Response to Hydrostatic Pressure of Bicellar Dispersions Containing an Anionic Lipid: Pressure-Induced Interdigitation

Bicellar dispersions of chain perdeuterated 1,2-dimyristoyl-<i>sn</i>-glycero-3-phosphocholine (DMPC-<i>d</i>₅₄), 1,2-dimyristoyl-<i>sn</i>-glycero-3-phospho-(1′-<i>rac</i>-glycerol) (DMPG), and 1,2-dihexanoyl-<i>sn</i>-glycero-3-phosphocholine (DHPC), with molar ratios of 3:1:1, were studied using variable-pressure ²H NMR spectroscopy at hydrostatic pressures up to 125 MPa. Upon warming of the dispersions, spectra at ambient pressure indicated a progressive coalescence from small bilayered disks undergoing isotropic reorientation to more extended micellar structures in which spectra indicated anisotropic reorientation and, under some conditions, magnetic orientation and finally to randomly oriented lamellae or multilamellar vesicles. Temperatures for the onsets of anisotropic reorientation and random lamellar orientation increased with pressure at rates of 0.22 and 0.15 °C/MPa, respectively. In the 3.5-T magnetic field used for this work, magnetic orientation within the intermediate phase was not observed at 83 MPa or higher pressures. Comparison of spectra obtained at fixed pressure showed significant asymmetry between behaviors upon warming and cooling. For samples of DMPC-<i>d</i>₅₄/DMPG/DHPC (3:1:1), but not DMPC-<i>d</i>₅₄/DHPC (4:1), a persistent interdigitated phase was formed after repeated cooling from high temperature at 83 MPa. This is likely a metastable phase and might reflect kinetic trapping of the short-chain lipid component, DHPC, in a nonequilibrium spatial distribution as temperature is lowered at high pressure. Bicellar dispersions typically behave differently upon warming and cooling, and these observations could provide some insight into the observed behaviors in such systems. This work also suggests the possibility of trapping bicellar dispersions in persistent nonequilibrium morphologies