Structural properties of a Ca2+-phosphatidic acid complex: small angle X-ray scattering and calorimetric results

The structures of Ca2+ and Cd2+ complexes with dipalmitoyl- and dimyristoylphosphatidic acids have been investigated by differential scanning calorimetry and small angle X-ray diffraction. The lipids are found to form complexes with Ca2+ which exhibit no thermal phase transitions between temperatures of 25 and 90 degrees C. Transition enthalpies of residual uncomplexed lipid extrapolate to zero near a 1 : 1 lipid to cation ratio indicating a 1 : 1 complex stoichiometry. Ca2+ and Cd2+ complexes appear isomorphous in the X-ray data and show lamellar phases with short repeat distances (51.5 A (5.15 nm) and 58.0 A (5.80 nm) for dimyristoyl and dipalmitoyl homologs, respectively). The data are discussed in terms of structures in which ions bridge phosphates on adjacent bilayers.