Optimized Fmoc solid-phase synthesis of Thymosin alpha1 by side-chain anchoring onto a PEG resin

Recent advances in the synthesis of C-terminally modified peptides

C-Terminally modified peptides are important for the development and delivery of peptide-based pharmaceuticals because they impact peptide activity, stability, hydrophobicity, and membrane permeability. Additionally, the vulnerability of C-terminal esters to cleavage by endogenous esterases makes them excellent pro-drugs. Methods for post-SPPS C-terminal functionalization potentially enable access to libraries of modified peptides, facilitating tailoring of their solubility, potency, toxicity, and uptake pathway. Apparently minor structural changes can significantly impact the binding, folding, and pharmacokinetics of the peptide. This review summarizes developments in chemical methods for C-terminal modification of peptides published since the last review on this topic in 2003.

Design of a substrate-tailored peptiligase variant for the efficient synthesis of thymosin-α1

The synthesis of thymosin-α₁, an acetylated 28 amino acid long therapeutic peptide, via conventional chemical methods is exceptionally challenging. The enzymatic coupling of unprotected peptide segments in water offers great potential for a more efficient synthesis of peptides that are difficult to synthesize. Based on the design of a highly engineered peptide ligase, we developed a fully convergent chemo-enzymatic peptide synthesis (CEPS) process for the production of thymosin-α₁via a 14-mer + 14-mer segment condensation strategy. Using structure-inspired enzyme engineering, the thiol-subtilisin variant peptiligase was tailored to recognize the respective 14-mer thymosin-α₁ segments in order to create a clearly improved biocatalyst, termed thymoligase. Thymoligase catalyzes peptide bond formation between both segments with a very high efficiency (>94% yield) and is expected to be well applicable to many other ligations in which residues with similar characteristics (e.g. Arg and Glu) are present in the respective positions P1 and P1'. The crystal structure of thymoligase was determined and shown to be in good agreement with the model used for the engineering studies. The combination of the solid phase peptide synthesis (SPPS) of the 14-mer segments and their thymoligase-catalyzed ligation on a gram scale resulted in a significantly increased, two-fold higher overall yield (55%) of thymosin-α₁ compared to those typical of existing industrial processes.

The road to the synthesis of "difficult peptides"

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.

Solid-phase peptide synthesis (SPPS), C-terminal vs. side-chain anchoring: a reality or a myth

Here we review the strategies for the solid-phase synthesis of peptides starting from the side chain of the C-terminal amino acid. Furthermore, we provide experimental data to support that C-terminal and side-chain syntheses give similar results in terms of purity. However, the stability of the two bonds that anchor the peptide to the polymer may determine the overall yield and this should be considered for the large-scale production of peptides. In addition, resins/linkers which do not subject to side reactions can be preferred for some peptides.

Making Ends Meet: Microwave-Accelerated Synthesis of Cyclic and Disulfide Rich Proteins Via In Situ Thioesterification and Native Chemical Ligation

The development of synthetic methodologies for cyclic peptides is driven by the discovery of cyclic peptide drug scaffolds such as the plant-derived cyclotides, sunflower trypsin inhibitor 1 (SFTI-1) and the development of cyclized conotoxins. Currently, the native chemical ligation reaction between an N-terminal cysteine and C-terminal thioester group remains the most robust method to obtain a head-to-tail cyclized peptide. Peptidyl thioesters are effectively generated by Boc SPPS. However, their generation is challenging using Fmoc SPPS because thioester linkers are not stable to repeated piperidine exposure during deprotection. Herein we describe a Fmoc-based protocol for synthesizing cyclic peptides adapted for microwave assisted solid phase peptide synthesis. The protocol relies on the linker Di-Fmoc-3,4-diaminobenzoic acid, and we demonstrate the use of Gly, Ser, Arg and Ile as C-terminal amino acids (using HBTU and HATU as coupling reagents). Following synthesis, an N-acylurea moiety is generated at the C-terminal of the peptide; the resin bound acylurea peptide is then deprotected and cleaved from the resin. The fully deprotected peptide undergoes thiolysis in aqueous buffer, generating the thioester in situ. Ultimately, the head-to-tail cyclized peptide is obtained via native chemical ligation. Two naturally occurring cyclic peptides, the prototypical Möbius cyclotide kalata B1 and SFTI-1 were synthesized efficiently, avoiding potential branching at the diamino linker, using the optimized protocol. In addition, we demonstrate the possibility to use the approach for the synthesis of long and synthetically challenging linear sequences, by the ligation of two truncated fragments of a 50-residue long plant defensin.

ChemMatrix(®) for complex peptides and combinatorial chemistry

CM resin is a totally PEG-based resin, made exclusively from primary ether bonds and therefore highly chemically stable. Compared to other PEG resins, it exhibits good loading and is user friendly because of its free-flowing form upon drying. It shows improved performance over PS resins for the preparation of hydrophobic, highly structured poly-Arg peptides. In combination with ψPros, it allows the synthesis of small proteins such as the chemokine RANTES. Like other PEG-based resins, CM resin swells well in biocompatible solvents such as water, thereby allowing on-bead screening. Furthermore, the high loading of this resin permits the use of a tiny quarter of a bead as a microreactor for HPLC and MALDI-TOF analysis, thus further extending its applications in the field of combinatorial chemistry.

Thymosin alpha 1: biological activities, applications and genetic engineering production

Thymosin alpha 1 (Tα1), a 28-amino acid peptide, was first described and characterized from calf thymuses in 1977. This peptide can enhance T-cell, dendritic cell (DC) and antibody responses, modulate cytokines and chemokines production and block steroid-induced apoptosis of thymocytes. Due to its pleiotropic biological activities, Tα1 has gained increasing interest in recent years and has been used for the treatment of various diseases in clinic. Accordingly, there is an increasing need for the production of this peptide. So far, Tα1 used in clinic is synthesized using solid phase peptide synthesis. Here, we summarize the genetic engineering methods to produce Tα1 using prokaryotic or eukaryotic expression systems. The effectiveness of these biological products in increasing the secretion of cytokines and in promoting lymphocyte proliferation were investigated in vitro studies. This opens the possibility for biotechnological production of Tα1 for the research and clinical applications.