The initial binding of Cu(II) to some amino acids and dipeptides: a 13C nuclear-magnetic-resonance study

The initial binding of Cu2+ ot L-lysine, L-histidine, glycyl-histidine and histidyl-glycine in aqueous solutions was examined by 13C nuclear magnetic resonance spectroscopy. The measurements were carried out in a substantially improved way employing the pulse Fourier transform technique. Spectra of both high quality and resolution were obtained. Cu2+ complex formation with L-lysine occurred with the alpha-amino and carboxyl group attributable to the well expressed broadening effect of the 13C signals of the alpha-carbon atom and the carboxyl atom. The epsilon-amino group was not involved. Measurements of the Cu chelates using L-histidine and glycyl-histidine and histidyl-glycine confirmed the ambidentate nature of the histidine residue. It was concluded that an equilibrium exists between two Cu-complex species designated as histamine-like and histamine-like/glycine-like species. In the homogeneous histamine-like Cu complex, the Cu2+ is exclusively bound with 4 nitrogens, while in the other species one oxygen of the glycyl carboxyl group is involved in the Cu2+ binding. Blocking of this carboxyl groups by peptide bonding as found in histidyl-glycine favoured the formation of a Cu complex where the imidazole carbons of the histidyl residue were the most influenced species.