Abstract
A theoretical approach to phase transition enthalpy change as a function of calcium ion concentration was studied in terms of calcium ion adsorption on a phospholipid surface. The phospholipids used in this study were dipalmitoylphosphatidylcholine (DPPC), dipalmitoylphosphatidylethanolamine (DPPE), and dipalmitoylphosphatidylglycerol (DPPG). The theory used here is based on the DLVO theory, taking into account electrostatic interaction caused by calcium ion adsorption on the membrane surface. Calcium ion effects on DPPC and DPPG phase behavior can be explained by Helmholtz free energy change. Experimental enthalpy change is dependent on the electrical contribution in the free energy equation. This contribution is related to the binding constant K and effective surface area a'. Binding parameters, binding constant K and effective surface area a' were K=100 M-1 and a'=60, 000 m2/g for DPPG, and K=40 M-1 and a'=750 m2/g for DPPC as optimal values. These values for DPPE could not be determined owing to the absence of enthalpy change.
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