1: Biophys J  1998 Jun;74(6):2973-82 

Formation of giant liposomes promoted by divalent cations: critical role of
electrostatic repulsion.

Akashi K, Miyata H, Itoh H, Kinosita K Jr.

Department of Physics, Faculty of Science and Technology, Keio University,
Yokohama, Japan.

Spontaneous formation of giant unilamellar liposomes in a gentle hydration
process, as well as the adhesion energy between liposomal membranes, has been
found to be dependent on the concentration of divalent alkali cations, Ca2+ or
Mg2+, in the medium. With electrically neutral phosphatidylcholine (PC), Ca2+ or
Mg2+ at 1-30 mM greatly promoted liposome formation compared to low yields in
nonelectrolyte or potassium chloride solutions. When negatively charged
phosphatidylglycerol (PG) was mixed at 10%, the yield was high in
nonelectrolytes but liposomes did not form at 3-10 mM CaCl2. In the adhesion
test with micropipette manipulation, liposomal membranes adhered to each other
only in a certain range of CaCl2 concentrations, which agreed with the range
where liposome did not form. The adhesion range shifted to higher Ca2+
concentrations as the amount of PG was increased. These results indicate that
the divalent cations bind to and add positive charges to the lipids, and that
membranes are separated and stabilized in the form of unilamellar liposomes when
net charges on the membranes produce large enough electrostatic repulsion. Under
the assumption that the maximum of adhesion energy within an adhesive range
corresponds to exact charge neutralization by added Ca2+, association constants
of PC and PG for Ca2+ were estimated at 7.3 M(-1) and 86 M(-1), respectively, in
good agreement with literature values.

PMID: 9635751 [PubMed - indexed for MEDLINE]