Water permeability and mechanical strength of polyunsaturated lipid bilayers

Micropipette aspiration was used to test mechanical strength and water permeability of giant-fluid bilayer vesicles composed of polyunsaturated phosphatidylcholine PC lipids. Eight synthetic-diacyl PCs were chosen with 18 carbon chains and degrees of unsaturation that ranged from one double bond (C18:0/1, C18:1/0) to six double bonds per PC molecule (diC18:3). Produced by increasing pipette pressurization, membrane tensions for lysis of single vesicles at 21 degrees C ranged from similar to 9 to 10 mN/m for mono- and dimono-unsaturated PCs (18:0/1, 18:1/0, and diC18:1) but dropped abruptly to similar to 5 mN/m when one or both PC chains contained two cia-double bonds (C18:0/2 and diC18:2) and even lower similar to 3 mN/m for diC18:3. Driven by osmotic filtration following transfer of individual vesicles to a hypertonic environment, the apparent coefficient for water permeability at 21 degrees C varied modestly in a range from similar to 30 to 40 mu m/s for mono- and dimono-unsaturated PCs. However, with two or more cia-double bonds in a chain, the apparent permeability rose to similar to 50 mu m/s for C18:0/2, then strikingly to similar to 90 mu m/s for diC18:2 and similar to 150 mu m/s for diC18:3. The measurements of water permeability were found to scale exponentially with the reduced temperatures reported for these lipids in the literature. The correlation supports the concept that increase in free volume acquired in thermal expansion above the main gel-liquid crystal transition of a bilayer is a major factor in water transport. Taken together, the prominent changes in lysis tension and water permeability indicate that major changes occur in chain packing and cohesive interactions when two or more cia-double bonds alternate with saturated bonds along a chain.