Influence of pH on phosphatidic acid multilayers. A rippled structure at high pH values.

The influence of pH on the structure of 1,2-(ditetradecyl)-phosphatidic acid was investigated by differential scanning calorimetry and freeze-fracture electron microscopy. At pH 13.5--14 (2.6 M K+), where phosphatidic acid has two negative charges, calorimetric scans show a small transition (pretransition) below the main phase transition temperature. Freeze-fracture studies of the same dispersions reveal regular band patterns (so-called ripples) in the plane of the bilayers, when the lipid is quenched from below the main phase transition temperature. This rippled structure is similar to the well-known rippled structure of phosphatidylcholines.