Thermodynamic Investigation of Interaction between Ca Cation and Negatively Charged Phospholipid Bilayers as Studied by DSC and Isothermal Titration Calorimetry

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The interaction of a negatively charged vesicle composed of dimyristoylphosphatidylglycerol (DMPG) and Ca²⁺ was investigated by isothermal titration calorimetry (ITC) and differential scanning calorimetry (DSC). A schematic diagram of relative enthalpy (ΔH) versus temperature (t) was constructed from two types of calorimetric enthalpies obtained by ITC and DSC and was compared for the fully neutralized vesicle (Ca²⁺-binding vesicle) and the charged vesicle (Ca²⁺-free vesicle). It was found that the stabilization enthalpy of the DMPG vesicle due to a binding of Ca²⁺ is larger for the liquid crystal phase than for the gel phase, so that the transition enthalpy of the gel-to-liquid crystal phase, i. e., the enthalpy difference between the two phases, is smaller for the Ca²⁺-binding vesicle than for its free vesicle although the transition temperature is higher for the former than for the latter.