Post-synthetic functionalization of tryptophan protected peptide sequences through indole (C-2) photocatalytic alkylation

Photoredox C(2)-Arylation of Indole- and Tryptophan-Containing Biomolecules

We introduce a novel and straightforward methodology for photoredox arylation of an indole scaffold using aryldiazonium salts under mild and metal-free conditions. Our approach enables the regioselective and chemoselective introduction of several aryl groups to the C(2) position of indoles and tryptophan, even in competition with other amino acids. This approach extends to the late-stage functionalization of peptides and lysozyme, heralding the unprecedented arylation of tryptophan residues in wild-type proteins and offering broad utility in chemical biology.

Photocatalytic Functionalization of Dehydroalanine-Derived Peptides in Batch and Flow

Unnatural amino acids, and their synthesis by the late-stage functionalization (LSF) of peptides, play a crucial role in areas such as drug design and discovery. Historically, the LSF of biomolecules has predominantly utilized traditional synthetic methodologies that exploit nucleophilic residues, such as cysteine, lysine or tyrosine. Herein, we present a photocatalytic hydroarylation process targeting the electrophilic residue dehydroalanine (Dha). This residue possesses an α,β-unsaturated moiety and can be combined with various arylthianthrenium salts, both in batch and flow reactors. Notably, the flow setup proved instrumental for efficient scale-up, paving the way for the synthesis of unnatural amino acids and peptides in substantial quantities. Our photocatalytic approach, being inherently mild, permits the diversification of peptides even when they contain sensitive functional groups. The readily available arylthianthrenium salts facilitate the seamless integration of Dha-containing peptides with a wide range of arenes, drug blueprints, and natural products, culminating in the creation of unconventional phenylalanine derivatives. The synergistic effect of the high functional group tolerance and the modular characteristic of the aryl electrophile enables efficient peptide conjugation and ligation in both batch and flow conditions.

Fluorescent coumarin-alkynes for labeling of amino acids and peptides via manganese(I)-catalyzed C-H alkenylation

The late-stage fluorescent labeling of structurally complex peptides bears immense potential for molecular imaging. Herein, we report on a manganese(I)-catalyzed peptide C-H alkenylation under exceedingly mild conditions with natural fluorophores as coumarin- and chromone-derivatives. The robustness and efficiency of the manganese(I) catalysis regime was reflected by a broad functional group tolerance and low catalyst loading in a resource- and atom-economical fashion.

Recent Advances in Photoinduced Modification of Amino Acids, Peptides, and Proteins

The chemical modification of biopolymers like peptides and proteins is a key technology to access vaccines and pharmaceuticals. Similarly, the tunable derivatization of individual amino acids is important as they are key building blocks of biomolecules, bioactive natural products, synthetic polymers, and innovative materials. The high diversity of functional groups present in amino acid-based molecules represents a significant challenge for their selective derivatization Recently, visible light-mediated transformations have emerged as a powerful strategy for achieving chemoselective biomolecule modification. This technique offers numerous advantages over other methods, including a higher selectivity, mild reaction conditions and high functional-group tolerance. This review provides an overview of the most recent methods covering the photoinduced modification for single amino acids and site-selective functionalization in peptides and proteins under mild and even biocompatible conditions. Future challenges and perspectives are discussed beyond the diverse types of photocatalytic transformations that are currently available.

Electrochemistry-enabled residue-specific modification of peptides and proteins

Selective chemical reactions at precise amino acid residues of peptides and proteins have become an exploding field of research in the last few decades. With the emerging utility of bioconjugated peptides and proteins as drug leads and therapeutic agents, the design of smart protocols to modulate and conjugate biomolecules has become necessary. During this modification, the most important concern of biochemists is to keep intact the structural integrity of the biomolecules. Hence, a soft and selective biocompatible reaction environment is necessary. Electrochemistry, a mild and elegant tunable reaction platform to synthesize complex molecules while avoiding harsh and toxic chemicals, can provide such a reaction condition. However, this strategy is yet to be fully exploited in the field of selective modification of polypeptides. With this possibility, the use of electrochemistry as a reaction toolbox in peptide and protein chemistry is flourishing day by day. Unfortunately, there is no suitable review article summarizing the residue-specific modification of biomolecules. The present review provides a comprehensive summary of the latest manifested electrochemical approaches for the modulation of five redox-active amino acid residues, namely cysteine, tyrosine, tryptophan, histidine and methionine, found in peptides and proteins. The article also highlights the incredible potential of electrochemistry for the regio- as well as chemoselective bioconjugation strategy of biomolecules.

Side-Selective Solid-Phase Metallaphotoredox N(in)-Arylation of Peptides

Postsynthetic diversification of peptides through selective modification of endogenous amino acid side chains has enabled significant advances in peptide drug discovery while expanding the biological and medical chemistry space. However, current tools have been focused on the modification of reactive polar and ionizable side chains, whereas the decoration of aromatic systems (e.g., the N(in) of the tryptophan) has been a long-standing challenge. Here, we introduce metallaphotocatalysis in solid-phase peptide synthesis for the on-resin orthogonal N-arylation of relevant tryptophan-containing peptides. The protocol allows the chemoselective introduction of a new C(sp2)-N bond at the N(in) of tryptophan in biologically active protected peptide sequences in the presence of native redox-sensitive side chains. The fusion of metallaphotocatalysis with solid-phase peptide synthesis opens new perspectives in diversifying native amino acid side chains.

Selective modification of tryptophan in polypeptides via C-N coupling with azoles using in situ-generated iodine-based oxidants in aqueous media

This study demonstrates the C-N coupling of tryptophan with azoles, promoted by an in situ-generated iodine-based oxidant. The protocol was successfully applied to the selective modification of tryptophan in nonprotected polypeptide bearing oxidatively sensitive residues in acidic aqueous media. The present method allows the attachment of reactive handles to polypeptides and the peptide stapling.

Chemoselective Late-Stage Functionalization of Peptides via Photocatalytic C2-Alkylation of Tryptophan

Across eukaryotic proteomes, tryptophan is the least abundant of the 20 canonical amino acids, which makes it an ideal chemical handle for the late-stage functionalization of peptide and protein scaffolds with minimal production of undesired isoforms. Herein, we report the photocatalytic C2-alkylation of tryptophan using bromodifluoroacetate/acetamide-derived radical precursors. This rapid visible-light-mediated reaction is additive-free, operationally simple, and tolerates diverse functionality. We demonstrate the late-stage modification of a variety of complex peptides, including examples of biological significance.

Photoredox catalysis enabling decarboxylative radical cyclization of γ,γ-dimethylallyltryptophan (DMAT) derivatives: formal synthesis of 6,7-secoagroclavine

An unusual photoredox-catalyzed radical decarboxylative cyclization cascade reaction of γ,γ-dimethylallyltryptophan (DMAT) derivatives containing unactivated alkene moieties has been developed, providing green and efficient access to various six-, seven-, and eight-membered ring 3,4-fused tricyclic indoles. This type of cyclization, which was hitherto very difficult to comprehend in ergot biosynthesis and to accomplish by more conventional procedures, enables the synthesis of ergot alkaloid precursors. In addition, this work describes a mild, environmentally friendly method to activate, reductively and oxidatively, natural carboxylic acids for decarboxylative C-C bond formation by exploiting the same photocatalyst.

Late-stage C-H Functionalization of Tryptophan-Containing Peptides with Thianthrenium Salts: Conjugation and Ligation

Bioorthogonal late-stage diversification of structurally complex peptides bears enormous potential for drug discovery and molecular imaging, among other applications. Herein, we report on a palladium-catalyzed C-H arylation of tryptophan-containing peptides with readily accessible and modular arylthianthrenium salts. Under exceedingly mild reaction conditions, the late-stage diversification of structurally complex peptides was accomplished. The tunability and ease of preparation of arylthianthrenium salts allowed the expedient stitching of tryptophan-containing peptides with drug, natural product, and peptidic scaffolds by forging sterically congested biaryl linkages. The robustness of the palladium catalysis regime was reflected by the full tolerance of a plethora of sensitive and coordinating functional groups. Hence, our manifold enabled efficient access to highly decorated, labelled, conjugated, and ligated linear and cyclic peptides.

Photocatalytic Late-Stage C-H Functionalization

The emergence of modern photocatalysis, characterized by mildness and selectivity, has significantly spurred innovative late-stage C-H functionalization approaches that make use of low energy photons as a controllable energy source. Compared to traditional late-stage functionalization strategies, photocatalysis paves the way toward complementary and/or previously unattainable regio- and chemoselectivities. Merging the compelling benefits of photocatalysis with the late-stage functionalization workflow offers a potentially unmatched arsenal to tackle drug development campaigns and beyond. This Review highlights the photocatalytic late-stage C-H functionalization strategies of small-molecule drugs, agrochemicals, and natural products, classified according to the targeted C-H bond and the newly formed one. Emphasis is devoted to identifying, describing, and comparing the main mechanistic scenarios. The Review draws a critical comparison between established ionic chemistry and photocatalyzed radical-based manifolds. The Review aims to establish the current state-of-the-art and illustrate the key unsolved challenges to be addressed in the future. The authors aim to introduce the general readership to the main approaches toward photocatalytic late-stage C-H functionalization, and specialist practitioners to the critical evaluation of the current methodologies, potential for improvement, and future uncharted directions.

Temperature-induced formation of Pd nanoparticles in heterogeneous nanobiohybrids: application in C-H activation catalysis

The effect of the temperature in the synthesis of Pd nanoparticles in the metal-enzyme biohybrids is evaluated. The effect on the formation, size, and morphology of nanoparticles was evaluated using C. antarctica B lipase as the protein scaffold. XRD analyses confirmed the formation of crystalline Pd(0) as the metal species in all cases. TEM analyses revealed spherical crystalline nanoparticles with average diameter size from 2 nm at 4 °C synthesis to 10 nm obtained at 50 °C synthesis. The thermal phenomenon was also critical in the final hybrid formation using more complex enzymes, where the relation of the protein structure and temperature and the influence of the latter has been demonstrated to be critical in the reducing efficiency of the enzyme in the final Pd nanoparticle formation, in the metal species, or even in the final size of the nanoparticles. Different Pd biohybrids were evaluated as catalysts in the C-H activation of protected l-tryptophan under mild conditions. Pd@CALB4 showed the best results, with >99% conversion for C-2 arylation in methanol at room temperature with a TOF value of 64 min^-1, being 2 or 4 times higher than that of the other synthesized hybrids. This catalyst showed a very high stability and recyclability, maintaining >95% activity after three cycles of use.

Donor-Acceptor Pyridinium Salts for Photo-Induced Electron-Transfer-Driven Modification of Tryptophan in Peptides, Proteins, and Proteomes Using Visible Light

Tryptophan (Trp) plays a variety of critical functional roles in protein biochemistry; however, owing to its low natural frequency and poor nucleophilicity, the design of effective methods for both single protein bioconjugation at Trp as well as for in situ chemoproteomic profiling remains a challenge. Here, we report a method for covalent Trp modification that is suitable for both scenarios by invoking photo-induced electron transfer (PET) as a means of driving efficient reactivity. We have engineered biaryl N-carbamoyl pyridinium salts that possess a donor-acceptor relationship that enables optical triggering with visible light whilst simultaneously attenuating the probe's photo-oxidation potential in order to prevent photodegradation. This probe was assayed against a small bank of eight peptides and proteins, where it was found that micromolar concentrations of the probe and short irradiation times (10-60 min) with violet light enabled efficient reactivity toward surface exposed Trp residues. The carbamate transferring group can be used to transfer useful functional groups to proteins including affinity tags and click handles. DFT calculations and other mechanistic analyses reveal correlations between excited state lifetimes, relative fluorescence quantum yields, and chemical reactivity. Biotinylated and azide-functionalized pyridinium salts were used for Trp profiling in HEK293T lysates and in situ in HEK293T cells using 440 nm LED irradiation. Peptide-level enrichment from live cell labeling experiments identified 290 Trp modifications, with 82% selectivity for Trp modification over other π-amino acids, demonstrating the ability of this method to identify and quantify reactive Trp residues from live cells.

Visible-Light Mediated Tryptophan Modification in Oligopeptides Employing Acylsilanes

A method for the selective tryptophan modification and labelling of tryptophan-containing peptides is described. Photoirradiation of acylsilanes generates reactive siloxycarbenes which undergo H-N-insertion into the indole moiety of tryptophan to give stable silyl protected hemiaminals. This method is successfully applied to chemically modify various tryptophan containing oligopeptides. The method enables the selective introduction of alkynes to peptides that are eligible for further alkyne-azide click chemistry. In addition, the dansyl fluorophore can be conjugated to a peptide using this approach.

Visible-Light-Mediated Modification and Manipulation of Biomacromolecules

Chemically modified biomacromolecules-i.e., proteins, nucleic acids, glycans, and lipids-have become crucial tools in chemical biology. They are extensively used not only to elucidate cellular processes but also in industrial applications, particularly in the context of biopharmaceuticals. In order to enable maximum scope for optimization, it is pivotal to have a diverse array of biomacromolecule modification methods at one's disposal. Chemistry has driven many significant advances in this area, and especially recently, numerous novel visible-light-induced photochemical approaches have emerged. In these reactions, light serves as an external source of energy, enabling access to highly reactive intermediates under exceedingly mild conditions and with exquisite spatiotemporal control. While UV-induced transformations on biomacromolecules date back decades, visible light has the unmistakable advantage of being considerably more biocompatible, and a spectrum of visible-light-driven methods is now available, chiefly for proteins and nucleic acids. This review will discuss modifications of native functional groups (FGs), including functionalization, labeling, and cross-linking techniques as well as the utility of oxidative degradation mediated by photochemically generated reactive oxygen species. Furthermore, transformations at non-native, bioorthogonal FGs on biomacromolecules will be addressed, including photoclick chemistry and DNA-encoded library synthesis as well as methods that allow manipulation of the activity of a biomacromolecule.