Thermodynamics of mixed complexes of Cu(II) and Zn(II) with ATP and some aromatic aminoacids

Abstract

Up to no mainly simple complexes of divalent metal ions with nucleotides have been studied in solution [1-4]; it is only for a few years that the stoichiometry, the formation constants and the structure of mixed complexes of M(II)- NT ( NT = nucleotide) with biofunctional ligands have been reported [4-6]. By means of spectroscopic investigations two different types of ternary complexes have been investigated; these complexes can be represented as L- NT-M(II) and L-M(II)- NT ( L = biogenic amines, aminoacids, etc...)[7]. In such mixed complexes an interesting ‘secondary’ bonding, due to formation of stacking adduct between the base of the nucleotide and the ‘aromatic’ moiety of other ligand, has been described [6]. Recently [8,9], we have determined, by direct calorimetry, the effect of stacking interaction on the thermodynamic parameters concerning the formation of copper(II) and zinc(II) mixed complexes with adenosine 5′-triphosphate ( ATP) and some biofunctional ligands as L-tryptophan, histamine and L-histidine. We now reports the thermodynamic properties of ternary complexes of Cu(II) and Zn(II) with ATP and aromatic aminoacids as phenylalanine and tyrosine. The formation of stacking adducts has been checked by spectroscopic measurements. The thermodynamic data obtained by means of potentiometric and calorimetric measurements are compared with those of the corresponding parent complexes measured under the same experimental conditions (25 °C and 0.1 mol dm −3 in K[NO 3]). The [Cu( ATP) 2] 6− and [Zn( ATP) 2] 6− species were also found to exist; their overall stability constants were determined.