The role of calcium in membrane condensation and spontaneous curvature variations in model lipidic systems

In this study, the dynamical behaviour of calcium-induced disordered to well-ordered structural transitions has been investigated by time-resolved synchrotron small-angle X-ray scattering (SAXS) in the milliseconds to seconds range. The in situ monitoring of the formed non-equilibrium self-assembled structures was achieved by the successful combination of synchrotron SAXS with stopped flow measurements. The effect of the rapid mixing of aqueous dispersions of dioleoylphosphatidyalglycerol (DOPG)/monoolein (MO) with low concentrations of Ca2+ ions is reported. Under static conditions and in the absence of Ca2+ ions, the evaluation of SAXS data for DOPG/MO aqueous dispersions prepared with three different DOPG/MO molar ratios indicates the formation of either a sponge-like L3 phase or uncorrelated bilayers. Clearly, the lipid composition plays a vital role in modulating the structural behaviour of these aqueous dispersions in the absence and also in the presence of Ca2+ ions. The rapid-mixing experiments revealed that the fast and strong interactions of Ca2+ ions with the negatively charged DOPG/MO membranes triggers the transformation from the L3 phase or the uncorrelated bilayers to the well-ordered dehydrated Lα phase or to inverted type bicontinuous cubic phases, V2, with either a symmetry of Pn3m or Im3m. Additionally, we recently reported (A. Yaghmur, P. Laggner, B. Sartori and M. Rappolt, PLoS ONE, 2008, 3, e2072) that low concentrations of Ca2+ ions trigger the formation of the inverted type hexagonal (H2) phase in DOPG/MO aqueous dispersions with a molar DOPG/MO ratio of 30/70. These are also temperature-sensitive structural transitions. Intriguingly, the strong association of Ca2+ ions with the negatively charged DOPG/MO membranes leads to fast re-organization of the two lipids and simultaneously induces fast tuning of the curvature.