Charge mapping in 3(10)-helical peptide chains by oxidation of the terminal ferrocenyl group

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

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 Cα,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.

Copper-Catalyzed Single C-H Amination of 8-Aminoquinoline-Directed Ferrocenes

An unprecedented copper-catalyzed C-H monoamination of ferrocenes directed by an 8-aminoquinoline amide directing group is described. This reaction proceeds in the presence of a catalytic amount of copper catalyst with both cyclic and acyclic amines to afford the various aminoferrocenes. The C-H amination of ortho-substituted ferroceneamides was also achieved, enabling rapid access to multisubstituted ferrocenes that are useful for developing new functional molecules.

Flat, Cα,β -Didehydroalanine Foldamers with Ferrocene Pendants: Assessing the Role of α-Peptide Dipolar Moments

The foldamer field is continuously expanding as it allows to produce molecules endowed with 3D-structures and functions never observed in nature. We synthesized flat foldamers based on the natural, but non-coded, Cα,β -didehydroalanine α-amino acid, and covalently linked to them two ferrocene (Fc) moieties, as redox probes. These conjugates retain the flat and extended conformation of the 2.05 -helix, both in solution and in the crystal state (X-ray diffraction). Cyclic voltammetry measurements agree with the adoption of the 2.05 -helix, characterized by a negligible dipole moment. Thus, elongated α-peptide stretches of this type are insulators rather than charge conductors, the latter being constituted by peptide α-helices. Also, our homo-tetrapeptide has a N-to-C length of about 18.2 Å, almost double than that (9.7 Å) of an α-helical α-tetrapeptide.

New bis-ferrocenyl end-capped peptides: synthesis and charge transfer properties

In this article, the successful preparation of a new series of 3(10) -helical peptides of different length containing two terminal ferrocenyl (Fc) units and based on the strongly foldameric α-aminoisobutyric (Aib) acid is reported. The synthesis of the Fc-CO-(Aib)(n) -NH-Fc (n = 1-5) homo-peptides was performed by solution methods. Moderate to good yields (26-85%) were obtained in each of the difficult coupling steps of Fc-COOH and the corresponding H-(Aib)(n)-NH-Fc compounds by C-activation with the 1-(3-dimethylaminopropyl)-3-ethylcarbodiimide/7-aza-1-hydroxy-1,2,3-benzotriazole method. Information on the C=O···H-N intramolecularly hydrogen-bonded networks was initially obtained from FT-IR absorption measurements. The N-H stretching (amide A) region allowed us to distinguish which amide protons are involved in intramolecular hydrogen bonds and indicates the formation of an incipient 3(10) -helix structure for peptides containing at least two Aib residues. This conclusion was confirmed by (1)H NMR titrations of the N-H groups of the peptides in CDCl(3) with dimethylsulfoxide and by crystallographic analysis of the N(α) -acylated Fc-CO-(Aib)(5)-NH-Fc pentapeptide amide. The two redox-active Fc groups covalently bound to the termini of the foldameric peptide architectures were used as electrochemical probes. The end-to-end effects of electron holes generated by single and double oxidations were analyzed by means of electrochemical and spectroelectrochemical techniques. The results of these studies indicate that charge transfer across the peptide main chain does occur in the five peptides. In particular, in the pentapeptide 5, charge is transferred through an intramolecular Fe···Fe separation of 14 Å.

A New Type of Electron Relay Station in Proteins: Three-Piece S:Π∴S↔S∴Π:S Resonance Structure

A type of relay station for electron transfer in proteins, three-piece five-electron bonding, is introduced in this paper, which is also first proposed here. The ab initio calculations predict the formation of S:Π∴S↔S∴Π:S resonance binding with an aromatic ring located in the middle of two sulfur-containing groups, which may participate in electron-hole transport in proteins. These special structures can lower the local ionization energies to capture electron holes efficiently and may be easily formed and broken because of their proper binding energies. In addition, the UV-vis spectra provide evidence of the formations of the three-piece five-electron binding. The cooperation of three adjacent pieces may be advantage to promote electron transfer a longer distance.

Key multi(ferrocenyl) complexes in the interplay between electronic coupling and electrostatic interaction

In this review, the properties of the most significant examples of multi(ferrocenyl) cations containing a number of ferrocenyl units from two to six are discussed and the results are compared with the outcomes of some of our recent studies on conjugated ferrocenyl complexes.

Designing foldamer-foldamer interactions in solution: the roles of helix length and terminus functionality in promoting the self-association of aminoisobutyric acid oligomers

The biological activity of antibiotic peptaibols has been linked to their ability to aggregate, but the structure-activity relationship for aggregation is not well understood. Herein, we report a systematic study of a class of synthetic helical oligomer (foldamer) composed of aminoisobutyric acid (Aib) residues, which mimic the folding behavior of peptaibols. NMR spectroscopic analysis was used to quantify the dimerization constants in solution, which showed hydrogen-bond donors at the N terminus promoted aggregation more effectively than similar modifications at the C terminus. Elongation of the peptide chain also favored aggregation. The geometry of aggregation in solution was investigated by means of titrations with [D6]DMSO and 2D NOE NMR spectroscopy, which allowed the NH protons most involved in intermolecular hydrogen bonds in solution to be identified. X-ray crystallography studies of two oligomers allowed a comparison of the inter- and intramolecular hydrogen-bonding interactions in the solid state and in solution and gave further insight into the geometry of foldamer-foldamer interactions. These solution-based and solid-state studies indicated that the preferred geometry for aggregation is through head-to-tail interactions between the N and C termini of adjacent Aib oligomers.

Conjugates of 1'-aminoferrocene-1-carboxylic acid and proline: synthesis, conformational analysis and biological evaluation

Our previous studies showed that alteration of dipeptides Y-Fca-Ala-OMe (III) into Y-Ala-Fca-OMe (IV) (Y = Ac, Boc; Fca = 1'-aminoferrocene-1-carboxylic acid) significantly influenced their conformational space. The novel bioconjugates Y-Fca-Pro-OMe (1, Y = Ac; 2, Y = Boc) and Y-Pro-Fca-OMe (3, Y = Boc; 4, Y = Ac) have been prepared in order to investigate the influence of proline, a well-known turn-inducer, on the conformational properties of small organometallic peptides with an exchanged constituent amino acid sequences. For this purpose, peptides 1–4 were subjected to detailed spectroscopic analysis (IR, NMR, CD spectroscopy) in solution. The conformation of peptide 3 in the solid state was determined. Furthermore, the ability of the prepared conjugates to inhibit the growth of estrogen receptor-responsive MCF-7 mammary carcinoma cells and HeLa cervical carcinoma cells was tested.

Redox-dependent conformational switching of diphenylacetylenes

Herein we describe the design and synthesis of a redox-dependent single-molecule switch. Appending a ferrocene unit to a diphenylacetylene scaffold gives a redox-sensitive handle, which undergoes reversible one-electron oxidation, as demonstrated by cyclic voltammetry analysis. ¹H-NMR spectroscopy of the partially oxidized switch and control compounds suggests that oxidation to the ferrocenium cation induces a change in hydrogen bonding interactions that results in a conformational switch.