IR spectroscopic study of phospholipid emulsions containing cholesteryl oleate

As models for the lipid organization of low density lipoproteins (LDL), protein-free aqueous emulsions are prepared from dimyristoyl phosphatidyl choline (DMPC), dipalmitoyl phosphatidyl choline (DPPC), and cholesteryl oleate (CO). Aqueous dispersions containing these lipids are sonicated and yield stable particles with diameters varying between 20 and 40 nm as measured through electron microscopy. IR spectroscopy shows that emulsions consisting of DMPC, DPPC, and CO at 3/1/1 and 1/1/1 ratios undergo specific thermal transitions, depending on their composition, that can be assigned to the phospholipids forming the surface layer of the emulsion particles and to core-located CO. However, at the 1/3/1 DMPC/DPPC/CO ratio this lipid system exhibits an order-disorder transition of the mixed phospholipids with no significant transition associated with core-located CO. Observation of the methylene C&bond;H and C&bond;D stretching modes of nondeuterated and deuterated lipids enables the packing characteristics and conformational order of each lipid to be monitored separately. The transition temperature changes compared to the temperatures for the analogous transitions in neat CO and CO-free phospholipid vesicles suggest the existence of interactions between CO and the above phospholipids in the ternary emulsion particles; these interactions are stronger at the 1/3/1 DMPC/DPPC/CO ratio. The results show that interactions between core and surface phases are dependent on the emulsion lipid composition and that these findings may be extended to native lipoproteins.