Peg-stabilized Bilayer Disks: Fundamentals and Pharmaceutical/biotechnical Applications of a Novel Model Membrane

Open membrane structures, best described as bilayer disks, may be found in mixtures of phospholipids and various micelle-forming components. In many cases the disks are merely short-lived intermediates formed during a phase-or structural transition. Examples are the circular, or sometimes more irregular membrane patches often observed during surfactant mediated solubilization of liposomes [Silvander, M. et al. In other cases the disks appear to be in thermodynamic equilibrium. Stable disk-shaped objects, commonly termed " bicelles " , may for instance be found at certain compositions in mixtures of the long-chain phospholipid dimyristoylphosphatidylcholine (DMPC) and the short-chain phospholipid dihexanoylphosphatidylcholine (DHPC) [van Dam, L. et al. Biochim. Biophys. Acta 1664 (2004) 241]. The disks formed in the DMPC/DHPC-system are rather small, with diameters of 20 nm or less, and form only within a limited temperature interval. Large discs, with diameters of 200 nm or more, have, on the other hand, been discovered in dilute aqueous solutions containing a mixture of phospholipids, cholesterol and polyethyleneglycol-lipids (PEG-lipids) [Edwards, K. et al. PEG-lipids are routinely used in order to prolong the blood circulation time of liposomes used for drug delivery applications. It is well established that upon inclusion of sufficiently high concentrations of PEG-lipids a transition from bilayer to micellar phase occurs. Further, systematic studies of the phase behaviour and aggregate structure in various lipid/PEG-lipid systems have revealed that either cylindrical or disk-shaped micelles may form as the bilayer saturation concentration is exceeded [Edwards, K. et al. Biophys. J. 73 (1997) 258]. The cylindrical micelles, which often adapt a long worm-like structure, are observed when PEG-lipids are mixed with the unsaturated phospholipid egg-phosphatidylcholine (EPC). Disks-shaped structures appear, on the other hand, in mixtures of PEG-lipids and saturated lipid analogues like DSPC and DPPC. Importantly, upon inclusion of 40 mol% cholesterol in the lipid mixtures the PEG-lipids induce formation of disk-shaped structures in all the investigated systems. The size of the disks is critically dependent on the PEG-lipid concentration. Large disks, perhaps better described as circular membrane patches, are found at PEG-lipid concentrations just above the bilayer saturation limit. Investigations based on a combination of dynamic light scattering and cryo-TEM