Hydrocarbon chains dominate coupling and phase coexistence in bilayers of natural phosphatidylcholines and sphingomyelins.

The structure and thermotropic phase behaviour of aqueous dispersions of egg phosphatidylcholine, egg sphingomyelin, bovine brain sphingomyelin and binary mixtures of phosphatidylcholine and sphingomyelins have been examined by synchrotron X-ray diffraction methods. Small-angle lamellar Bragg peaks and wide-angle X-ray scattering bands have been subjected to peak fitting procedures to identify coexisting gel and fluid as well as fluid-fluid bilayer structures. Molecular species of egg phosphatidylcholine exhibit fluid-fluid immiscibility throughout heating scans from 20 degrees to 50 degrees C. Egg and brain sphingomyelins exhibit gel-fluid bilayer coexistence at temperatures below the main phase transition temperature and fluid-fluid phase coexistence at higher temperatures. Binary mixtures of equimolar proportions of egg phosphatidylcholine and either of the sphingomyelins show gel-fluid phase coexistence at temperatures below the gel phase transition temperature of the respective sphingomyelin. Binary mixtures containing egg sphingomyelin show fluid-fluid immiscibility at all temperatures of the heating scans whereas the fluid phase of mixtures comprising brain sphingomyelin are apparently miscible at all temperatures. An analysis of binary mixtures containing egg sphingomyelin and egg phosphatidylcholine in molar ratios 50:50, 67:33 and 83:17 at 50 degrees C to identify the composition of the lamellar phases indicated that the two phospholipids are immiscible in bilayers in the fluid phase. The results are discussed in terms of the role of intermolecular hydrogen bonds and hydrocarbon chain composition of sphingomyelins in maintaining coupling across fluid bilayers.