Formation of cubic phases from large unilamellar vesicles of dioleoylphosphatidylglycerol/monoolein membranes induced by low concentrations of Ca2+

We developed a new method for the transformation of large unilamellar vesicles (LUVs) into the cubic phase. We found that the addition of low concentrations of Ca2+ to suspensions of multilamellar vesicles (MLVs) of membranes of monoolein (MO) and dioleoylphosphatidylglycerol (DOPG) mixtures I DOPG/MO) changed their L. phase to the cubic phases. For instance, the addition of 15-25 mM Ca2+ to.30%-DOPG/ 70%-MO-MLVs induced the Q(229) phase, whereas the addition of >= 28 mM Ca2+ induced the Q(224) phase. LUVs of DOPG/MO membranes containing >= 25 mol% DOPG were prepared easily. Low concentrations of Ca2+ transformed these LUVs in excess buffer into the Q(224) or the Q(229) phase, depending on the Ca2+ concentration. For example, 15 and 50 mM Ca2+ induced the Q(224) and Q(229) phase in the 30%-DOPG/ 70%-MO-LUVs at 25 degrees C, respectively. This finding is the first demonstration or transformation of LLTVs of lipid membranes into the cubic phase under excess water condition.