Model Membrane Structure, Molecular Order, Fluctuations and Dynamics

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The overall aim of this research program is to improve our understanding of how the physical properties of membranes affect their biological function. A more immediate goal is to understand how these physical properties affect and determine the approaches which can be used to study membranes. Our ability to perform accurate experimental measurements depends on the physical properties of the membrane. For example, in fluorescence microscopy or in nuclear magnetic resonance (NMR) spectroscopy the rates of molecular motion determine whether or not we can actually observe anything at all. Cholesterol has long been known to thicken the membrane bilayer and to make it somewhat more resilient. More recently it has been found that the high cholesterol content of plasma membranes places them close to a critical point such that the local membrane composition fluctuates. This may turn out to have a significant impact on membrane function. We are also studying how the inclusion of peptides, as model proteins, affect this behaviour in model membranes and how the peptides themselves are affected by the critical behaviour. Furthermore, we want to learn more about the interaction between small membrane active peptides and the membrane.**One of the principal aims of our research is to find conditions under which we can make accurate physical measurements. We outline in this proposal our program for studying the interactions of peptides, lipids and cholesterol in model membranes. Part of this program involves the development of new NMR techniques which take advantage of the special nature of the membrane components: i) cholesterol which induces the formation of two fluid membrane phases; ii) short chain lipids whose inclusion in a model membrane can cause the system to spontaneously orient within an applied magnetic field; iii) polyunsaturated lipids which are much more highly 'fluid' than the saturated lipids, resulting in more rapid molecular reorientation and, consequently, higher resolution in NMR experiments. All of these factors can be used to optimize experimental design. An important illustration of this principle is that we are able to obtain high resolution 1H magic angle spinning NMR spectra in order to study an interesting class of small peptides interacting with membranes under physiological conditions. This is possible because the molecules in a fluid membrane undergo rapid axially symmetric reorientation about the local bilayer normal. Even though the spectral linewidths are still broader than those observed in solution, we can analyse them to obtain information about the overall phase behaviour and the molecular structure and dynamics.**