Native Chemical Ligation-Photodesulfurization in Flow

Protein desulfurization and deselenization

Methods enabling the dechalcogenation of thiols or selenols have been investigated and developed for a long time in fields of research as diverse as the study of prebiotic chemistry, the engineering of fuel processing techniques, the study of biomolecule structures and function or the chemical synthesis of biomolecules. The dechalcogenation of thiol or selenol amino acids is nowadays a particularly flourishing area of research for being a pillar of modern chemical protein synthesis, when used in combination with thiol or selenol-based chemoselective peptide ligation chemistries. This review offers a comprehensive and scholarly overview of the field, emphasizing emerging trends and providing a detailed and critical mechanistic discussion of the dechalcogenation methods developed so far. Taking advantage of recently published reports, it also clarifies some unexpected desulfurization reactions that were observed in the past and for which no explanation was provided at the time. Additionally, the review includes a discussion on principal desulfurization methods within the framework of newly introduced green chemistry metrics and toolkits, providing a well-rounded exploration of the subject.

Expressed Protein Ligation in Flow

The development of a flow chemistry platform for the generation of modified protein targets via expressed protein ligation (EPL) is described. The flow EPL platform enables efficient ligation reactions with high recoveries of target protein products and superior reaction rates compared to corresponding batch processes. The utility of the flow EPL technology was first demonstrated through the semisynthesis of the tick-derived chemokine-binding protein ACA-01 containing two tyrosine sulfate modifications. Full-length, sulfated ACA-01 could be efficiently assembled by ligating a recombinantly expressed C-terminal protein fragment and a synthetic sulfopeptide thioester in flow. Following folding, the semisynthetic sulfoprotein was shown to exhibit potent binding to a variety of pro-inflammatory chemokines. In a second modified protein target, we employed an in-line flow EPL-photodesulfurization strategy to generate both unmodified and phosphorylated forms of human β-synuclein by fusing a recombinant protein thioester, generated through cleavage of an intein fusion protein, and a synthetic (phospho)peptide. The semisynthetic proteins were assembled in 90 min in flow, a significant improvement over corresponding batch protein assembly, and enabled access to tens of milligrams of high purity material. Flow EPL has the potential to serve as a robust technology to streamline access to homogeneously modified proteins for a variety of applications in both academia, as well as in the pharmaceutical and biotechnology sector.

Late-Stage Desulfurization Enables Rapid and Efficient Solid-Phase Synthesis of Cathepsin-Cleavable Linkers for Antibody-Drug Conjugates

The synthesis of linker-payloads is a critical step in developing antibody-drug conjugates (ADCs), a rapidly advancing therapeutic approach in oncology. The conventional method for synthesizing cathepsin B-labile dipeptide linkers, which are commonly used in ADC development, involves the solution-phase assembly of cathepsin B-sensitive dipeptides, followed by the installation of self-immolative para-aminobenzyl carbonate to facilitate the attachment of potent cytotoxic payloads. However, this approach is often low yield and laborious, especially when extending the peptide chain with components like glutamic acid to improve mouse serum stability or charged amino acids or poly(ethylene glycol) moieties to enhance linker hydrophilicity. Here, we introduce a novel approach utilizing late-stage desulfurization chemistry, enabling safe, facile, and cost-effective access to the cathepsin B-cleavable linker, Val-Ala-PABC-MMAE, on resin for the first time.

Peptide macrocyclisation via intramolecular interception of visible-light-mediated desulfurisation

Synthetic methods that enable the macrocyclisation of peptides facilitate the development of effective therapeutic and diagnostic tools. Herein we report a peptide cyclisation strategy based on intramolecular interception of visible-light-mediated cysteine desulfurisation. This method allows cyclisation of unprotected peptides in an aqueous solution via the installation of a hydrocarbon linkage. We explore the limits of this chemistry using a range of model peptides of increasing length and complexity, including peptides of biological/therapeutic relevance. The method is applied to replace the native disulfide of the peptide hormone, oxytocin, with a proteolytically/redox-stable hydrocarbon, and internal macrocyclisation of an MCL-1-binding peptide.

Azole reagents enabled ligation of peptide acyl pyrazoles for chemical protein synthesis

Native chemical ligation (NCL) has been playing an increasingly important role in chemical protein synthesis (CPS). More efficient ligation methods that circumvent the requirement of a peptidyl thioester and thiol additive-which allow the following desulfurization or refolding in one pot-are urgently needed for the synthesis of more complex protein targets and in large quantities. Herein, we discover that the weak acyl donor peptidyl N-acyl pyrazole can be activated by azole reagents like 3-methylpyrazole or imidazole to facilitate its ligation directly with an N-terminal cysteine peptide. As it requires no thioester or thiol additive, this ligation strategy can be conveniently combined with metal-free desulfurization (MFD) or oxidative protein folding to allow various one-pot protocols. The utility and generality of the strategy are showcased by the total synthesis of ubiquitin via an N-to-C sequential ligation-MFD strategy, the semi-synthesis of the copper protein azurin, and the efficient assembly of a sulfated hirudin variant and the cyclotide kalata B1, all in a one-pot fashion.

Peptide and Protein Desulfurization with Diboron Reagents

In this Letter, we report a direct and robust desulfurization method employing water-soluble phosphine, specifically tris(2-carboxyethyl)phosphine hydrochloride (TCEP), and tetrahydroxydiboron (B2(OH)4), which serves as a radical initiator. This innovative reaction exhibits compatibility with a diverse array of substrates, including cysteine residues in chemically synthesized oligopeptides and cyclic peptides, alkyl thiols in bioactive molecules, disulfides in commercial proteins, and selenocysteine. We optimized the reaction conditions to minimize the formation of undesired oxidized and borylated byproducts. Furthermore, the refined desulfurization process is executed after native chemical ligation (NCL) in a single pot, streamlining the existing synthetic approaches. This demonstrates its potential applications in the synthesis of complex peptides and proteins, showcasing a significant advancement in the field.

Synthesis and applications of mirror-image proteins

The homochirality of biomolecules in nature, such as DNA, RNA, peptides and proteins, has played a critical role in establishing and sustaining life on Earth. This chiral bias has also given synthetic chemists the opportunity to generate molecules with inverted chirality, unlocking valuable new properties and applications. Advances in the field of chemical protein synthesis have underpinned the generation of numerous 'mirror-image' proteins (those comprised entirely of D-amino acids instead of canonical L-amino acids), which cannot be accessed using recombinant expression technologies. This Review seeks to highlight recent work on synthetic mirror-image proteins, with a focus on modern synthetic strategies that have been leveraged to access these complex biomolecules as well as their applications in protein crystallography, drug discovery and the creation of mirror-image life.

Thiocholine-Mediated One-Pot Peptide Ligation and Desulfurization

Thiol catalysts are essential in native chemical ligation (NCL) to increase the reaction efficiency. In this paper, we report the use of thiocholine in chemical protein synthesis, including NCL-based peptide ligation and metal-free desulfurization. Evaluation of thiocholine peptide thioester in terms of NCL and hydrolysis kinetics revealed its practical utility, which was comparable to that of other alkyl thioesters. Importantly, thiocholine showed better reactivity as a thiol additive in desulfurization, which is often used in chemical protein synthesis to convert Cys residues to more abundant Ala residues. Finally, we achieved chemical synthesis of two differently methylated histone H3 proteins via one-pot NCL and desulfurization with thiocholine.

Phosphine-Dependent Photoinitiation of Alkyl Thiols under Near-UV Light Facilitates User-Friendly Peptide Desulfurization

Peptides are steadily gaining importance as pharmaceutical targets, and efficient, green methods for their preparation are critically needed. A key deficiency in the synthetic toolbox is the lack of an industrially viable peptide desulfurization method. Without this tool, the powerful native chemical ligation reaction typically used to assemble polypeptides and proteins remains out of reach for industrial preparation of drug targets. Current desulfurization methods require very large excesses of phosphine reagents and thiol additives or low-abundance metal catalysts. Here, we report a phosphine-only photodesulfurization (POP) using near-UV light that is clean, high-yielding, and requires as little as 1.2 equiv phosphine. The user-friendly reaction gives complete control to the chemist, allowing solvent and reagent selection based on starting material and phosphine solubility. It can be conducted in a range of solvents, including water or buffers, on protected or unprotected peptides, in low or high dilution and on gram scale. Oxidation-prone amino acids, π-bonds, aromatic rings, thio-aminal linkages, thioesters, and glycans are all stable to the POP reaction. We highlight the utility of this approach for desulfurization of industrially relevant targets including cyclic peptides and glucagon-like peptide 1 (GLP-1(7-36)). The method is also compatible with NCL buffer, and we highlight the robustness of the approach through the one-pot disulfide reduction/multidesulfurization of linaclotide, aprotinin, and wheat protein.

Flavin catalyzed desulfurization of peptides and proteins in aqueous media

A biomimetic method has been established for the chemo-selective desulfurization of cysteinyl peptides and proteins in aqueous media. The derivatives of biocatalytic cofactors, flavins, were found to be efficient photosensitizers in a thiyl-radical-mediated desulfurization of Cys. The reaction was conducted in an ultrafast manner with both polypeptides and proteins.

Pedal to the Metal: The Homogeneous Catalysis of the Native Chemical Ligation Reaction

The native chemical ligation reaction of peptide thioesters with cysteinyl peptides is a pivotal chemical process in the production of native or modified peptides and proteins, and well beyond in the preparation of various biomolecule analogs and materials. To benefit from this reaction at its fullest and to access all the possible applications, the experimentalist needs to know the factors affecting its rate and how to control it. This concept article presents the fundamental principles underlying the rate of the native chemical ligation and its homogeneous catalysis by nucleophiles. It has been prepared to serve as a quick guide in the search for an appropriate catalyst.

Total Chemical Synthesis of Correctly Folded Disulfide-Rich Proteins Using a Removable O-Linked β-N-Acetylglucosamine Strategy

Disulfide-rich proteins are useful as drugs or tool molecules in biomedical studies, but their synthesis is complicated by the difficulties associated with their folding. Here, we describe a removable glycosylation modification (RGM) strategy that expedites the chemical synthesis of correctly folded proteins with multiple or even interchain disulfide bonds. Our strategy comprises the introduction of simple O-linked β-N-acetylglucosamine (O-GlcNAc) groups at the Ser/Thr sites that effectively improve the folding of disulfide-rich proteins by stabilization of their folding intermediates. After folding, the O-GlcNAc groups can be efficiently removed using O-GlcNAcase (OGA) to afford the correctly folded proteins. Using this strategy, we completed the synthesis of correctly folded hepcidin, an iron-regulating hormone bearing four pairs of disulfide-bonds, and the first total synthesis of correctly folded interleukin-5 (IL-5), a 26 kDa homodimer cytokine responsible for eosinophil growth and differentiation.

Removable Backbone Modification (RBM) Strategy for the Chemical Synthesis of Hydrophobic Peptides/Proteins

Chemical synthesis can provide hydrophobic proteins with natural or man-made modifications (e.g. S-palmitoylation, site-specific isotope labeling and mirror-image proteins) that are difficult to obtain through the recombinant expression technology. The difficulty of chemical synthesis of hydrophobic proteins stems from the hydrophobic nature. Removable backbone modificaiton (RBM) strategy has been developed for solubilizing the hydrophobic peptides/proteins. Here we take the chemical synthesis of a S-palmitoylated peptide as an example to describe the detailed procedure of RBM strategy. Three critical steps of this protocol are: (1) installation of Lys6-tagged RBM groups into the peptides by Fmoc (9-fluorenylmethyloxycarbonyl) solid-phase peptide synthesis, (2) chemical ligation of the peptides, and (3) removal of the RBM tags by TFA (trifluoroacetic acid) cocktails to give the target peptide.

Cysteinyl radicals in chemical synthesis and in nature

Nature harnesses the unique properties of cysteinyl radical intermediates for a diverse range of essential biological transformations including DNA biosynthesis and repair, metabolism, and biological photochemistry. In parallel, the synthetic accessibility and redox chemistry of cysteinyl radicals renders them versatile reactive intermediates for use in a vast array of synthetic applications such as lipidation, glycosylation and fluorescent labelling of proteins, peptide macrocyclization and stapling, desulfurisation of peptides and proteins, and development of novel therapeutics. This review provides the reader with an overview of the role of cysteinyl radical intermediates in both chemical synthesis and biological systems, with a critical focus on mechanistic details. Direct insights from biological systems, where applied to chemical synthesis, are highlighted and potential avenues from nature which are yet to be explored synthetically are presented.

The Chemical Synthesis of Site-Specifically Modified Proteins Via Diselenide-Selenoester Ligation

Peptide ligation techniques enable the controlled chemical synthesis of native and engineered proteins, including examples that display site-specific post-translational modifications (PTMs) and non-proteinogenic functionality. Diselenide-selenoester ligation (DSL) is a recent addition to the synthetic methodology that offers several advantages over existing strategies. The standard DSL reaction involves the additive-free ligation of a peptide carrying an N-terminal selenocysteine (Sec) residue with a fragment bearing a C-terminal selenoester. This operationally simple ligation proceeds rapidly at sterically hindered junctions and is efficient across a broad pH range. The incorporation of deselenization and oxidative deselenization techniques into the DSL protocol enables conversion of the Sec residue at the ligation site to alanine (Ala) and serine (Ser), respectively, thus enhancing the scope and versatility of the method. In this chapter, we describe the application of DSL to the one-pot chemical synthesis of proteins via both two-component and three-component ligation pathways.

Rapid one-pot iterative diselenide-selenoester ligation using a novel coumarin-based photolabile protecting group

The development of an iterative one-pot peptide ligation strategy is described that capitalises on the rapid and efficient nature of the diselenide–selenoester ligation reaction, together with photodeselenisation chemistry. This ligation strategy hinged on the development of a novel photolabile protecting group for the side chain of selenocysteine, namely the 7-diethylamino-3-methyl coumarin (DEAMC) moiety. Deprotection of this DEAMC group can be effected in a mild, reagent-free manner using visible light (λ = 450 nm) without deleterious deselenisation of selenocysteine residues, thus enabling a subsequent ligation reaction without purification. The use of this DEAMC-protected selenocysteine in iterative DSL chemistry is highlighted through the efficient one-pot syntheses of 60- and 80-residue fragments of mucin-1 as well as apolipoprotein CIII in just 2–4 hours.

A method for the rapid one-pot iterative assembly of proteins via diselenide–selenoester ligation (DSL) chemistry is described that capitalises on a novel coumarin-based photolabile protecting group for selenocysteine.

Bioorthogonal Reactions of Triarylphosphines and Related Analogues

Bioorthogonal phosphines were introduced in the context of the Staudinger ligation over 20 years ago. Since that time, phosphine probes have been used in myriad applications to tag azide-functionalized biomolecules. The Staudinger ligation also paved the way for the development of other phosphorus-based chemistries, many of which are widely employed in biological experiments. Several reviews have highlighted early achievements in the design and application of bioorthogonal phosphines. This review summarizes more recent advances in the field. We discuss innovations in classic Staudinger-like transformations that have enabled new biological pursuits. We also highlight relative newcomers to the bioorthogonal stage, including the cyclopropenone-phosphine ligation and the phospha-Michael reaction. The review concludes with chemoselective reactions involving phosphite and phosphonite ligations. For each transformation, we describe the overall mechanism and scope. We also showcase efforts to fine-tune the reagents for specific functions. We further describe recent applications of the chemistries in biological settings. Collectively, these examples underscore the versatility and breadth of bioorthogonal phosphine reagents.

Late-Stage Solubilization of Poorly Soluble Peptides Using Hydrazide Chemistry

A novel late-stage solubilization of peptides using hydrazides is described. A solubilizing tag was attached through a selective N-alkylation at a hydrazide moiety with the aid of a 2-picoline-borane complex in 50% acetic acid-hexafluoro-2-propanol. The tag, which tolerates ligation and desulfurization conditions, can be detached by a Cu-mediated selective oxidative hydrolysis of the N-alkyl hydrazide. This new method was validated through the synthesis of HIV-1 protease.

Insights into the Mechanism and Catalysis of Peptide Thioester Synthesis by Alkylselenols Provide a New Tool for Chemical Protein Synthesis

While thiol-based catalysts are widely employed for chemical protein synthesis relying on peptide thioester chemistry, this is less true for selenol-based catalysts whose development is in its infancy. In this study, we compared different selenols derived from the selenocysteamine scaffold for their capacity to promote thiol-thioester exchanges in water at mildly acidic pH and the production of peptide thioesters from bis(2-sulfanylethyl)amido (SEA) peptides. The usefulness of a selected selenol compound is illustrated by the total synthesis of a biologically active human chemotactic protein, which plays an important role in innate and adaptive immunity.

General synthetic strategy for regioselective ultrafast formation of disulfide bonds in peptides and proteins

Despite six decades of efforts to synthesize peptides and proteins bearing multiple disulfide bonds, this synthetic challenge remains an unsolved problem in most targets (e.g., knotted mini proteins). Here we show a de novo general synthetic strategy for the ultrafast, high-yielding formation of two and three disulfide bonds in peptides and proteins. We develop an approach based on the combination of a small molecule, ultraviolet-light, and palladium for chemo- and regio-selective activation of cysteine, which enables the one-pot formation of multiple disulfide bonds in various peptides and proteins. We prepare bioactive targets of high therapeutic potential, including conotoxin, RANTES, EETI-II, and plectasin peptides and the linaclotide drug. We anticipate that this strategy will be a game-changer in preparing millions of inaccessible targets for drug discovery.

Flow Photochemistry as a Tool in Organic Synthesis

Photochemical transformations of molecular building blocks have become an important and widely recognized research field in the past decade. Detailed and deep understanding of novel photochemical catalysts and reaction concepts with visible light as the energy source has enabled a broad application portfolio for synthetic organic chemistry. In parallel, continuous-flow chemistry and microreaction technology have become the basis for thinking and doing chemistry in a novel fashion with clear focus on improved process control for higher conversion and selectivity. As can be seen by the large number of scientific publications on flow photochemistry in the recent past, both research topics have found each other as exceptionally well-suited counterparts with high synergy by combining chemistry and technology. This review will give an overview on selected reaction classes, which represent important photochemical transformations in synthetic organic chemistry, and which benefit from mild and defined process conditions by the transfer from batch to continuous-flow mode.

Chemical Protein Synthesis in Medicinal Chemistry

Although the majority of proteins used for biomedical research are produced using living systems such as bacteria, biological means for producing proteins can be advantageously complemented by protein semisynthesis or total chemical synthesis. The latter approach is particularly useful when the proteins to be produced are toxic for the expression system or show unusual features that cannot be easily programmed in living organisms. The aim of this review is to provide a wide overview of the use of chemical protein synthesis in medicinal chemistry with a special focus on the production of post-translationally modified proteins and backbone cyclized proteins.

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.

N-Terminal speciation for native chemical ligation

Native chemical ligation (NCL) enables the chemical synthesis of peptides via reactions between N-terminal thiolates and C-terminal thioesters under mild, aqueous conditions at pH 7-8. Here we demonstrate quantitatively how thiol speciation at N-terminal cysteines and analogues varies significantly depending upon structure at typical pH values used in NCL.

Challenges and Perspectives in Chemical Synthesis of Highly Hydrophobic Peptides

Solid phase peptide synthesis (SPPS) provides the possibility to chemically synthesize peptides and proteins. Applying the method on hydrophilic structures is usually without major drawbacks but faces extreme complications when it comes to "difficult sequences." These includes the vitally important, ubiquitously present and structurally demanding membrane proteins and their functional parts, such as ion channels, G-protein receptors, and other pore-forming structures. Standard synthetic and ligation protocols are not enough for a successful synthesis of these challenging sequences. In this review we highlight, summarize and evaluate the possibilities for synthetic production of "difficult sequences" by SPPS, native chemical ligation (NCL) and follow-up protocols.

The Journey for the Total Chemical Synthesis of a 53 kDa Protein

Chemical protein synthesis has been proved as an efficient way to afford medium-sized proteins with high homogeneity in workable quantities for various biochemical, structural, and functional studies. In particular, chemical protein synthesis has enabled access to proteins that are difficult or impossible to prepare by molecular biology approaches, such as those with post-translational modifications and mirror-image proteins. One prominent example is related to ubiquitination, a well-known modification that mediates a variety of cellular processes (e.g., proteasomal degradation). Ubiquitination is considered as a modification that is difficult to introduce into proteins in a test tube to generate ubiquitin (Ub) conjugates with high homogeneity with respect to the chain length and the anchored Lys residue in workable quantities to perform the biochemical and biophysical studies. Chemical protein synthesis has emerged as a powerful approach to prepare Ub conjugates for studies aiming to understand ubiquitination in great detail and decipher its roles in cell processes. Nevertheless, in order to answer more challenging questions in this field, it has been clear that researchers must also prepare Ub conjugates with increased size and complexity. Employing solid-phase peptide synthesis and chemoselective ligation, chemical protein synthesis offers a powerful way to furnish polypeptides composed of 100-200 residues. However, to synthesize larger proteins such as Ub conjugates, longer and more segments are required. This on the other hand leads to difficulties related to solubility, purification, ligation, and late-stage modifications. These challenges have encouraged us to explore more practical synthetic tools to facilitate the synthesis of complex Ub conjugates. In this Account, we summarize the synthetic tools that we have developed to achieve these goals. These include (1) δ-mercaptolysine-mediated isopeptide chemical ligation, (2) chemical synthesis of Ub building blocks, (3) palladium-mediated deprotection of key side chains during protein synthesis, (4) one-pot ligation and desulfurization, and (5) improving the solubility of peptide segments. The developed chemical toolbox has been a key for our successes in the synthesis of diverse and complex Ub conjugates. In this Account, we describe our approaches for generating various Ub conjugates, including (1) the K48 tetra-Ub chain composed of 304 amino acids, (2) the ubiquitinated histones and their analogues made of >200 amino acids, (3) the di-Ub-SUMO-2 hybrid chain composed of 245 amino acids, and (4) the 53 kDa tetra-Ub-α-globin composed of 472 amino acids, which represents the largest protein composed of natural amino acids ever made using chemical protein synthesis. The last target, Flag-Ub-Ub-Ub-Myc-Ub-(HA-α-globin), was prepared in the labeled form where the proximal Ub and distal Ub in the chain were labeled with Myc and Flag tags, respectively, while the α-globin was labeled with the HA tag. Applying the tetra-Ub-α-globin in proteasomal degradation studies assisted us to shed light on the proteolytic signal and the fates of the Ub moieties in the chains. Although these developments have contributed to the synthesis of interesting and challenging targets related to Ub signaling, several other targets may enforce new synthetic challenges. Hence, there is still a need to optimize the current synthetic tools and explore novel synthetic approaches to facilitate this process.

A sequential native chemical ligation – thiol-Michael addition strategy for polymer–polymer ligation

Native Chemical Ligation (NCL) between cysteine-terminated polymers and functional thioesters has been employed to prepare functional (co)polymers. The retained thiol functionality at the NCL junction can be exploited for thiol-Michael addition.

Novel protein science enabled by total chemical synthesis

Chemical synthesis is a well-established method for the preparation in the research laboratory of multiple-tens-of-milligram amounts of correctly folded, high purity protein molecules. Chemically synthesized proteins enable a broad spectrum of novel protein science. Racemic mixtures consisting of d-protein and l-protein enantiomers facilitate crystallization and determination of protein structures by X-ray diffraction. d-Proteins enable the systematic development of unnatural mirror image protein molecules that bind with high affinity to natural protein targets. The d-protein form of a therapeutic target can also be used to screen natural product libraries to identify novel small molecule leads for drug development. Proteins with novel polypeptide chain topologies including branched, circular, linear-loop, and interpenetrating polypeptide chains can be constructed by chemical synthesis. Medicinal chemistry can be applied to optimize the properties of therapeutic protein molecules. Chemical synthesis has been used to redesign glycoproteins and for the a priori design and construction of covalently constrained novel protein scaffolds not found in nature. Versatile and precise labeling of protein molecules by chemical synthesis facilitates effective application of advanced physical methods including multidimensional nuclear magnetic resonance and time-resolved FTIR for the elucidation of protein structure-activity relationships. The chemistries used for total synthesis of proteins have been adapted to making artificial molecular devices and protein-inspired nanomolecular constructs. Research to develop mirror image life in the laboratory is in its very earliest stages, based on the total chemical synthesis of d-protein forms of polymerase enzymes.

Recent advances in the chemical synthesis of N-linked glycoproteins

Glycoproteins have many biological roles. Due to the heterogeneity of natural glycoproteins in the sugar part resulting in glycoforms the evaluation of the biochemical roles of individual glycans remains difficult to investigate. Since pure glycoforms are still not accessible via recombinant or chromatographic methods, the synthesis of proteins with uniform posttranslational modifications using ligation methods or glycan remodeling are currently the best options for accessing these targets. Recent developments in chemical protein synthesis, the assembly of N-glycans and the use of enzymatic procedures have provided access to many glycoproteins with modifications as well as their analogs.

Native chemical ligation in protein synthesis and semi-synthesis

Combining modern synthetic and molecular biology toolkits, native chemical ligation and expressed protein ligation enables robust access to modified proteins.