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

Abstract

Bicellar dispersions of chain perdeuterated 1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC-d54), 1,2-dimyristoyl-sn-glycero-3-phospho-(1'-rac-glycerol) (DMPG), and 1,2-dihexanoyl-sn-glycero-3-phosphocholine (DHPC), with molar ratios of 3:1:1, were studied using variable-pressure (2)H 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-d54/DMPG/DHPC (3:1:1), but not DMPC-d54/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.

Divide & Conquer: Surfactant Protein SP-C and Cholesterol Modulate Phase Segregation in Lung Surfactant

“…The superposition of spectral components corresponding to a liquid-crystalline phase and a highly ordered phase for this sample suggest an SP-C-induced phase separation. Spectra similar to the highly ordered phase component have been attributed to interdigitated lipid bilayers at high pressures (36)(37)(38)(39), but the interpretation of the broad spectral component should be approached cautiously, since other bilayer phases can produce a comparably high orientational order. However, it must be noted that the ordered spectrum does not represent a typical gel phase, since intensity does not monotonically decrease with increased splitting.…”

“…In the absence of cholesterol, the SP-C-induced ordered phase exhibited spectral features consistent with interdigitated structures (36)(37)(38). Nevertheless, as discussed previously, we cannot discard other lipid organizations that FIGURE 4 ESR spectra of DPPC/POPC/POPG membranes incorporating no protein (dotted line), 5 wt% (1 mol%) (gray line), or 10 wt% SP-C (2 mol%) (black line) labeled with 1 mol% of the probe 12-PCSL at different temperatures.…”

Effect of an Anionic Lipid on the Barotropic Behavior of a Ternary Bicellar Mixture

“…15 Bicellar mixture properties have been reviewed in more detail elsewhere. 15,16 Phase changes in lipid dispersions can also be driven by changes in hydrostatic pressure. Some bilayer mechanical properties, such as linear compressibility, depend on whether stress is applied along the plane of the bilayer or perpendicular to it.…”

“…A number of recent reviews have described the responses of lipid dispersions to changes in hydrostatic pressure 17−19 and the range of techniques used to observe such effects. 16 Phase diagrams for bilayers of phosphatidylcholines with saturated acyl chain lengths ranging from 12 to 22 carbons have been determined using differential scanning calorimetry and light transmittance. 20 In a number of lipid dispersions, elevated hydrostatic pressure induces the formation of an interdigitated gel phase.…”

Comprehensive characterization of temperature- and pressure-induced bilayer phase transitions for saturated phosphatidylcholines containing longer chain homologs

“…In the L ␤ I phase, hydrocarbon chains of the CnPC molecules in one of the monolayers constituting a bilayer extend beyond the region of the bilayer midplane and alternately interpenetrate into the other opposing monolayer. Most cases where the pressure-induced interdigitation can be observed involve phospholipids with saturated linear acyl chains like CnPC [19][20][21][22][23][24][25].…”