Influence of retinoids on phosphatidylethanolamine lipid polymorphism

The interaction of all-trans-retinoic acid and all-trans-retinol with dielaidoylphosphatidylethanolamine has been studied by differential scanning calorimetry and 31P-NMR spectroscopy. Increasing concentrations of all-trans-retinoic acid up to a mol fraction of 0.09 were found to induce shifts to lower temperatures of both the L beta to L alpha and L alpha to hexagonal-HII phase transitions, with a slight decrease in the enthalpy change of the transitions. At higher concentrations no further effects on the transitions were observed, and this is interpreted as indicative of a limited miscibility of retinoic acid with the phospholipid. 31P-NMR spectroscopy confirmed that the L alpha to hexagonal-HII phase transition was shifted to lower temperatures in the presence of retinoic acid. On the other hand increasing concentrations of all-trans-retinol up to a mol fraction of 0.166, induced a progressive shift of the L beta to L alpha and the L alpha to hexagonal-HII phase transitions to lower temperatures. At higher concentrations the main gel to liquid-crystalline phase transition was further displaced to lower temperatures and the lamellar to hexagonal-HII phase transition was not observed in the thermograms. 31P-NMR spectroscopy indicated that retinol was able of inducing the phospholipid to adopt the hexagonal-HII phase at temperatures even below the main gel to liquid-crystalline phase transition temperature of the pure phospholipid.