Bilayer Discs - Fundamental Investigations and Applications of Nanosized Membrane Models

The bilayer disc is a flat, lipid aggregate structure in the nanometre regime. It is composed of a bilayer of amphiphilic molecules with micelle-forming amphiphilic molecules supporting the rim, which prevent disc fusion and self-closure. Stable discs have been found in lipid mixtures containing polyethylene glycol (PEG)-lipids as a rim-stabilizing component. One of the aims of the work described in this thesis was to increase the fundamental knowledge and understanding of the systems in which these discs are formed. Other micelle-forming surfactants apart from PEG-lipids were also explored to see if they could be used to stabilize the disc aggregate structure. Due to the similarities of these lipid discs with natural membranes it was hypothesized that they could be used as models for biological membranes. It was demonstrated that discs are formed in PEG-lipid/lipid systems when the lipid mixture contains components that reduce the spontaneous curvature and increase the monolayer bending rigidity. Discoidal structures are furthermore preferred when the lipids are in the gel phase, probably due to a combination of high bending rigidity and reduced PEG-lipid/lipid miscibility. The disc size could be varied by changing the PEG-lipid concentration. The size and size homogeneity of the discs could also be varied by changing the preparation path. Generally, the preferences of certain lipid systems to form discs remained when the PEG-lipid was replaced by more conventional surfactants. However, discs prepared in PEG-lipid/lipid systems are more useful as model membranes because of their relatively large size and good temperature, dilution and long-term stability. Data obtained with isothermal titration calorimetry and drug partition chromatography indicate that these bilayer discs may serve as an attractive and sometimes superior alternative to liposomes in studies of drug-membrane interactions