Recent Advances in the Solid- and Solution-Phase Synthesis of Peptides and Proteins Using Microflow Technology

Effect of Brønsted Acids on the Activation of Mixed Anhydride/Mixed Carbonic Anhydride and C-Terminal-Free N-Methylated Peptide Synthesis in a Micro-Flow Reactor

Amidations employing mixed (carbonic) anhydrides have long been favoured in peptide synthesis because of their cost-effectiveness and less waste generation. Despite their long history, no study has compared the effects of additives on the activation of mixed anhydrides and carbonic anhydrides. In this study, we investigated the amidation of mixed (carbonic) anhydride in the presence of a base and/or Brønsted acids. The use of NMI⋅HCl significantly improved the conversion of the mixed carbonic anhydride, while expediting nucleophilic attacks on the desired carbonyl group. In contrast, in the case of mixed anhydrides, neither the conversion nor the desired nucleophilic attack improved significantly. We developed a C-terminus-free N-methylated peptide synthesis method using mixed carbonic anhydrides in a micro-flow reactor. Fourteen N-alkylated peptides were synthesized in moderate to high yields (55-99 %) without severe racemization (<1 %). Additionally, a significant enhancement in the amidation between mixed carbonic anhydrides and bis-TMS-protected N-methyl amino acids with the inclusion of NMI⋅HCl was observed for the first time. In addition, we observed unexpected C-terminal epimerization of the C-terminus-free N-methyl peptides.

Rational Design of Antifungal Peptides Based on the γ-Core Motif of a Neosartorya (Aspergillus) fischeri Antifungal Protein to Improve Structural Integrity, Efficacy, and Spectrum

Antifungal peptides offer promising alternative compounds for the treatment of fungal infections, for which new antifungal compounds are urgently needed. Constant and broad antifungal spectra of these peptides play essential roles in their reliable therapeutic application. It has been observed that rationally designed peptides using the evolutionarily conserved γ-core region (GXC-X3-9-C) of an antifungal protein from Neosartorya (Aspergillus) fischeri highly inhibit the growth of fungi. The cysteines in these peptides have free sulfhydryl groups, which allow cyclization and dimerization under oxidative conditions, thereby impairing antifungal efficacy. To overcome this problem, one or two cysteine residues were substituted by serines or S-tert-butyl was applied as a cysteine-protecting group. Furthermore, structural integrity and antifungal efficacy investigations before and after oxidative exposure revealed that substituting both cysteines with serines and S-tert-butylation helped maintain the structural integrity. However, it slightly decreased the antifungal efficacy against a yeast, Candida albicans. Interestingly, S-tert-butylation maintained the efficacy and could extend the antifungal activity to a mold, Aspergillus fumigatus. Usually, cyclization and dimerization did not influence the antifungal efficacy of most peptides. Additionally, hemolysis tests and Galleria mellonella toxicity model experiments indicated that none of the applied modifications made the peptides harmful to animals.

Quantifying Intermolecular Interactions in Asymmetric Peptide Organocatalysis as a Key toward Understanding Selectivity

The kinetic resolution of trans-cyclohexane-1,2-diol with a lipophilic oligopeptide catalyst shows extraordinary selectivities. To improve our understanding of the factors governing selectivity, we quantified the Gibbs free energies of interactions of the peptide with both enantiomers of trans-cyclohexane-1,2-diol using nuclear magnetic resonance (NMR) spectroscopy. For this, we use advanced methods such as transverse relaxation (R2), diffusion measurements, saturation transfer difference (STD), and chemical shift (δ) analysis of peptide-diol mixtures upon varying their composition (NMR titrations). The methods employed give comparable and consistent results. The molecular recognition by the catalyst is approximately 3 kJ mol-1 in favor of the preferentially acetylated (R,R)-enantiomer in the temperature range studied. Interestingly, the difference of 3 kJ mol-1 is also confirmed by results from reaction monitoring of the acylation step under catalytic conditions, indicating that this finding is true regardless of whether the investigation is performed on the acetylated species or on the free catalyst. To arrive at these conclusions, the self-association of both the catalyst and the substrate in toluene was found to play an important role and thus needs to be taken into account in reaction screening.

Peptaibiotics: Harnessing the potential of microbial secondary metabolites for mitigation of plant pathogens

Agricultural systems are in need of low-cost, safe antibiotics to protect crops from pests and diseases. Peptaibiotics, a family of linear, membrane-active, amphipathic polypeptides, have been shown to exhibit antibacterial, antifungal, and antiviral activity, and to be inducers of plant resistance against a wide range of phytopathogens. Peptaibiotics belong to the new generation of alternatives to agrochemicals, aligned with the United Nations Sustainable Development Goals and the One Health approach toward ensuring global food security and safety. Despite that, these fungi-derived, non-ribosomal peptides remain surprisingly understudied, especially in agriculture, where only a small number has been tested against a reduced number of phytopathogens. This lack of adoption stems from peptaibiotics' poor water solubility and the difficulty to synthesize and purify them in vitro, which compromises their delivery and inclusion in formulations. In this review, we offer a comprehensive analysis of peptaibiotics' classification, biosynthesis, relevance to plant protection, and mode of action against phytopathogens, along with the techniques enabling researchers to extract, purify, and elucidate their structure, and the databases holding such valuable data. It is also discussed how chemical synthesis and ionic liquids could increase their solubility, how genetic engineering and epigenetics could boost in vitro production, and how omics can reduce screenings' workload through in silico selection of the best candidates. These strategies could turn peptaibiotics into effective, ultra-specific, biodegradable tools for phytopathogen control.

An identification method to distinguish monomeric sugar isomers on glycopeptides

A one-step protocol for the automated flow synthesis of protected glycosylated amino acids is described using pumps with open-source controls in overall yields of 21-50%. The resulting glycosylated amino acids could be used directly in solid-phase peptide synthesis (SPPS) protocols to quickly produce glycopeptide standards. Access to a variety of stereoisomers of the sugar enabled the development of an LC-MS/MS protocol that can distinguish between peptides modified with carbohydrates having the same exact mass. This method could definitively identify fucose in an O-glycosylation site on the transmembrane protein, Notch1.

Development of a nitrogen-bound hydrophobic auxiliary: application to solid/hydrophobic-tag relay synthesis of calpinactam

In the last couple of decades, technologies and strategies for peptide synthesis have advanced rapidly. Although solid-phase peptide synthesis (SPPS) and liquid-phase peptide synthesis (LPPS) have contributed significantly to the development of the field, there have been remaining challenges for C-terminal modifications of peptide compounds in SPPS and LPPS. Orthogonal to the current standard approach that relies on installation of a carrier molecule at the C-terminus of amino acids, we developed a new hydrophobic-tag carbonate reagent which facilitated robust preparation of nitrogen-tag-supported peptide compounds. This auxiliary was easily installed on a variety of amino acids including oligopeptides that have a broad range of noncanonical residues, allowing simple purification of the products by crystallization and filtration. We demonstrated a de novo solid/hydrophobic-tag relay synthesis (STRS) strategy using the nitrogen-bound auxiliary for total synthesis of calpinactam.

Current research status of anti-cancer peptides: Mechanism of action, production, and clinical applications

The escalating rate of cancer cases, together with treatment deficiencies and long-term side effects of currently used cancer drugs, has made this disease a global burden of the 21st century. The number of breast and lung cancer patients has sharply increased worldwide in the last few years. Presently, surgical treatment, radiotherapy, chemotherapy, and immunotherapy strategies are used to cure cancer, which cause severe side effects, toxicities, and drug resistance. In recent years, anti-cancer peptides have become an eminent therapeutic strategy for cancer treatment due to their high specificity and fewer side effects and toxicity. This review presents an updated overview of different anti-cancer peptides, their mechanisms of action and current production strategies employed for their manufacture. In addition, approved and under clinical trials anti-cancer peptides and their applications have been discussed. This review provides updated information on therapeutic anti-cancer peptides that hold great promise for cancer treatment in the near future.

Rapid mild macrocyclization of depsipeptides under continuous flow: total syntheses of five cyclodepsipeptides

A systematic investigation of the flow macrocyclization approaches for five destruxin analogues natural products at three different cyclization point has been reported.