Relationship between the unbinding and main transition temperatures of phospholipid bilayers under pressure

Using neutron diffraction and a specially constructed high pressure cell suitable for aligned multibilayer systems, we have studied, as a function of pressure, the much observed anomalous swelling regime in dimyristoyl- and dilauroyl-phosphatidylcholine bilayers, DMPC and DLPC, respectively. We have also reanalyzed data from a number of previously published experiments and have arrived at the following conclusions. (a) The power law behavior describing anomalous swelling is preserved in all PC bilayers up to a hydrostatic pressure of 240 MPa. (b) As a function of increasing pressure there is a concomitant decrease in the anomalous swelling of DMPC bilayers. (c) For PC lipids with hydrocarbon chains greater than or equal to13 carbons the theoretical unbinding transition temperature T-star is coupled to the main gel-to-liquid crystalline transition temperature T-M. (d) DLPC is intrinsically different from the other lipids studied in that its T-star is not coupled to T-M. (e) For DLPC bilayers we predict a hydrostatic pressure (>290 MPa) where unbinding may occur.