Binding of zinc cations to pectin and its oligomeric fragments

Abstract

The activity of Zn2+ ions bound to carboxyl groups of pectin of esterification degree E 0 to 90% and to oligomeric potassium galacturonates of polymerization degree 1 to 7 was estimated by the metallochromic indicator (tetramethylmurexide) method. The zinc ions are stoichiometrically bound to pectin, one Zn2+ cation to two free carboxyl groups. The stability constant K of zinc pectinates strongly decreases with the increasing esterification degree in a relationship close to a linear logarithmic function, log K = f(E) similarly, as with the binding of Ca2+ ions to pectin. Pectin reveals a little higher selectivity to Zn2+ ions when considering this couple of cations. The zinc ions are bound to the monomeric D-galacturonic acid to a very low extent in contrast to some toxic cations as e.g. Pb2+, Cu2+, and Cd2+. A continuous increase of the degree of association of Zn2+ ions with carboxyl groups of oligomers takes place with the increasing polymerization degree. The validity of the increment additivity of terminal and inner uronic acid units to the final activity of zinc counterions bound to these oligomers was proved. The results are discussed from the standpoint of application of pectin as an active component of prophylactic diets against poisoning with cations of toxic metals with respect to the excretion of Zn2+ ions from the human body. Findings concerning the interaction of Zn2+ ions with pectin in dependence on its esterification degree (E) constitute a theoretical basis for a potential application of pectin as a useful ligand of Zn2+ ions in medical treatment of zinc deficiency in the human body.