Calcium-induced fusion and lateral phase separations in phosphatidylcholine-phosphatidylserine vesicles. Correlation by calorimetric and fusion measurements

High-sensitivity differential scanning calorimetry has been used to determine the phase diagrams for mixtures of dielaidoyl- and dimyristoylphosphatidylserine with the corresponding phosphatidylcholines (PC's) in the presence or absence of 30 mM calcium ion. The PC and phosphatidylserine (PS) species of like acyl composition are miscible in the absence of calcium but exhibit a limited lateral phase separation at high calcium levels. At higher temperatures, mixtures containing less than or approximate to 40% or greater than or approximate to 70% PS in PC are not phase separated at equilibrium in the presence of calcium, while mixtures of intermediate PS content will separate into two phases of these limiting compositions. At lower temperatures, where hydrated solid phases are present, phase separation is more extensive.