Interactions between cations and sugars. Part 5.—Enthalpy and entropy of interaction of the calcium ion with some aldopentoses and aldohexoses in water at 298.15 K

Abstract

The enthalpies of interaction of ribose, arabinose, lyxose, xylose, glucose, mannose, galactose, talose and allose with CaCl₂ and KCl have been determined in water at 298.15 K. From the free energies of interaction determined previously, the corresponding entropies have been calculated and an enthalpy–entropy compensation has been observed. The pair interaction parameters can be corrected for the anion–sugar interaction and for the non-specific cation–sugar interaction by comparing the Ca²⁺–sugar interactions either with the interactions between the non-complexed cation K⁺ and the same sugar or with the interactions between Ca²⁺ and an inactive epimer of the studied sugar. For the sugars that have specific interactions with Ca²⁺, namely ribose, talose, and allose, this allows the thermodynamic properties for the reaction of association of Ca²⁺ with the ‘ complexing ’ sequences of hydroxyl groups to be estimated as : Δ_RG^⊖≈– 2 kJ mol^–1, Δ_RH^⊖≈– 17 kJ mol^–1 and TΔ_RS^⊖≈– 15 kJ mol^–1.