Metal coordination of ferrocene–histidine conjugates
This study presents a few bis(histidine) ligands working to build a small peptidic model system of zinc structural sites. Ferrocene–peptide conjugates Fc[CO-His(Trt)-His(Trt)-OMe]2 (3), Fc[CO-His(Trt)-Asp(OMe)-OMe]2 (4), and Fc[CO-His(Trt)-Glu(OMe)-OMe]2 (5) were synthesized and characterized spectroscopically. 1H-NMR and IR spectroscopic studies reveal hydrogen bonding interactions and while more detailed circular dichroism studies show a 1,2′-P helical “Herrick conformation” for Fc-conjugates 4 and 5, we discovered M-helical chirality in Fc–peptide 3. The half-wave potentials (E1/2) of ferrocene–peptides follow the sequence 3 < 5 < 4 which is rationalized by the capability of the peptide side chains to stabilize the oxidized ferrocene–peptide form. The diffusion coefficient (D) and electron transfer rate constant (ksh) values for all Fc-conjugates were determined by their resultant cyclic voltammetry data. The interactions for all Fc-conjugates were probed with metal ions Zn2+, Cd2+, Ni2+, Cu2+, Mn2+, and Mg2+ which were detected to interact at 1:1 ratio between the ligand and metal ion verified by 1H-NMR and UV titration studies, electrochemical investigations, and ESI-MS experiments. Electrochemical studies for all Fc-conjugates exhibit anodic potential shifts upon the addition of metal ions, which follow the order Cu2+ > Zn2+ > Ni2+ > Cd2+ > Mn2+ > Mg2+. NMR spectroscopic experiments show that the two nitrogen atoms present on each imidazole ring of His residues are the site of metal coordination. There is a strong indication that peptide conjugates 4 and 5 in the presence of Zn2+ enforce a coordination number of four as the CD spectra of Fc-conjugates 4 and 5 exhibited a red shift which corresponds to the third and fourth coordination sites occupied by neutral carbonyl oxygen donor atoms, in addition, carbonyl amide appears downward shifted in wavenumber upon metal addition.