Effects of Various Molecules on the Structure and Dynamics of Lipid Membranes

Abstract

In my time at the Laboratory of Membrane and Protein Dynamics at McMaster University, it has been our goal to investigate the fundamental properties of model membranes and how some common membrane molecules, namely water, ethanol, and cholesterol, interact with the bilayer. Our studies employ highly-oriented, solid-supported membranes in order to extract unambiguous structural information perpendicular to and in the plane of the membranes, with the exception of the hydrated powder samples used in probing the effects on ethanol. Both X-ray and neutron scattering were employed to investigate the structural properties of the membranes and neutron scattering was used to infer the dynamical properties. A variety of neutron scattering techniques were used to determine the properties of hydrated lipid bilayers, as described in the first two publications listed. Instruments including a neutron backscattering spectrometer, reflectometer, and time-of-flight spectrometer were used to observe bilayer structure, lipid/water coupling, and water diffusion. We found that hydrated, solid-supported single-bilayers showed no strong coupling between hydration water and lipid tails and the out-of-plane structure of stacked fluid bilayers as well as the anisotropic and anomalous behaviour of hydration water compared to bulk water. Both X-ray and neutron scattering experiments were done to determine the effect of a 2mol% concentration of ethanol on a hydrated lipid powder. X-ray scattering was used to determine the structural changes due to the addition of ethanol and the location of ethanol within the bilayer. This was accomplished by determining areas of increased electron density in the head group and among the acyl tails. The presence of ethanol also attributed to a decrease in lateral lipid diffusion constant in the gel phase, while no significant change was found in fluid bilayers. In the final study outlined in this thesis, the result of a 32.5% concentration of cholesterol in a hydrated, fluid phospholipid membrane is discussed. Coarse-grained molecular simulations and measurements of the lateral structure of the membrane via neutron spectrometry were able to determine the heterogeneous nature of the liquid-ordered phase and the structure of each of the domains in the membrane. The following thesis will introduce model membranes, their relevant components and the scattering of X-rays and neutrons from such matter. Next, experimental techniques, sample constituents, sample preparations, and instruments used in experiments will be described. Then, each study will be introduced and discussed which will showcase the progress made in the field of model membranes. Lastly, an overview of the studies will lead in to future directions for each model system in terms of suggested experiments and general path.