Control of chirality-organized structures of ferrocene–dipeptide bioconjugates

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.

Ferrocene-containing non-interlocked molecular machines

Ferrocene is chemically robust and readily functionalized which enables its facile incorporation into more complex molecular systems. This coupled with ferrocene's reversible redox properties and ability to function as a “molecular ball bearing” has led to the use of ferrocene as a component in wide range of non-interlocked synthetic molecular machine systems.

The Knoevenagel product of indolin-2-one and ferrocene-1,1'-dicarbaldehyde

Indolin-2-one (oxindole), (I), undergoes a Knoevenagel condensation with ferrocene-1,1'-dicarbaldehyde, (II), to afford the title complex 3,3'-[(E,E)-ferrocene-1,1'-diyldimethylidyne]diindolin-2-one dichloromethane disolvate, [Fe(C(28)H(20)N(2)O(2))]·2CH(2)Cl(2), (IV). The structure of (IV) contains two ferrocene complex molecules in the asymmetric unit and displays, as expected, intermolecular hydrogen bonding (N-H···O=C) between the indolin-2-one units. Intermolecular π-π stacking interactions are also observed.

Electrochemistry of redox-active self-assembled monolayers

Redox-active self-assembled monolayers (SAMs) provide an excellent platform for investigating electron transfer kinetics. Using a well-defined bridge, a redox center can be positioned at a fixed distance from the electrode and electron transfer kinetics probed using a variety of electrochemical techniques. Cyclic voltammetry, AC voltammetry, electrochemical impedance spectroscopy, and chronoamperometry are most commonly used to determine the rate of electron transfer of redox-activated SAMs. A variety of redox species have been attached to SAMs, and include transition metal complexes (e.g., ferrocene, ruthenium pentaammine, osmium bisbipyridine, metal clusters) and organic molecules (e.g., galvinol, C(60)). SAMs offer an ideal environment to study the outer-sphere interactions of redox species. The composition and integrity of the monolayer and the electrode material influence the electron transfer kinetics and can be investigated using electrochemical methods. Theoretical models have been developed for investigating SAM structure. This review discusses methods and monolayer compositions for electrochemical measurements of redox-active SAMs.

1,1′-Bis[1-(Hydroxyimino)Alkyl]Ferrocenes and their Antioxidant Activity

Condensation reactions of 1,1′-diacylferrocenes with hydroxylamine afford a series of 1,1′-bis[1-(hydroxyimino)alkyl]ferrocenes, [2(a–d)] which have antioxidant activity towards DPPH· This is enhanced with an increase of the compound concentration, but reduced as the bulkiness of the α-substituent hydroximino group increases. The antioxidant activity of [2(a–d)] is much higher than that of diacylferrocenes.