Spin‐Label Characterisation of the Lamellar‐to‐Hexagonal (HII) Phase Transition in Egg Phosphatidylethanolamine Aqueous Dispersions

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Abstract
The thermotropic behavior of egg yolk phosphatidylethanolamine [PE] dispersions in excess aqueous phase was investigated by spin label ESR spectroscopy and differential thermal analysis. PE isomers spin-labeled at 6 different positions along the acyl chain, and steroid spin labels, indicate both gel-fluid lamellar and lamellar-reverse hexagonal (HII) phase transitions, in agreement with complementary calorimetric studies. Analysis of spin label data shows that the transition to theHII phase is accompanied by an increase in conformational freedom of the acyl chain, more pronounced towards the methyl terminus, and representing an increase in the population of gauche isomers which can only be accomodated by a transition to the non-bilayer phase. Raising the bulk pH to, and above, pH 8.5 results in stabilization of the bilayer phase and no transition to the HII phase is observed. PE spin labels also indicate a polarity profile which is characteristic of each phase.