TEMPERATURE-DEPENDENCE OF THE REPULSIVE PRESSURE BETWEEN PHOSPHATIDYLCHOLINE BILAYERS

Bilayer structure and interbilayer repulsive pressure were measured from 5 to 50 degrees C by the osmotic stress/x-ray diffraction method for both gel and liquid crystalline phase lipid bilayers. For gel phase dibehenoylphosphatidylcholine (DBPC) the bilayer thickness and pressure-distance relations were nearly temperature-independent, and at full hydration the equilibrium fluid spacing increased similar to 1 Angstrom, from 10 Angstrom at 5 degrees C to 11 Angstrom at 50 degrees C. In contrast, for liquid crystalline phase egg phosphatidylcholine (EPC), the bilayer thickness, equilibrium fluid spacing, and pressure-distance relation were all markedly temperature-dependent. As the temperature was increased from 5 to 50 degrees C the EPC bilayer thickness decreased similar to 4 Angstrom, and the equilibrium fluid spacing increased from 14 to 21 Angstrom. Over this temperature range there was little change in the pressure-distance relation for fluid spacings less than similar to 10 Angstrom, but a substantial increase in the total pressure for fluid spacings greater than 10 Angstrom. These data show that for both gel and liquid crystalline bilayers there is a short-range repulsive pressure that is nearly temperature-independent, whereas for liquid crystalline bilayers there is also a longer-range pressure that increases with temperature. From analysis of the energetics of dehydration we argue that the temperature-independent short-range pressure is consistent with a hydration pressure due to polarization or electrostriction of water molecules by the phosphorylcholine moiety. For the liquid crystalline phase, the 7 Angstrom increase in equilibrium fluid spacing with increasing temperature can be predicted by an increase in the undulation pressure as a consequence of a temperature-dependent decrease in bilayer bending modulus.