Influence of dioxane on the extent of intramolecular hydrophobic ligand-ligand interactions in the binary Cu 2+ 1:2 complexes of L-leucinate, L-valinate and L-norvalinate

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, NaNO 3; 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 K AA * = log K Cu(AA) 2 Cu(AA) - log K Cu(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 Cu 2+/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.