BENDING, HYDRATION AND INTERSTITIAL ENERGIES QUANTITATIVELY ACCOUNT FOR THE HEXAGONAL-LAMELLAR-HEXAGONAL REENTRANT PHASE-TRANSITION IN DIOLEOYLPHOSPHATIDYLETHANOLAMINE

We have accounted for the unusual structural change wherein dioleoylph osphatidylethanolamine undergoes a hexagonal-lamellar-hexagonal transi tion sequence as the water content is reduced systematically. We descr ibe the role played by the energies of bending, hydration, voids in he xagonal interstices, and van der Waals interaction in this transition sequence. We have used the x-ray diffraction and osmotic stress experi ments on the two phases to derive the structural parameters and all of the force constants defining the energetics of the hexagonal and lame llar phases. We have calculated the chemical potentials of lipid and w ater in both phases and derived the phase diagram of the lipid with no free, adjustable parameters. The calculated temperature/osmotic stres s and temperature/composition diagrams quantitatively agree with exper iment. The reentrant transition appears to be driven by a delicate bal ance between the hydration energy in the lamellar phase and bending en ergy in the hexagonal phase, whereas the energy of voids in hexagonal interstices defines its energy scale and temperature range. Van der Wa als attraction between the bilayers in the lamellar phase does not app ear to be important in this transition.