Efficient synthesis of a side-chain extended diaminodiacid for solid-phase synthesis of peptide disulfide bond mimics

Synthesis of Asp-based lactam cyclic peptides using an amide-bonded diaminodiacid to prevent aspartimide formation

Asp-based lactam cyclic peptides are considered promising drug candidates. However, using Fmoc solid-phase peptide synthesis (Fmoc-SPPS) for these peptides also causes aspartimide formation, resulting in low yields or even failure to obtain the target peptides. Here, we developed a diaminodiacid containing an amide bond as a β-carboxyl-protecting group for Asp to avoid aspartimide formation. The practicality of this diaminodiacid has been illustrated by the synthesis of lactam cyclic peptide cyclo[Lys9,Asp13] KIIIA7-14 and 1Y.

Chemical synthesis of disulfide surrogate peptides by using beta-carbon dimethyl modified diaminodiacids

The replacement of disulfide bridges with metabolically stable isosteres is a promising strategy to improve the stability of disulfide-rich polypeptides towards reducing agents and isomerases. A diaminodiacid-based strategy is one of the most effective methods to construct disulfide bond mimics, but modified diaminodiacids have not been developed till now. Inspired by the fact that alkylation of disulfide bonds can regulate the activity of polypeptides, herein, we report the first example of thioether bridged diaminodiacids incorporating Cys C_β dimethyl modification, obtained by penicillamine (Pen)-based thiol alkylation. The utility of these new diaminodiacids was demonstrated by the synthesis of disulfide surrogates of oxytocin containing a short-span disulfide bond and of KIIIA with large-span disulfide bonds. This new type of synthetic bridge further extends the diaminodiacid toolbox to facilitate the study of the structure-activity relationship of disulfide-rich peptides.

An overview of recent progress in modern synthetic approach—combinatorial synthesis

Background

In recent times, a powerful tool of combinatorial synthesis has been used for the preparation of large chemical entities through a small set up of reactions between different building blocks using solid-phase and solution-phase techniques. This method reduced the time and cost of the drug discovery process substantially.

Main text

Thousands of compounds are synthesised in a few reactions through combinatorial synthesis instead of getting a few compounds in the traditional method. This method also helps to identify chemical lead of the compounds and optimise them through the biological screening using a high-throughput method. There is no review concerning the recent research finding of combinatorial synthesis. Hence, an attempt had been made on the latest research findings (2002–2020) of newly synthesised compounds using combinatorial synthesis and their biological activities.

Conclusion

To the best of our knowledge, the current review has completely analysed the importance of combinatorial synthesis and furnished an overview of solid-phase and solution-phase techniques as well as helped mankind by improving higher productivity at low cost, lead identification and optimization and preventing environmental pollution.

Graphical abstract

Chemical synthesis and biological activity of peptides incorporating an ether bridge as a surrogate for a disulfide bond

Disulfide bridges contribute to the definition and rigidity of polypeptides, but they are inherently unstable in reducing environments and in the presence of isomerases and nucleophiles. Strategies to address these deficiencies, ideally without significantly perturbing the structure of the polypeptide, would be of great interest. One possible surrogate for the disulfide bridge is a simple thioether, but these are susceptible to oxidation. We report the introduction of an ether linkage into the biologically active, disulfide-rich peptides oxytocin, tachyplesin I, and conotoxin α-ImI, using an ether-containing diaminodiacid as the key building block, obtained by the stereoselective ring-opening addition reaction of an aziridine skeleton with a hydroxy group. NMR studies indicated that the derivatives with an ether surrogate bridge exhibited very small change of their three-dimensional structures. The analogs obtained using this novel substitution strategy were found to be more stable than the original peptide in oxidative and reductive conditions; without a loss of bioactivity. This strategy is therefore proposed as a practical and versatile solution to the stability problems associated with cysteine-rich peptides.

We report the first introduction of an ether linkage as surrogate into the disulfide-rich peptides using ether-containing diaminodiacid.