Phase polymorphism, molecular interactions, and miscibility of binary mixtures of...

The phase diagram of hydrated binary mixtures of dimyristoyl-N-biotinylphosphatidylethanolamine with dimyristoylphosphatidylcholine in 1 M NaCl has been established by differential scanning calorimetry. Identification of the structures of the phases involved has been made by using X-ray diffraction, spin label ESR spectroscopy, and 31P NMR spectroscopy. On the composition axis, the phase diagram is divided into three regions corresponding to formation of compounds in the gel phase with biotinyl lipid to phosphatidylcholine stoichiometries of approximately 1:1 and 1:3 mol/mol. For the first two regions (up to 75 mol % phosphatidylcholine), the lipids in the gel phase have interdigitated chains (L beta i), whereas in the third region the gel phase is not interdigitated (L beta' or L beta). For the first region (up to 50 mol % phosphatidylcholine), the fluid phase is of the novel isotropic type (IMI) composed of aggregated normal micelles that is characteristic of shorter chainlength biotinylated lipids [Swamy, M.J., Würz, U., & Marsh, D. (1993) Biochemistry 32, 9960-9967], whereas for the other two regions a normal fluid lamellar (L alpha) phase obtains. The equimolar mixture, which lies at a stoichiometric phase boundary, melts isothermally and then undergoes a transition from the isotropic IMI structure to the lamellar L alpha structure with increasing temperature in the fluid phase.