Ion-induced changes in head group conformation of lecithin bilayers

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 surface^1-4. 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.