DIRECT DETERMINATION OF HYDRATION IN THE LAMELLAR TO INVERTED HEXAGONAL TRANSITION OF PHOSPHATIDYLETHANOLAMINE

The bound water associated with phosphatidylethanolamine (PtdEtn) in t he lamellar and inverted hexagonal structures is determined directly. Bound water is considered as that water which is unavailable for solva tion of the polar solute sucrose. In the fluid lamellar (L(alpha)) sta te of dioleoyl PtdEtn (at 2 degrees C), 7.2 water molecules per phosph olipid are bound and unavailable as a solvent for sucrose. In the inve rted hexagonal structure (16 degrees C and 30 degrees C), 5.4 and 5.6 water molecules per phospholipid, respectively, are unavailable for so lvation. Similar results are obtained for egg PtdEtn (L alpha, 15 degr ees C, 7.5 water per Lipid; L alpha, 28 degrees C 6.9; H-Pi, 40 degree s C, 5.1). Weakly binding polar solutes(glycine and acetate) yield com parable trends that support a dehydration at the lamellar to inverted hexagonal phase transition of approximately 2 water molecules per PtdE tn. This is the first direct determination of the changes in hydration that occur in the lamellar to inverted hexagonal transition.