Session A8 - Focus Session: Neutron Scattering and Other Studies of Phospholipids.
FOCUS session, Monday morning, March 22
510A, Palais des Congres
Near the main liquid-crystalline to gel transition of smectic stacks of phospholipids in excess water, the inter-layer distance increases in a critical-like manner. This so-called anomalous swelling phenomenon has been attributed to a hidden unbinding transition of the stack due to density fluctuations of the chains. We have studied the influence of hydrostatic pressure on anomalous swelling in phosphocholine bilayers using neutron diffraction and a specially constructed pressure cell, as a function of temperature, pressure and acyl-chain length. The power law behavior of anomalous swelling is preserved up to 240 MPa, and for PC lipids with hydrocarbon chains more than 12 carbons, the theoretical unbinding transition temperature is coupled to the main transition temperature. Lipids with shorter chains exhibit a qualitatively different trend, with the difference between these two transition temperatures decreasing with increasing pressure. We predict that at high hydrostatic pressure, complete unbinding may occur for the smaller chain lipids.