Thermodynamic Selectivity of Mixed-Ligand Cu(II) Complexes of Dipeptides and Phenoxy Acid Herbicides: Structure-Stability-Activity Correlations

Abstract

Non-covalent peptide-herbicide intramolecular interactions have been examined in solution in ternary complexes of the type CuAL⁺ or Cu(HA)L, where A^– = anion of dipeptide like Gly-Phe, HA^– = anion of Gly-Tyr and L^– = anion of phenoxy acid herbicide like phenoxyacetic acid (PAA); 2-chloro phenoxyacetic acid (2-CPA); 2,4-dichlorophenoxyacetic acid (2,4-D); 2,5-dichlorophenoxyacetic acid (2,5-D); 2,4,5- trichlorophenoxyacetic acid (2,4,5-T); 4-chloro-2-methylphenoxyacetic acid (MCPA); 2-(2,4-dichloro-phenoxy)propionic acid (2,4-DP); 4-(4- chloro-2-methylphenoxy)butyric acid (MCPB) or 4-(2,4-dichlorophenoxy) butyric acid (2,4-DB). The protonation constants of ligands and stability constants of binary and ternary complexes have been determined by potentiometric pH-titration at 25 ºC and μ = 0.1 M (KNO₃) together with the thermodynamic data. From these parameters and the UV-Visible and ESR spectra of the complexes, it has been established that, besides Cu(II)-ligand coordination characteristics of simple dipeptides and phenoxy acid ligands, there are interactions between the d-electron orbitals of Cu(II) and the 6π-electron systems of aromatic rings and between hydrophobic parts of the molecules. A linear correlation between pK_a/or stability constants of binary/ternary complexes of chlorophenoxyacetic acids and Hammett σ-values indicates that they share a common mode of action.