Contributions of hydration and steric (entropic) pressures to the interactions between phosphatidylcholine bilayers: experiments with the subgel phase.
The total repulsive interaction between electrically neutral, fluid bilayer membranes is thought to have a number of components, including a hydration pressure, due to the reorientation of water by the bilayer, and steric (entropic) pressures due to bilayer undulations, head group motion, and molecular protrusions. For fully hydrated, crystalline bilayers these three steric pressures should be relatively small, and the major repulsive pressure present should be the hydration pressure. Therefore, to isolate the contribution of hydration pressure to the total interbilayer interaction, we have measured pressure-distance data by X-ray diffraction analysis of osmotically stressed dipalmitoylphosphatidylcholine (DPPC) multilayers in the subgel phase, where wide-angle and low-angle X-ray data show the bilayers are crystalline. As applied pressure was increased from 0 to 1 x 10(6) dyn/cm2, the interbilayer fluid space (df) decreased less than 1 A from its value at full hydration of 8.4 A. As the pressure was increased from 1 x 10(6) to 3 x 10(7) dyn/cm2, df decreased from about 8 to 4 A. For this range of df, the repulsive pressure decayed exponentially with df with a decay length of 1.4 A. Further increases in applied pressure did not appreciably decrease df, so that there was a sharp upward break in the pressure-distance curve at an interbilayer spacing of about 3 A. In contrast, pressure-distance relations for gel (L beta') phase and liquid-crystalline (L alpha) phase bilayers had much softer upward breaks at df < 5 A and extended to larger df at zero applied pressure. However, the pressure-distance curves for all three phases decayed exponentially with approximately the same decay length for 4 < df < 8 A. We interpret these data to mean the following: (1) the repulsion observed for df < 5 A is primarily a steric pressure whose range depends on head group motion; (2) the steric undulation pressure plays an important role in determining the hydration properties and the range of the total repulsive pressure for fluid membranes; (3) the same underlying mechanisms govern the repulsive pressure for all phases for 4 < df < 8 A; (4) these mechanisms include a pressure due to reorientation of water molecules; and (5) the hydration pressure component extents a maximum of about two water molecules from the bilayer surface for the subgel phase.
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