Inorganic Chemistry, Vol.40, No.17, 4409-4419, 2001
Interaction of ferrocenoyl-dipeptides with 3-aminopyrazole derivatives: beta-sheet models? A synthetic, spectroscopic, structural, and electrochemical study
The use of 3-aminopyrazole derivatives as P-sheet templates is investigated using a series of ferrocenoyl (Fc)dipeptides (Fc-Gly(2)-OEt, Fc-Ala(2),-OBzl, Fc-Leu-Phe-OMe, Fc-Val-Phe-OMe, Fc-Phe(2)-OMe, Fc-Leu(2)-OMe, Fe-Val(2)-OMe). The synthesis and full characterization are reported. The solid-state structures of Fc-Gly(2)-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-Gly(2)-OMe, this can be described as a parallel beta -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-aniino-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 Fc-C=O and the ester C=O 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 P-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 Fe 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 Fe-peptide resulted in 2-arr ino acid substituted 3-AMP derivatives. Substitution at the 2-position blocks the binding site, and no interactions with Fc-dipeptides are observed.