Slow change in the repeat period of multilamellar dimyristoyl phosphatidylcholine accompanied by thermal phase transition.

The slow change of the repeat period of multilamellar structures accompanied by a thermal phase transition in dimyristoyl phosphatidylcholine containing 50% water was measured by a small-angle X-ray diffraction method employing the temperature-jump technique. The repeat period immediately after the application of a temperature jump from 25 degrees C (fluid state, L alpha) to 22.5 degrees C (crystalline state, P' beta) was observed to be about 1 A larger than that at the steady state (65.1 A), and was then followed by a decrease to a steady value with a relaxation time of 70 min. The slow decrease in the repeat period seems to be due to the transformation from loosely stacked to closely stacked multilamellar structures, the looseness being caused by the formation of contracted domains in each lamella at an early stage of the phase transition from the fluid to the crystalline state.