Abstract: The thermodynamic functions of the reactions of D-glucose and D-fructose with Ca²⁺, Mg²⁺, Cu²⁺, Zn²⁺, and Cd²⁺ were determined by titration calorimetry combined with computer fit program, respectively.The results show the following :(1) The stable constants of the metallic-fructose complexes are greater than those of corresponding glucose complexes.(2) The entropy of the system decreases during the formation of the complexes except for Ca²⁺.(3) Enthalpy and entropy changes for fructose or glucose-Ca²⁺complexes formation are considerably different from the corresponding hexose-Mg²⁺. The results are explained on the basis of the hexose structure and the metal ion radii. Because in aqueous solutions fructose exists mainly in the much more flexible acyclic form, and in this there are three pairs of hydroxyl groups favorably oriented for metal ion complexes formation with the lactol form without bond strain or steric hindrance, than with glucose. As a result, the fructose complexes are generally stable than the glucose complexes. The formation of the complexes results in the loss of rotational freedom for the -CH₂OHgroup, and is reflected in the negative entropy changes. The positive entropy changes for the formation of Ca²⁺ complexes may results from its larger radius by the facile mutual loss of the water molecules. It is interesting to note that there is considerable difference in thermodynamic behavior between Ca²⁺ and Mg²⁺.

Key words: D-Glucose, D-Fructose, Metal ions, Thermodynamic functions, Titration calorimetry