Are the carboxyl groups of pectin polymers the only metal-binding sites in plant cell walls?

Are the carboxyl groups of pectin polymers the only metal-binding sites in plant cell walls? To answer this question we investigated Cu2+ and Ni2+ -exchange capacities of isolated root cell walls of seven plant species. As the ionization degree of carboxyl groups and consequently their ability to bind metal ions is dependent upon the solution pH it is possible to reveal the predominant sites of metal binding at a certain pH. Amounts of different ionogenic groups in the isolated cell walls was determined by potentiometric titration, Cu2+ and Ni2+ binding capacities - by measuring the amount of adsorbed metal ions after incubation in 1 mM CuCl2 or NiCl2 at different pHs. Carboxyl groups of polygalacturonic acid (PGA) are the only Cu2+ and Ni2+ binding sites at pH < 5. At NEuroDe a parts per thousand yenaEuro parts per thousand 5 the ratio between the amount of the metal bound and the amount of PGA carboxyl groups is > 1 in some plant species, indicating the involvement of carboxyl groups of hydroxycinnamic acids (HCA) in Cu2+ and Ni2+ binding. HCA carboxyl groups in cell walls of grasses bind 40-85 % of total cell wall Cu2+ and Ni2+ at NEuroDeaEuro parts per thousand a parts per thousand yenaEuro parts per thousand 5, but in dicotyledons their contribution does not exceed 45 % and varies widely between species. Carboxyl groups of both polygalacturonic and hydroxycinnamic acids provide Cu2+ and Ni2+ adsorption sites in root cell walls. Their relative content depends on plant species, and ability to bind Cu2+ and Ni2+ - on the solution pH.