Polymorphism of phosphatidylethanolamine–phosphatidylserine model systems: influence of cholesterol and Mg2+ on Ca2+-triggered bilayer to hexagonal (HII) transitions

Abstract

Previous work has shown that Ca²⁺ can trigger bilayer to hexagonal (H_II) polymorphic phase transitions in (unsaturated) phosphatidylserine (PS) – phosphatidylethanolamine (PE) model systems. In this work we examine the influence of cholesterol and Mg²⁺ on the phase preferences of PS–PE systems. Subsequently, the influence of cholesterol and Mg²⁺ on the levels of Ca²⁺ required to trigger bilayer–H_II transitions in these mixed systems is studied. It is shown that at 30 °C the presence of equimolar (with respect to phospholipid) levels of cholesterol engenders formation of the H_II phase for PE–PS systems containing 15 and 30 mol% PS, whereas bilayer structure is maintained for PE–PS–cholesterol (1:1:2) dispersions. However, the polymorphic phase preferences of the latter system are much more sensitive to the presence of monovalent and divalent cations. In the absence of cholesterol, Mg²⁺ and high salt concentrations do not affect the polymorphic phase preferences of PE–PS (1:1) systems. In contrast, 8 mM or higher Mg²⁺ levels or salt concentrations greater than 1.0 M induce H_II-phase formation in PE–PS–cholesterol (1:1:2) systems. Further, lower Mg²⁺ concentrations (2 mM) act as a powerful adjunct to Ca²⁺ triggering of H_II-phase structure in such systems, reducing the Ca²⁺ concentration required from 4 to 0.25 mM. These results are discussed in terms of Ca²⁺ concentrations required for fusion events and the influence of cholesterol on the structural preferences of the inner monolayer lipids of the erythrocyte membrane.