HYDRATION PROPERTIES OF LAMELLAR AND NON-LAMELLAR PHASES OF PHOSPHATIDYLCHOLINE AND PHOSPHATIDYLETHANOLAMINE

Two of the most common phospholipids in biological membranes are phosp hatidylcholine (PC) and phosphatidylethanolamine (PE). Over a wide ran ge of temperatures the PCs found in biological membranes form lamellar (bilayer) phases when dispersed in excess water, whereas PEs form eit her lamellar or hexagonal phases depending on their hydrocarbon chain composition. This paper details the hydration properties of lamellar a nd hexagonal phases formed by PCs and PEs, focusing on the energetics of hydration of these phases. For the hexagonal phase, the energy of b ending the lipid monolayer is a critical term, with other contribution s arising from the energies of hydrating the lipid headgroups and fill ing voids in the interstices in the hydrocarbon region. For the lamell ar phase of PC, the water content is determined by a balance between t he attractive van der Waals pressure and repulsive hydration and entro pic (steric) pressures. In the case of PE bilayers, recent experiments demonstrate the presence of an additional strong, short-range attract ive interaction, possibly due to hydrogen-bonded water interactions be tween N+H3 groups in one bilayer and the PO4- groups in the apposing b ilayer. This additional attractive pressure causes apposing PE bilayer s to adhere strongly and to imbibe considerably less water than PC bil ayers.