How Useful Is Ferrocene as a Scaffold for the Design of β-Sheet Foldamers?

Chiral molecular nanosilicas

Molecular nanoparticles including polyoxometalates, proteins, fullerenes and polyhedral oligosiloxane (POSS) are nanosized objects with atomic precision, among which POSS derivatives are the smallest nanosilicas. Incorporation of molecular nanoparticles into chiral aggregates either by chiral matrices or self-assembly allows for the transfer of supramolecular chirality, yet the construction of intrinsic chirality with atomic precision in discrete molecules remains a great challenge. In this work, we present a molecular folding strategy to construct giant POSS molecules with inherent chirality. Ferrocenyl diamino acids are conjugated by two or four POSS segments. Hydrogen bonding-driven folding of diamino acid arms into parallel β-sheets facilitates the chirality transfer from amino acids to ferrocene and POSS respectively, disregarding the flexible alkyl spacers. Single crystal X-ray structures, density functional theory (DFT) calculations, circular dichroism and vibrational circular dichroism spectroscopy clearly verify the preferential formation of one enantiomer containing chiral molecular nanosilicas. The chiral orientation and chiroptical properties of POSS show pronounced dependence on the substituents of α-amino acids, affording an alternative way to control the folding behavior and POSS chirality in addition to the absolute configuration of amino acids. Through the kinetic nanoprecipitation protocol, one-dimensional aggregation enables chirality transfer from the molecular scale to the micrometer scale, self-assembling into helices in accordance with the packing propensity of POSS in a crystal phase. This work, by illustrating the construction of chiral molecular nanosilicas, paves a new way to obtain discrete chiral molecular nanoparticles for potential chiroptical applications.

Bispidine as a β-strand nucleator: from a β-arch to self-assembled cages and vesicles

The development of synthetic scaffolds that nucleate well-folded secondary structures is highly challenging. Herein, we designed and synthesized a series of core-modified peptides (F1, F2, F3, and F4) that fold into β-strand structures. These bispidine-scaffolded peptides were studied by CD, IR, NMR, single crystal XRD, and Molecular Dynamics (MD) simulations to investigate their conformational preferences. Solid-state and solution studies revealed that bispidine is a versatile scaffold that could be placed either at the terminal or at the middle of the peptide strand for nucleating the β-strand structure. Scaffolds that nucleate an isolated β-strand conformation are rare. Bispidine placed at the C-terminus of the peptide chain could nucleate a β-strand conformation, while bispidine placed at the middle resulted in a β-arch conformation. This nucleation activity stems from the ability to restrict the psi torsion angle (ψ) through intramolecular C5 hydrogen bonding between the equatorial hydrogen(s) of bispidine and the carbonyl oxygen(s) of the amino acid close to the scaffold. Furthermore, the bispidine peptidomimetic with a super secondary structure, namely β-arch, assembled into single-hole submicron cages and spherical vesicles as evident from microscopic studies. The design logic defined here will be a significant strategy for the development of β-strand mimetics and super secondary structures.

Water-Mediated Folding Behaviors and Chiroptical Inversion of Ferrocene-Conjugated Dipeptides

The hydration effect on the folding behavior of oligopeptides is of vital importance both in the structure basis of biomolecules and in the rational design of peptide-based materials, which however has rarely been addressed. Here we present the hydration impact on the spontaneous folding of dipeptides conjugated by the ferrocene spacer. In organic phase, the ferrocene-glycine-phenylalanine dipeptide formed a parallel β-sheet structure and Herrick's conformation, which underwent conformational transformation encountering aqueous media, by significantly switching dipeptide arm angles around the ferrocene axis up to 72°. The conformational transformation behavior aroused inversion of the chiroptical activity. Solid X-ray structures, proton nuclear magnetic resonance, chiroptical spectroscopy, and the density functional theory calculation were employed to unveil the hydration effect in the secondary structure transition, in which the rearrangement of hydrogen bonds played the vital role. This work deepens the understanding of water functioning in the structure modulation of biomolecules and also provides an alternative protocol in designing novel chiroptical switches and adaptive peptide-based biomaterials.

Aromatic foldamers as scaffolds for metal second coordination sphere design

As metalloproteins exemplify, the chemical and physical properties of metal centers depend not only on their first but also on their second coordination sphere. Installing arrays of functional groups around the first coordination sphere of synthetic metal complexes is thus highly desirable, but it remains a challenging objective. Here we introduce a novel approach to produce tailored second coordination spheres. We used bioinspired artificial architectures based on aromatic oligoamide foldamers to construct a rigid, modular and well-defined environment around a metal complex. Specifically, aza-aromatic monomers having a tethered [2Fe-2S] cluster have been synthesized and incorporated in conical helical foldamer sequences. Exploiting the modularity and predictability of aromatic oligoamide structures allowed for the straightforward design of a conical architecture able to sequester the metal complex in a confined environment. Even though no direct metal complex-foldamer interactions were purposely designed in this first generation model, crystallography, NMR and IR spectroscopy concurred to show that the aromatic oligoamide backbone alters the structure and fluxional processes of the metal cluster.

Structure-Dependent Guest Recognition with Flexible Ferrocene-Based Aromatic Oligoamide β-Sheet Mimics

A series of aromatic oligoamides incorporating an inherently flexible ferrocene dicarboxylic acid unit was synthesized. Solid state, solution, and computational studies on these systems indicated that the aromatic strands can adopt a syn parallel stacked conformation. This results in modular β-sheet-like molecular clefts that display structure-dependent recognition of small polar molecules. NMR and theoretical studies of the host-guest interaction support an in cleft binding mode and allowed the selectivity of the oligomers to be rationalized on the basis of minor changes in functional-group presentation on the edge of the aromatic strands.

New 1,1'-Ferrocene Bis(sulfonyl) Reagents

Several new 1,1'-bis(sulfonyl)ferrocenes designed for the synthesis of sulfonamide linked biological conjugates have been prepared. 1,1'-Bis(sulfonylbromide)ferrocene can be produced from the corresponding sulfonylchloride via a bis(sulfonylhydrazide) intermediate. Bis(sulfonyl-N-hydroxybenzotriazole)ferrocene can also be synthesized from the sulfonyl chloride, and reaction of glycine methyl ester with the sulfonyl chloride affords a [3]ferrocenophane complex. All new compounds have been structurally characterized by X-ray crystallography.

The synthesis and structures of 1,1'-bis(sulfonyl)ferrocene derivatives

A series of 1,1'-bis(sulfonyl)ferrocene compounds were produced via the 1,1'-bis(sulfonate)ferrocene ammonium salt. This compound can be readily converted to 1,1' bis(sulfonylchloride)ferrocene. By varying stoichiometry and reaction times, both mono- and bis-sulfonamide derivatives can be synthesized. All new compounds presented in this report have been structurally characterized. The structures of the bis-sulfonamide systems are similar to the well-studied bis(amide) ferrocene compounds. Intermolecular hydrogen bonding is observed, typically between NH and SO groups of neighboring sulfonamides. However in the bis(GABA) derivative, intermolecular NH to CO hydrogen bonding interactions are present.

Engineering β-sheets employing N-methylated heterochiral amino acids

There is a lack of functional group diversity in the reverse turn motifs nucleating a β-sheet conformation in designed peptides, proteins and foldamers. The majority of these sequences consist of d-Pro-l-Pro, d-Pro-Gly or Asn-Gly as the turn inducing motif restricting their biological application and physicochemical modulation. In this report, for the first time we elucidate that N-methylation of heterochiral amino acids in linear peptides nucleates β-sheet conformation without the necessity of having a ring or covalent constraint at the reverse turn. Our results show that d-Pro can be conveniently substituted by any other N-methylated d-amino acid followed by an N-methylated l-amino acid or sarcosine to adopt a βII' turn inducing the β-sheet folding. Furthermore, we reveal that a single amino acid either at the i + 1 or i + 2 site of the reverse turn can modulate the right-handed twist, which eventually dictates the extent of the foldedness of the β-hairpin.

The conjugates of ferrocene-1,1'-diamine and amino acids. A novel synthetic approach and conformational analysis

A novel synthetic approach toward a poorly explored bioorganometallic consisting of ferrocene-1,1'-diamine bearing structurally and chirally diverse amino acid sequences is reported. Until now, ferrocene-1,1'-diamine was suitable for accommodating only identical amino acid sequences at its N-termini, leading to the symmetrically disubstituted homochiral products stabilized through a 14-membered intramolecular hydrogen-bonded ring as is seen in antiparallel β-sheet peptides. The key step of the novel synthetic pathway is the transformation of Ac-Ala-NH-Fn-COOH (5) (Fn = 1,1'-ferrocenylene) to orthogonally protected Ac-Ala-NH-Fn-NHBoc (7). The spectroscopic analysis (IR, NMR, CD) of the novel compounds, corroborated with DFT studies, suggests the interesting feature of the ferrocene-1,1'-diamine scaffold. The same hydrogen-bonding pattern, i.e. a 14-membered hydrogen-bonded ring, was determined both in solution and in the solid state, thus making them promising, yet simple scaffolds capable of mimicking β-sheet peptides. In vitro screening of potential anticancer activity in Hep G2 human liver carcinoma cells and Hs 578 T human breast cancer cells revealed a cytotoxic pattern for novel compounds (150-500 μM) with significantly decreased cell proliferation.

Synthesis of ferrocene amides and esters from aminoferrocene and 2-substituted ferrocenecarboxylic acid and properties thereof

The X-ray diffraction structure of the ferrocene ester FcE-1 showed the presence of an intermolecular hydrogen bond between the amide functional groups.

Structure-Based Design of Inhibitors of Protein-Protein Interactions: Mimicking Peptide Binding Epitopes

Protein-protein interactions (PPIs) are involved at all levels of cellular organization, thus making the development of PPI inhibitors extremely valuable. The identification of selective inhibitors is challenging because of the shallow and extended nature of PPI interfaces. Inhibitors can be obtained by mimicking peptide binding epitopes in their bioactive conformation. For this purpose, several strategies have been evolved to enable a projection of side chain functionalities in analogy to peptide secondary structures, thereby yielding molecules that are generally referred to as peptidomimetics. Herein, we introduce a new classification of peptidomimetics (classes A-D) that enables a clear assignment of available approaches. Based on this classification, the Review summarizes strategies that have been applied for the structure-based design of PPI inhibitors through stabilizing or mimicking turns, β-sheets, and helices.

Diphenylacetylene-linked peptide strands induce bidirectional β-sheet formation

In the search for synthetic mimics of protein secondary structures relevant to the mediation of protein-protein interactions, we have synthesized a series of tetrasubstituted diphenylacetylenes that display β-sheet structures in two directions. Extensive X-ray crystallographic and NMR solution phase studies are consistent with these proteomimetics adopting sheet structures, displaying both hydrophobic and hydrophilic amino acid side chains.

Discovery of linear receptors for multiple dihydrogen phosphate ions using dynamic combinatorial chemistry

We describe the use of dynamic combinatorial chemistry to discover a new series of linear hydrazone-based receptors that bind multiple dihydrogen phosphate ions. Through the use of a template-driven, selection-based approach to receptor synthesis, dynamic combinatorial chemistry allows for the identification of unexpected host structures and binding motifs. Notably, we observed the unprecedented selection of these linear receptors in preference to competing macrocyclic hosts. Furthermore, linear receptors containing up to nine building blocks and three different building blocks were amplified in the dynamic combinatorial library. The receptors were formed using a dihydrazide building block based on an amino acid-disubstituted ferrocene scaffold. A detailed study of the linear pentamer revealed that it forms a helical ditopic receptor that employs four acylhydrazone hydrogen-bond donor motifs to cooperatively bind two dihydrogen phosphate ions.

Synthetic mimetics of protein secondary structure domains

Proteins modulate the majority of all biological functions and are primarily composed of highly organized secondary structural elements such as helices, turns and sheets. Many of these functions are affected by a small number of key protein-protein contacts, often involving one or more of these well-defined structural elements. Given the ubiquitous nature of these protein recognition domains, their mimicry by peptidic and non-peptidic scaffolds has become a major focus of contemporary research. This review examines several key advances in secondary structure mimicry over the past several years, particularly focusing upon scaffolds that show not only promising projection of functional groups, but also a proven effect in biological systems.

Heterobimetallic metal-complex assemblies constructed from the flexible arm-like ligand 1,1'-bis[(3-pyridylamino)carbonyl]ferrocene: structural versatility in the solid state

The bidentate ferrocenyl sandwich molecule 1,1'-bis[(3-pyridylamino)carbonyl]ferrocene (3-BPFA) has been employed as an organometallic ligand in reactions with a series of transition metal salts to construct heterobimetallic architectures. X-ray crystallographic characterization reveals that the crystal packing of free ligand 3-BPFA induces spontaneous resolution of helical chains via intermolecular hydrogen bonds. By combining the flexibility from the arm-like molecule (3-BPFA) with the variation of the coordination property from different metal ions and/or the different counteranions, five different types of architectures are prepared: one octahedral coordination cage (copper(II) complex 1); two discrete pseudocapsules for combination of chlorine anions (nickel(II) complex 2 and cobalt(II) complex 3); two dimers with metal-metal interactions (silver(I) complexes 4 and 5); one macrocyclic complex (mercury(II) complex 6); and five two-dimensional mixed-metal-organic frameworks (M'-MOFs) (zinc(II), cadmium(II), and mercury(II) complexes 7-11). The structures of all complexes are characterized in detail by IR, elementary analysis, and single-crystal X-ray diffraction analysis. The factors inducing the structure variation among the complexes are discussed by taking account of the coordination geometry of different metal ions, the span angle between the two "arms", and the coordination mode of the 3-BPFA ligand.