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 Ca(2+) ions is reported. Under static conditions and in the absence of Ca(2+) 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 L(3) 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 Ca(2+) ions. The rapid-mixing experiments revealed that the fast and strong interactions of Ca(2+) ions with the negatively charged DOPG/MO membranes triggers the transformation from the L(3) phase or the uncorrelated bilayers to the well-ordered dehydrated L(α) phase or to inverted type bicontinuous cubic phases, V(2), 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 Ca(2+) ions trigger the formation of the inverted type hexagonal (H(2)) 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 Ca(2+) 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.
Department of Pharmaceutics and Analytical Chemistry, Faculty of Pharmaceutical Sciences, University of Copenhagen, Universitetsparken 2, DK-2100 Copenhagen, Denmark. aya@farma.ku.dk
Anan
A
Yaghmur
Barbara
B
Sartori
Michael
M
Rappolt
D002118:Calcium / Q000737:chemistry* / Q000502:physiology; D005989:Glycerides / Q000737:chemistry; D008958:Models, Molecular; D010715:Phosphatidylglycerols / Q000737:chemistry; D053838:Scattering, Small Angle; D013696:Temperature; D014961:X-Ray Diffraction
D005989:Glycerides; D010715:Phosphatidylglycerols; C051388:1,2-dioleoyl-sn-glycero-3-phosphoglycerol; C005953:monoolein; D002118:Calcium