Structural Mimicry of Two Cytochrome b562 Interhelical Loops Using Macrocycles Constrained by Oxazoles and Thiazoles

Strategies for Fine-Tuning the Conformations of Cyclic Peptides

Cyclic peptides are promising scaffolds for drug development, attributable in part to their increased conformational order compared to linear peptides. However, when optimizing the target-binding or pharmacokinetic properties of cyclic peptides, it is frequently necessary to "fine-tune" their conformations, e.g., by imposing greater rigidity, by subtly altering certain side chain vectors, or by adjusting the global shape of the macrocycle. This review systematically examines the various types of structural modifications that can be made to cyclic peptides in order to achieve such conformational control.

Efficient synthesis of novel conjugated 1,3,4-oxadiazole–peptides

In this study, we developed an efficient approach for the synthesis of 2-amino-1,3,4-oxadiazoles that are bioisosteres of the amide functional group. The synthesized oxadiazoles were conjugated to octa- and nonapeptides through the C- or N-terminus as precursors of leuprolide acetate. The synthesized compounds are peptidomimetics of leuprolide acetate.

Macrocyclic Peptidomimetics Prepared by Ring-Closing Metathesis and Azide–Alkyne Cycloaddition

Macrocycles are finding increasing use as a means to define the backbone geometries of peptides and peptidomimetics. Ring-closing metathesis and CuI-catalyzed azide–alkyne cycloaddition are particularly useful for introducing such rings and they do so in high yield and with a good functional group tolerance and compatibility. Here, we present an overview of the use of these two methods, with reference to selected examples and particular reference to β-strand peptidomimetics for use as protease inhibitors.

Oxadiazole grafts in peptide macrocycles

Synthetic methods that provide control over macrocycle conformation and, at the same time, mitigate the polarity of peptide bonds represent valuable tools for the discovery of new bioactive molecules. Here, we report a macrocyclization reaction between a linear peptide, an aldehyde and (N-isocyanimino)triphenylphosphorane. This process generates head-to-tail cyclic peptidomimetics in a single step. This method is tolerant to variation in the peptide and aldehyde components and has been applied for the synthesis of 15-, 18-, 21- and 24-membered rings. The resulting peptide macrocycles feature a 1,3,4-oxadiazole and a tertiary amine in their scaffolds. This non-canonical backbone region acts as an endocyclic control element that promotes and stabilizes a unique intramolecular hydrogen-bond network and can lead to macrocycles with conformationally rigid turn structures. Oxadiazole-containing macrocycles can also display a high passive membrane permeability, an important property for the development of bioavailable peptide-based therapeutics.

Reconstructing the discontinuous and conformational β1/β3-loop binding site on hFSH/hCG by using highly constrained multicyclic peptides

Making peptide-based molecules that mimic functional interaction sites on proteins remains a challenge in biomedical sciences. Here, we present a robust technology for the covalent assembly of highly constrained and discontinuous binding site mimics, the potential of which is exemplified for structurally complex binding sites on the "Cys-knot" proteins hFSH and hCG. Peptidic structures were assembled by Ar(CH2 Br)2-promoted peptide cyclizations, combined with oxime ligation and disulfide formation. The technology allows unprotected side chain groups and is applicable to peptides of different lengths and nature. A tetracyclic FSH mimic was constructed, showing >600-fold improved binding compared to linear or monocyclic controls. Binding of a tricyclic hCG mimic to anti-hCG mAb 8G5 was identical to hCG itself (IC50 =260 vs. 470 pM), whereas this mimic displayed an IC50 value of 149 nM for mAb 3468, an hCG-neutralizing antibody with undetectable binding to either linear or monocyclic controls.

Total synthesis, structure, and oral absorption of a thiazole cyclic peptide, sanguinamide A

The first total synthesis and three-dimensional solution structure are reported for sanguinamide A, a thiazole-containing cyclic peptide from the sea slug H. sanguineus. Solution phase fragment synthesis, solid phase fragment assembly, and solution macrocyclization were combined to give (1) in 10% yield. Spectral properties were identical for the natural product, requiring revision of its structure from (2) to (1). Intramolecular transannular hydrogen bonds help to bury polar atoms, which enables oral absorption from the gut.

Noncovalent template-assisted mimicry of multiloop protein surfaces: assembling discontinuous and functional domains

We report here a novel noncovalent synthetic strategy for template-assembled de novo protein design. In this approach, a peptide was first conjugated with two oligoguanosine strands via click chemistry and the conjugates were then self-assembled in the presence of metal ions. G-quadruplex formation directs two peptide strands to assemble on one face of the scaffold and form an adjacent two loop surface. This approach can be used to rapidly prepare multiple two-loop structures with both homo- and heterosequences.

Cyclooligomerization of a helix-bearing template into macrocycles bearing multiple helices

Cyclooligomerization was investigated for separating and spatially arranging helical peptides as discontinuous surfaces. Tetrapeptide H-[Ile-Ser-Lys(Ox)]-OH, containing a turn-inducing oxazole constraint, was connected through its lysine side chain via a beta-alanine linker to the C-terminus of a two-turn helical nonapeptide Ac-(cyclo-4,8)-LRL [KARAD](Aib). The resulting helix-appended template was self-condensed and cyclized to a library of macrocycles ( n = 2-6) containing multiple (2-6) helices. An NMR structure shows retention of alpha helicity in the cyclotrimer ( n = 3).

Self-condensation of a thiazole-peptide bearing a 21-membered loop into a library of giant macrocycles with multiple orthogonal loops

[reaction: see text] Tetrapeptide analogue H-[Glu-Ser-Lys(Thz)]-OH, containing a turn-inducing thiazole constraint, was used as a template to produce a 21-membered structurally characterized loop by linking Glu and Lys side chains with a Val-Ile dipeptide. This template was oligomerized in one pot to a library (cyclo-[1](n)(), n = 2-10) of giant symmetrical macrocycles (up to 120-membered rings), fused to 2-10 appended loops that were carried intact through multiple oligomerization (chain extension) and cyclization (chain terminating) reactions of the template. A three-dimensional solution structure for cyclo-[1](3) shows all three appended loops projecting from the same face of the macrocycle. This is a promising approach to separating peptide motifs over large distances.