Head group modulation of membrane fluidity in sonicated phospholipid dispersions

The mobility of the polar head group, as well as of the ~ydrocarbon region, OfPhospHatidYl~thanolamine (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 nega~ively charged than at pO levels where it is zwitterionic.At physiological values of pO, the molecular motions of PE are more restricted than are those of phosphatidylcholine (PC).These results were obtained by preparing, at various pO values, sonicated disperions of PE, of PC, and of equimolar mixtures of PC and PE, and by studying the proton and phosphorus 11Ia~ne'tic resonance spectru of these dispersions.At pO levels below the pK of the head group amine of PE the spectra are broader and of a lower intensity than at pO levels above the pK.In addition, we found that the PE and PC molecules are s,Yll1l1etri cally di s tributed between the inner and outer surface of the single-shelled co-sonicated PC:PE vesicles buffered at pO 11.0.This finding was obtained by exploiting the effects of the paramagnetic ions Fe(CN)6-3 and Mn+2 on the proton and phosphorus magnetic resonance spectra.I At the concentrations employed, these ions do not permeate the vesicles, • I thus permitting a distinction between the signals of PE and PC which origi-.nate on the outer and on the inner surfaces.The observed bilayer symmetry and head group modulation of the fluidity of sonicated PE and co-sonicated .PC:PE dispersions are disbussed in the context of the structure and fluidity of biological membranes.