Structural preferences of phosphatidylinositol and phosphatidylinositol-phosphatidylethanolamine model membranes influence of Ca2+ and Mg2+

The structural preferences of soya phosphatidylinositol in isolation and in mixtures with soya phosphatidylethanolamine, and the influence of Ca²⁺ and Mg²⁺ on these preferences, have been examined employing ³¹P-NMR and freeze-fracture techniques. It is shown that phosphatidylinositol assumes the bilayer organization on hydration both in the presence and absence of Ca²⁺ and Mg²⁺. In mixed systems with H_II phase) phosphatidylethanolamine, phosphatidylinositol induces lipidic particle structure at low (<10 mol%) concentrations and bilayer structure at higher levels. In systems containing 15 or 20 mol% phosphatidylinositol, Ca²⁺ (but not Mg²⁺) can induce H_II phase structure. The results indicate that phosphatidylinositol is a more effective agent than other acidic phospholipids for stabilizing bilayer structure, particularly when high levels of divalent cations are present. These findings are discussed in terms of functional roles of phosphatidylinositol and mechanisms whereby Ca²⁺ induces structural reorganization in mixed systems containing acidic phospholipids and phosphatidylethanolamine.