Abstract
Five small, completely protected, L‐histidyl(N^τ^Bzl)‐containing peptides, were synthesized and characterized by NMR spectroscopy, i.e., R‐Ala‐His(N^τ^‐Bzl)‐Ala‐R′ (1), R‐His(N^τ^‐Bzl)‐Ala‐Ala‐Ala‐Met‐R′ (2), R‐His(N^τ^‐Bzl)‐Ala‐Ala‐Ala‐His(N^τ^‐Bzl)‐R′ (3), R‐His(N^τ^‐Bzl)‐Ala‐His(N^τ^‐Bzl)‐Ala‐His(N^τ^‐Bzl)‐R′ (4), R‐His(N^τ^‐Bzl)‐Ala‐His(N^τ^‐Bzl)‐R′ (5), in which: R is phenylacetyl (PhCH~2~CO‐), R′ is phenylamino (‐NHPh); Bzl is benzyl (PhCH~2~‐).
It has been shown that benzylation of histidine residues takes place at the N^τ^ atom of the imidazole. The resulting peptide derivatives appear to be good ligands for Co^II^, Zn^II^ and Cu^I^. Titrations of zinc chloride solutions towards peptide solutions (in dmso‐d~6~, dimethyl sulfoxide followed by NMR spectroscopy show the formation of the two species [Zn(N^π^His)~2~Cl~2~] and [Zn(N^π^His)(dmso)Cl~2~]. Copper(I)‐chloride titrations result in formation of trigonal planar and tetrahedral complexes (in dmso‐d~6~). Zinc, copper and cobalt trifluoromethanesulfonates were also investigated, and corresponding tetrahedral species with additional dmso ligands replacing chloride were obtained. In addition, octahedral complexes have been found, as shown for cobalt(II) using ligand‐field spectroscopy.
Conductivity experiments of zinc chloride complexes in dmso and chloroform indicate non‐electrolytes. Their very low conductivity in methanol indicates only slight dissociation of chloride, in agreement with a tetrahedral geometry for zinc(II). For [Co(N^π^His)~2~Cl~2~], the UV‐VIS adsorption spectra in dmso and in the solid state (diffuse reflection) are comparable, indicating tetrahedral geometry, with a CoN~2~Cl~2~ chromophore.