Complex-forming properties of tyrosine isomers with transition metal ions

Abstract

The thermodynamic quantities relating to the formation of the manganese(II), cobalt(II), nickel(II), copper(II), and zinc(II) complexes of tyrosine, o-tyrosine, and m-tyrosine have been determined pH-metrically and calorimetrically at 25 °C and at an ionic strength of 0.2 mol dm^–3(KCl). From the thermodynamic data and the spectral behaviour of the complexes formed, it was demonstrated that tyrosine and m-tyrosine do not interact directly via their phenolic hydroxy groups with the metal ions, even after deprotonation of these groups. However, this interaction occurs with o-tyrosine. The microconstants of the stepwise deprotonation processes taking place by various pathways for the tyrosine isomer (H₂A) complexes of type [Zn(HA)_n] were also determined with a combined pH-spectrophotometric method. It was found that loss of proton from the complexes [ZnHA]⁺, [ZnH₂A₂], and [ZnHA₂]^– occurs to practically the same extent from the phenolic hydroxy group and from the co-ordinated water molecule. However, the second phenolic hydroxy group deprotonates only to a slight extent in the complex [ZnHA₂]^–.