid packing density and hence the membrane order may vary with time and, in turn, may modulate the function of membrane proteins. A conformational change in a membrane protein may further be induced by an outof- plane rotation of the phospholipid headgroup dipole resulting in the development of a storage electric field across the membrane, which changes the protein structure. NMR measurements have further demonstrated that a fast exchange of lipid molecules is observed between the boundary of transmembrane proteins and the bulk lipid phase. At present, no physical–chemical evidence for the formation of domains or microdomains with lifetimes >10−4 s has been obtained under physiological conditions.
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