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The factors which govern the stability of lyotropic amphiphilic nematic phases are delineated and then used to design mesogens which give rise to stable N, and N, phases on dissolution in water. The synthesis and phase behaviour of novel discoid amphiphiles, designed to form N, phases, are described.

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2H NMR methods have been used to study how bilayer-forming phospholipids interact with each other at the membrane surface. Aqueous dispersions of dimyristoyl-sn-phosphatidylcholine (Myr2-PtdCho), dimyristoyl-sn-phosphatidylethanolamine (Myr2-PtdEtn), and dimyristoyl-sn-phosphatidyl-3-glycerol, specifically deuterated at different positions in their headgroups, give well-resolved 2H NMR powder spectra. These spectra are characteristic of a lipid bilayer with quadrupole splittings that are sensitive to the amplitude of headgroup motion. In binary mixed bilayers of deuterated lipids with an unlabeled component, all parts of the deuterated headgroup monitor the presence of the second lipid from changes in the measured quadrupole splittings. The headgroups of the charged lipids, dimyristoyl-sn-phosphatidylserine and dimyristoyl-mn-phosphatidyl-3-glycerol, interact to the largest extent with the choline moiety of Myr2-PtdCho and the ethanolamine moiety of Myr2-PtdEtn, whereas a somewhat smaller but still marked alteration in headgroup motion was observed in Myr2-PtdCho/Myr2-PtdEtn mixtures. The large changes in the deuterium quadrupole splittings for the zwitterionic lipids after addition of a charged lipid suggest that either a strong perturbation in the hydrogen bonding occurs or changes take place in the water structure at the membrane surface, or possibly both.The lipid component of biological membranes is usually composed of a limited number of phospholipid types, which are identified by the chemical nature of their polar headgroup. Both the conformational and motional properties of lipid headgroups are well characterized for model membranes containing single phospholipid species, mainly on the basis of proton, deuterium, and phosphorus NMR and neutron diffraction studies (1)(2)(3)(4)(5). In particular, 2H NMR has been used to investigate conformational changes induced by ions, cholesterol, and anesthetics (6-8). However, limited information is available on lipid structure at membrane surfaces if more than one naturally occurring phospholipid type is present (9-12).Here we present results from 2H;NMR studies of fully hydrated bilayers of phospholipids deuterated in their headgroups, which suggest that strong interactions occur at the membrane surface between different lipids in binary mixed bilayers. In particular, phospholipids that carry a residual negative charge significantly change the measured quadrupole splittings for zwitterionic lipids. These observations imply that headgroup reorientations and motional amplitude changes can be induced in mixed lipid bilayers by alterations in the structured water layer at the membrane surface. MATERIALS AND METHODSMaterials. The following specifically deuterated dimyristoyl derivatives of sn-phosphatidylcholine (Myr2-PtdCho), phosphatidyl-3-glycerol (Myr2-PtdGro), and phosphatidylethanolamine (Myr2-PtdEtn) were synthesized as described (9): R-O-C2H2-C2Hr-N+(CH3)3 = Myr2-PtdCho-d4; R-O-CH2-CH2-N+(C2H3)3 = Myr2-PtdCho-d9; R-O-C2H2-C2Hg-N+H3 = Myr2-PtdEtn-d4; and ...

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