Slow motions in bilayers containing anionic phospholipid

Lung surfactant monolayer formation and maintenance requires substantial reorganization of phospholipid assemblies containing up to 10% anionic lipid. Nuclear magnetic resonance was used to study chain order and slow reorientation in bilayer mixtures of dipalmitoylphosphatidylcholine (DPPC) and an anionic phospholipid, dipalmitoylphosphatidylglycerol (DPPG), with one or both lipids chain-perdeuterated and in the presence and absence of calcium in the aqueous medium. Anionic lipid broadens the chain-melting transition in the presence of calcium but has little effect on phase behaviour or chain order in its absence. Chain spectra suggest that any calcium-induced DPPG aggregation in the liquid crystalline phase is short-lived. While bilayers containing DPPG can exist in a metastable hydration state, their stable hydration state is found to be one in which the contribution of slow collective or diffusive bilayer motions to liquid crystal phase quadrupole echo decay is substantially greater than observed for DPPC bilayers.