Membrane Orientation Studies with Polarized Light Spectroscopy

Lipid vesicles are versatile tools in modern biophysical science. They can be used as models of the cell membrane, to examine permeability, stability and behavior of molecules in the bilayer. They may act as transporters in a pharmacological context, designed to deliver their cargo to a certain place in the patient. The thesis has a methodological approach to the problem of assessing the orientation of guest molecules in a bilayer environment, a problem pertinent to drug development as well as to the understanding of biochemical and biological processes connected to cell membranes. Lipid vesicles can be deformed in a shear flow into ellipsoidal shapes and these macroscopically oriented samples studied with polarized light. The differential absorption between light polarized parallel and perpendicular to the flow orientation direction tells whether a transition moment in the molecule of interest is oriented preferentially parallel or perpendicular to the lipid chains of the vesicle bilayer. Molecules studied range from small aromatic molecules (pyrene, anthracene) over membrane-spanning substituted aromatic groups to large polypyridyl ruthenium complexes, for which small changes of peripheral substituents are found to cause dramatic changes in the orientational behavior. Refractive index matching of the liposomes, by adding sucrose in the surrounding solution, reduces light scattering dramatically, and for the first time enables study of transitions in the ultraviolet region of the spectrum. An alternative way to achieve a membrane host with anisotropic features is to orient a lamellar lyotropic liquid crystal, consisting of planar bilayers with interstitial water layers, between glass plates. Upon introducing a guest molecule into this system, its orientation can be assessed by tilting the sample and measuring linear dichroism. At normal incidence, the circular dichroism of the oriented molecule can be used to reveal the absolute configuration and as an aid for the assignment of electronic transitions.