Pressure induced cubic-to-cubic phase transition in monoolein hydrated system

Synchrotron X-ray diffraction has been used to investigate structure, stabi lity, and transformation of the Pn3m bicontinuous cubic phase in the monool ein-water system under hydrostatic pressure. As a first result, it appears that the full-hydration properties of monoolein are strongly related to the pressure. Moreover, the experimental results show the occurrence of a Pn3m to Ia3d cubic phase transition when the mechanical pressure increases to 1 -1.2 kbar, depending on the water concentration. The underlying mechanism f or the phase transition has been then explored in searching for relationshi ps between the structural parameters derived from the two cubic phases. The emerging picture is a change in the basic geometrical shape of the monoole in molecule during compression. Moreover, the analysis of the position of t he pivotal surface indicates that the interface is bending and stretching s imultaneously as a function of pressure. Because the lipid concentration is rather low and the external pressure increases the cell sizes, thus reduci ng the principal curvatures, a tentative analysis of the pressure effects o n the energetics of these structures has been exploited. A simple theoretic al model based on curvature elastic contributions has been used: calculatio ns show that increasing the pressure the spontaneous curvature Ho of the mo noolein tends to zero, whereas the ratio between the monolayer saddle splay modulus and the monolayer splay modulus k(G)/k increases td 1. Moreover, t he curvature elastic energy appears to reduce progressively as a function o f pressure, indicating that in these conditions, the curvature elasticity d oes not dominate the total free energy.