PHASE-TRANSITION CHARACTERIZATION IN LOW HYDRATION PHASES OF DILAUROYLPHOSPHATIDYLCHOLINE

Phase transitions in slightly hydrated samples of dilauroylphosphatidylcholine (DLPC) were studied using fixed temperature and real time X-ray diffraction. Only a single transition was observed involving, in all cases, the crystalline (Lc) and liquid crystalline (Lα) bilayer phases. Real time X-ray diffraction measurements indicated that this transition proceeded via a two-state or first-order transition mechanism involving both mesophases and acyl chain packings. Transition temperatures, structural parameters, and transition mechanisms obtained from real time X-ray diffraction measurements of DLPC bilayers in limited water samples were the same as those derived from fixed temperature X-ray experiments comparing samples of the same lipid/water concentration using stepwise temperature changes. These results differ from previous reports on slightly hydrated phospholipids, which show different transition mechanisms for the gel to liquid crystalline phase transition, which depended on whether real time or static X-ray measurements were used. © 1990.