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

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Abstract
Previous work has shown that Ca2+ can trigger bilayer to hexagonal (HII) 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–HII 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 HII 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 HII-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 HII-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.

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