STRUCTURE OF POLYMERIZABLE LIPID BILAYERS .3. 2 HEPTACOSADIYNOYL PHOSPHATIDYLCHOLINE ISOMERS

The structures of hydrated bilayers of 1,2-bis(10,12-heptacosadiynoyl)-sn-glycero-3-phosphocholine (DC8, 13PC) and 1,2-bis-(11,13-heptacosadiynoyl)-sn-glycero-3-phosphocholine (DC9, 12PC) were investigated using low-angle X-ray diffraction. Multilayer stacks of these diacetylenic phosphatidylcholine derivatives were highly ordered, typically yielding up to 16 orders of lamellar diffraction. The unit cell repeat was smaller than expected for lipids of this size. The electron density profiles of both 27 carbon lipid bilayers contained peaks at 10 angstrom and 16 angstrom from the bilayer center due to the diacetylene moieties. The electron density profile for DC8, 13PC was similar to that of the previously studied polymerizable lipid, DC8, 9PC, except that in the DC8, 13PC profile, the distance between diacetylene electron density maxima on opposing sides of the bilayer was approximately 4 angstrom further apart. The electron density profile of DC9, 12PC showed that the peaks corresponding to the diacetylenic regions were at approximately the same positions as in the DC8, 13PC bilayers, but were less well resolved. Acyl chains in model structures inferred from the electron density data are tilted relative to the bilayer normal and may not be as well ordered as previously believed.