Small angle scattering studies of phospholipids in excess water

Abstract

<p>Model membrane systems composed of homogeneous suspensions of phospholipids in an aqueous environment have been of great interest for their biological relevance and for their intriguing physical properties. When placed in excess water, phospholipids will spontaneously self-assembly into multilamellar vesicles consisting of repeating bilayer-water layer units which are ideally suited for examination via scattering techniques. Small angle scattering techniques have played an integral role in the study of the morphology and phase behaviour of model membrane systems. This thesis focuses on small angle neutron scattering (SANS) work on two different model membrane systems, dipalmitoylphosphatidylcholine (DPPC) and 1-palmitoyl-2-oleoylphosphatidylethanolomine (POPE). Three main results, which contribute new information or ideas, are presented. SANS and X-ray diffraction studies of DPPC and DPPC in which the quaternary ammonium hydrogens have been replaced with deuterium (DPPC-d 9 ) have been performed as a function of temperature in a range from 20°C to 50°C. DPPC will display three distinct phases in this range: a low temperature gel (Lβ' ) phase, an intermediate ripple (Pβ' ) phase, and a high temperature liquid crystalline (Lα ) phase. The Pβ' phase, which spans an eight degree range from 35°C to 42°C, is known to exhibit non-equilibrium behaviour. On cooling from the Lα phase, the Pβ' phase formed is metastable but long lived. We present complementary SANS and X-ray diffraction work which shows a complex multipeak pattern for the metastable Pβ' phase. A simple, single wavelength model is proposed that explains our data and also previously published synchrotron X-ray data which was interpreted in terms of a more complex model of competing rippled domains. We also show that the metastability persists into the Lβ' phase. High resolution SANS work on large unilamellar vesicles (LUVs) of DPPC shows that we are able to extract such parameters as vesicle radius and bilayer thickness from the small angle scattering profile. We present, for the first time, diffraction evidence for the development of a ripple modulation of the bilayer surface in LUVs. Calorimetric work done in conjunction with the SANS offers supporting evidence for this claim. An examination of the high resolution SANS profiles of multilamellar samples of POPE, DPPC, and DPPC-d9 is presented which shows that the interfacial properties of model membrane systems can be inferred from scattering techniques. Results show that POPE, which has a relatively small headgroup, displays a smooth water-lipid interface, which can be described using Porod's law of surface scattering, in both its gel and liquid crystalline phases. DPPC and DPPC-d9 , on the other hand, are consistent in showing that the interface between the phosphatidylcholine (PC) headgroups and the water in the Lβ ' and Pβ' phases is rough and disordered. On raising the temperature into the Lα phase of DPPC, this interface becomes smooth.</p>