General Considerations Concerning Carbohydrate Structure

Indeed, the size of the metal ion has been found to be a factor in binding. The a-e-a site on a six membered ring complexes only cations having ionic radii 80 pm. The triaxial site, found on cu-inositol, prefers cat-ions that have ionic radii 60 pm, but 100 pm. 

In the binding of the metal ion to the carbohydrate moiety, charge also appears to play a role, and this is related to a difference in solvation enthalpy in polar solvents. Oxygen donor atoms displace with more difficulty the solvation shell for a trivalent ion in a polar solvent than for a divalent ion. This difference is removed when the study is performed in a nonpolar solvent. Consequently, it would appear that, for studies of carbohydrates in aqueous solution, the charge on the molecule must be considered. 

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