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Abstract
Of the individual potentials which comprise the potential profile of a membrane, the
least well understood is the dipole potential. In general, the dipole potential is
manifested between the hydrocarbon interior of the membrane and the first few water
layers adjacent to the lipid head groups. Changes in dipole potential caused by spreading
a lipid at an air- or oil-water interface can be measured directly and the dipole
potential of bilayers can be estimated from the conductances of hydrophobic ions.
For a typical phospholipid, like phosphatidylcholine, its measured value is approximately
400 mV in monomolecular films and approximately 280 mV in bilayer membranes, with
the hydrocarbon region being positive relative to the aqueous phase. The difference
between dipole potentials measured in monolayers and bilayer membranes appears to
arise from the use of the lipid-free air- or oil-water interface as the reference
point for monolayer measurements and can be corrected for. The species-specific correction
term is a lipid concentration-independent potential, the existence of which suggests
the ability of lipid headgroups to globally reorganize water structure at the interface.
The dipole potential arises from the functional group dipoles of the terminal methyl
groups of aliphatic chains, the glycerol-ester region of the lipids and the hydrated
polar head groups. Classical methods for obtaining partial dipole moments for each
of the three contributing regions are all based on questionable assumptions and give
conflicting results. More sophisticated mean-field models of dipole potential origin
recognize the important role of interfacial water in determining its value but still
cannot adequately describe the microscopic nature of the interactions from which it
arises. In part this is because the dipole potential develops in a region over which
the dielectric constant of the medium is changing from 2 to 80. Despite of our limited
understanding of the dipole potential, it is an important regulator of membrane structure
and function. Membrane-membrane and membrane-ligand interactions are regulated by
the hydration force, the value of which can be related to the dipole potential of
the membrane. For thermotropically phase-separated or multicomponent membranes the
size and shape of lipid domains is controlled by the balance between the line tension
at the domain borders and the difference in dipole density between the domains. Line
tension tends to make the domains compact and circular whereas dipole repulsion promotes
transitions to complex domain shapes with larger perimeters.(ABSTRACT TRUNCATED AT
400 WORDS)