ESR study of the structure and bonding parameters in binary copper(II) complexes of some α-amino acids and dipeptides

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Abstract
Binary 1 : 1 copper(II)-amino acid and copper(II)-dipeptide complexes, [CuL] and [CuL]+ (H2L = +H3N-CHR-CO-NH-CHR'-COO-, HL' = +H3N-CHR-COO-), have been investigated in aqueous solution by means of ESR and electron absorption spectroscopy. Molecular orbital coefficients characteristic of the metal-ligand bonds have been derived for an effective D4h local symmetry. It is suggested that at the pH near to the physiological conditions both histidine and tryptophan coordinate as a tridentate ligand via O(carboxyl), N(heterocyclic ring) and N(amino) atoms. ESR investigation at room temperature and in frozen aqueous solutions, and visible spectral evidence suggest histamine-like coordination of histidine and tryptamine-like coordination of tryptophan in the quatorial plane of the binary complexes. Since proline contains the imino group, there is no ionizable amide-NH-proton when it is inserted in a peptide chain, hence the proline-nitrogen is unable to bind metal ions in peptides.

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