Sugar-induced stabilization of the monoolein Pn3m bicontinuous cubic phase during dehydration

To explore the molecular mechanism of the protective function of sugars on cubic lipidic systems, the mesomorphic properties of the monoolein-water system, dehydrated in the presence of a series of sugars, have been studied by osmotic stress experiments. Two bicontinuous inverse cubic structures (Pn3m and Ia3d) and a lamellar L(alpha) phase form under dehydration in pure water. In sugar solutions, the Pn3m phase shows an extraordinary stability: as a function of sugar concentration, the lattice parameter decreases to very low values, but no phase transitions occur. Instead, the Pn3m to Ia3d phase transition is obtained by equilibrating the lipid phase with aqueous polymer solutions of increasing osmotic pressure. As a result, the pressure at which the phase transition occurs strongly depends on sugar concentration. The free-energy curves obtained from the osmotic-pressure unit-cell data show that the sugar exerts an additional stabilization on both the cubic phases. The analysis of the structural parameters indicates that sugars alter the interface geometry. We suggest that a consequent release of stretching contributions in the chain packing or a reduction of the inhomogeneity in molecular splay mainly stabilize the Pn3m phase and prevent the transition to the Ia3d phase on dehydration.