THE structure and orientation of the phosphocholine polar group in lecithin-containing membranes has been discussed extensively. Studies of egg-yolk lecithin and 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC), using neutron diffraction and 2H and 31P nuclear magnetic resonance (NMR), have led to the conclusion that the phosphocholine dipole is orientated parallel to the membrane surface1-4. On the other hand, 1H and 31P NMR spectra of sonicated DPPC vesicles in the presence of trivalent ions suggested that the choline group is extended approximately perpendicular5 or folded parallel6 to the bilayer surface. We have, therefore, repeated our previous 2H and 31P NMR measurements in the presence of trivalent ions and report here that the addition of these shift reagents induces large changes in the spectral parameters, that is, the 2H quadrupole splittings and the 31P chemical shift anisotropy. These observations can only be interpreted in terms of specific ion-induced changes in the choline head group conformation of DPPC.
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