A thermodynamic model for the formation of the ripple phase of hydrated phospholipid bilayers is proposed based on the idea of a spontaneous local curvature free energy. It is suggested that this arises from electrostatic coupling between water dipoles and the dipolar head groups of the lipid molecules. A bilayer profile for the ripple phase is calculated using the model and agrees qualitatively with xhyphen;ray structure measurements. The premelting transition is discussed in terms of a repulsive coupling between the spontaneous curvature free energy and the free energy of ordering of the head group array in theLbgr;prime; phase. This leads to a mechanism for the premelting transition based on the competition between head group ordering, which softens with increasing temperature, and the free energy of ripple formation, which is favored by an increase in the bilayer polarisability when the head groups become disordered.
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