Aligning Nanodiscs at the Air-Water Interface, a Neutron Reflectivity Study

In English Nanodiscs are self-assembled nanostructures composed of a belt protein and a small patch of lipid bilayer, which can solubilize membrane proteins in a lipid bilayer environment. We present a method for the alignment of a well-defined two-dimensional layer of nanodiscs at the air-water interface by careful design of an insoluble surfactant monolayer at the surface. We used neutron reflectivity to demonstrate the feasibility of this approach and to elucidate the structure of the nanodisc layer. The proof of concept is hereby presented with the use of nanodiscs composed of a mixture of two different lipid (DMPC and DMPG) types to obtain a net overall negative charge of the nanodiscs. We find that the nanodisc layer has a thickness or 40.9 +/- 2.6 angstrom with a surface coverage of 66 +/- 4%. This layer is located about 15 angstrom below a cationic surfactant layer at the air water interface. The high level of organization within the nanodiscs layer is reflected by a low interfacial roughness (similar to 4.5 angstrom) found. The use of the nanodisc as a biomimetic model of the cell membrane allows for studies of single membrane proteins isolated in a confined lipid environment. The 2D alignment of nanodiscs could therefore enable studies of high-density layers containing membrane proteins that, in contrast to membrane proteins reconstituted in a continuous lipid bilayer, remain isolated front influences of neighboring membrane proteins within the layer.