SMALL-ANGLE X-RAY-SCATTERING STUDIES OF MAGNETICALLY ORIENTED LIPID BILAYERS

Magnetically oriented lipid/detergent bilayers are potentially useful for studies of membrane-associated molecules and complexes using x-ray scattering and nuclear magnetic resonance (NMR). To establish whether the system is a reasonable model of a phospholipid bilayer, we have s tudied the system using x-ray solution scattering to determine the bil ayer thickness, interparticle spacing, and orientational parameters fo r magnetically oriented lipid bilayers. The magnetically orientable sa mples contain the phospholipid L-alpha-dilauroylphosphatidylcholine (D LPC) and the bile salt analog nio]-2-hydroxy-1-propanesulfonate-propan esulfonate (CHAPSO) in a 3:1 molar ratio in 70% water (w/v) and are si milar to magnetically orientable samples used as NMR media for structu ral studies of membrane-associated molecules. A bilayer thickness of 3 0 Angstrom was determined for the DLPC/CHAPSO particles, which is the same as the bilayer thickness of pure DLPC vesicles, suggesting that t he CHAPSO is not greatly perturbing the lipid bilayer. These data, as well as NMR data on molecules incorporated in the oriented lipid parti cles, are consistent with the sample consisting of reasonably homogene ous and well dispersed lipid particles. Finally, the orientational ene rgy of the sample suggests that the size of the cooperatively orientin g unit in the samples is 2 x 10(7) phospholipid molecules.