Molecular dynamics and conformation in the gel and liquid-crystalline phases of phosphatidylethanolamine bilayers

Solid-state deuterium and carbon-13 nuclear magnetic resonance (NMR) spectra have been used to study the molecular dynamics and conformation of dipalmitoylphosphatidylethanolamine (DPPE) in both the gel (L beta) and liquid-crystalline (L alpha) phases. For this purpose DPPE was labeled with 13C in the carbonyl group of the sn-2 chain and with 2H at three different positions--4, 8, and 12--of the sn-2 chain, at the 2 position of the glycerol backbone, and at the 1 position of the ethanolamine head group. The 13C carbonyl and 2H chain spectra indicate that in the gel phase the DPPE molecules are diffusing about their long axes at rates of 10(5)-10(6) s-1 and the acyl chains are in an approximately all-trans conformation. The glycerol backbone spectra suggest that the backbone is in a gauche conformation in the gel state, rather than a trans conformation such as found in single crystals. The head group spectra in the gel phase are broad, featureless lines of about 20-kHz width. At the L beta leads to L alpha phase transition several changes take place. As is well-known, the chains disorder, and fast long-axis rotational diffusion begins, which results in the sharp, axially symmetric L alpha phase 2H spectra, which are a factor of 2 narrower than those observed in the L beta phase. The head group spectra also sharpen substantially at the transition, although their total width remains approximately constant. The invariance of the spectral width suggests that the average head group conformation is similar in both phases. However, the sharper spectra seen in the L alpha phase indicate that the rates of the head group motions in this phase are at least 3 orders of magnitude faster than those in the L beta phase. Thirdly, the 2H spectra of the glycerol backbone labeled DPPE narrow by a factor of about 4, and we believe this is due to a conformational change in this region of the molecule. Consistent with this interpretation is the fact that the powder pattern exhibited by the sn-2 13C = O in the L beta phase collapses to an isotropic-like line at the phase transition.