Dependence of the conformation of the polar head groups of phosphatidylcholine on its packing in bilayers. Nuclear magnetic resonance studies on the effect of the binding of lanthanide ions

Proton magnetic resonance spectra of vesicles of various sizes composed of egg phosphatidylcholine (PC) with varying concentrations of cholesterol differed in the apparent line width of the signal of the methylene protons of PC (delta v1/2). They also varied in the extent of lanthanide-induced shifts of the 31P and 1H NMR signals of the corresponding nuclei of the polar head groups located on the outer surface of the vesicles (delta delta). The differences in the lanthanide-induced shifts of the 31P signals are fully accounted for by the ratio between the externally added lanthanide and the number of PC head groups available for interaction with the lanthanide ions. This was not the case ofr the changes in the 1H NMR spectra. Here delta delta decreased with increasing delta v1/2, suggesting that the packing of the PC paraffinic chaings in the bilayer affects the conformation of the polar head groups; tightening of the packing probably results in a more extended conformation of the head groups. This conclusion is also supported by the larger effect lanthanides have on the 1H chemical shift of the choline head groups on the outer surface of small unilamellar vesicles as compared to groups on the inner, tighter packed layer.