title: Effect of an Anionic Lipid on the Barotropic Behavior of a Ternary Bicellar Mixture.
abstract: Dispersions of lipid mixtures comprising long- and short-chain phospholipids (bicellar mixtures) can form small isotropically reorienting particles (bilayered micelles), magnetically orientable stuctures, or unorientable lamellar structures. Application of hydrostatic pressure can also induce interdigitation of the long-chain lipid components. In this work, variable-pressure2H NMR was used to study the effect of head group charge on the barotropic behavior of bicellar mixtures. Observations at pressures up to 152 MPa and temperatures up to 64 °C were combined with earlier observations at lower pressure and lower temperature to obtain a pressure-temperature phase diagram for DMPC-d54/DMPG/DHPC (3:1:1). In this phase diagram, a region corresponding to small, isotropically reorienting particles at lower temperature and higher pressure is separated from a region corresponding to unorientable lamellar organization, at higher temperature and lower pressure, by a band in which the magnetically orientable phase is stable below ∼100 MPa and in which an interdigitated gel phase is stable above ∼120 MPa. From ∼46 to ∼52 °C, the dispersion transforms directly from the unorientable lamellar to isotropically reorienting particle phases upon isothermal pressurization. The extent to which this behavior reflects the presence of anionic lipid in the long-chain fraction of this mixture is illustrated by comparison with spectral series obtained during isothermal pressurization of DMPC-d54/DHPC (4:1) and DMPC-d54/DMPG/DHPC (2.7:1.3:1) at selected temperatures. These observations show how electrostatic interactions at a bilayer surface can affect the balance between hydrophobic and hydrophilic interactions that is reflected by a dispersion's barotropic phase behavior.
authors.affiliation: Department of Physics and Physical Oceanography, Memorial University of Newfoundland , St. John's, Newfoundland and Labrador, Canada A1B 3X7.
authors.forename: Collin
authors.initials: C
authors.lastname: Knight
authors.affiliation: Department of Physics and Physical Oceanography, Memorial University of Newfoundland , St. John's, Newfoundland and Labrador, Canada A1B 3X7.
authors.forename: Ashkan
authors.initials: A
authors.lastname: Rahmani
authors.affiliation: Department of Physics and Physical Oceanography, Memorial University of Newfoundland , St. John's, Newfoundland and Labrador, Canada A1B 3X7.
authors.forename: Michael R
authors.initials: MR
authors.lastname: Morrow