Studies on Biologically Relevant Binary and Ternary Metal Comlexes II. Cu(II) Ternary Complexes Involving Bipyridyl and a Series of Terdentate Amino Acids

Abstract

Abstract Formation constants of Cu(II) ternary complexes involving bipyridyl as the primary ligand and the amino acids asparagine, citrulline, aspartic acid, glutamic acid, histidine, arginine, lysine and 3,4-dihydroxyphenylalanine as the secondary ligands are reported at 35.0°C and μ = 0.2 M (KNO3). The difference between the stability of the ternary complexes and the corresponding binary complexes has been quantitatively expressed in terms of the parameter ΔlogKT. With the exception of the protonated ternary complex of 3,4-dihydroxyphenylalanine the ΔlogKT values for all other ternary complexes are negative. In the case of the normal ternary complexes involving aspartic acid, histidine and 3,4-dihydroxyphenylalanine the ΔlogKT values are appreciably negative and provide substantial evidence for these amino acids to be bidentate in the ternary complexes. This is in contrast to the normal terdentate behaviour of these ligands in corresponding binary complexes. The ternary complexes involving protonated 3,4-dihydroxyphenylalanine show considerably positive ΔlogKT values. The enhanced stability of this ternary complex as compared to the corresponding protonated binary complex of 3,4-dihydroxyphenylalanine is attributed to intramolecular stacking interactions between the phenyl ring of 3,4-dihydroxyphenylalanine and the Cu(II)-bound bipyridyl. Key Words: copperaminoacid complexesstability constantshydrophobic bonding