Role of the position of unsaturation on the phase behavior and intrinsic curvature of phosphatidylethanolamines

The bilayer-to-hexagonal phase transition temperatures (T-H) of di-18:1(c) phosphatidylethanolamine with double bonds at positions 6, 9, and 11 are 37 degrees C, 8 degrees C, and 28 degrees C, respectively, as measured by differential scanning calorimetry and x-ray diffraction. Thus T-H exhibits a minimum when the C=C is around position 9, similar to what has been found for the gel-to-liquid crystalline phase transition temperature in other lipids. Factors that may contribute to the dependence of T-H on double bond position were studied by x-ray diffraction of the hexagonal phases in the presence and absence of added alkane: with or without the osmotic stress of polyethylene glycol, and over a wide temperature range, The lattice dimensions show that the intrinsic radius of lipid monolayer curvature increases as the double bond is moved toward the tail ends. A measure of the bending moduli of these lipid monolayers shows a higher value for the 9 position, and lower values for the other two, Consideration of the bilayer-to-hexagonal transition in terms of bending and interstitial energies provides a rationale for the relative values of T-H.