Bis-amino Acid Derivatives of 1,1′-Ferrocenedicarboxylic Acid: Structural, Electrochemical, and Metal Ion Binding Studies

We report on the structural and electrochemical
investigation of
1,1′-ferrocenedicarboxylic acid derivatives of tryptophan (Fc­[CO-Trp-OMe]2 - Fc-conjugate 1), threonine (Fc­[CO-Thr-OMe]2 – Fc-conjugate 2), aspartic acid (Fc­[CO-Asp-OMe]2 - Fc-conjugate 3) and glutamic acid (Fc­[CO-GluOMe]2 - Fc-conjugate 4) and their hydrolyzed analogues 1a–4a respectively (Scheme ). CD and NMR spectroscopy established 1,2′-“Herrick
conformation” in solution, having intramolecular interstrand
hydrogen bonds for all Fc-conjugates. However, in solid state, Fc­[CO-Trp-OMe]2 exists in “Herrick conformation” whereas Fc­[CO-Thr-OMe]2 is present in anti conformation. In solution, the involvement
of indole NH of Trp and alcoholic proton of Thr in intermolecular
hydrogen bonding has been explored by temperature- and concentration-dependent
NMR studies. The half-wave potentials (E1/2) of ferrocene-conjugates follow the sequence 1 < 2 <
4 < 3 which is explained by the contribution of amino acid
side chain functionalities toward the stability of ferrocenium ion.
The CV of the Fc-conjugate 1/1a (having Trp moiety) displays
two redox processes, one of which is assigned to the Fc group, and
the other being related to the indole group. The oxidation peak potential
of indole was found to depend strongly on the pH of the medium. The
values of diffusion coefficient (D) and electron
transfer rate constant (ksh) for all Fc-conjugates
were determined from their corresponding cyclic voltammograms. In
addition, metal ion interactions were studied with hydrolyzed Fc-conjugates 2a–4a using CV and DPV. Upon binding to
metal ions, the electrochemical changes associated with the hydrolyzed
Fc-conjugates correlated to the charge density of the binding metal
ion.