Structural changes in lipid bilayers and biological membranes caused by hydrostatic pressure

Abstract

By use of neutron diffraction, the structural parameters of oriented multilayers of 1,2-dimyristoyl-sn-glycero-3-phosphatidylcholine with deuteriocarbon chains/cholesterol (molar ratio 70:30), multilamellar lipid vesicles composed of pure lipids and lipid/cholesterol mixtures, and crystalline purple membrane patches from Halobacterium halobium have been measured at pressures up to 2 kbar. Pressurization of the oriented 1,2-dimyristoyl-sn-glycero-3-phosphatidylcholine/cholesterol multilayers results in an in-plane compression with the mean deuterocarbon chain spacing of 4.44 .ANG. obtained under ambient conditions decreasing by 3-7% at 1.9 kbar. The thickness for this bilayer increases by .simeq. 1.5 .ANG., but the net bilayer volume decreases and the isothermal compressibility is estimated to be in the range (-0.1 to -0.6) .times. 10-4/bar at 19.0.degree. C. The d spacings for multilamellar vesicles composed of lipids in the liquid crystalline state and lipid/cholesterol mixtures increased linearly as a function of pressure, suggesting that these bilayers are also compressed in the membrane plane. For 1,2-dipalmitoyl-sn-glycero-3-phosphatidylcholine and 1,2-distearoyl-sn-glycero-3-phosphatidylcholine MLVs in the gel state, the d spacing decreases with pressure. For 1,2-dipalmitoyl-sn-glycerol-3-phosphatidylcholine, the hexagonally packed chains are anisotropically compressed in the bilayer plane, resulting in a pseudohexagonal chain packing at 1.9 kbar. The bilayer compressibility is (-0.4 or -0.5) .times. 10-4/bar depending on whether the chain tilt increases with pressure or terminal methyl groups or apposing lipid monolayers approach each other. Crystalline purple membrane patches composed of protein and lipid in the volume ratio 2:1 are compressed in the plane, with the unit cell area decreasing by .simeq. 4% on pressurization to 2.0 kbar.