Pressure-induced ordering in mixed-lipid bilayers.

Isothermal application of hydrostatic pressure to liquid crystalline phospholipid bilayers increases chain segment orientational order and thus chain extension. By using pressure to perturb chain order in single-component bilayers and bilayers comprising a binary mixture of lipids, it is possible to compare the relative influences of intrinsic lipid properties and collective bilayer properties on chain orientational order. Deuterium nuclear magnetic resonance was used to investigate the response of saturated chain orientational order to pressure in single-component and two-component liquid crystal bilayers of lipids having saturated chains of different lengths (dipalmitoyl phosphatidylcholine and dimyristoyl phosphatidylcholine) or having one saturated chain and one unsaturated chain (1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine). For bilayers containing a single lipid species, the isothermal response of average chain orientational order to pressure decreased with increasing difference between measurement temperature and that lipid's ambient-pressure gel-to-liquid-crystalline phase transition temperature. For all of the lipids observed, the range of orientational order displayed by the saturated chain methylene groups was approximately conserved as pressure was applied. In binary mixtures, the difference between the average saturated chain orientational orders of the two bilayer components was approximately conserved as pressure was increased. These observations provide some insight into how the response of the bilayer to pressure is distributed over interacting components with different intrinsic properties and illustrate the sensitivity of the effective chain ordering potential in the bilayer interior to bilayer composition.