Structure and dynamics of dimyristoylphosphatidic acid/calcium complexes by 2H NMR, infrared, spectroscopies and small-angle x-ray diffraction

The structural and dynamic properties of complexes of dimyristoylphosphatidic acid (DMPA) and calcium ions have been characterized by 2H NMR, Raman, and infrared spectroscopies and small-angle X-ray diffraction. All techniques used show that these complexes do not undergo a cooperative thermotropic phase transition. Small-angle X-ray diffraction unambiguously demonstrates that the structure of the lipid molecules of the DMPA/Ca2+ complexes remains lamellar even at a temperature as high as 85 degrees C. Raman results indicate that within this temperature range, only a few trans-gauche isomerizations of the C-C bonds of the phospholipid acyl chains arise in this system. The 2H NMR spectra indicate that the DMPA chains are highly motionally restricted up to 65 degrees C and that higher temperatures might activate some low-frequency overall motions of entire lamellar domains. Small-angle X-ray scattering and 2H NMR spectroscopy of 2H2O also show that the interaction of calcium with DMPA promotes an important dehydration of the lipid assembly, even though the latter technique clearly demonstrates that some water molecules remain strongly bond in the DMPA/Ca2+ complexes. The carbonyl stretching mode region of the infrared spectrum of DMPA/Ca2+ complexes suggests that these water molecules are trapped near the interfacial region of the lipid membrane and are hydrogen bonded with the carbonyl groups of the lipid. Finally, comparison of the phosphate stretching mode region of the infrared spectra of complexes of DMPA with calcium ions with those of model compounds provides strong evidence that calcium ions bind to both charges of the phosphate group of DMPA and form bridges between adjacent bilayers.