Solution Equilibria of Ternary α-Amino Acid-Copper(II) Complexes with Electrostatic Ligand-Ligand Interactions

Abstract

Abstract The stability constants for the parent and mixed copper (II) complexes involving an acidic α-amino acid (aspartic acid or glutamic acid, abbreviated as A) and/or a basic α-amino acid (arginine, lysine, or ornithine, abbreviated as B) have been determined by potentiometric titration at 25 °C in 0.1 and 0.03 M KNO3 (M=mol dm−3). The protonated ternary species Cu(l-A)(l-B)(H) and Cu(d-A)(l-B)(H) (species 1111) have essentially the same stability constants, the log β1111 values being in the range 25.0–27.5. The species distribution curves for the systems Cu(II)-l- or d-glutamic acid-l-lysine at the ionic strength I=0.1 and 0.03 reveal that species 1111 is more predominant at I=0.03 than at I=0.1, suggesting that electrostatic ligand-ligand interactions between the carboxylate group in the side chain of A and the ammonium or the guanidinium group in the side chain of B are reinforced in solution at lower ionic strength to favor the ternary complex formation. The constants for deprotonation from species 1111 to form 1110, e.g. Cu(l-A)(l-B), are given by the difference log β1111−log β1110, which is higher for the Cu(II)–A–B systems than for the systems containing alanine in place of A. The results indicate that intramolecular electrostatic interactions are present in the ternary species 1111, stabilizing them under favorable conditions.