Ion-exchange equilibria with aluminum pectinates

Pectins, which play an important role in the structure of the plant cell wall, is used in medical treatment and for the prevention of metal intoxication, and also as a gelling component in the food industry. The ions of various metals (iron, zinc, copper, manganese, calcium and aluminum) are involved in biological reactions and can bind to pectins for transport through the cell wall into the cytoplasm; Al3+ ions, however, are toxic to plants. Despite the serious problems caused by such aluminum toxicity, little is known about the interaction of the Al3+ ions and pectins, especially those demethylated by pectin methylesterase (PME). In the present paper, the ion-exchange equilibrium (K-c) between solid aluminum pectinates (obtained from enzymatic hydrolysis) with differing degrees of demethylation (DM) and aqueous solutions of iron, zinc, copper, manganese and calcium nitrates was studied. The order of preference for PME demethylated pectins (Fe3+ > Al3+ > Cu2+ congruent to Mn2+ > Zn2+ congruent to Ca) shows that aluminum has a greater affinity for the carboxyls of the pectins, an affinity that can be related to the Al toxicity in plants sensitive to the Al3+ ion. In the ionic exchange with Fe, Cu and Mn small variations in K-c with DM was observed whereas those with Zn and Ca remained constant. A cooperative effect for the ion exchange between the aluminum ions and those of Fe, Cu and Mn was observed, whereas a competitive one was found for the exchange with Zn and Ca. Possibly the cooperative effect is due to the greater affinities of Fe, Cu and Mn for the carboxyls, whereas the competitive effect was due to the lesser affinities of Ca and Zn. These results were compared with those of a prior study of the ion-exchange process of aluminum pectinates with differing DM obtained through alkaline hydrolysis. (C) 2002 Elsevier Science B.V. All rights reserved.