Spectroscopic and Electrochemical Investigations into the Interactions of Metal Ions with a Ferrocenoyl–Histidine Peptide Conjugate

Abstract

AbstractA new ferrocene–peptide conjugate Fc‐[His(DNP)‐Gly‐OMe]2 (2) was synthesized and characterized spectroscopically. The conjugate displays the familiar 1,2′‐P helical Herrick conformation in solution and exhibits a reversible one‐electron oxidation at 380 mV versus Fc/Fc+. The interactions of the metal ions Na+, Mg2+, Fe2+, Cu2+, Zn2+, and Cd2+ with conjugate 2 were probed by using a range of techniques, including cyclic voltammetry, and NMR and CD spectroscopies. Electrochemical studies showed that the system exhibits a rare cathodic potential shift upon the addition of metal ions, which followed the order Cu2+ (–436 mV) > Fe2+ (–284 mV) > Zn2+ (–270 mV) > Cd2+ (–238 mV) > Mg2+ (–180 mV) > Na+ (–75 mV). The formation of a 1:1 metal complex was postulated on the basis of CV and NMR spectroscopic titration experiments. The complexes themselves were characterized by electrospray mass spectrometry. NMR spectroscopic studies show that the His imidazole is the site of metal coordination. As is confirmed by CD spectroscopy, metal coordination itself results in conformational changes of the ferrocene core that are dependent on the particular metal ion and its coordination preferences. Whereas P‐helicity of the conjugate is maintained during the interactions with Mg2+, Zn2+, and Cd2+, M‐helicity is adopted for Fe2+ and Cu2+.