Infrared studies of fully hydrated saturated phosphatidylserine bilayers. Effect of Li+ and Ca2+

The thermotropic phase behavior of fully hydrated Na+ and/or NH4+ salts of 1,2-dimyristoyl-sn-glycero-3-phospho-L-serine (DMPS) was determined by temperature-dependent infrared spectra. The molecular level properties and thermal phase behavior of DMPS-Li+ complexes were also characterized by infrared spectroscopy. With increasing concentrations of Li+, the infrared spectra reveal the appearance of a second, more ordered, lipid phase which shows a gel to liquid-crystal transition at significantly higher temperatures (75-95 degrees C) than the Na+ or NH4+ salts of DMPS (39 degrees C). Li+ binds to the phosphate and carboxylate groups of DMPS, resulting in the following changes: (1) water of hydration is lost from both the carboxylate and phosphate groups; (2) there are changes in the conformation of the glycerol backbone but not in the P-O ester bonds of the phosphate group which remain in the gauche-gauche conformation; and (3) the packing of the fatty acyl chains becomes more ordered. In addition, the properties of the DMPS-Ca2+ complex were studied by infrared spectroscopy. While the DMPS-Ca2+ complex is also characterized by rigidly packed, well-ordered fatty acyl chains, the mode of Ca2+ binding to the DMPS head groups differs significantly from that of Li+ binding. By comparison, with dry DMPS-Ca2+ [Casal, H. L., Mantsch, H. H., Paltauf, F., & Hauser, H. (1987) Biochim. Biophys. Acta (in press)], the phosphate group undergoes a conformational change, probably to the antiplanar-antiplanar conformation, and loses its water of hydration.(ABSTRACT TRUNCATED AT 250 WORDS)