PHASE POLYMORPHISM, MOLECULAR-INTERACTIONS, AND MISCIBILITY OF BINARY-MIXTURES OF DIMYRISTOYL-N-BIOTINYLPHOSPHATIDYLETHANOLAMINE WITH DIMYRISTOYLPHOSPHATIDYLCHOLINE

The phase diagram of hydrated binary mixtures of dimyristoyl-N-biotiny lphosphatidylethanolamine with dimyristoylphosphatidylcholine in 1 M N aCl has been established by differential scanning calorimetry. Identif ication of the structures of the phases involved has been made by usin g X-ray diffraction, spin label ESR spectroscopy, and P-31 NMR spectro scopy. On the composition axis, the phase diagram is divided into thre e regions corresponding to formation of compounds in the gel phase wit h biotinyl lipid to phosphatidylcholine stoichiometries of approximate ly 1:1 and 1:3 mol/mol. For the first two regions (up to 75 mol % phos phatidylcholine), the lipids in the gel phase have interdigitated chai ns (L(beta)(i)), whereas in the third region the gel phase is not inte rdigitated (L(beta') or L(beta)) For the first region (up to 50 mol % phosphatidylcholine), the fluid phase is of the novel isotropic type ( I-MI,) composed of aggregated normal micelles that is characteristic of shorter chainlength biotinylated lipids [Swamy, M. J., Wurz, U., an d Marsh, D. (1993) Biochemistry 32, 9960-9967], whereas for the other two regions a normal fluid lamellar (L(alpha)) phase obtains. The equi molar mixture, which lies at a stoichiometric phase boundary, melts is othermally and then undergoes a transition from the isotropic I-MI str ucture to the lamellar L(alpha) structure with increasing temperature in the fluid phase.