Calcium induced phospholipid ordering depends on surface pressure

The effect of sodium and calcium ions on zwitterionic and anionic phospholipids monolayers is investigated using vibrational sum-frequency generation in conjunction with surface pressure measurements and fluorescence microscopy. Sodium ions only subtly affect the monolayer structure, while the effect of calcium is large and depends strongly on the surface pressure. At low surface pressures (~5 mN/m), the presence on Ca²⁺ results in the unexpected appearance of ordered domains. For pressures between ~5 and ~25 mN/m, Ca²⁺ ions induce disorder in the monolayer. For pressures exceeding 25 mN/m, calcium cations expand the monolayer, while simultaneously ordering the lipid chains. Interestingly, effects are similar for both zwitterionic lipids and negatively charged lipids. In both vibrational sum-frequency generation and surface tension measurements, the molecular signature of the association of Ca²⁺ with the lipids is evident from Ca²⁺-induced changes in the signals corresponding to area changes of 4 Ã…2/lipidprecisely the surface area of a Ca²⁺ ion, with evidence for a change in lipid Ca²⁺ complexation at high pressures.