(95g) Phase Behavior of a Mixed Bilayered System of Lipids

Lipid mixtures of short and long amphiphile chains self-assemble in water to form a wide variety of structures. The morphology of these structures include phases composed of extended flexible bilayer membranes that may display order by stacking in a lamellar phase with a periodicity on the order of hundreds of Angstroms. They may also form bilayered micelles (or bicelles), which are bilayered disks formed of a long lipid chain with their edges stabilized by short chain lipid, with diameter of a few hundred Angstroms.

The lipid mixtures have a great potential in the study of membrane proteins and peptides. These mixtures imitate the physical properties of biological membranes and they are stable over a wide range of temperatures, pH and ionic strength. A detailed description of the morphology of the lipid mixtures and an understanding of their phases will help increase their use in various structural biology techniques.

SANS (Small ? Angle Neutron Scattering), reflectometry, and crossed polarizers were use to study the transition from the bicelle to lamellar phase. Our studies focused on a 10% total lipid mixture consisting of the long chain lipid, DMPC, and the short chain lipid, DHPC. The goal was to study the effect on the transition between the bicelle to lamellar phase by changing the Q value of the mixture, which is defined as the molar ratio of the DMPC with respect to the DHPC. The Q values used were 3, 5, and 7. A charged lipid (POPG or DMPG) was also added to the system in various molar ratios with respect to the DMPC. We found that the transition temperature of the solution is dependent on the Q value. Also, the amount of the charged lipid affects the phase diagram. With these results, the structure of the phase boundary between the bicelles and lamellar morphology was better understood and characterized.