Helical versus Flat Bis-Ferrocenyl End-Capped Peptides: The Influence of the Molecular Skeleton on Redox Properties

Abstract

Despite the fact that peptide conjugates with a pendant ferrocenyl (Fc) have been widely investigated, bis-ferrocenyl end-capped peptides are rarely synthetized. In this paper, in addition to the full characterization of the Fc-CO-[_L-Dap(Boc)]_n-NH-Fc series, we report a comparison of the three series of bis-ferrocenyl homopeptides synthesized to date, to gain insights into the influence of α-amino isobutyric (Aib), 2,3-diamino propionic (Dap) and C^α,β-didehydroalanine (ΔAla) amino acids on the peptide secondary structure and on the ferrocene redox properties. The results obtained by 2D NMR analysis and X-ray crystal structures, and further supported by electrochemical data, evidence different behaviors depending on the nature of the amino acid; that is, the formation of 3₁₀-helices or fully extended (2.0₅-helix) structures. In these foldamers, the orientation of the carbonyl groups in the peptide helix yields a macrodipole with the positive pole on the N-terminal amino acid and the negative pole on the C-terminal amino acid, so that oxidation of the Fc moieties takes place more or less easily depending on the orientation of the macrodipole moment as the peptide chain grows. Conversely, the fully extended conformation adopted by ΔAla flat peptides neither generates a macrodipole nor affects Fc oxidation. The utilization as electrochemical and optical (Circular Dichroism) probes of the two terminal Fc groups, bound to the same peptide chain, makes it possible to study the end-to-end effects of the positive charges produced by single and double oxidations, and to evidence the presence "exciton-coupled" CD among the two intramolecularly interacting Fc groups of the _L-Dap(Boc) series.

Keywords: 2,3-diamino propionic acid; Cα,β-didehydroalanine; electrochemistry; exciton-coupled Circular Dichroism; ferrocene; helical peptides; macrodipole; α-amino isobutyric acid.

Figure 1

Chemical structures of bis-ferrocenyl end-capped peptides (1–4) of the amino acid (Dap) Fc–CO–[_L-Dap(Boc)]_n–NH–Fc (n = 1–4).

Figure 2

X-ray diffraction structure of 2. H-atoms and the disordered, co-crystallized CDCl₃ molecule are omitted for clarity. Anisotropic displacement ellipsoids are drawn at the 30% probability level. The three intramolecular N–H…O=C H-bonds are indicated by dashed lines.

Scheme 1

Functional groups of 2 with labels.

Figure 3

Region of the NOESY spectrum of 2 in CDCl₃ solution, T = 25 °C.

Scheme 2

Functional groups of 3 with labels.

Figure 4

Region of the NOESY spectrum of 3 solution, T = 25 °C.

Figure 5

IR absorption spectra in the amide A region of (a) 1–4 in CDCl₃ solution at 0.5 mM concentration and (b) 4 at 0.5 and 0.1 mM concentration.

Figure 6

Cyclic voltammetry of the 1–4 series in CH₂Cl₂ containing 0.1 M nBu₄NPF₆ as the supporting electrolyte (Au disk electrode (d = 0.5 mm); scan rate 0.2 Vs⁻¹; T = 20 °C). The current i is normalized according to the equation iv^−1/2c⁻¹, to account for diffusion-controlled current variation, with concentration c in the range 0.9–2.7 mM.

Figure 7

Variation in oxidation potentials of the 1–4 series. For comparison, the values for Bz–[_L-Dap(Boc)]_n–NH–Fc, (n = 1–4) and Fc–CO–[_L-Dap(Boc)]_n–NH–iPr (n = 1–3) are reported [23]. Each potential is presented as the mean value of manifold measures (error bars ± 3 mV).

Figure 8

Comparison of variation in oxidation potential of _L-Dap(Boc) (red), Aib (green) and ΔAla (blue) bis-ferrocenyl end-capped peptides. The data are normalized to the potentials of 1.

Figure 9

Visible absorption spectra in CH₂Cl₂/0.1 M nBu₄NPF₆ of 4 obtained by oxidation of the neutral parent compound with up to 1 equivalent of ferrocenium[BF₄] followed by stepwise addition of a second equivalent of acetylferrocenium[BF₄]; c = 0.5 mM.

Figure 10

IR absorption spectra (left: amide A region, right: carbonyl region) in CH₂Cl₂/0.1 M nBu₄NPF₆ of (a,d) 1–4; (b,e) 1⁺–4⁺ obtained by addition of 1 equivalent of ferrocenium[BF₄] to 1–4; (c,f) 1²⁺–4²⁺ obtained by addition of 1 further equivalent of acetylferrocenium[BF₄] to 1⁺–4⁺; c = 0.7–2.4 mM.

Figure 11

CD spectra in CH₂Cl₂ solutions of compounds (a) 1–4; (b) 3 and Z-[_L-Dap(Boc)]₃–NH–Fc; c = 1.0 mM.