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

Abstract

Solid-state 2H and 13C NMR spectra were 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 3 different positions.sbd.4, 8, and 12.sbd.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 in DPPE molecules are diffusing about their long axes at rates of 105-106 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. .fwdarw. L.alpha. phase transition several changes take plase. As is well-known, the chains disorganize and fast long-axis rotational diffusion begins; this 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 became narrower by a factor of about 4, which may be 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.dbd.O in the L.beta. phase collapses to an isotropic-like line at the phase transition.