Structural evaluation of phospholipid bicelles for solution-state studies of membrane-associated biomolecules

Several complementary physical techniques have been used to characterize th e aggregate structures formed in solutions containing dimyristoylphosphatid ylcholine (DMPC)/dihexanoylphosphatidylcholine (DHPC) at ratios of less tha n or equal to0.5 and to establish their morphology and lipid organization a s that of bicelles. P-31 NMR studies showed that the DMPC and DHPC componen ts were highly segregated over a wide range of DMPC/DHPC ratios (q = 0.05-0 .5) and temperatures (15 degreesC and 37 degreesC). Only at phospholipid co ncentrations below 130 mM did the bicelles appear to undergo a change in mo rphology. These results were corroborated by fluorescence data, which demon strated the inverse dependence of bicelle size on phospholipid concentratio n as well as a distinctive change in phospholipid arrangement at low concen trations. In addition, dynamic light scattering and electron microscopy stu dies supported the hypothesis that the bicellar phospholipid aggregates are disk-shaped. The radius of the planar domain of the disk was found to be d irectly proportional to the ratio of DMPC/DHPC and inversely proportional t o the total phospholipid concentration when the DMPC/DHPC ratio was held co nstant at 0.5. Taken together, these results suggest that bicelles with low q retain the morphology and bilayer organization typical of their liquid-c rystalline counterparts, making them useful membrane mimetics.