Dilution thermodynamics of the biologically relevant cation mixtures

• Dilution energetics of Ca2+ can be altered by the aqueous phase ionic composition.
• Dissipated heat upon Ca2+ dilution is drastically reduced in the K+ presence.
• Reduction of the enthalpy change upon Ca2+ dilution is K+ concentration dependent.
• The cooperativity of Ca2+ hydration might be of great biological relevance providing a thermodynamic argument for the specific ionic composition of the intracellular environment.

The ionic composition of intracellular space is rigorously controlled by a variety of processes consuming large quantities of energy. Since the energetic efficiency is an important evolutional criterion, therefore the ion fluxes within the cell should be optimized with respect to the accompanying energy consumption.In the paper we present the experimental evidence that the dilution enthalpies of the biologically relevant ions; i.e. calcium and magnesium depend on the presence of monovalent cations; i.e. sodium and potassium. The heat flow generated during the dilution of ionic mixtures was measured with the isothermal titration calorimetry. When calcium was diluted together with potassium the dilution enthalpy was drastically reduced as the function of the potassium concentration present in the solution. No such effect was observed when the potassium ions were substituted with sodium ones. When the dilution of magnesium was investigated the dependence of the dilution enthalpy on the accompanying monovalent cation was much weaker. In order to interpret experimental evidences the ionic cluster formation is postulated. The specific organization of such cluster should depend on ions charges, sizes and organization of the hydration layers.

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