Ultraviolet spectrophotometric characterization of copper(II) complexes with imidazole N-methyl derivatives of l-histidine in aqueous solution

Abstract

In this study we considered π-methyl- l-histidine (π-methis) and τ-methyl- l-histidine (τ-methis) as ligands for copper(II) ion, in order to clarify, by means of ultraviolet (UV) spectroscopy in aqueous solution ( T=25 °C, I=0.1 M), some aspects of the co-ordination mode with respect to other ligands of a previous study in which copper(II) complexes of l-histidine, N-acetyl- l-histidine, histamine, l-histidine methyl ester or carnosine were investigated. Particularly, UV spectra (300–400 nm) were recorded on solutions at various pH values, containing each binary system Cu–L; afterwards, an UV absorption spectrum for single complexes was calculated, taking into account the chemical model previously assessed, in order to fulfil a correct spectrum–structure correlation. The problem related to the eventual superimposition of the CT shoulder (≈330 nm) to copper(II) of OH − and imidazole pyridine nitrogen groups were now solved by means of a comparison of the UV spectra of dimer species formed by both π-methis or τ-methis. Finally, copper(II) complex formation with 2,2′-bipyridine was taken into account to compare the behaviour of pyridine (from 2,2′-bipyridine) and pyridine imidazole nitrogens (from π-methis or τ-methis) with respect to the UV charge transfer process to copper(II) ion.