EXPERIMENTAL TESTS FOR PROTRUSION AND UNDULATION PRESSURES IN PHOSPHOLIPID-BILAYERS

Theoretical treatments predict that strong entropic pressures between adjacent bilayer membranes can arise from out of plane motions caused by either thermally induced bending undulations of the entire bilayer [Harbich, W., & Helfrich, W. (1984) Chem, Phys. Lipids 36, 39-63; Evan s. E. A., and Parsegian, V. A. (1986) Proc, Natl, Acad. Sci. U.S.A. 83 , 7132-7136] or protrusions of individual lipid molecules from the bil ayer surface [Israelachvili, J. N., and Wennerstrom, H. (1992) J. Phys . Chem. 96, 520-531]. To determine the relative contributions of these motions to the repulsive pressure between phospholipid bilayers, the osmotic stress/X-ray diffraction method was used to measure the range and magnitude of the total repulsive pressure, and micropipet methods were used to measure the bending moduli of phosphatidylcholine bilayer s containing lysophosphatidylcholine and polyunsaturated diarachidonoy lphosphatidylcholine (DAPC) bilayers. In the gel phase, incorporation of equimolar lysophosphatidylcholine into phosphatidylchoiine bilayers caused the hydrocarbon chains from apposing monolayers to interdigita te, but did not appreciably change the equilibrium fluid spacing in ex cess buffer from its control value of 12 Angstrom. In contrast, the in corporation of equimolar lysophosphatidylcholine into liquid-crystalli ne phase phosphatidylcholine bilayers markedly increased the range of the repulsive pressure so that equilibrium fluid separation increased from 15 to 28 Angstrom, and also decreased the bilayer bending modulus from 5.1 x 10(-13) to 1.3 x 10(-13) erg, Liquid-crystalline DAPC bila yers had intermediate values of both equilibrium fluid separation (20 Angstrom) and bending modulus (2.8 x 10(13) erg). Analysis of these da ta indicates that (1) the relative importance of entropic pressures co mpared to the hydration pressure depends strongly on the composition a nd structure of the bilayer, (2) the protrusion pressure may contribut e to the total repulsive pressure at large pressures or small fluid sp acings, and (3) the repulsive undulation pressure, together with the a ttractive van der Waals pressure, is a primary factor in determining t he fluid spacing at low and/or zero applied pressures in liquid-crysta lline bilayers.