ChemInform Abstract: Influence of Dioxane on the Extent of Intramolecular Hydrophobic Ligand‐Ligand Interactions in the Binary Cu2+ 1:2 Complexes of L‐Leucinate, L‐Valinate and L‐Norvalinate

Abstract

Abstract The stability constants of the binary Cu(AA)+ and Cu(AA)2 complexes, where AA− = L-alaninate (Ala−), L-leucinate (Leu−), L-valinate (Val−) or L-norvalinate, have been determined by potentiometric pH titrations in water, and in 30, 50, 70 and 80% (v/v) dioxane-water mixtures (I = 0.1 M, NaNO3; 25 °C). The overall stability of Cu(AA)+ and Cu(AA)2 is governed for all four amino acetates (AA−) by the polarity of the solvent, while the extent of the intramolecular hydrophobic ligand-ligand interaction between the aliphatic side-chains in Cu(Val)2, Cu(Leu)2 and Cu(Nva)2 is obviously influenced by the hydrophobic solvation properties of the organic solvent molecules. Based on the stability difference Δ log KAA* = log KCu(AA)2Cu(AA) - log KCu(AA)Cu it is shown that Cu(Val)2, Cu(Leu)2 and Cu(Nva)2 are more stable than Cu(Ala)2, and this increased stability is taken as evidence for hydrophobic side- chain interactions in Cu(Val)2, Cu(Leu)2 and Cu(Nva)2; such interactions are not possible in Cu(Ala)2 due to the small size of the methyl side-chain. By using the stability data of the Cu2+/Ala− system as a basis for the evaluation, the extent of the hydrophobic ligand-ligand interaction (= closed form) in the other three Cu(AA)2 complexes is calculated: the percentages of the closed forms vary between about 10 and 30% (based on Cu(AA)2/tot). The formation degree of the closed species is influenced by the solvent: addition of some dioxane to an aqueous solution favors their formation, contrary to the experience with simple unbridged hydrophobic adducts which are destabilized. Such a destabilization of the closed Cu(AA)2 species occurs only at high concentrations of the organic solvent (usually more than 70%). The general relevance of the present results, especially with regard to biological systems, is indicated.