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