Thermodynamic stereoselectivity assisted by weak interactions in metal complexes. Chiral recognition of L/D-amino acids by the copper(II) complex of 6-deoxy-6-[4-(2-aminoethyl)imidazolyl]cyclomaltoheptaose

Abstract

Potentiometric and spectroscopic methods were used to investigate the stability and bonding details of the copper(II) complexes of β-cyclodextrin (β-CD) functionalized by histamine through the imidazole N-1 (CDmh) and a number of enantiomeric pairs of amino acidates (AaO⁻) where AaO⁻ is alaninate (AlaO⁻), phenylalaninate (PheO⁻) or tryptophanate (TrpO⁻). Stability constant values were determined at 25 °C and I = 0.1 mol dm⁻³ (KNO₃). Enantioselectivity was observed in the copper(II) mixed complex formation of L/D-TrpO⁻ in aqueous solution and [Cu(CDmh)(L-TrpO)]⁺ was seen as being favoured over [Cu(CDmh)(D-TrpO)]⁺. In contrast, the AlaO⁻ systems did not show any differences in the stability of copper(II) diastereomeric ternary complexes. The c.d. spectra of the complexes containing aromatic L-amino acids showed much higher intensity (Δε) compared with the spectra for the corresponding D-amino acids, the difference Δ(Δε) increasing in proportion to the size of the side chain. Subtle differences were found to exist between the ESR parameters of the diastereomeric complexes with TrpO⁻. However, these tended to disappear upon the addition of a less polar solvent. The results obtained are consistent with a model where the complexation of the L enantiomer to copper already bound to the histamine residue is favoured by the inclusion of the aromatic side chain in the cyclodextrin cavity, this in turn being due to the preferential cis arrangement of the amino nitrogens in the two ligands.