Thermal transitions in dimyristoylphosphatidylcholine foam bilayers.

Thermal transitions in the system dimyristoylphosphatidylcholine/water/ethanol/sodium chloride were studied in the temperature range 10-31 degrees C. The water-ethanol dispersions were investigated by differential scanning calorimetry and the foam bilayers by the microinterferometric method for investigation of thin liquid films. Calorimetry showed that an increase in ethanol content (up to 47.5 vol.%-the concentration used in the experiments with foam bilayers) did not significantly influence the temperature of the main phase transition and led to the disappearance of the pretransition. The microinterferometric study of the foam bilayer thickness showed that there were two thermal transitions-at 13 and 23 degrees C. An Arrhenius type dependence was obtained for the critical concentration of dimyristoylphosphatidyl-choline (DMPC) in the solution, which was necessary for the formation of the foam bilayer. A steep change in the slope of the linearized Arrhenius dependence was found at 23 degrees C. Values of the binding energy of a DMPC molecule in the foam bilayers were calculated using the hole-nucleation theory of stability and permeability of bilayers. It was proved that the phase transition at 23 degrees C was due to melting of the hydrocarbon tails of phospholipid molecules. The low-temperature phase transition was assumed to be due to a change in the tilt of the hydrocarbon tails. These experiments demonstrate for the first time the occurrence of phase transitions in foam bilayers.