New methods for the production and analysis of nanostructured self-assembled lipid mesophases with bicontinuous cubic topology as supported thin films

This research aims to extend our understanding of a newly-emerging class of materials known as QII or inverse bicontinuous cubic phases, by producing them in the form of thin films on flat substrates.When lipid molecules are mixed with water, they spontaneously assemble into a variety of ordered structures, including three different shapes of QII phase. These each contain branching networks of water channels, billionths of a metre in size, separated by a single lipid bilayer which is just like the one in a biological cell. We can precisely control the size of the water channels by varying the water content or temperature of the sample. By changing conditions further, we can also induce a phase transition from one shape into another.The controllable nanometer-scale size of the water channels, the large area of lipid bilayer packed into a small volume, and the fact that the bilayer contains an environment similar to a biological cell membrane, give QII phases a wide range of applications. In some, the bilayer is templated with another material, to make molecular sieves, electrodes or sensors. In other cases they may be used directly, as a vehicle for drug delivery, in biosensors based on membrane-bound proteins, or as a method of crystallizing membrane proteins in order to solve their structure. Furthermore, QII phases exist in nature, performing various biological roles; understanding their formation can help us to understand more generally what happens when cell membranes divide or fuse.There is much current research towards understanding the processes that produce and inter-convert QII phases, and towards developing new materials to exploit their properties. However, this research is hampered by the fact that experiments on QII materials are carried out on polydomain samples, where the regular 3D ordering only extends within a single micron-sized domain . A QII sample will contain billions of these domains, all oriented in random directions. This reduces the information obtained from experiments, introduces additional effects due to the boundaries between domains, and limits the technological potential of the material.Here, we aim to develop ways to produce a new form of QII sample, as thin films between 20 and 200nm thick. To achieve this we will begin by making a stack of bilayers supported on an extremely flat surface, using proven methods. Guided by phase diagrams that are already known for polydomain lipid samples, we will then change the sample environment to a different temperature and/or humidity, where it will undergo a transition into a QII phase. This will be a single domain in depth, and we will be able to apply, for the first time, a range of techniques that can investigate an area only one domain across. These include atomic force microscopy, which probes the surface of the sample with a resolution high enough to visualize single water channels in the QII phase, and x-ray scattering, which tells us the geometry, orientation and repeat spacing of the regular structures adopted. These new methods of sample preparation and analysis will produce a wealth of information on QII phases.First, we will be able to test unconfirmed models and predictions for the geometric pathways by which one phase turns into another. Secondly, we will find out how to control the sizes of the domains, and see the role that domain boundaries play in phase transitions. Thirdly, we will be able to produce and analyse asymmetric QII phases, structures that so far have never been made in a laboratory, where the two monolayers making up the bilayer differ in lipid composition. Such materials would have new properties and applications, and would offer better analogs of cell membranes. Finally, the work will form the basis for further projects, using supported thin films of QII as a better controlled system for electrochemistry, membrane protein research,and a range of other nanotechnological applications based on QII phases.