Tight turn in dipeptide bridged ferrocenes: Synthesis, X-ray structural, theoretical and spectroscopic studies

Dinuclear ferrocene-based conjugates comprising a small homochiral sequence of two aminoacids (FcCO-Pro-Ala-NHFc) were prepared using stepwise solution-phase coupling techniques. The propensity of these bioorganometallics to form tight turns that resemble those in natural peptides was investigated by means of spectroscopy and the crystal structure was determined by X-ray diffraction analysis. The experimental data were corroborated by DFT calculations. The intramolecular NHFc⋯OCFc hydrogen bond keeps the molecule in the β-turn conformation. Molecules are further organized into bilayers with the inner region containing amino acid parts with relatively strong NHAla⋯OCAla hydrogen bonds, and the outer region linked by relatively weaker ferrocene⋯ferrocene interactions. The current research again confirms the robustness of the β-turn and its persistence in solution and in the solid state of small dinuclear ferrocene peptidomimetics.