Interactions of divalent cations with phosphatidylserine bilayer membranes

The interaction of divalent cations with a homologous series of diacylphosphatidylserines (diacyl-PS) has been studied by differential scanning calorimetry and X-ray diffraction. Hydrated di-C14-PS (DMPS) exhibits a gel leads to liquid-crystal bilayer transition at 39 degrees C (delta H = 7.2 kcal/mol of DMPS). With increasing MgCl2 concentration, progressive conversion to a phase exhibiting a high melting (98 degrees C), high enthalpy (delta H congruent to 11.0 kcal/mol of DMPS) transition is observed. Similar behavior is observed for DMPS with increasing CaCl2 concentration. In this case, the high-temperature transition of the Ca2+-DMPS complex occurs at approximately 155 degrees C and is immediately followed by an exothermic transition probably associated with PS decomposition. For di-C12-, di-C14-, di-C16- (DPPS), and di-C18-PS, the transition temperatures of the Ca2+-PS complexes are in the range 151-155 degrees C; only di-C10-PS exhibits a significantly lower value, 142 degrees C. A different pattern of behavior is exhibited by DPPS in the presence of Sr2+ or Ba2+, with transitions in the range 70-80 degrees C being observed. X-ray diffraction of the Ca2+-PS complexes at 20 degrees C provides evidence of structural homology. All Ca2+-PS complexes exhibit bilayer structures, the bilayer periodicity increasing linearly from 35.0 A for di-C10-PS to 52.5 A for di-C18-PS. Wide-angle X-ray diffraction data indicate that hydrocarbon chain "crystallization" occurs on Ca2+-PS complex formation.(ABSTRACT TRUNCATED AT 250 WORDS)