| Abstract | The use of 3-aminopyrazole derivatives as β-sheet templates is investigated using a series of ferrocenoyl (Fc)-dipeptides (Fc-Gly2-OEt, Fc-Ala2-OBzl, Fc-Leu-Phe-OMe, Fc-Val-Phe-OMe, Fc-Phe2-OMe, Fc-Leu2-OMe, Fc-Val2-OMe). The synthesis and full characterization are reported. The solid-state structures of Fc-Gly2-OMe and Fc-Leu-Phe-OMe show extensive hydrogen bonding of the podand peptide substituents, resulting in the formation of supramolecular Fc-dipeptide assemblies. For Fc-Gly2-OMe, this can be described as a parallel β-sheet, whereas intermolecular interactions in Fc-Leu-Phe-OMe result in the formation of supramolecular helical structures. The saturation titrations of Fc-dipeptides with 3-amino-5-methylpyrazole (3-AMP) and 3-trifluoroacetylamido-5-methylpyrazole (3-TFAc-AMP) show a 1:1 interaction of the Fc-peptide with the aminopyrazole derivatives. IR measurements in solution confirm binding to the top face of the Fc-dipeptide and the involvement of the FcCO and the ester CO groups in establishing H-bonding interactions with the 3-TFAc-AMP. However, binding constants in chloroform are low and range from 8 to 27 M-1, which correspond to binding energies of 5−7 kJ mol-1. In higher polarity solvents, such as acetonitrile or acetone, the binding constants are below 5 M-1, emphasizing the limited utility of 3-AMP derivatives as β-sheet templates. Electrochemical measurements confirm the weak interactions between the various Fc-dipeptides and 3-TFAc-AMP. Typical shifts in the redox potential of the Fc moiety are in the range 0−20 mV. Attempts to modify 3-AMP at the 3-position by carbodiimide coupling with amino acid derivatives and, thus, enhance the binding to the Fc-peptides resulted in 2-amino acid substituted 3-AMP derivatives. Substitution at the 2-position blocks the binding site, and no interactions with Fc-dipeptides are observed. |
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