The dipole potential of phospholipid membranes and methods for its detection

The dipole potential is an electrical potential within phospholipid membranes, which arises because of the alignment of dipolar residues of the lipids and/or water dipoles in the region between the aqueous phases and the hydrocarbon-like interior of the membrane. For a fully saturated phosphatidylcholine membrane, its value is believed to be in the range 220-280 mV, positive in the membrane interior. This results in an enormous electric field strength within the membrane of 10(8)-10(9) Vm(-1). The dipole potential is thus likely to have great significance in controlling the conformation of ion-translocating membrane proteins and so in regulating enzyme function. Because of its location within the membrane, quantification of the dipole potential is extremely difficult and presents a great challenge to the experimentalist and theoretician alike. Both electrical and spectroscopic methods developed for the determination of the dipole potential on lipid bilayers and monolayers are presented and possible causes for differences in the values derived are discussed.