Rigidity and spontaneous curvature of lipidic monolayers in the presence of trehalose: a measurement in the DOPE inverted hexagonal phase

Trehalose is a sugar which plays an important protectant role in organisms against damage due to dehydration. To explore the basic molecular mechanism which governs the protective function exerted on lipid membranes, X-ray diffraction and osmotic stress experiments have been performed on l--dioleoyl-phosphatidyl-ethanolamine (DOPE) in trehalose solutions of different concentrations. In pure water, DOPE forms an inverted hexagonal (H_II) phase; in sugar solutions, a strong dehydration, which induces a large reduction of the H_II lattice parameter, has been detected, but nevertheless no phase transitions occur. Structural data, directly obtained from reconstructed electron density maps, show that the bending of the lipid monolayer induced by the sugar is coupled to changes in the DOPE molecular shape. By osmotic stress, the work required to dehydrate the monolayer has been obtained and the overall free energy described as a function of trehalose concentration. Three results should be stressed: (1) dehydration experiments performed in the presence of sugar demonstrate that the protective effect cannot be purely osmotic; (2) the pivotal surface, that location on the molecule whose area is invariant upon isothermal bending, has been analyzed by two different methods: the approach by Rand and co-workers and the approach by Templer and co-workers; in both cases its presence along the DOPE molecule has been revealed and its position estimated; (3) the spontaneous radius of curvature and the rigidity constant of the lipid monolayer, measured at the pivotal plane, changes from 3.06 nm (in pure water) to 2.82 nm (in 1.4 M trehalose), and from 0.55×10^–19 to 0.74×10^–19 J, respectively. We assume that these modifications are related to direct interactions between trehalose and DOPE that alter the interface geometry, reducing the repulsion between the polar heads. However, the increased bending rigidity also accounts for the changes of the property of the aqueous compartment, reflecting the rigidity and stiffness of the sugar matrix around and inside the lipid phase.