Characterization by infrared spectroscopy of the bilayer to nonbilayer phase transition of phosphatidylethanolamines

A Fourier-transform infrared spectroscopic study of the thermotropic behavior of egg yolk phosphatidylethanolamines is reported. Two phase changes were monitored, the gel to liquid-crystalline acyl chain melting transition, centered at 12 degrees C, and a transition from the liquid-crystalline to be inverted hexagonal phase, centered at 28 degrees C. It is demonstrated that the gel to liquid-crystalline phase transition results in a large increase in the conformational disorder of the acyl chains in the bilayer and that the nonbilayer phase contains a still higher degree of conformational disorder. It is shown that the transition to the inverted hexagonal phase is promoted by highly unsaturated acyl chains. A model is developed for the bilayer to nonbilayer phase transition in which it is proposed that the driving force which triggers this phase transition is the introduction of a degree of conformational disorder so high that the integrity of the bilayer surface can no longer be maintained, due to the volume requirements of the acyl chains. A number of previously reported data are rationalized in terms of this hypothesis.