Divalent cations and chlorpromazine can induce non-bilayer structures in phosphatidic acid-containing model membranes

(1) The structural organization of aqueous dispersions of 1,2-dioleoylphosphatidic acid has been investigated by freeze-fracture electron microscopy in relation to variations in pH, divalent cations and the local anaesthetic chlorpromazine. (2) In the pH range 4–8 in the presence of 100 mM Na⁺, dioleoylphosphatidic acid is organized in bilayers. (3) At pH 6 and not at pH 4 and 8.5 addition of Ca²⁺, Mg²⁺, Mn²⁺ and chlorpromazine results in the formation of the hexagonal H_II phase. (4) Ca²⁺ and chlorpromazine addition to mixed phosphatidylcholine-dioleoylphosphatidic acid bilayers at pH 6 results in the formation of lipidic particles.