Sugar interaction with metal ions. FT-IR study on the structure of crystalline galactaric acid and its K +, NH 4 +, Ca 2+, Ba 2+, and La 3+ complexes

Abstract

The FT-IR spectra of galactaric acid and its K +, NH 4 +, Ca 2+, Ba 2+, and La 3+ salts have been recorded and interpreted. Spectroscopic evidence shows that the dimeric carboxylic groups of the free acid are dissociated upon formation of the salt, and the asymmetric and symmetric stretching vibrations of the anionic COO − group in these salts are observed at about 1600 and 1400 cm −1, respectively. The two carboxylic groups of the galactarate coordinate with Ca 2+ ions in a monodentate form. One of the carboxylic groups in the Ba 2+ salt coordinates in a monodentate state; another group interacts with three cations in a tetradentate form. In the K +, NH 4 +, and La 3+ salts, the COO − groups coordinate in a polydentate manner with the cations. By comparison of the spectra of the salts with that of the free acid, it is concluded that the hydroxyl groups of the galactarate skeleton take part in metal–oxygen interaction, and the hydrogen-bonding network is rearranged upon sugar metalation. The degree of participation of the sugar OH groups in metal–galactarate interaction is varied from the K + and NH 4 + salts to the Ca 2+, Ba 2+, and La 3+ salts.