Structure of divalent cation-phosphatidic acid complexes as determined by 31P-NMR

A comparative study of the structure of various phosphatidic acid-divalent cation complexes has been completed using 31P solids NMR methods. These complexes had been implicated as important intermediates in the fusion of phospholipid vesicles and several pieces of evidence had suggested that differences in activity of various ions may stem from structural differences among the complexes. Solids NMR studies using spin one-half nuclei reflect structural properties of molecules through partial averaging of chemical shift tensors. We have found significant differences in the chemical shift tensors observed for Mg, Ca and Cd complexes. Low angle X-ray scattering data were used to assure comparison of similar phases. At low temperatures Ca and Cd complexes exhibit unique phases prohibiting comparison with Mg complexes. At higher temperatures, all complexes exhibit a hexagonal phase, and 31P tensors of the complexes in the hexagonal phase can be interpreted using headgroup geometries similar to those found in crystal structures of phospholipids and assuming motional averaging by simple axial motions. Tensors of Ca and Cd complexes are very similar to one another but are significantly broader than those observed for Mg complexes suggesting a more erect headgroup structure. The differences parallel the fusion activities of the ions for which Ca and Cd are similar and significantly enhanced over that of Mg, supporting a structural link to fusion activity.