A comparison of the headgroup conformation and dynamics in synthetic analogs of dipalmitoylphosphatidylcholine.

14N-NMR spectra and relaxation times for dipalmitoylphosphatidylcholine and three analogs were obtained in both the liquid crystal and gel phases. The analogs either changed the PO-4 to N+ (CH3)3 distance (P-N) within the headgroup by increasing the number of CH2 groups from two in the phosphocholine headgroup (PN-2) to six in the phospho-(N',N',N'-trimethyl)hexanolamine headgroup (PN-6), or replaced the ester linkages to the hydrocarbon chains with either linkages. 31P-NMR spectra were obtained for the four samples in the liquid-crystal phase. (1) The 14N- and 31P-NMR spectra and 14N relaxation times all indicate that increasing the P-N distance within the headgroup causes changes in both the average orientation of the C-N bond and its dynamics. (2) The 14N-NMR spectra provide evidence for a change in orientational order of the headgroup as a result of changing the linkage to the acyl chains. On the other hand, the relaxation time measurements indicate that the molecular motion for the headgroup is independent of the type of linkage. (3) The thermal behaviour of the four samples is clearly reflected in the 14N-NMR spectra. The second moments of the spectra show distinct changes at each of the phase transitions. (4) The 14N-NMR spectra show that the average conformation of the headgroups is not significantly altered by the main phase transition. For the PN-2 samples, T2e, the decay of the quadrupolar echo, decreases discontinuously in the P beta, phase, which is evidence for a possible exchange process between two molecular states within this phase.