The road to the synthesis of “difficult peptides”

Abstract

The last decade has witnessed a renaissance of peptides as drugs. This progress, together with advances in the structural behavior of peptides, has attracted the interest of the pharmaceutical industry in these molecules as potential APIs. In the past, major peptide-based drugs were inspired by sequences extracted from natural structures of low molecular weight. In contrast, nowadays, the peptides being studied by academic and industrial groups comprise more sophisticated sequences. For instance, they consist of long amino acid chains and show a high tendency to form aggregates. Some researchers have claimed that preparing medium-sized proteins is now feasible with chemical ligation techniques, in contrast to medium-sized peptide syntheses. The complexity associated with the synthesis of certain peptides is exemplified by the so-called "difficult peptides", a concept introduced in the 80's. This refers to sequences that show inter- or intra-molecular β-sheet interactions significant enough to form aggregates during peptide synthesis. These structural associations are stabilized and mediated by non-covalent hydrogen bonds that arise on the backbone of the peptide and-depending on the sequence-are favored. The tendency of peptide chains to aggregate is translated into a list of common behavioral features attributed to "difficult peptides" which hinder their synthesis. In this regard, this manuscript summarizes the strategies used to overcome the inherent difficulties associated with the synthesis of known "difficult peptides". Here we evaluate several external factors, as well as methods to incorporate chemical modifications into sequences, in order to describe the strategies that are effective for the synthesis of "difficult peptides". These approaches have been classified and ordered to provide an extensive guide for achieving the synthesis of peptides with the aforementioned features.

A Helping Hand to Overcome Solubility Challenges in Chemical Protein Synthesis

“…88 We aim to expand its application beyond the introduction of solubilizing tags, e.g., to immobilize a peptide for solid phase ligations, introduce affinity purification tags, or temporarily link protein subunits in order to facilitate difficult folding processes.…”

Structure–mechanical property correlations of hydrogel forming β-sheet peptides

“…120 In another study, peptides amidated in the C-terminus and containing alternating Val-Lys residues from 9 to 13 residues in length were reported. 122,123 Thus, the fact that Ac-(VKVK) 2 -NH 2 or (VKVK) 2 -NH 2 , a b-sheet component of MAX1, fails to form hydrogels could be attributed to the sequence length. The longest peptide; (VKVK) 3 V-NH 2 formed the stiffest gels, which was attributed to the formation of a greater number of physical crosslinks.…”

Disulfide Bond Mimetics: Strategies and Challenges

“…Conveniently, on‐resin metathesis also allows the removal of the catalyst by simple resin washing steps. Limitations to on‐resin metathesis approach come from the potential for aggregation that is well known to plague resin‐bound fully protected peptides in a sequence‐dependent manner . The use of microwave irradiation during on‐resin RCM, also in combination with chaotropic agents, has contributed to improving the success rate of solid supported metathesis .…”