PHOSPHOLIPID PHASE-TRANSITIONS AS REVEALED BY NMR

Aqueous dispersions of phospholipids can adopt a range of polymorphic phases which include bilayer and non-bilayer forms. Within the bilayer form, laterally separated phases may be induced as a result of surface electrostatic associations, thermotropic behaviour, lipid-protein interactions or because of molecular mismatch between chemically distinct phospholipids. Nuclear magnetic resonance (NMR) methods, designed to exploit the properties of either indigenous nuclei or isotopic labels introduced specifically into a phospholipid, can be used in some cases to describe the molecular properties and behaviour of phospholipids in both macroscopically distinct phases and in molecularly distinct phases within the same polymorphic state. If the molecular motion of phospholipids in co-existing phases is sufficiently different, NMR methods can, in principle, give estimates of the life-time of the phases and the rate of molecular exchange between the phases.