Repulsive interactions between uncharged bilayers. Hydration and fluctuation pressures for monoglycerides

Abstract

Pressure versus distance relations have been obtained for solid (gel) and neat (liquid-crystalline) phase uncharged lipid bilayers by the use of x-ray diffraction analysis of osmotically stressed monoglyceride aqueous dispersions and multilayers. For solid phase monoelaidin bilayers, the interbilayer repulsive pressure decays exponentially from a bilayer separation of approximately 7 A at an applied pressure of 3 x 10(7) dyn/cm2 to a separation of approximately 11 A at zero applied pressure, where an excess water phase forms. The decay length is approximately 1.3 A, which is similar to the value previously measured for gel phase phosphatidylcholine bilayers. This implies that the decay length of the hydration pressure does not depend critically on the presence of zwitterionic head groups in the bilayer surface. For liquid-crystalline monocaprylin, the repulsive pressure versus distance curve has two distinct regions. In the first region, for bilayer separations of approximately 3-8 A and applied pressures of 3 x 10(8) to 4 x 10(6) dyn/cm2, the pressure decays exponentially with a decay length of approximately 1.3 A. In the second region, for bilayer separations of approximately 8-22 A and applied pressures of 4 x 10(6) to 1 x 10(5) dyn/cm2, the pressure decays much more gradually and is inversely proportional to the cube of the distance between bilayers. These data imply that two repulsive pressures operate between liquid-crystalline monocaprylin bilayers, the hydration pressure, which dominates at small (3-8 A) bilayer separations, and the fluctuation pressure, which dominates at larger bilayer separations (greater than 8 A) and strongly influences the hydration properties of the liquid-crystalline bilayers. Thus, due primarily to thermally induced fluctuations, monocaprylin bilayers imbibe considerably more water than do monoelaidin bilayers. For both monoelaidin andmonocaprylin, the measured magnitude of the hydration pressure is found to be proportional to the square of the dipole potential.

Hydration and Steric Pressures between Phospholipid Bilayers

“…Theoretical treatments (11,13,23) have indicated that the undulation pressure should extend the range of the underlying repulsive pressure (in this case the hydration pres sure), as shown experimentally in Figure 4. This behavior is also ob served between monoglycerides whose bending moduli differ greatly (56). However, because the data for liquid crystalline bilayers such as EPC ( Figure 2) can be fit to a single exponentionai of dr > 5 A , ex tracting information about the undulation pressure from a single data set is very difficult.…”

Structural Dynamics of the S4 Voltage-Sensor Helix in Lipid Bilayers Lacking Phosphate Groups

“…The simulations were carried out in excess water, which denotes a bilayer containing more water molecules than the minimal amount needed for hydration. For a POPC bilayer, a minimal amount of ~9.5 water molecules per lipid are needed for hydration at 93% relative humidity (Mihailescu et al unpublished and 35,36 ) In these simulations, water-lipid ratios of 44.8 and 44.1 were used for POPC and DOTAP, respectively. The sequence of the capped S4mut peptide in our simulations is similar to the wild-type S4:…”

Magnitude of the solvation pressure depends on dipole potential.

“…The data used in this paper have been previously published (4,6,20,23,24) and hence the methods will be described briefly. The lipids used were egg phosphatidylcholine (EPtdCho), E-PtdCho/cholesterol of various mole ratios, and dipalmitoylphosphatidylcholine (Pam2-PtdCho).…”