A proton NMR moment study of the gel and liquid-crystalline phases of dipalmitoyl phosphatidylcholine.

Measurements of the proton second and fourth moments have been undertaken for multilamellar dispersions, in excess water, of protiated and chain-perdeuterated dipalmitoyl phosphatidylcholine (DPPC) in the temperature range -20 to 50 degrees C. The comparison of the measured moment with the rigid lattice M2 and the calculated second moment values in the presence of certain motions gives insight into the dynamic structure of the methylene chains of DPPC. This study demonstrates that at -15 degrees C there is still a significant amount of methylene chain motion or disorder in DPPC. At 35 degrees C the moment values indicate that the methylene chains are not in the fully extended all-trans conformation and they may also be rotationally disordered. At the pretransition there is a decrease in magnitude of the proton second moments, which can be accounted for by an increase in the lateral diffusion rate to a value greater than 10(-11) cm2/s. This work suggests that at temperatures just below the main transition the chain conformation is considerably different from the common model structure (P beta') in which the chains are fully extended. In the liquid-crystalline phase the proton moment data are in agreement with data from other techniques on the liquid-like nature of the methylene chains. It is demonstrated how the values of the moment ratio M4r/(M2r)2, which are relatively constant within each phase, can be used to calculate the molar fractions of coexisting liquid-crystalline and gel-phase phospholipid at temperatures near the main transition.