| Abstract | Infrared spectroscopy has been used to characterize the thermal-phase behavior of fully hydrated 1-palmitoyl-2-oleoyl-sn-glycero-3-phospho-~-serine (POPS) and 1,2-dioleoyl-sn-glycero-3-phospho-~-serine (DOPS) as well as their interaction with Li⁺ and Ca2⁺. The order-disorder transition of POPS-NH₄⁺ is at 17ºC; in the presence of Li+ a POPS-Li⁺ complex is formed, and the transition temperature of this complex is 40ºC. DOPS-NH₄+ has an order-disorder transition at -11ºC, and unlike POPS the addition of Li⁺ has no effect on the thermal behavior of DOPS-NH₄⁺. This indicates that the binding of Li⁺ to DOPS is negligible or very weak. Li⁺ binds to the phosphate and carboxylate groups of POPS, and as a result these groups lose their water of hydration. Li⁺ binding induces a conformational change, probably in the glycerol backbone of POPS; however, the conformation of the two P-0 ester bonds remains gauche-gauche as in POPS-NH4⁺. Both POPS and DOPS form crystalline complexes with Ca²⁺. As a result of Ca²⁺ binding to the phosphate, this group loses its water of hydration and there is a conformational change in the P-0 ester bonds from gauche-gauche to antiplanar-antiplanar. In contrast to the POPS-Li⁺ complex, the carboxylate group remains hydrated in the Ca²⁺ complexes. Furthermore, in these PS-Ca²⁺ complexes a new hydrogen bond is formed between one of the ester C=O groups and probably water. Such a situation is not found in the NH₄⁺ and Li⁺ salts of phosphatidylserine. |
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