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Theoretical and experimental characterization of ferrocene tripeptides

Organic chromophores can be used as the sensing elements for determination of the helicity of small peptides.[1, 2] The screw sense of the helix is correlated with the sign of the Cotton effect in circular dichroism (CD) spectra. Our group’s studies have shown that even small monosubstituted peptidomimetics derived from ferrocenamine show a signal in the CD spectra near 470 nm related to the ferrocene chromophore region.[3] Two novel stereoisomeric ferrocene-based tripeptides, Boc- Pro-Pro-Ala-NHFc, were synthesized and characterized experimentally (IR, NMR, CD). Theoretical conformational analysis performed in implicitly modelled solvent, nicely corroborates with the experimental findings about the hydrogen bond patterns. Varying in the chirality of only one amino acid, two stereoisomers show a significantly different distribution of the conformers. In both stereoisomers dominates only P-helix, although the unique orientation of the ferrocene subunit pointing away from the peptide chain is obtained in stereoisomer with heterochiral Pro-Pro amino acid sequence. A strong correlation was also noticed between the sign of the Cotton effect and dihedral angle describing the deviation between the amide and cyclopentadienyl ring plane.

ferrocene chromophore, circular dichroism, Cotton effect, ferrocene-based tripeptides, molecular mechanics, DFT, QTAIM

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