Sugar-induced stabilization of the monoolein Pn3m bicontinuous cubic phaseduring dehydration - art. no. 040902

To explore the molecular mechanism of the protective function of sugars on cubic lipidic systems, the mesomorphic properties of the monoolein-water sy stem, dehydrated in the presence of a series of sugars, have been studied b y osmotic stress experiments. Two bicontinuous inverse cubic structures (Pn 3m and Ia3d) and a lamellar L-alpha phase form under dehydration in pure wa ter. In sugar solutions, the Pn3m phase shows an extraordinary stability: a s a function of sugar concentration, the lattice parameter decreases to ver y low values, but no phase transitions occur. Instead, the Pn3m to Ia3d pha se transition is obtained by equilibrating the lipid phase with aqueous pol ymer 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 s how that the sugar exerts an additional stabilization on both the cubic pha ses. 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 m olecular splay mainly stabilize the Pn3m phase and prevent the transition t o the Ia3d phase on dehydration.