Effects of calcium and neomycin on phase behavior of phospholipid bilayers

Calcium ion (Ca2+, 5 mM) caused a large upward shift in the transition temperature (Tc) of dipalmitoyl phosphatidylglycerol liposomes, apparently interacting with external and internal lipid in the bilayer. Neomycin (1 mM) caused only a small shift, apparently not penetrating the internal lipid domains. When liposomes were first incubated with Ca2+ followed by neomycin, the shift in Tc indicated displacement of Ca2+ by the drug. Liposomes of dipalmitoyl phosphatidylcholine or distearoyl phosphatidylcholine did not interact significantly with either cation. In mixed dipalmitoyl phosphatidylcholine-dipalmitoyl phosphatidylglycerol and distearoyl phosphatidylcholine-dipalmitoyl phosphatidylglycerol liposomes, addition of Ca2+ or neomycin resulted in phase separation.