Electrostatic ligand-ligand interactions in ternary copper(II) complexes with 3, 5-diiodo-l-tyrosine and polar amino acids

Electrostatic ligand-ligand interactions in ternary copper(II) complexes, Cu(AA)(PA), where AA refers to an aromatic amino acid (AA = 3, 5-diiodo- l -tyrosinate ( l -I 2 tyr), l -tyrosinate ( l -Tyr), or l -phenylalaninate ( l -Phe)) and PA refers to an amino acid with a polar or charged side-chain (PA = l -argininate ( l -Arg), ω-protonated l -lysinate ( l -LysH), l -asparaginate ( l -Asn), l -glutaminate ( l -Gln), or l -alaninate ( l -Ala)), have been studied by potentiometric and spectroscopic methods. The stability constants for Cu(AA)(PA) were determined by pH titrations at 25°C and I = 0.1M (KNO 3 ). The stability enhancement due to electrostatic interactions in Cu(AA)(PA) has been evaluated by several methods including the constant K for the following hypothetical equilibrium calculated from the determined overall stability constants of the relevant ternary complexes: Cu(AA)( l -Ala) + Cu( l -Phe)(PA) ⇄ Cu(AA)(PA) + Cu( l -Phe)( l -Ala) While the log K values were small or negative for Cu( l -I 2 tyr)( l -Asn), Cu( l -I 2 tyr)( l -Gln), and Cu( l -Tyr)(PA) with the protonated phenol OH group, large positive log K values were obtained for Cu( l -I 2 tyrO − )( l -Arg) (0.40) and Cu( l -I 2 tyrO − )( l -LysH) (0.41) with the deprotonated phenol moiety, which indicates that they are stabilized by intramolecular electrostatic interactions between the oppositely charged side groups of the coordinated ligands. Comparison of the 1 H NMR spectra of Pd( l -I 2 tyrO − )( l -LysH) and Pd( l -Ala)( l -LysH) in D 2 O indicated the interaction between the diiodophenolate moiety of I 2 tyrO − and the protonated ω-amino group of LysH. The circular dichroism spectra observed in the d-d region for these complexes exhibited an anomalously large negative maximum near 600 nm, supporting the existence of such interactions in the complexes in solution. Contribution of the iodo groups to the interactions was discussed.