Phase equilibria, molecular conformation, and dynamics in phosphatidylcholine/phosphatidylethanolamine bilayers.

Solid-state 13C and 2H NMR experiments have been used to examine the phase equilibria and dynamic structure of binary mixtures of dipalmitoylphosphatidylcholine (DPPC) and dipalmitoylphosphatidylethanolamine (DPPE). The experiments rely on changes in the 13C and 2H spectra of sn-2 13C = O labeled and 2H chain and head group labeled lipids at phase boundaries. In particular, broad powder patterns are observed in the gel state, and these patterns narrow dramatically in the liquid-crystalline phase. In the two-phase region a superposition of the gel-phase and liquid-crystalline-phase spectra is observed, which results from the presence of large domains in the DPPC/DPPE system. The appearance of the liquid-crystalline component and the disappearance of the gel component permit the solidus and liquidus curves of the phase diagram to be located accurately. Furthermore, a comparison of the temperature dependence of the 13C and 2H spectra shows the phase transition mechanism to be a function of composition. This result, together with other evidence, supports the hypothesis that the gel-state lattice changes from the L beta or P beta to the L beta configuration with increasing DPPE content. Finally, a detailed examination of the composition dependence of the spectra provides evidence for nonideal mixing.