Complex formation and stereoselectivity in the ternary systems copper(II)–D/L-histidine–L-amino-acids

Abstract

Formation constants of the parent and ternary complexes of general formula [Cu^II(D/L-HisO)(L-A)](HisO = histidinate; HA = phenylalanine, tryptophan, valine, proline, methionine, leucine, serine, threonine, 2,4-diaminobutyric acid, ornithine, lysine, arginine, glutamic acid, aspartic acid, glycylvaline, glycylphenylalanine, or valyl-L-valine) have been measured potentiometrically at 25.0 °C and I= 0.10 mol dm^–3(K[NO₃]). The ternary systems of Cu^II and the substituted histidines N³-benzyl-L-histidine and N^αN³-dibenzyl-L-histidine with D- and L-tryptophan, phenylalanine, valine, and glutamic acid have also been studied. The ternary complexes containing tryptophan and phenylalanine are unusually stable, complexes containing ligands of opposite chirality being significantly more stable than those with ligands of the same chirality. With ornithine, lysine, and arginine, stereoselectivity is significant in monoprotonated ternary complexes, those with ligands of the same chirality being more stable. This stereoselectivity is at a maximum at ca. pH 6 and vanishes when the proton is ionized. With aspartic acid, stereoselectivity is significant in the non-protonated ternary complex, that with ligands of opposite chirality being more stable. The stereoselectivity found may be explained by simple electrostatic interactions.