Binary and ternary copper(II) complexes of Nτ- and Nπ-methyl-L-histidine in aqueous solution

Abstract

Protonation, binary and ternary copper(II) complex formation constants of N^τ-methyl-L-histidine, His(τMe) and N^π-methyl-L-histidine, His(πMe), have been measured through potentiometric titrations. The UV/VIS spectra have been recorded for both the binary systems and the Cu^II–His(πMe) complexes have been investigated by means of fast atom bombardment mass spectroscopy. The data on the protonation equilibria of the histidine derivatives show the existence of a difference in basicity between the respective π- and τ-nitrogen atoms, thus giving information on the tautomerism involving the imidazole residue of L-histidine (L-His) in the physiological pH range. The complexation behaviour of His(τMe) is quite similar to that of underivatized L-His: the same species are formed and their log β values are very similar. On the contrary, the presence of a methyl substituent on the N^π atom deeply affects the complexing properties of the amino acid; His(πMe) cannot act as a tridentate ligand for steric reasons and tends to form polynuclear complexes.