Contemporary approaches to site-selective protein modification

Cleavable and tunable cysteine-specific arylation modification with aryl thioethers

Cysteine represents an attractive target for peptide/protein modification due to the intrinsic high nucleophilicity of the thiol group and low natural abundance. Herein, a cleavable and tunable covalent modification approach for cysteine containing peptides/proteins with our newly designed aryl thioethers via a S N Ar approach was developed. Highly efficient and selective bioconjugation reactions can be carried out under mild and biocompatible conditions. A series of aryl groups bearing different bioconjugation handles, affinity or fluorescent tags are well tolerated. By adjusting the skeleton and steric hindrance of aryl thioethers slightly, the modified products showed a tunable profile for the regeneration of the native peptides.

Site-selective modification strategies in antibody-drug conjugates

Antibody-drug conjugates (ADCs) harness the highly specific targeting capabilities of an antibody to deliver a cytotoxic payload to specific cell types. They have garnered widespread interest in drug discovery, particularly in oncology, as discrimination between healthy and malignant tissues or cells can be achieved. Nine ADCs have received approval from the US Food and Drug Administration and more than 80 others are currently undergoing clinical investigations for a range of solid tumours and haematological malignancies. Extensive research over the past decade has highlighted the critical nature of the linkage strategy adopted to attach the payload to the antibody. Whilst early generation ADCs were primarily synthesised as heterogeneous mixtures, these were found to have sub-optimal pharmacokinetics, stability, tolerability and/or efficacy. Efforts have now shifted towards generating homogeneous constructs with precise drug loading and predetermined, controlled sites of attachment. Homogeneous ADCs have repeatedly demonstrated superior overall pharmacological profiles compared to their heterogeneous counterparts. A wide range of methods have been developed in the pursuit of homogeneity, comprising chemical or enzymatic methods or a combination thereof to afford precise modification of specific amino acid or sugar residues. In this review, we discuss advances in chemical and enzymatic methods for site-specific antibody modification that result in the generation of homogeneous ADCs.

Systematic Activity Maturation of a Single-Domain Antibody with Non-canonical Amino Acids through Chemical Mutagenesis

Great advances have been made over the last four decades in therapeutic and diagnostic applications of antibodies. The activity maturation of antibody candidates, however, remains a significant challenge. To address this problem, we present a method that enables the systematic enhancement of the activity of a single-domain antibody through the post-translational installation of non-canonical side chains by chemical mutagenesis. We illustrate this approach by performing a structure-activity relationship study beyond the 20 naturally occurring amino acids on a single-domain antibody designed in silico to inhibit the aggregation of the amyloid-β peptide, a process closely linked to Alzheimer's disease. We found that this approach can improve, by five orders of magnitude, the anti-aggregation activity of the starting single-domain antibody, without affecting its stability. These results show that the expansion of the chemical space available to antibodies through chemical mutagenesis can be exploited for the systematic enhancement of the activity of these molecules.

1,3-Dipolar Cycloaddition between Dehydroalanines and C,N-Cyclic Azomethine Imines: Application to Late-Stage Peptide Modification

A non-catalytic, mild, and easy-to-handle protecting group switched 1,3-dipolar cycloaddition (1,3-DC) between bi- or mono-N-protected Dha and C,N-cyclic azomethine imines, which afford various quaternary amino acids with diverse scaffolds, is disclosed. Specifically, normal-electron-demand 1,3-DC reaction occurs between bi-N-protected Dha and C,N-cyclic azomethine imines, while inverse-electron-demand 1,3-DC reaction occurs between mono-N-protected Dha and C,N-cyclic azomethine imines. Above all, the reactions can be carried out between peptides with Dha residues at the position of interest and C,N-cyclic azomethine imines, both in homogeneous phase and on resins in SPPS. It provides a new toolkit for late-stage peptide modification, labeling, and peptide-drug conjugation. To shed light on the high regioselectivity of the reaction, DFT calculations were carried out, which were qualitatively consistent with the experimental observations.

"Doubly Customizable" Unit for the Generation of Structural Diversity: From Pure Enantiomeric Amines to Peptide Derivatives

Readily available, low-cost 4R-hydroxy-l-proline (Hyp) is introduced as a "doubly customizable" unit for the generation of libraries of structurally diverse compounds. Hyp can be cleaved at two points, followed by the introduction of new functionalities. In the first cycle, the removal and replacement of the carboxylic group are carried out, followed (second cycle) by the scission of the 4,5-position and manipulation of the resulting chains. In this way, three new chains are generated and can be transformed independently to afford a diversity of products with tailored substituents, such as β-amino aldehydes, diamines, β-amino acid derivatives, including N-alkylated ones, or modified peptides. Many of these products are high-profit compounds but, in spite of their commercial value, are still scarce. Moreover, the process takes place with stereochemical control, and either pure R or S isomers can be obtained with small variations of the synthetic route.

Photo-induced radical thiol-ene chemistry: a versatile toolbox for peptide-based drug design

While the global market for peptide/protein-based therapeutics is witnessing significant growth, the development of peptide drugs remains challenging due to their low oral bioavailability, poor membrane permeability, and reduced metabolic stability. However, a toolbox of chemical approaches has been explored for peptide modification to overcome these obstacles. In recent years, there has been a revival of interest in photoinduced radical thiol-ene chemistry as a powerful tool for the construction of therapeutic peptides.

Light-induced efficient and residue-selective bioconjugation of native proteins via indazolone formation

Chemical modification of proteins has emerged as a powerful tool to realize enormous applications, such as development of novel biologics and functional studies of individual protein. We report a light-induced lysine-selective native protein conjugation approach via indazolone formation, conferring reliable chemoselectivity, excellent efficiency, temporal control and biocompatibility under operationally simple and mild conditions, in vitro and in living systems. This straightforward protocol demonstrates the generality and accessibility for direct and rapid functionalization of diverse native proteins, which suggests a new avenue of great importance to bioconjugation, medicinal chemistry and chemical biology.

Photocatalytic methods for amino acid modification

This tutorial review introduces photocatalysis for amino acid modification and summarises recent advances in the field.

Site-selective modification of peptide backbones

Exciting developments in the site-selective modification of peptide backbones are allowing an outstanding fine-tuning of peptide conformation, folding ability, and physico-chemical and biological properties.

Site-Selective Modification of Peptides and Proteins via Interception of Free-Radical-Mediated Dechalcogenation

The development of site‐selective chemistry targeting the canonical amino acids enables the controlled installation of desired functionalities into native peptides and proteins. Such techniques facilitate the development of polypeptide conjugates to advance therapeutics, diagnostics, and fundamental science. We report a versatile and selective method to functionalize peptides and proteins through free‐radical‐mediated dechalcogenation. By exploiting phosphine‐induced homolysis of the C−Se and C−S bonds of selenocysteine and cysteine, respectively, we demonstrate the site‐selective installation of groups appended to a persistent radical trap. The reaction is rapid, operationally simple, and chemoselective. The resulting aminooxy linker is stable under a variety of conditions and selectively cleavable in the presence of a low‐oxidation‐state transition metal. We have explored the full scope of this reaction using complex peptide systems and a recombinantly expressed protein.

In Vivo Pretargeting Based on Cysteine-Selective Antibody Modification with IEDDA Bioorthogonal Handles for Click Chemistry

Pretargeted imaging has emerged as an effective multistep strategy aiming to improve imaging contrast and reduce patient radiation exposure through decoupling of the radioactivity from the targeting vector. The inverse electron-demand Diels-Alder (IEDDA) reaction between a trans-cyclooctene (TCO)-conjugated antibody and a labeled tetrazine holds great promise for pretargeted imaging applications due to its bioorthogonality, rapid kinetics under mild conditions, and formation of stable products. Herein, we describe the use of functionalized carbonylacrylic reagents for site-specific incorporation of TCO onto a human epidermal growth factor receptor 2 (HER2) antibody (THIOMAB) containing an engineered unpaired cysteine residue, generating homogeneous conjugates. Precise labeling of THIOMAB-TCO with a fluorescent or radiolabeled tetrazine revealed the potential of the TCO-functionalized antibody for imaging the HER2 after pretargeting in a cellular context in a HER2 positive breast cancer cell line. Control studies with MDA-MD-231 cells, which do not express HER2, further confirmed the target specificity of the modified antibody. THIOMAB-TCO was also evaluated in vivo after pretargeting and subsequent administration of an ¹¹¹In-labeled tetrazine. Biodistribution studies in breast cancer tumor-bearing mice showed a significant activity accumulation on HER2⁺ tumors, which was 2.6-fold higher than in HER2^- tumors. Additionally, biodistribution studies with THIOMAB without the TCO handle also resulted in a decreased uptake of ¹¹¹In-DOTA-Tz on HER2⁺ tumors. Altogether, these results clearly indicate the occurrence of the click reaction at the tumor site, i.e., pretargeting of SK-BR-3 HER2-expressing cells with THIOMAB-TCO and reaction through the TCO moiety present in the antibody. The combined advantages of site-selectivity and stability of TCO tagged-antibodies could allow application of biorthogonal chemistry strategies for pretargeting imaging with minimal side-reactions and background.

Patterning of protein-based materials

Micro- and nanopatterning of proteins on surfaces allows to develop for example high-throughput biosensors in biomedical diagnostics and in general advances the understanding of cell-material interactions in tissue engineering. Today, many techniques are available to generate protein pattern, ranging from technically simple ones, such as micro-contact printing, to highly tunable optical lithography or even technically sophisticated scanning probe lithography. Here, one focus is on the progress made in the development of protein-based materials as positive or negative photoresists allowing micro- to nanostructured scaffolds for biocompatible photonic, electronic and tissue engineering applications. The second one is on approaches, which allow a controlled spatiotemporal positioning of a single protein on surfaces, enabled by the recent developments in immobilization techniques coherent with the sensitive nature of proteins, defined protein orientation and maintenance of the protein activity at interfaces. The third one is on progress in photolithography-based methods, which allow to control the formation of protein-repellant/adhesive polymer brushes.

Late-stage peptide and protein modifications through phospha-Michael addition reaction

We developed a late-stage modification strategy by a phospha-Michael addition reaction between various functional phosphines and unprotected dehydroalanine (Dha) peptides and proteins under mild conditions. This strategy was applied to generate a staple peptide to enhance its cell membrane penetrability, and it was also able to regulate α-synuclein aggregation properties and morphological characteristics with the addition of different charges.

Serine-Selective Bioconjugation

This Communication reports the first general method for rapid, chemoselective, and modular functionalization of serine residues in native polypeptides, which uses a reagent platform based on the P(V) oxidation state. This redox-economical approach can be used to append nearly any kind of cargo onto serine, generating a stable, benign, and hydrophilic phosphorothioate linkage. The method tolerates all other known nucleophilic functional groups of naturally occurring proteinogenic amino acids. A variety of applications can be envisaged by this expansion of the toolbox of site-selective bioconjugation methods.

A Visible and Near-Infrared Light Activatable Diazocoumarin Probe for Fluorogenic Protein Labeling in Living Cells

Chemical modification of proteins in living cells permits valuable glimpses into the molecular interactions that underpin dynamic cellular events. While genetic engineering methods are often preferred, selective labeling of endogenous proteins in a complex intracellular milieu with chemical approaches represents a significant challenge. In this study, we report novel diazocoumarin compounds that can be photoactivated by visible (430-490 nm) and near-infrared light (800 nm) irradiation to photo-uncage reactive carbene intermediates, which could subsequently undergo an insertion reaction with concomitant fluorescence "turned on". With these new molecules in hand, we have developed a new approach for rapid, selective, and fluorogenic labeling of endogenous protein in living cells. By using CA-II and eDHFR as model proteins, we demonstrated that subcellular localization of proteins can be precisely visualized by live-cell imaging and protein levels can be reliably quantified in multiple cell types using flow cytometry. Dynamic protein regulations such as hypoxia-induced CA-IX accumulation can also be detected. In addition, by two-photon excitation with an 800 nm laser, cell-selective labeling can also be achieved with spatially controlled irradiation. Our method circumvents the cytotoxicity of UV light and obviates the need for introducing external reporters with "click chemistries". We believe that this approach of fluorescence labeling of endogenous protein by bioorthogonal photoirradiation opens up exciting opportunities for discoveries and mechanistic interrogation in chemical biology.

Deciphering protein post-translational modifications using chemical biology tools

Proteins carry out a wide variety of catalytic, regulatory, signalling and structural functions in living systems. Following their assembly on ribosomes and throughout their lifetimes, most eukaryotic proteins are modified by post-translational modifications; small functional groups and complex biomolecules are conjugated to amino acid side chains or termini, and the protein backbone is cleaved, spliced or cyclized, to name just a few examples. These modifications modulate protein activity, structure, location and interactions, and, thereby, control many core biological processes. Aberrant post-translational modifications are markers of cellular stress or malfunction and are implicated in several diseases. Therefore, gaining an understanding of which proteins are modified, at which sites and the resulting biological consequences is an important but complex challenge requiring interdisciplinary approaches. One of the key challenges is accessing precisely modified proteins to assign functional consequences to specific modifications. Chemical biologists have developed a versatile set of tools for accessing specifically modified proteins by applying robust chemistries to biological molecules and developing strategies for synthesizing and ligating proteins. This Review provides an overview of these tools, with selected recent examples of how they have been applied to decipher the roles of a variety of protein post-translational modifications. Relative advantages and disadvantages of each of the techniques are discussed, highlighting examples where they are used in combination and have the potential to address new frontiers in understanding complex biological processes.

Unlocking the Full Evolutionary Potential of Artificial Metalloenzymes Through Direct Metal-Protein Coordination : A review of recent advances for catalyst development

Generation of artificial metalloenzymes (ArMs) has gained much inspiration from the general understanding of natural metalloenzymes. Over the last decade, a multitude of methods generating transition metal-protein hybrids have been developed and many of these new-to-nature constructs catalyse reactions previously reserved for the realm of synthetic chemistry. This perspective will focus on ArMs incorporating 4d and 5d transition metals. It aims to summarise the significant advances made to date and asks whether there are chemical strategies, used in nature to optimise metal catalysts, that have yet to be fully recognised in the synthetic enzyme world, particularly whether artificial enzymes produced to date fully take advantage of the structural and energetic context provided by the protein. Further, the argument is put forward that, based on precedence, in the majority of naturally evolved metalloenzymes the direct coordination bonding between the metal and the protein scaffold is integral to catalysis. Therefore, the protein can attenuate metal activity by positioning ligand atoms in the form of amino acids, as well as making non-covalent contributions to catalysis, through intermolecular interactions that pre-organise substrates and stabilise transition states. This highlights the often neglected but crucial element of natural systems that is the energetic contribution towards activating metal centres through protein fold energy. Finally, general principles needed for a different approach to the formation of ArMs are set out, utilising direct coordination inspired by the activation of an organometallic cofactor upon protein binding. This methodology, observed in nature, delivers true interdependence between metal and protein. When combined with the ability to efficiently evolve enzymes, new problems in catalysis could be addressed in a faster and more specific manner than with simpler small molecule catalysts.

Cu(II)-Catalysed β-silylation of dehydroalanine residues in peptides and proteins

We report the efficient and selective Cu(ii)-catalysed β-silylation of naturally occurring dehydroalanine (Dha) residues in various ribosomally synthesized and post-translationally modified peptides (RiPPs). The method is also applicable to proteins, as was shown by the modification of a Dha residue that was chemically introduced into Small Ubiquitin-like Modifier (SUMO).

Chemical modifications of tryptophan residues in peptides and proteins

Chemical protein modifications facilitate the investigation of natural posttranslational protein modifications and allow the design of proteins with new functions. Proteins can be modified at a late stage on amino acid side chains by chemical methods. The indole moiety of tryptophan residues is an emerging target of such chemical modification strategies because of its unique reactivity and low abundance. This review provides an overview of the recently developed methods of tryptophan modification at the peptide and protein levels.

Site-Specific and Trigger-Activated Modification of Proteins by Means of Catalytic Hemin/G-quadruplex DNAzyme Nanostructures

Catalytic nanostructures have the potency to mimic enzymatic features. In this paper, we show that the complex between hemin and G-quadruplex DNA efficiently catalyzes the modification of proteins with N-methyl luminol derivatives. Final conversions are reached within 15–30 min, and LC-MS analysis of tryptic digests of the proteins shows that the reaction proceeds with chemoselectivity for electron-rich aromatic residues (Tyr ≫ Trp), and the site-specificity of the modification depends on the sequence and secondary structure folding of the G-quadruplex nanostructure. Furthermore, the modification can be applied on proteins with different biomedical functions, and the nanostructure can be designed to contain a regulatory element in order to regulate protein modification by an external stimulus.

The Chemical Biology of Reversible Lysine Post-translational Modifications

Lysine (Lys) residues in proteins undergo a wide range of reversible post-translational modifications (PTMs), which can regulate enzyme activities, chromatin structure, protein-protein interactions, protein stability, and cellular localization. Here we discuss the "writers," "erasers," and "readers" of some of the common protein Lys PTMs and summarize examples of their major biological impacts. We also review chemical biology approaches, from small-molecule probes to protein chemistry technologies, that have helped to delineate Lys PTM functions and show promise for a diverse set of biomedical applications.

Activity-Based Sensing: A Synthetic Methods Approach for Selective Molecular Imaging and Beyond

Emerging from the origins of supramolecular chemistry and the development of selective chemical receptors that rely on lock-and-key binding, activity-based sensing (ABS)-which utilizes molecular reactivity rather than molecular recognition for analyte detection-has rapidly grown into a distinct field to investigate the production and regulation of chemical species that mediate biological signaling and stress pathways, particularly metal ions and small molecules. Chemical reactions exploit the diverse chemical reactivity of biological species to enable the development of selective and sensitive synthetic methods to decipher their contributions within complex living environments. The broad utility of this reaction-driven approach facilitates application to imaging platforms ranging from fluorescence, luminescence, photoacoustic, magnetic resonance, and positron emission tomography modalities. ABS methods are also being expanded to other fields, such as drug and materials discovery.

Transition metal catalyzed site-selective cysteine diversification of proteins

Site-specific protein conjugation is a critical step in the generation of unique protein analogs for a range of basic research and therapeutic developments. Protein transformations must target a precise residue in the presence of a plethora of functional groups to obtain a well-characterized homogeneous product. Competing reactive residues on natural proteins render rapid and selective conjugation a challenging task. Organometallic reagents have recently emerged as a powerful strategy to achieve site-specific labeling of a diverse set of biopolymers, due to advances in water-soluble ligand design, high reaction rate, and selectivity. The thiophilic nature of various transition metals, especially soft metals, makes cysteine an ideal target for these reagents. The distinctive reactivity and selectivity of organometallic-based reactions, along with the unique reactivity and abundancy of cysteine within the human proteome, provide a powerful platform to modify native proteins in aqueous media. These reactions often provide the modified proteins with a stable linkage made from irreversible cross-coupling steps. Additionally, transition metal reagents have recently been applied for the decaging of cysteine residues in the context of chemical protein synthesis. Orthogonal cysteine protecting groups and functional tags are often necessary for the synthesis of challenging proteins, and organometallic reagents are powerful tools for selective, rapid, and water-compatible removal of those moieties. This review examines transition metal-based reactions of cysteine residues for the synthesis and modification of natural peptides and proteins.

Application of High-Throughput Competition Experiments in the Development of Aspartate-Directed Site-Selective Modification of Tyrosine Residues in Peptides

Herein we report a Cu-catalyzed, site-selective functionalization of peptides that employs an aspartic acid (Asp) as a native directing motif, which directs the site of O-arylation at a proximal tyrosine (Tyr) residue. Through a series of competition studies conducted in high-throughput reaction arrays, effective conditions were identified that gave high selectivity for the proximal Tyr in Asp-directed Tyr modification. Good levels of site-selectivity were achieved in the O-arylation at a proximal Tyr residue in a number of cases, including a peptide-small molecule hybrid.

Quantification of protein delivery in live cells using fluorescence correlation spectroscopy

Fluorescence correlation spectroscopy (FCS) is a quantitative single-molecule method that measures the concentration and rate of diffusion of fluorophore-tagged molecules, both large and small, in vitro and within live cells, and even within discrete cellular compartments. FCS is exceptionally well-suited to directly quantify the efficiency of intracellular protein delivery-specifically, how well different "cell-penetrating" proteins and peptides guide proteinaceous materials into the cytosol and nuclei of live mammalian cells. This article provides an overview of the procedures necessary to execute robust FCS experiments and evaluate endosomal escape efficiencies: preparation of fluorophore-tagged proteins, incubation with mammalian cells and preparation of FCS samples, setup and execution of an FCS experiment, and a detailed discussion of and custom MATLAB® script for analyzing the resulting autocorrelation curves in the context of appropriate diffusion models.

Kinetics and Optimization of the Lysine-Isopeptide Bond Forming Sortase Enzyme from Corynebacterium diphtheriae

Site-specifically modified protein bioconjugates have important applications in biology, chemistry, and medicine. Functionalizing specific protein side chains with enzymes using mild reaction conditions is of significant interest, but remains challenging. Recently, the lysine-isopeptide bond forming activity of the sortase enzyme that builds surface pili in Corynebacterium diphtheriae (CdSrtA) has been reconstituted in vitro. A mutationally activated form of CdSrtA was shown to be a promising bioconjugating enzyme that can attach Leu-Pro-Leu-Thr-Gly peptide fluorophores to a specific lysine residue within the N-terminal domain of the SpaA protein (NSpaA), enabling the labeling of target proteins that are fused to NSpaA. Here we present a detailed analysis of the CdSrtA catalyzed protein labeling reaction. We show that the first step in catalysis is rate limiting, which is the formation of the CdSrtA-peptide thioacyl intermediate that subsequently reacts with a lysine ε-amine in NSpaA. This intermediate is surprisingly stable, limiting spurious proteolysis of the peptide substrate. We report the discovery of a new enzyme variant (CdSrtAΔ) that has significantly improved transpeptidation activity, because it completely lacks an inhibitory polypeptide appendage ("lid") that normally masks the active site. We show that the presence of the lid primarily impairs formation of the thioacyl intermediate and not the recognition of the NSpaA substrate. Quantitative measurements reveal that CdSrtAΔ generates its cross-linked product with a catalytic turnover number of 1.4 ± 0.004 h-1 and that it has apparent KM values of 0.16 ± 0.04 and 1.6 ± 0.3 mM for its NSpaA and peptide substrates, respectively. CdSrtAΔ is 7-fold more active than previously studied variants, labeling >90% of NSpaA with peptide within 6 h. The results of this study further improve the utility of CdSrtA as a protein labeling tool and provide insight into the enzyme catalyzed reaction that underpins protein labeling and pilus biogenesis.

Expanding the Scope of Antibody Rebridging with New Pyridazinedione-TCO Constructs

Disulfide rebridging methods have recently emerged as a route to hinge region-specific antibody modification, and there exist numerous examples of successful rebridging chemistry applied to clinically relevant human IgG1 antibodies. Here, dibromopyridazinedione disulfide rebridging is adapted to fast trans-cyclooctene/tetrazine (TCO/Tz) bioorthogonal ligations and extended beyond therapeutic human IgG1 antibodies for the first time to include mouse and rat monoclonal antibodies integral to multiplexed analytical diagnostics. In spite of a common architecture, only a subset of antibody host species and IgG isotype subclasses can be rebridged, highlighting the intricate relationship between hinge region sequence, structure, biological activity, and the conjugation chemistry of IgG antibodies.

General dual functionalisation of biomacromolecules via a cysteine bridging strategy

Site-selective modification of peptides and proteins has resulted in the development of a host of novel tools for the study of cellular systems or the synthesis of enhanced biotherapeutics. There is a need for useful methodologies that enable site-selective modification of native peptides or proteins, which is even more prevalent when modification of the biomolecule with multiple payloads is desired. Herein, we report the development of a novel dual functional divinylpyrimidine (dfDVP) platform that enables robust and modular modification of peptides, antibody fragments and antibodies. These biomacromolecules could be easily functionalised with a range of functional payloads (e.g. fluorescent dyes, cytotoxic warheads or cell-penetrating tags). Importantly, the dual functionalised peptides and antibodies demonstrated exquisite bioactivity in a range of in vitro cellular assays, showcasing the enhanced utility of these bioactive conjugates.

Alkynyl Benzoxazines and Dihydroquinazolines as Cysteine Targeting Covalent Warheads and Their Application in Identification of Selective Irreversible Kinase Inhibitors

With a resurgence in interest in covalent drugs, there is a need to identify new moieties capable of cysteine bond formation that are differentiated from commonly employed systems such as acrylamide. Herein, we report on the discovery of new alkynyl benzoxazine and dihydroquinazoline moieties capable of covalent reaction with cysteine. Their utility as alternative electrophilic warheads for chemical biological probes and drug molecules is demonstrated through site-selective protein modification and incorporation into kinase drug scaffolds. A potent covalent inhibitor of JAK3 kinase was identified with superior selectivity across the kinome and improvements in in vitro pharmacokinetic profile relative to the related acrylamide-based inhibitor. In addition, the use of a novel heterocycle as a cysteine reactive warhead is employed to target Cys788 in c-KIT, where acrylamide has previously failed to form covalent interactions. These new reactive and selective heterocyclic warheads supplement the current repertoire for cysteine covalent modification while avoiding some of the limitations generally associated with established moieties.

N-Terminal selective modification of peptides and proteins using 2-ethynylbenzaldehydes

Selective modification of the N-terminus of peptides and proteins is a promising strategy for single site modification methods. Here we report N-terminal selective modification of peptides and proteins by using 2-ethynylbenzaldehydes (2-EBA) for the production of well-defined bioconjugates. After reaction screening with a series of 2-EBA, excellent N-terminal selectivity is achieved by the reaction in slightly acidic phosphate-buffered saline using 2-EBA with electron-donating substituents. Selective modification of a library of peptides XSKFR (X = either one of 20 natural amino acids) by 2-ethynyl-4-hydroxy-5-methoxybenzaldehyde (2d) results in good-to-excellent N-terminal selectivity in peptides (up to >99:1). Lysozyme, ribonuclease A and a therapeutic recombinant Bacillus caldovelox arginase mutant (BCArg mutant) are N-terminally modified using alkyne- and fluorescein-linked 2-EBA. Alkyne-linked BCArg mutant is further modified by rhodamine azide via copper(I)-catalyzed [3 + 2] cycloaddition indicating that the reaction has high functional group compatibility. Moreover, the BCArg mutant modified by 2-ethynyl-5-methoxybenzaldehyde (2b) exhibits comparable activity in enzymatic and cytotoxic assays with the unmodified one.

Triazolecarbaldehyde Reagents for One-Step N-Terminal Protein Modification

Site-specific modification of peptides and proteins is a key aspect of protein engineering. We developed a method for modification of the N terminus of proteins using 1H-1,2,3-triazole-4-carbaldehyde (TA4C) derivatives, which can be prepared in one step. The N-terminal specific labeling of bioactive peptides and proteins with the TA4C derivatives proceeds under mild reaction conditions in excellent conversion (angiotensin I: 92 %, ribonuclease A: 90 %). This method enables site-specific conjugation of various functional molecules such as fluorophores, biotin, and polyethylene glycol attached to the triazole ring to the N terminus. Furthermore, a functional molecule modified with a primary amine moiety can be directly converted into a TA4C derivative through a Dimroth rearrangement reaction with 1-(4-nitrophenyl)-1H-1,2,3-triazole-4-carbaldehyde. This method can be used to obtain N-terminal-modified proteins via only two steps: 1) convenient preparation of a TA4C derivative with a functional group and 2) modification of the N terminus of the protein with the TA4C derivative.