Peptide Synthesis Utilizing Micro-flow 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.

Convergent approach for direct cross-coupling enabled by flash irreversible generation of cationic and anionic species

In biosynthesis multiple kinds of reactive intermediates are generated, transported, and reacted across different parts of organisms, enabling highly sophisticated synthetic reactions. Herein we report a convergent synthetic approach, which utilizes dual intermediates of cationic and carbanionic species in a single step, hinted at by the ideal reaction conditions. By reactions of unsaturated precursors, such as enamines, with a superacid in a flow microreactor, cationic species, such as iminium ions, are generated rapidly and irreversibly, and before decomposition, they are transported to react with rapidly and independently generated carbanions, enabling direct C-C bond formation. Taking advantage of the reactivity of these double reactive intermediates, the reaction take place within a few seconds, enabling synthetic reactions which are not applicable in conventional reactions.

Peptide-Mediated Nanocarriers for Targeted Drug Delivery: Developments and Strategies

Effective drug delivery is essential for cancer treatment. Drug delivery systems, which can be tailored to targeted transport and integrated tumor therapy, are vital in improving the efficiency of cancer treatment. Peptides play a significant role in various biological and physiological functions and offer high design flexibility, excellent biocompatibility, adjustable morphology, and biodegradability, making them promising candidates for drug delivery. This paper reviews peptide-mediated drug delivery systems, focusing on self-assembled peptides and peptide-drug conjugates. It discusses the mechanisms and structural control of self-assembled peptides, the varieties and roles of peptide-drug conjugates, and strategies to augment peptide stability. The review concludes by addressing challenges and future directions.

Considerations for the genotoxicity assessment of middle size peptide drugs containing non-canonical amino acid residues

BACKGROUND

Middle size peptides (MSPs) have emerged as a promising new pharmaceutical modality. We are seeking the best way to assess the non-clinical safety of MSPs.

CONSIDERATION

The requirements for assessing the genotoxicity of pharmaceuticals differ between small molecule drugs and biotherapeutics. Genotoxicity tests are necessary for small molecule drugs but not for biotherapeutics. MSPs, however, share similarities with both small molecule drugs and biotherapeutics. Here, we describe important points to consider in assessing the genotoxicity of MSP drugs. The current standard of genotoxicity assessment for small molecules may not be entirely appropriate for MSP drugs. MSP drugs need genotoxicity assessment mostly according to the current standard of small molecule drugs.

CONCLUSION

We propose a few modifications to the standard test battery of genotoxicity tests, specifically, the inclusion of an in vitro gene mutation test using mammalian cells, and exclusion of (Q)SAR assessment on MSP-related impurities.

Research Progress on the Preparation and Function of Antioxidant Peptides from Walnuts

Food-derived peptides have good antioxidant activity and are highly safe for humans; consequently, there has been continuous growth in research on antioxidants, with potential applications in food, medicine, cosmetics, and other fields. Among food-derived peptides, walnut-derived peptides have attracted increasing attention as food-derived peptides rich in eight essential amino acids. This review summarizes the progress made in the development and identification of antioxidant peptides in walnut proteins. This article mainly describes the interaction between reactive oxygen species and cellular antioxidant products, modulation of enzyme content and activity, and regulation of the redox signaling pathways and analyzes the mechanisms of reduction in oxidative stress. Finally, the complex structure-activity relationships of walnut-derived peptides are analyzed based on their amino acid composition and secondary structure of the polypeptides. This review provides a theoretical basis for the production of walnut-derived antioxidant peptides and could help promote the development of the walnut industry.

The cyclic peptide G4CP2 enables the modulation of galactose metabolism in yeast by interfering with GAL4 transcriptional activity

Genetically-encoded combinatorial peptide libraries are convenient tools to identify peptides to be used as therapeutics, antimicrobials and functional synthetic biology modules. Here, we report the identification and characterization of a cyclic peptide, G4CP2, that interferes with the GAL4 protein, a transcription factor responsible for the activation of galactose catabolism in yeast and widely exploited in molecular biology. G4CP2 was identified by screening CYCLIC, a Yeast Two-Hybrid-based combinatorial library of cyclic peptides developed in our laboratory. G4CP2 interferes with GAL4-mediated activation of galactose metabolic enzymes both when expressed intracellularly, as a recombinant peptide, and when provided exogenously, as a chemically-synthesized cyclic peptide. Our results support the application of G4CP2 in microbial biotechnology and, additionally, demonstrate that CYCLIC can be used as a tool for the rapid identification of peptides, virtually without any limitations with respect to the target protein. The possible biotechnological applications of cyclic peptides are also discussed.

Amide Bonds Meet Flow Chemistry: A Journey into Methodologies and Sustainable Evolution

Formation of amide bonds is of immanent importance in organic and synthetic medicinal chemistry. Its presence in "traditional" small-molecule active pharmaceutical ingredients, in linear or cyclic oligo- and polypeptidic actives, including pseudopeptides, has led to the development of dedicated synthetic approaches for the formation of amide bonds starting from, if necessary, suitably protected amino acids. While the use of solid supported reagents is common in traditional peptide synthesis, similar approaches targeting amide bond formation in continuous-flow mode took off more significantly, after a first publication in 2006, only a couple of years ago. Most efforts rely upon the transition of traditional approaches in flow mode, or the combination of solid-phase peptide synthesis principles with flow chemistry, and advantages are mainly seen in improving space-time yields. This Review summarizes and compares the various approaches in terms of basic amide formation, peptide synthesis, and pseudopeptide generation, describing the technological approaches and the advantages that were generated by the specific flow approaches. A final discussion highlights potential future needs and perspectives in terms of greener and more sustainable syntheses.

A microfluidic valve with bubble trap and zero dead volume

We present a technique to swiftly change the contents of a small sample chamber using only a few times the chamber volume. Our design has no dead volume and functions as a manifold that minimizes mixing between consecutive liquids at one inlet. Thereby, it is ideal for minimizing sample consumption. In addition, our fluidic circuit works as an efficient bubble trap. These properties make our design an exciting alternative to standard solutions using multiple valves and junctions.

Development of an amino acid sequence-dependent analytical method for peptides using near-infrared spectroscopy

We aimed to develop an amino acid sequence-dependent analytical method using near-infrared (NIR) spectroscopy. The possibility for monitoring peptide synthesis was proved with very high accuracies.

Rapid and Mild Lactamization Using Highly Electrophilic Triphosgene in a Microflow Reactor

Lactams are cyclic amides that are indispensable as drugs and as drug candidates. Conventional lactamization includes acid-mediated and coupling-agent-mediated approaches that suffer from narrow substrate scope, much waste, and/or high cost. Inexpensive, less-wasteful approaches mediated by highly electrophilic reagents are attractive, but there is an imminent risk of side reactions. Herein, a methods using highly electrophilic triphosgene in a microflow reactor that accomplishes rapid (0.5-10 s), mild, inexpensive, and less-wasteful lactamization are described. Methods A and B, which use N-methylmorpholine and N-methylimidazole, respectively, were developed. Various lactams and a cyclic peptide containing acid- and/or heat-labile functional groups were synthesized in good to high yields without the need for tedious purification. Undesired reactions were successfully suppressed, and the risk of handling triphosgene was minimized by the use of microflow technology.

Peptide-Based Electrospun Fibers: Current Status and Emerging Developments

Electrospinning is a well-known, straightforward, and versatile technique, widely used for the preparation of fibers by electrifying a polymer solution. However, a high molecular weight is not essential for obtaining uniform electrospun fibers; in fact, the primary criterion to succeed is the presence of sufficient intermolecular interactions, which function similar to chain entanglements. Some small molecules able to self-assemble have been electrospun from solution into fibers and, among them, peptides containing both natural and non-natural amino acids are of particular relevance. Nowadays, the use of peptides for this purpose is at an early stage, but it is gaining more and more interest, and we are now witnessing the transition from basic research towards applications. Considering the novelty in the relevant processing, the aim of this review is to analyze the state of the art from the early 2000s on. Moreover, advantages and drawbacks in using peptides as the main or sole component for generating electrospun nanofibers will be discussed. Characterization techniques that are specifically targeted to the produced peptide fibers are presented.

Method of Monitoring the Number of Amide Bonds in Peptides Using Near-Infrared Spectroscopy

Using near-infrared (NIR) spectroscopy, we aimed to develop a method of monitoring the increasing number of amide bonds with the elongation of the chain length of peptides. Because peptide synthesis can be monitored by evaluating the increasing number of amide bonds with dehydration occurring between amino acids, polyglycine, which has the simplest structure among polyamino acids, was studied, and the key bands whose absorption intensities increased with the elongation of the chain length, such as the bands attributed to glycine, diglycine, triglycine, and tetraglycine, were searched. The bands due to the combinations of the amide A and amide II/III modes in the region of 5000-4500 cm^-1 were revealed to be good candidates for key bands, their second derivative intensities increased as the number of amide bonds increased, regardless of pH, solvent species, and the presence of protecting groups. The number of amide bonds was evaluated by a partial least square regression using the abovementioned combination bands, and a calibration model with a high determination coefficient (≥0.99) was constructed. These results not only have demonstrated the usefulness of NIR spectroscopy as a process analytical technology tool for the process of synthesizing the peptide in a microflow reactor but also have provided basic knowledge for analyzing amide bonds in the NIR spectra of proteins, polyamino acids, polypeptides, and polyamides.

Recent Updates on Antifungal Peptides

The current trend of increment in the frequency of antifungal resistance has brought research into an era where new antifungal compounds with novel mechanisms of action are required. Natural antimicrobial peptides, which are ubiquitous components of innate immunity, represent their candidature for novel antifungal peptides. Various antifungal peptides have been isolated from different species ranging from small marine organisms to insects and from various other living species. Based on these peptides, various mimetics of antifungal peptides have also been synthesized using non-natural amino acids. Utilization of these antifungal peptides is somehow limited due to their toxic and unstable nature. This review discusses recent updates and future directions of antifungal peptides, for taking them to the shelf from the bench.

Greening the synthesis of peptide therapeutics: an industrial perspective

Solid-phase peptide synthesis (SPPS) is generally the method of choice for the chemical synthesis of peptides, allowing routine synthesis of virtually any type of peptide sequence, including complex or cyclic peptide products. Importantly, SPPS can be automated and is scalable, which has led to its widespread adoption in the pharmaceutical industry, and a variety of marketed peptide-based drugs are now manufactured using this approach. However, SPPS-based synthetic strategies suffer from a negative environmental footprint mainly due to extensive solvent use. Moreover, most of the solvents used in peptide chemistry are classified as problematic by environmental agencies around the world and will soon need to be replaced, which in recent years has spurred a movement in academia and industry to make peptide synthesis greener. These efforts have been centred around solvent substitution, recycling and reduction, as well as exploring alternative synthetic methods. In this review, we focus on methods pertaining to solvent substitution and reduction with large-scale industrial production in mind, and further outline emerging technologies for peptide synthesis. Specifically, the technical requirements for large-scale manufacturing of peptide therapeutics are addressed.

Recent Advances in Continuous-Flow Reactions Using Metal-Free Homogeneous Catalysts

Developments that result in high-yielding, low-cost, safe, scalable, and less-wasteful processes are the most important goals in synthetic organic chemistry. Continuous-flow reactions have garnered much attention due to many advantages over conventional batch reactions that include precise control of short reaction times and temperatures, low risk in handling dangerous compounds, and ease in scaling up synthesis. Combinations of continuous-flow reactions with homogeneous, metal-free catalysts further enhances advantages that include low-cost and ready availability, low toxicity, higher stability in air and water, and increased synthetic efficiency due to the avoidance of the time-consuming removal of toxic metal traces. This review summarizes recently reported continuous-flow reactions using metal-free homogeneous catalysts and classifies them either as acidic catalysts, basic catalysts, or miscellaneous catalysts. In addition, we compare the results between continuous-flow conditions and conventional batch conditions to reveal the advantages of using flow reactions with metal-free homogeneous catalysts.

Novel Methodologies for Chemical Activation in Organic Synthesis under Solvent-Free Reaction Conditions

One central challenge for XXI century chemists is the development of sustainable processes that do not represent a risk either to humanity or to the environment. In this regard, the search for more efficient and clean alternatives to achieve the chemical activation of molecules involved in chemical transformations has played a prominent role in recent years. The use of microwave or UV-Vis light irradiation, and mechanochemical activation is already widespread in many laboratories. Nevertheless, an additional condition to achieve "green" processes comes from the point of view of so-called atom economy. The removal of solvents from chemical reactions generally leads to cleaner, more efficient and more economical processes. This review presents several illustrative applications of the use of sustainable protocols in the synthesis of organic compounds under solvent-free reaction conditions.

Synthesis of proteins by automated flow chemistry

Ribosomes can produce proteins in minutes and are largely constrained to proteinogenic amino acids. Here, we report highly efficient chemistry matched with an automated fast-flow instrument for the direct manufacturing of peptide chains up to 164 amino acids long over 327 consecutive reactions. The machine is rapid: Peptide chain elongation is complete in hours. We demonstrate the utility of this approach by the chemical synthesis of nine different protein chains that represent enzymes, structural units, and regulatory factors. After purification and folding, the synthetic materials display biophysical and enzymatic properties comparable to the biologically expressed proteins. High-fidelity automated flow chemistry is an alternative for producing single-domain proteins without the ribosome.

Assessing the correlation of microscopy-based and volumetry-based measurements for resin swelling in a range of potential greener solvents for SPPS

The degree of resin swelling in a particular solvent system is one of the critical parameters for solid-phase peptide synthesis (SPPS) and for solid-phase synthesis in general. Methods used for measuring the degree of resin swelling include microscopy-based and volumetry-based methods. This study describes and compares the use of both methods for a number of commercially available resins commonly used in SPPS, with a range of solvents, which have been identified in the literature as 'greener' than DCM, DMF and NMP. The results were analysed by statistical methods, and a significant correlation between the two distinct methods has been demonstrated for the first time. The results will likely be used, in conjunction with other literature methods, to help in choosing both the resin and solvent system for greener SPPS, as well as for continuous flow SPPS, which is of growing importance.

Acyl azide generation and amide bond formation in continuous-flow for the synthesis of peptides

Acyl azides were safely generated by using nitrous acid in water and reactedin situwithin a flow system. The acyl azide was efficiently extracted into the organic phase containing an amine nucleophile for a highly enantioselective peptide coupling.

Continuous Flow Synthesis of ACE Inhibitors From N-Substituted l-Alanine Derivatives

A strategy for the continuous flow synthesis of angiotensin converting enzyme (ACE) inhibitors is described. An optimization effort guided by in situ IR analysis resulted in a general amide coupling approach facilitated by N-carboxyanhydride (NCA) activation that was further characterized by reaction kinetics analysis in batch. The three-step continuous process was demonstrated by synthesizing 8 different ACE inhibitors in up to 88 % yield with throughputs in the range of ≈0.5 g h^-1 , all while avoiding both isolation of reactive intermediates and process intensive reaction conditions. The process was further developed by preparing enalapril, a World Health Organization (WHO) essential medicine, in an industrially relevant flow platform that scaled throughput to ≈1 g h^-1 .