Spectroscopic and Electrochemical Studies of Ferrocenyl Triazole Amino Acid and Peptide Bioconjugates Synthesized by Click Chemistry

Ferrocenyl triazole amino acid (L-leucine methyl ester (2, 5), L-phenylalanine methyl ester (3, 7), L-alanine methyl ester (4), L-valine tert-butyl ester (6), L-proline methyl ester (8)) and peptide derivatives ([Leu(5)]-enkephalin (9, 10)) were prepared by Cu(I)-catalyzed [3+2] cycloaddition of azidoferrocene and 1,1'-diazidoferrocene with the corresponding alkyne-modified amino acids and peptide. A purity of > 95% for the peptide conjugates was confirmed by HPLC. All new compounds were comprehensively characterized by elemental analysis, mass spectrometry (FAB and ESI-MS, including high-resolution MS), IR, and multinuclear 1D and 2D NMR spectroscopy. Solution structures were studied by circular dichroism (CD) and NMR spectroscopy, showing that compounds 5, 6, and 7 form intramolecular hydrogen bonds (IHBs) in noncoordinating solvents. Electrochemical studies show reversible processes of the redox couple Fc(0)/Fc(+) (Fc = ferrocenyl) for compounds 2-9, whereas compound 10 exhibits an irreversible oxidation. A good correlation between the diffusion coefficients as determined by electrochemical methods and the molecular weight was established.