13C CP-MAS study of the gel phases of 1,2-dipalmitoylphosphatidylcholine

The multilamellar liposomes of the racemic 1,2-dipalmitoylphosphatidylcholine (DL-DPPC) existing in the various gel phases were investigated by means of the cross-polarization/magic angle spinning (CP-MAS) 13C-NMR. The intensity and the width of the 13C-NMR signals were found to depend to a large extent on temperature. In the metastable gel phase (L beta) signals from all carbon atoms are apparent, with the signals arising from the glycerol backbone significantly broader than those of the choline function. The signal from C-2 of the glycerol backbone undergoes additional broadening between 298 K and 307 K, and at 307 K its coalescence is observed. In the P beta' phase all carbon atoms give rise to relatively sharp separate resonance lines. In the liquid crystalline phase (L alpha) the signals from the choline function and from the terminal methyl groups of hydrocarbon are not observed in the spectrum. The recorded spectral changes are explained in terms of the rate of motional processes occurring in the lipid bilayer in different phases.