X-Ray Diffraction Study of the Lamellar-Hexagonal Phase Transition in Phospholipid/Surfactant Mixtures

Thermodynamic and structural behaviour of equimolar POPC/CnE4 (n=8, 12, 16) mixtures by sorption gravimetry, 2H NMR spectroscopy and X-ray diffraction

The hydration behaviour of equimolar mixtures of phospholipids and nonionic surfactants with different chain length was investigated by gravimetric sorption, NMR spectroscopy and X-ray diffraction. At the most hydration degrees investigated, the incorporation of nonionic surfactants in a phospholipid bilayer leads to an increase of the hydrophilicity, which can be shown by the presence of excess hydration. The increased hydrophilicity could be explained by the excavation of additional water binding sites due to the "dilution" of the dipole field of the phospholipid bilayer. Another related contribution arises from the increase of the accessible surface area due to the increase of gauche conformers that result from the steric mismatch when surfactants are incorporated into the phospholipid matrix. (2)H NMR spectroscopy was used to determine the quadrupolar splitting representing a measure of the order state of water. The swelling behaviour could be assessed by small-angle X-ray diffraction. (31)P NMR spectroscopy was applied for the assignment of phase structures to the respective hydration range.

Interactions of surfactants with lipid membranes

Surfactants are surface-active, amphiphilic compounds that are water-soluble in the micro- to millimolar range, and self-assemble to form micelles or other aggregates above a critical concentration. This definition comprises synthetic detergents as well as amphiphilic peptides and lipopeptides, bile salts and many other compounds. This paper reviews the biophysics of the interactions of surfactants with membranes of insoluble, naturally occurring lipids. It discusses structural, thermodynamic and kinetic aspects of membrane-water partitioning, changes in membrane properties induced by surfactants, membrane solubilisation to micelles and other phases formed by lipid-surfactant systems. Each section defines and derives key parameters, mentions experimental methods for their measurement and compiles and discusses published data. Additionally, a brief overview is given of surfactant-like effects in biological systems, technical applications of surfactants that involve membrane interactions, and surfactant-based protocols to study biological membranes.

Membrane domain formation, interdigitation, and morphological alterations induced by the very long chain asymmetric C24:1 ceramide

Ceramide (Cer) is involved in the regulation of several biological processes, such as apoptosis and cell signaling. The alterations induced by Cer in the biophysical properties of membranes are thought to be one of the major routes of Cer action. To gain further knowledge about the alterations induced by Cer, membrane reorganization by the very long chain asymmetric nervonoylceramide (NCer) was studied. The application of an established fluorescence multiprobe approach, together with x-ray diffraction, differential scanning calorimetry, and confocal fluorescence microscopy, allowed the characterization of NCer and the determination of the phase diagram of palmitoyloleoylphosphatidylcholine (POPC)/NCer binary mixtures. Nervonoylceramide undergoes a transition from a mixed interdigitated gel phase to a partially interdigitated gel phase at approximately 20 degrees C, and a broad main transition to the fluid phase at approximately 52 degrees C. The solubility of NCer in the fluid POPC is low, driving gel-fluid phase separation, and the binary-phase diagram is characterized by multiple and large coexistence regions between the interdigitated gel phases and the fluid phase. At 37 degrees C, the relevant phases are the fluid and the partially interdigitated gel. Moreover, the formation of NCer interdigitated gel phases leads to strong morphological alterations in the lipid vesicles, driving the formation of cochleate-type tubular structures.

The impact of lipid distribution, composition and mobility on xylem water refilling of the resurrection plant Myrothamnus flabellifolia

•  Lipids play a crucial role in the maintenance of the structural and functional integrity of the water-conducting elements and cells of the resurrection plant Myrothamnus flabellifolia during complete dehydration. •  Lipid composition, mobility and distribution within the internodal and nodal xylem regions (including short shoots and leaves) were investigated in the presence and absence of water by using various nuclear magnetic resonance (NMR) spectroscopy and imaging techniques differing greatly in the level of spatial resolution and acquisition of lipid parameters. •  Significant findings include: a discontinuity in the branch xylem between an inner zone where no water moves and an outer zone where the water moves; the blocking of water movement in the inner zone by lipids that are not dispersed by water, and the facilitation of water advance in the xylem elements and pits of the outer zone by water-dispersed lipids; the relative impermeability of leaf trace xylem to the rehydrating water and, hence, the relative hydraulic isolation of the leaves. •  These results elucidated part of the strategy used by the resurrection plant to cope with extreme drought and to minimize transpirational water loss upon hydration.

Lyotropic phase transitions in phospholipids as evidenced by small-angle synchrotron X-ray scattering

Hydration is an important factor in regulating the phase behaviour of lipids and besides affects their interactions with other compounds relevant for biological membranes. We present a reliable and fast method to detect and characterise hydration-induced phase transitions in phospholipids by means of small-angle synchrotron X-ray scattering. Films consisting of aggregations of representatives of the two important lipid classes lecithins (DPPC a, POPC and OPPC,a for abbreviations, see below) and cephalins (DPPE and DOPE) were investigated at room temperature in dependence on relative humidity. Qualitative changes in the sets of the diffraction patterns obtained in dynamic hydration/dehydration scans were taken as markers indicating the existence of lyotropic phase transitions. The efficiency of this methodology is demonstrated by illustrating the course of hydration-driven phase transitions between lamellar as well as nonlamellar phases. In detail, this was realised for chain melting in the mixed-chain lipids, POPC and OPPC, and for a novel nonlamellar-phase transition for DOPE between a disordered inverted ribbon phase designated as Palpha and the canonical H(II), phase, respectively.