Kinetics of the lamellar and hexagonal phase transitions in phosphatidylethanolamine. Time-resolved x-ray diffraction study using a microwave-induced temperature jump.

Abstract

The kinetics of the thermotropic lamellar gel (L)/lamellar liquid crystal (La) and La/inverted hexagonal (H,,) phase transitions in fully hydrated dihexadecylphosphatidylethanolamine (DHPE) have been studied. Measurements were made by using time-resolved x-ray diffraction (TRXRD) to monitor progress of the transitions. In these studies microwave energy Ot 2.5 GHz was used to increase the sample temperature rapidly and uniformly through the phase transition regions. The L.,/La, and La/Hi1 transitions of DHPE were examined under active microwave heating and passive cooling. The transitions were found to be repeatable and reversible, and to have an upper bound on the time required to complete the transition of <3 s. Regardless of the direction of the transition, both phase transitions appeared to be two-state with no accumulation of intermediates to within the sensitivity limits of the TRXRD method. The rate and amplitude of the temperature jump can be controlled by regulating microwave radiation input power. A temperature jump rate of 290C/s was obtained at a final microwave power setting of 120 W. Comparisons between previously reported fluid flow (Caffrey, M. 1985. Biochemistry. 24:4826-4844) and microwave heating studies suggest that the determination of limiting transit times will require faster heating.