ANOMALOUS PHASE-TRANSITION IN DIPALMITOYLPHOSPHATIDYLETHANOLAMINE PALMITOYLPHOSPHATIDYLCHOLINE/WATER SYSTEM/

An anomalous phase transition with a marked rise in specific heat, the isobaric thermal expansion coefficient, and the compressibility coeff icient at 62.5 degrees C for an equimolar mixture of 1,2-dipalmitoyl-s n-glycero-3-phosphoethanolamine (DPPE) and 1-palmitoyl-sn-glycero-3-ph osphocholine (PLPC), in water (34 wt.%) has been shown by differential scanning calorimetry, scanning dilatometry and isothermal compressibi lity measurements. This transition occurs 15 degrees C above a first-o rder transition observed in the same system, P-31 and H-2 nuclear magn etic resonance results are consistent with the occurrence of 'defects' in the bilayer in the temperature range between the first and the ano malous phase transitions, It is proposed that conically, PLPC molecule s prefer regions with high curvature in the defective bilayer, while D PPE molecules are mostly confined to the flat regions of the bilayers. (C) 1998 Elsevier Science B.V.