Head Group Modulation of Membrane Fluidity in Sonicated Phospholipid Dispersions

The mobility of the polar head group, as well as of the hydrocarbon region, of phosphatidylethanolamine (PE) molecules arranged in sonicated dispersions is markedly dependent on the charge of the ethanolamine head group: there is more molecular motion when PE is negatively charged than at pD levels where it is zwitterionic. At physiological values of pD, the molecular motions of PE are more restricted than are those of phosphatidylcholine (PC). These results were obtained by preparing, at various pD values, sonicated dispersions of PE, of PC, and of equimolar mixtures of PC and PE, and by studying the proton and phosphorus magnetic resonance spectra of these dispersions. At pD levels below the pK of the head group amine of PE the spectra are broader and of a lower intensity than at pD levels above the pK. In addition, we found that the PE and PC molecules are symmetrically distributed between the inner and outer surface of the singleshelled cosonicated PC: PE vesicles buffered at pD 11.0. This finding was obtained by exploiting the effects of the paramagnetic ions Fe(CN)₆^3- and Mn²⁺ on the proton and phosphorus magnetic resonance spectra. At the concentrations employed, these ions do not permeate the vesicles, thus permitting a distinction between the signals of PE and PC which originate on the outer and on the inner surfaces. The observed bilayer symmetry and head group modulation of the fluidity of sonicated PE and cosonicated PC:PE dispersions are discussed in the context of the structure and fluidity of biological membranes.