Ca2+-induced lateral phase separations in phosphatidic acid-phosphatidylcholine membranes

The effects of Ca²⁺ on phosphatidic acid-phosphatidylcholine membranes have been studied using phospholipid spin labels. ESR spectra of spin-labeled phosphatidic acid-phosphatidylcholine membranes and phosphatidic acid-spin-labeled phosphatidylcholine membranes are exchange-broadened immediately upon addition of CaCl₂. These changes directly and conclusively indicate Ca²⁺-induced clustering of spin-labeled phosphatidylcholine and aggregation of spin-labeled phosphatidic acid bridged by Ca²⁺-chelation in the binary phopholipid membranes. In the Ca²⁺-chelated aggregates, the motions of the alkyl chains of phosphatidic acid are greatly reduced and the lipid molecules are more closely packed. The clusters and aggregates are formed in patches and the sizes are dependent on the fractions. Ba²⁺ and Sr²⁺ induce the lateral phase separations to the same extent as Ca²⁺. Mg²⁺ is also effective but to a lesser extent. In acid solutions (pH 5.5), the Ca²⁺-induced lateral phase separations are of slightly lesser extent than in alkaline solution (pH 7.9). These results are compared with those for phosphatidylserine-phosphatidylcholine membranes reported previously and necessary conditions for the lateral phase separations are discussed.