The effect of protons or calcium ions on the phase behavior of phosphatidylserine-cholesterol mixtures.

The influence of protons or calcium ions on the miscibility of cholesterol in phosphatidylserine has been examined using differential scanning calorimetry and X-ray diffraction. At pH 2.6, where the carboxyl group of the serine moiety is protonated, two endothermic transitions are observed in cholesterol-phosphatidylserine mixtures. The midpoint of the first is at 35 degrees C in the absence of cholesterol and decreases to approx. 15 degrees C for molar fraction of cholesterol 0.5. The second transition is centered at approx. 44 degrees C, almost independent of cholesterol content. The two lower temperature phases are lamellar and the high temperature phase has hexagonal symmetry. Cholesterol is more miscible in protonated phosphatidylserine than in the sodium form: cholesterol crystals are detected at a molar ratio of phosphatidylserine to cholesterol of about 1.7:1 as compared to about 2.3:1 at neutral pH. In the presence of calcium ions (1.3 Ca2+ per phosphatidylserine), a lamellar phase is observed with layer spacing 53 A which is independent of temperature (25 degrees C-65 degrees C) and of cholesterol content. Calcium ions cause reduced cholesterol solubility: crystallites are detected already at a molar ratio of 4:1.