X-ray Diffraction Study of Three lgF-Labeled Dimyristoylphosphatidylcholines

Low-angle and wide-angle X-ray diffraction was used to examine the phase behavior, as a function of temperature, of fully hydrated multilamellar vesicles of dimyristoylphosphatidylcholine (DMPC) labeled with a difluoromethylene group in the sn-2 acyl chain at position 4,8, or 12. At temperatures near 5 OC, DMPC and 2-[ 12,12-FzIDMPC are both in the Lb (gel) phase with a D-spacing of 59.5 i 0.5 A, 2-[8,8-F2]DMPC is in the Pb (rippled) phase with a D-spacing of 65.0 f 0.5 A, and 2-[4,4-F2]DMPC is in the L, (subgel) phase with a D-spacing of 56.5 f 0.5 A. Thus the greatest bilayer perturbation is caused by labeling with fluorine closest to the head group region. It was found that 2-[4,4-F2]DMPC converted from the La (fluid) phase to the subgel phase at temperatures as high as 19.3 OC when incubated for 16 h at 19.3 OC. A hypothesis is presented that high concentrations of phospholipid in water (25% w/w) facilitate the rapid formation of the subgel phase in 2-[4,4-F2]DMPC. This hypothesis is consistent with the nucleation and growth mechanism of subgel formation as was shown for subgel formation in dipalmitoylphosphatidylcholine (DPPC).' When the subgel phase of 2-[4,4-F2]DMPC was melted over the course of several days, phase coexistence of the subgel and fluid phases was observed a t 23.1 OC without observation of intermediary gel or rippled phases.