'Building' asymmetric model cell membranes

Large area lipid bilayers, on solid surfaces, are useful tools for the biophysical study of biological membranes. There exists the problem that the presence of the substrate can impinge upon the properties of the bilayer. For example it has been observed that the phase behaviour of single bilayers on solid supports is different to that seen in vesicles in solution. It is therefore advantageous to minimise interactions between the bilayer and the substrate. This can be achieved via the formation of a Floating Supported Bilayer (FAB) upon either a surface bound phospholipid bilayer or monolayer. The independence of the FSB is enabled by the continuous water layer (> 15 Å) that separates the lower surface bound layer and the floating bilayer. This presentation will firstly introduce the FSB and describe how we have addressed previous issues with FSBs, such as limited stability and low completeness. We have produced very dense (> 96 %) symmetric FSBs of saturated phosphatidylcholines formed by sequential Langmuir Blodgett and Langmuir Schaefer procedures. This system reproduces the phase behaviour expected from vesicles and the bilayer parameters, albeit at lower resolution, are in agreement with that obtained by diffraction from multi-lamellar stacks. Neutron reflection experiments have enabled the characterisation of the FSB fully hydrated, as a function of solution ionic strength, pH and temperature plus allow for the application of partial deuteration strategies. Finally I will describe the steps that we have made towards producing stable asymmetric models of the gram negative bacteria outer membrane. The outer leaflet of the bilayer is constructed from Lipopolysaccharide while, at this stage, 1,2-dipalmitoyl-glycero-3-phoshocholine has been used for the inner leaflet.