β-Turn Induction by a Diastereopure Azepane-Derived Quaternary Amino Acid

β-Turns are one of the most common secondary structures found in proteins. In the interest of developing novel β-turn inducers, a diastereopure azepane-derived quaternary amino acid has been incorporated into a library of simplified tetrapeptide models in order to assess the effect of the azepane position and peptide sequence on the stabilization of β-turns. The conformational analysis of these peptides by molecular modeling, NMR spectroscopy, and X-ray crystallography showed that this azepane amino acid is an effective β-turn inducer when incorporated at the i + 1 position. Moreover, the analysis of the supramolecular self-assembly of one of the β-turn-containing peptide models in the solid state reveals that it forms a supramolecular helical arrangement while maintaining the β-turn structure. The results here presented provide the basis for the use of this azepane quaternary amino acid as a strong β-turn inducer in the search for novel peptide-based bioactive molecules, catalysts, and biomaterials.

Miniature β-Hairpin Mimetic by Intramolecular Hydrogen Bond and C-H···π Interactions

Canonically, protein β-hairpin motifs are stabilized by intramolecular hydrogen bonds. Here, we attempt to develop a rational design recipe for a miniature hairpin structure stabilized by hydrogen bonding as well as C-H···π interaction and try to understand how such a stabilization effect varies with different functional groups at each terminus. Database analysis shows that the α-amino acids with an aromatic side chain will not favor that kind of C-H···π stabilized hairpin structure. However, hybrid tripeptides with an N-terminal Boc-Trp-Aib corner residue and C-terminal aromatic ω-amino acids fold into the hairpin conformation with a central β-turn/open-turn that is reinforced by a C-H···π interaction. The CCDC database analysis further confirms that this C-H···π stabilized hairpin motif is general for Boc-protected tripeptides containing Aib in the middle and aromatic functionality at the C-terminus. The different α-amino acids like Leu/Ala/Phe/Pro/Ser at the N-terminus have a minor influence on the C-H···π interaction and stabilities of the folded structures in solid-state. However, the hybrid peptides exhibit different degrees of conformational heterogeneity both in the solid and solution phase, which is common for this kind of flexible small molecule. Conformational heterogeneity in the solution phase including the C-H···π stabilized β-hairpin structures are characterized by the molecular dynamics (MD) simulations explaining their plausible origin at an atomistic level.

Unique supramolecular assembly of a synthetic achiral α, γ-hybrid tripeptide

An achiral tripeptide, namely, Boc-γ-Abu-m-ABA-Aib-OMe (γ-Abu: γ−amino butyric acid; m-ABA: meta-aminobenzoic acid) was synthesized by solution phase procedure. The α, γ-hybrid peptide was designed in such a way that two dissimilar γ−amino acids, one flexible and another rigid, were positioned sidewise along with α-amino isobutyric acid (Aib) as C-terminal residue. The single crystal X-ray diffraction analysis revealed that two kinks were generated around centrally placed m-ABA. Interestingly, the peptide self-assembled via three intermolecular N–H···O and one intermolecular C–H···O hydrogen bonding interactions to supramlecular helical architecture.

Supramolecular helical self-assembly of small peptides

In this highlight, we describe the construction of supramolecular single/double/triple-helical assemblies from small di/tri/tetrapeptides and their applications.

Supramolecular Antiparallel β-Sheet Formation by Tetrapeptides Based on Amyloid Sequence

Self-assembly of short peptides has emerged as an interesting research field for a wide range of applications. Recently, several truncated fragments of long-chain peptides or proteins responsible for different neurodegenerative diseases were studied to understand whether they can mimic the property and function of native peptides or not. It was reported that such a kind of peptide adopts a β-sheet structure in the disease state. It was observed that aromatic amino acid-rich peptide fragments possess a high tendency to adopt a β-sheet conformation. In this article, we are first time reporting the crystal structure of two tetrapeptides: Boc-GAII-OMe (Peptide 1) and Boc-GGVV-OMe (Peptide 2), composed of aliphatic amino acids, and the sequences are similar to the Aβ-peptide fragments Aβ_29-32 and Aβ_37-40 , respectively. In the solid-state, they are self-assembled in an antiparallel β-sheet fashion. The peptide units are connected by the strong amide hydrogen-bonding (N-H···O) interactions. Apart from that, other noncovalent interactions are also present, which help to stabilize the cross-β-sheet arrangement. Interestingly, in the crystal structure of Peptide 1, noncovalent C···C interaction between the electron-deficient carbonyl carbon, and the electron-rich sp³-carbon atom is observed, which is quite rare in the literature. The calculated torsion angles for these peptides are lying in the β-sheet region of the Ramachandran plot. FT-IR studies also indicate the formation of an antiparallel β-sheet structure in the solid-state. Circular dichroism of the peptides in the aqueous solution also suggests the presence of predominantly β-sheet-like conformation in the aqueous solution. Under cross-polarized light, Congo Red stained both peptides showed green-gold color due to birefringence indicating their amyloidogenic nature. This result indicates that the short peptide composed of aliphatic amino acid is capable of forming a β-sheet structure in the absence of aromatic amino acid and also can mimic the function of the native amyloid peptide.

Conformational Heterogeneity and Self-Assembly of α,β,γ-Hybrid Peptides Containing Fenamic Acid: Multistimuli-Responsive Phase-Selective Gelation

The effect of fenamic acid-α-aminoisobutyric acid corner motif in α,β,γ-hybrid peptides has been reported. From X-ray single-crystal diffraction studies, it is observed that Phe-containing peptide 1 has an "S"-shaped conformation that is stabilized by two consecutive intramolecular N-H···N hydrogen bonds. However, the tyrosine analogue peptide 2 has an "S"-shaped conformation, which is stabilized by consecutive intramolecular six-member N-H···N and seven-member N-H···O hydrogen bonds. The asymmetric unit of peptide 3 containing m-aminobenzoic acid has two molecules which are stabilized by multiple intermolecular hydrogen-bonding interactions. There are also π-π stacking interactions between the aromatic rings of fenamic acid. The peptides 1 and 2 have a polydisperse microsphere morphology, but peptide 3 has an entangled fiber-like morphology. Peptides 1-3 do not form organogels. However, in the presence of water, the peptide 3 forms a phase-selective instant gel in xylene. The gel exhibits high stability and thermal reversibility. The phase-selective gel of peptide 3 is highly responsive to H₂SO₄.

Multiplex optical detection and quantification of DNA fragments by metallo-peptide assemblies

Rapid detection of infectious agents such as bacteria and viruses are important for proper health management, agriculture and homeland security. This paper presents a multiplex DNA detection system self-assembled by a metallo-peptide complex. Within five minutes, the system can simultaneously detect multiple DNA fragments, without any need for their separation. The presence of proteins in the sample does not harm the detection capabilities of the system, which can discriminate even between one base-pair mismatch and can perform at concentrations as low as 200 pM.

Intertwined gababutin-based supramolecular double helix

A dimer-assembly driven supramolecular double helix is observed for the gababutin-based short peptide sequence and this architecture exhibits electrochemical features.

Formation of supramolecular single and double helix-like structures from designed tripeptides

The conformation and self-assembly of N- and C-protected tripeptides, Boc-Gly-l-Phg-d-Phe-OMe (1, Phg: phenylglycine) and Boc-Gly-l-Phg-d-Phg-OMe (2), have been investigated.

Unique crystallographic signatures of Boc-Gly-Phe-Phe-OMe and Boc-Gly-Phg-Phe-OMe and their self-association

The crystal structures and morphology of Boc-Gly-Phe-Phe-OMe (1) and Boc-Gly-Phg-Phe-OMe (2) are reported. While 1 forms a rare open turn conformation, 2 forms a β-sheet conformation.

Self-assembly of Functional Nanostructures by Short Helical Peptide Building Blocks

The self-assembly of short peptide building blocks into well-ordered nanostructures is a key direction in bionanotechnology. The formation of β -sheet organizations by short peptides is well explored, leading to the development of a wide range of functional assemblies. Likewise, many natural proteinaceous materials, such as silk and amyloid fibrils, are based on β-sheet structures. In contrast, collagen, the most abundant protein in mammals, is based on helical arrangement. Similar to β-sheet structures, short helical peptides have been recently discovered to possess a diverse set of functionalities with the potential to fabricate artificial self-assembling materials. Here, we outline the functional roles of self-assembled nanostructures formed by short helical peptides and their potential as artificial materials. We focus on the association between self-assembled mesoscale structures and their material function and demonstrate the way by which this class of building blocks bears the potential for diverse applications, such as the future fabrication of smart devices.

A self-assembled peptide mimetic of a tubular host and a supramolecular polymer

A tripeptide self-assembles in a helical manner to form a tubular host-like supramolecular nanotube. The tube with a hydrophobic core has been used to develop a supramolecular polymer.

Self-assembled peptide microspheres for sustainable release of sulfamethoxazole

Porous peptide microspheres have been used for the loading and sustained release of the bacteriostatic antibiotic sulfamethoxazole.

Thionoglycine as a multifunctional spectroscopic reporter of screw-sense preference in helical foldamers

A single thionoglycine (glycine thioamide, -HNCH2C(=S)-) residue inserted into a peptide foldamer provides both a pair of germinal protons for use as a (1)H NMR stereochemical probe and a chromophore giving rise to a well defined Cotton effect in CD. Comparison of the response of these two features to a local helically chiral environment validates them as independent methods for quantifying the conformational screw-sense preference of a helical oligomer, in this case a peptide made of repeated Aib units. The sign of the Cotton effect provides a measure of the sign of the screw-sense preference, while both the chemical shift separation of the anisochronous signals of the glycine CH2 group and the magnitude of the Cotton effect give an estimate of the helicity excess of the oligomer. The thionoglycine unit is readily introduced synthetically by a thionation of a BocGlyAibOMe dipeptide.

1H-Azepine-4-amino-4-carboxylic acid: a new α,α-disubstituted ornithine analogue capable of inducing helix conformations in short Ala-Aib pentapeptides

A very efficient synthesis of orthogonally protected 1H-azepine-4-amino-4-carboxylic acid, abbreviated as Azn, a conformationally restricted analogue of ornithine, was realized. It was obtained on a gram scale in good overall yield in five steps, three of which did not require isolation of the intermediates, starting from the readily available 1-amino-4-oxo-cyclohexane-4-carboxylic acid. Both enantiomers were used for the preparation of pentapeptide models containing Ala, Aib, and Azn. Conformational studies using both spectroscopic techniques (NMR, CD) and molecular dynamics on model 5-mer peptides showed that the (R)-Azn isomer possesses a marked helicogenic effect.