Binding of cadmium and copper(II) ions to oligogalacturonic acids

Abstract

The binding of Cu2+ and Cd2+ ions to potassium oligogalacturonates of polymerization degree DP 1 to 9 was investigated by ion-specific electrodes and by the metallochromic indicator method. The interaction was evaluated on the basis of degree of association β of cations (M2+) with carboxyl groups of the compounds examined and the single-ion activity coefficients of counterions γM2+. The results were compared with values corresponding to an electrostatic bond of Ca2+ ions to these oligomers. The Cd2+, and especially Cu2+ ions were bound in a considerable measure already to mono- and oligogalacturonates of a low polymerization degree very probably by a chelate bond. Precipitation of Cu- and Cd-oligogalacturonates takes place at a polymerization degree DP 5 and 8, respectively. Interpretation of results by the multiple equilibria theory evidenced that the Cu2+ ions were stoichiometrically bound to mono- and oligogalacturonates (one cation to two carboxyl groups) similarly, as with polymeric D-galacturonan. The results revise the early conception concerning the formation of a positively charged complex of Cu2+ ions (similarly also of Cd2+ and Pb2+ ions) with monogalacturonic acid of (MA)+ type, where A- is the anion of uronic acid. The results are a contribution to the elucidation of the elimination mechanism of these toxic cations from human body involving the oligomeric fragments of pectin as products of its enzymic degradation in the digestive tract.