Concurrent and orthogonal gold(I) and ruthenium(II) catalysis inside living cells

Copper-Catalyzed Sulfimidation in Aqueous Media: a Fast, Chemoselective and Biomolecule-Compatible Reaction

Performing transition metal-catalyzed reactions in cells and living systems has equipped scientists with a toolbox to study biological processes and release drugs on demand. Thus far, an impressive scope of reactions has been performed in these settings, but many are yet to be introduced. Nitrene transfer presents a rather unexplored new-to-nature reaction. The reaction products are frequently encountered motifs in pharmaceuticals, presenting opportunities for the controlled, intracellular synthesis of drugs. Hence, we explored the transition metal-catalyzed sulfimidation reaction in water for future in vivo application. Two Cu(I) complexes containing trispyrazolylborate ligands (Tpx ) were selected, and the catalytic system was evaluated with the aid of three fitness factors. The excellent nitrene transfer reactivity and high chemoselectivity of the catalysts, coupled with good biomolecule compatibility, successfully enabled the sulfimidation of thioethers in aqueous media. We envision that this copper-catalyzed sulfimidation reaction could be an interesting starting point to unlock the potential of nitrene transfer catalysis in vivo.

Modular Fabrication of Bioorthogonal Nanozymes for Biomedical Applications

The incorporation of transition metal catalysts (TMCs) into nanoscaffolds generates nanocatalysts that replicate key aspects of enzymatic behavior. The TMCs can access bioorthogonal chemistry unavailable to living systems. These bioorthogonal nanozymes can be employed as in situ "factories" for generating bioactive molecules where needed. The generation of effective bioorthogonal nanozymes requires co-engineering of the TMC and the nanometric scaffold. This review presents an overview of recent advances in the field of bioorthogonal nanozymes, focusing on modular design aspects of both nanomaterial and catalyst and how they synergistically work together for in situ uncaging of imaging and therapeutic agents.

Enhanced Efficiency of Pd(0)-Based Single Chain Polymeric Nanoparticles for in Vitro Prodrug Activation by Modulating the Polymer's Microstructure

Bioorthogonal catalysis employing transition metal catalysts is a promising strategy for the in situ synthesis of imaging and therapeutic agents in biological environments. The transition metal Pd has been widely used as a bioorthogonal catalyst, but bare Pd poses challenges in water solubility and catalyst stability in cellular environments. In this work, Pd(0) loaded amphiphilic polymeric nanoparticles are applied to shield Pd in the presence of living cells for the in situ generation of a fluorescent dye and anticancer drugs. Pd(0) loaded polymeric nanoparticles prepared by the reduction of the corresponding Pd(II)-polymeric nanoparticles are highly active in the deprotection of pro-rhodamine dye and anticancer prodrugs, giving significant fluorescence enhancement and toxigenic effects, respectively, in HepG2 cells. In addition, we show that the microstructure of the polymeric nanoparticles for scaffolding Pd plays a critical role in tuning the catalytic efficiency, with the use of the ligand triphenylphosphine as a key factor for improving the catalyst stability in biological environments.

Dipeptide coacervates as artificial membraneless organelles for bioorthogonal catalysis

Artificial organelles can manipulate cellular functions and introduce non-biological processes into cells. Coacervate droplets have emerged as a close analog of membraneless cellular organelles. Their biomimetic properties, such as molecular crowding and selective partitioning, make them promising components for designing cell-like materials. However, their use as artificial organelles has been limited by their complex molecular structure, limited control over internal microenvironment properties, and inherent colloidal instability. Here we report the design of dipeptide coacervates that exhibit enhanced stability, biocompatibility, and a hydrophobic microenvironment. The hydrophobic character facilitates the encapsulation of hydrophobic species, including transition metal-based catalysts, enhancing their efficiency in aqueous environments. Dipeptide coacervates carrying a metal-based catalyst are incorporated as active artificial organelles in cells and trigger an internal non-biological chemical reaction. The development of coacervates with a hydrophobic microenvironment opens an alternative avenue in the field of biomimetic materials with applications in catalysis and synthetic biology.

Target-Specific Bioorthogonal Reactions for Precise Biomedical Applications

Bioorthogonal chemistry is a promising toolbox for dissecting biological processes in the native environment. Recently, bioorthogonal reactions have attracted considerable attention in the medical field for treating diseases, since this approach may lead to improved drug efficacy and reduced side effects via in situ drug synthesis. For precise biomedical applications, it is a prerequisite that the reactions should occur in the right locations and on the appropriate therapeutic targets. In this minireview, we highlight the design and development of targeted bioorthogonal reactions for precise medical treatment. First, we compile recent strategies for achieving target-specific bioorthogonal reactions. Further, we emphasize their application for the precise treatment of different therapeutic targets. Finally, a perspective is provided on the challenges and future directions of this emerging field for safe, efficient, and translatable disease treatment.

Transition Metal Catalysis in Living Cells: Progress, Challenges, and Novel Supramolecular Solutions

The importance of transition metal catalysis is exemplified by its wide range of applications, for example in the synthesis of chemicals, natural products, and pharmaceuticals. However, one relatively new application is for carrying out new-to-nature reactions inside living cells. The complex environment of a living cell is not welcoming to transition metal catalysts, as a diverse range of biological components have the potential to inhibit or deactivate the catalyst. Here we review the current progress in the field of transition metal catalysis, and evaluation of catalysis efficiency in living cells and under biological (relevant) conditions. Catalyst poisoning is a ubiquitous problem in this field, and we propose that future research into the development of physical and kinetic protection strategies may provide a route to improve the reactivity of catalysts in cells.

Polymeric architecture as a tool for controlling the reactivity of palladium(II) loaded nanoreactors

Self-assembled systems, like polymeric micelles, have become great facilitators for conducting organic reactions in aqueous media due to their broad potential applications in green chemistry and biomedical applications. Massive strides have been taken to improve the reaction scope of such systems, enabling them to perform bioorthogonal reactions for prodrug therapy. Considering these significant advancements, we sought to study the relationships between the architecture of the amphiphiles and the reactivity of their PdII loaded micellar nanoreactors in conducting depropargylation reactions. Towards this goal, we designed and synthesized a series of isomeric polyethylene glycol (PEG)-dendron amphiphiles with different dendritic architectures but with an identical degree of hydrophobicity and hydrophilic to lipophilic balance (HLB). We observed that the dendritic architecture, which serves as the main binding site for the PdII ions, has greater influence on the reactivity than the hydrophobicity of the dendron. These trends remained constant for two different propargyl caged substrates, validating the obtained results. Density functional theory (DFT) calculations of simplified models of the dendritic blocks revealed the different binding modes of the various dendritic architectures to PdII ions, which could explain the observed differences in the reactivity of the nanoreactors with different dendritic architectures. Our results demonstrate how tuning the internal architecture of the amphiphiles by changing the orientation of the chelating moieties can be used as a tool for controlling the reactivity of PdII loaded nanoreactors.

Biomimetic and bioorthogonal nanozymes for biomedical applications

Nanozymes mimic the function of enzymes, which drive essential intracellular chemical reactions that govern biological processes. They efficiently generate or degrade specific biomolecules that can initiate or inhibit biological processes, regulating cellular behaviors. Two approaches for utilizing nanozymes in intracellular chemistry have been reported. Biomimetic catalysis replicates the identical reactions of natural enzymes, and bioorthogonal catalysis enables chemistries inaccessible in cells. Various nanozymes based on nanomaterials and catalytic metals are employed to attain intended specific catalysis in cells either to mimic the enzymatic mechanism and kinetics or expand inaccessible chemistries. Each nanozyme approach has its own intrinsic advantages and limitations, making them complementary for diverse and specific applications. This review summarizes the strategies for intracellular catalysis and applications of biomimetic and bioorthogonal nanozymes, including a discussion of their limitations and future research directions.

A narrative review: progress in transition metal-mediated bioorthogonal catalysis for the treatment of solid tumors

Background and Objective

Transition metals are commonly used catalysts in bioorthogonal chemistry and have attracted extensive attention in biochemistry because of their efficient catalytic performance. In recent years, transition metal-mediated cycloaddition reactions, bond cleavage, and formation reactions are being actively explored for tumor treatment. However, the direct application of transition metals in complex biological environments has several problems, including poor solubility, toxicity, and easy inactivation. The combination of transition metals and nanomaterials can solve those problems by playing a bioorthogonal catalytic role in tumor treatment. In this review, we summarize some research on the application of transition metals modified by nanomaterials in tumor therapy and discuss the potential and challenges of transition metal-mediated bioorthogonal therapy in comprehensive tumor therapy.

Methods

English literature on transition metal in cancer treatment was searched in PubMed and Web of Science. The main search terms were "cancer treatment", "bioorthogonal reaction", "transition metal", "bioorthogonal catalysis", etc.

Key Content and Findings

This review summarizes research on several major transition metals that can be used for bioorthogonal catalysis with the assistance of nanomaterials in anti-tumor therapy. In addition, bioorthogonal catalysis is a new supplement to antitumor therapy. We have compiled the potential challenges of the clinical application of transition metal-based nanocatalysts, which lays the foundation for future research related to medicinal chemistry and targeted cancer therapy.

Conclusions

Most of the transition metals still have a lot of room for exploration in cancer treatment research. We still need more research to confirm the feasibility of in vivo and clinical trials.

Bond-Dissociation Energies to Probe Pyridine Electronic Effects on Organogold(III) Complexes: From Methodological Developments to Application in π-Backdonation Investigation and Catalysis

In this work, we report on the synthesis of several organogold(III) complexes based on 4,4'-diterbutylbiphenyl (C^C) and 2,6-bis(4-terbutylphenyl)pyridine (C^N^C) ligands and bond with variously substituted pyridine ligands (pyrR). Altogether, 33 complexes have been prepared and studied with mass spectrometry using higher-energy collision dissociation (HCD) in an Orbitrap mass spectrometer. A complete methodology including the kinetic modeling of the dissociation process based on the Rice-Ramsperger-Kassel-Marcus (RRKM) statistical method is proposed to obtain critical energies E0 of the pyrR loss for all complexes. The capacity of these E0 values to describe the pyridine ligand effect is further explored, at the same time as more classical descriptors such as 1H pyridinic NMR shift variation upon coordination and Au-NpyrR bond length measured by X-ray diffraction. An extensive theoretical work, including density functional theory (DFT) and domain-based local pair natural orbital coupled-cluster theory (DLPNO-CCSD(T)) methods, is also carried out to provide bond-dissociation energies, which are compared to experimental results. Results show that dissociation energy outperforms other descriptors, in particular to describe ligand effects over a large electronic effect range as seen by confronting the results to the pyrR pKa values. Further insights into the Au-NpyrR bond are obtained through an energy decomposition analysis (EDA) study, which confirms the isolobal character of Au+ with H+. Finally, the correlation between the lability of the pyridine ligands toward the catalytic efficiency of the complexes could be demonstrated in an intramolecular hydroarylation reaction of alkyne. The results were rationalized considering both pre-catalyst activation and catalyst reactivity. This study establishes the possibility of correlating dissociation energy, which is a gas-phase descriptor, with condensed-phase parameters such as catalysis efficiency. It therefore holds great potential for inorganic and organometallic chemistry by opening a convenient and easy way to evaluate the electronic influence of a ligand toward a metallic center.

A photoresponsive gold catalyst based on azobenzene-functionalized NHC ligands

An azobenzene-bearing N-heterocyclic carbene-based gold catalyst is reported of which the reactivity in a cyclization reaction depends on the isomeric state of the azobenzene. The configurations of the catalyst can be reversibly switched by light and are stable during the reaction, effectively leading to a switchable catalyst system.

Ruthenium-catalyzed intermolecular alkene-alkyne couplings in biologically relevant media

Cationic cyclopentadienyl Ru(ii) catalysts can efficiently promote mild intermolecular alkyne-alkene couplings in aqueous media, even in the presence of different biomolecular components, and in complex media like DMEM. The method can also be used for the derivatization of amino acids and peptides, therefore proposing a new way to label biomolecules with external tags. This C-C bond-forming reaction, based on simple alkene and alkyne reactants, can now be added to the toolbox of bioorthogonal reactions promoted by transition metal catalysts.

Expanding Transition Metal-Mediated Bioorthogonal Decaging to Include C-C Bond Cleavage Reactions

The ability to control the activation of prodrugs by transition metals has been shown to have great potential for controlled drug release in cancer cells. However, the strategies developed so far promote the cleavage of C-O or C-N bonds, which limits the scope of drugs to only those that present amino or hydroxyl groups. Here, we report the decaging of an ortho-quinone prodrug, a propargylated β-lapachone derivative, through a palladium-mediated C-C bond cleavage. The reaction's kinetic and mechanistic behavior was studied under biological conditions along with computer modeling. The results indicate that palladium (II) is the active species for the depropargylation reaction, activating the triple bond for nucleophilic attack by a water molecule before the C-C bond cleavage takes place. Palladium iodide nanoparticles were found to efficiently trigger the C-C bond cleavage reaction under biocompatible conditions. In drug activation assays in cells, the protected analogue of β-lapachone was activated by nontoxic amounts of nanoparticles, which restored drug toxicity. The palladium-mediated ortho-quinone prodrug activation was further demonstrated in zebrafish tumor xenografts, which resulted in a significant anti-tumoral effect. This work expands the transition-metal-mediated bioorthogonal decaging toolbox to include cleavage of C-C bonds and payloads that were previously not accessible by conventional strategies.

Bioorthogonal Activation of TLR7 Agonists Provokes Innate Immunity to Reinforce Aptamer-Based Checkpoint Blockade

Although cancer immunotherapy based on immune checkpoint blockade has shown promising clinical responses, the limited host response rate and systemic side effects still restrict immunotherapy efficacy. To address these challenges, here, we construct an aptamer-functionalized metal-organic framework (MOF) catalyst for bioorthogonal activation of Toll-like receptors (TLR) 7 agonists and programmed death-ligand 1 (PDL1) blockade for enhanced antitumor immunotherapy. The catalyst contains ultrasmall Pd nanoparticles enabling the local activation of TLR7 agonists in native form, which results in the remodeling of the tumor microenvironment (TME). Meanwhile, the loaded PDL1 aptamers release in response to phosphate and block the PD1/PDL1 signaling pathway between T cells and cancer cells. Thus, synergy between TLR7 agonists and PDL1 blockade induces the infiltration and activation of immune cells to initiate a robust immune response, thereby simultaneously inhibiting primary and distant metastatic tumors. The immunotherapeutic effect of our design has been demonstrated in both single and bilateral subcutaneous colorectal cancer (CT26) models. In situ bioorthogonal activation of agonists may offer an alternative approach to improve the therapeutic efficacy of immunotherapy with minimized systemic toxicity. Our work will provide good inspiration for current checkpoint blockade-based immunotherapy.

Dual-Bioorthogonal Catalysis by a Palladium Peptide Complex

Artificial metalloenzymes (ArMs) enrich bioorthogonal chemistry with new-to-nature reactions while limiting metal deactivation and toxicity. This enables biomedical applications such as activating therapeutics in situ. However, while combination therapies are becoming widespread anticancer treatments, dual catalysis by ArMs has not yet been shown. We present a heptapeptidic ArM with a novel peptide ligand carrying a methyl salicylate palladium complex. We observed that the peptide scaffold reduces metal toxicity while protecting the metal from deactivation by cellular components. Importantly, the peptide also improves catalysis, suggesting involvement in the catalytic reaction mechanism. Our work shows how a palladium-peptide homogeneous catalyst can simultaneously mediate two types of chemistry to synthesize anticancer drugs in human cells. Methyl salicylate palladium LLEYLKR peptide (2-Pd) succeeded to simultaneously produce paclitaxel by depropargylation, and linifanib by Suzuki-Miyaura cross-coupling in cell culture, thereby achieving combination therapy on non-small-cell lung cancer (NSCLC) A549 cells.

A Dual-Metal-Catalyzed Sequential Cascade Reaction in an Engineered Protein Cage

Herein, we describe the creation of an artificial protein cage housing a dual-metal-tagged guest protein that catalyzes a linear, two-step sequential cascade reaction. The guest protein consists of a fusion protein of HaloTag and monomeric rhizavidin. Inside the protein capsid, we established a ruthenium-catalyzed allylcarbamate deprotection reaction followed by a gold-catalyzed ring-closing hydroamination reaction that led to indoles and phenanthridines with an overall yield of up to 66 % in aqueous solutions. Furthermore, we show that the encapsulation stabilizes the metal catalysts against deactivation by air, proteins and cell lysate.

In Cellulo Bioorthogonal Catalysis by Encapsulated Nanoalloys: Overcoming Intracellular Deactivation

Bioorthogonal metallocatalysis has opened up a xenobiotic route to perform nonenzymatic catalytic transformations in living settings. Despite their promising features, most metals are deactivated inside cells by a myriad of reactive biomolecules, including biogenic thiols, thereby limiting the catalytic functioning of these abiotic reagents. Here we report the development of cytocompatible alloyed AuPd nanoparticles with the capacity to elicit bioorthogonal depropargylations with high efficiency in biological media. We also show that the intracellular catalytic performance of these nanoalloys is significantly enhanced by protecting them following two different encapsulation methods. Encapsulation in mesoporous silica nanorods resulted in augmented catalyst reactivity, whereas the use of a biodegradable PLGA matrix increased nanoalloy delivery across the cell membrane. The functional potential of encapsulated AuPd was demonstrated by releasing the potent chemotherapy drug paclitaxel inside cancer cells. Nanoalloy encapsulation provides a novel methodology to develop nanoreactors capable of mediating new-to-life reactions in cells.

Metal complexes for catalytic and photocatalytic reactions in living cells and organisms

The development of organometallic catalysis has greatly expanded the synthetic chemist toolbox compared to only exploiting "classical" organic chemistry. Although more widely used in organic solvents, metal-based catalysts have also emerged as efficient tools for developing organic transformations in water, thus paving the way for further development of bio-compatible reactions. However, performing metal-catalysed reactions within living cells or organisms induces additional constraints to the design of reactions and catalysts. In particular, metal complexes must exhibit good efficiency in complex aqueous media at low concentrations, good cell specificity, good cellular uptake and low toxicity. In this review, we focus on the presentation of discrete metal complexes that catalyse or photocatalyse reactions within living cells or living organisms. We describe the different reaction designs that have proved to be successful under these conditions, which involve very few metals (Ir, Pd, Ru, Pt, Cu, Au, and Fe) and range from in cellulo deprotection/decaging/activation of fluorophores, drugs, proteins and DNA to in cellulo synthesis of active molecules, and protein and organelle labelling. We also present developments in bio-compatible photo-activatable catalysts, which represent a very recent emerging area of research and some prospects in the field.

Bioorthogonal nanozymes: an emerging strategy for disease therapy

Transition metal catalysts (TMCs), capable of performing bioorthogonal reactions, have been engineered to trigger the formation of bioactive molecules in a controlled manner for biomedical applications. However, the widespread use of TMCs based biorthogonal reactions in vivo is still largely limited owing to their toxicity, poor stability, and insufficient targeting properties. The emergence of nanozymes (nanomaterials with enzyme-like activity), especially bioorthogonal nanozymes that combine the bioorthogonal catalytic activity of TMCs, the physicochemical properties of nanomaterials, and the enzymatic properties of classical nanozymes potentially provide opportunities to address these challenges. Thus, they can be used as multifunctional catalytic platforms for disease treatment and will be far-reaching. In this review, we first briefly recall the classical TMC-based bioorthogonal reactions. Furthermore, this review highlights the diverse strategies for manufacturing bioorthogonal nanozymes and their potential for therapeutic applications, with the goal of facilitating bioorthogonal catalysis in the clinic. Finally, we present challenges and the prospects of bioorthogonal nanozymes in bioorthogonal chemistry.

Enhanced Design of Gold Catalysts for Bioorthogonal Polyzymes

Bioorthogonal chemistry introduces nonbiogenic reactions that can be performed in biological systems, allowing for the localized release of therapeutic agents. Bioorthogonal catalysts can amplify uncaging reactions for the in situ generation of therapeutics. Embedding these catalysts into a polymeric nanoscaffold can protect and modulate the catalytic activity, improving the performance of the resulting bioorthogonal "polyzymes". Catalysts based on nontoxic metals such as gold(I) are particularly attractive for therapeutic applications. Herein, we optimized the structural components of a metal catalyst to develop an efficient gold(I)-based polyzyme. Tailoring the ligand structure of gold phosphine-based complexes, we improved the affinity between the metal complex and polymer scaffold, resulting in enhanced encapsulation efficiency and catalytic rate of the polyzyme. Our findings show the dependence of the overall polyzyme properties on the structural properties of the encapsulated metal complex.

Exporting Homogeneous Transition Metal Catalysts to Biological Habitats

The possibility of performing designed transition-metal catalyzed reactions in biological and living contexts can open unprecedented opportunities to interrogate and interfere with biology. However, the task is far from obvious, in part because of the presumed incompatibly between organometallic chemistry and complex aqueous environments. Nonetheless, in the past decade there has been a steady progress in this research area, and several transition-metal (TM)-catalyzed bioorthogonal and biocompatible reactions have been developed. These reactions encompass a wide range of mechanistic profiles, which are very different from those used by natural metalloenzymes. Herein we present a summary of the latest progress in the field of TM-catalyzed bioorthogonal reactions, with a special focus on those triggered by activation of multiple carbon-carbon bonds.

Organometallic catalysis in aqueous and biological environments: harnessing the power of metal carbenes

Translating the power of transition metal catalysis to the native habitats of enzymes can significantly expand the possibilities of interrogating or manipulating natural biological systems, including living cells and organisms. This is especially relevant for organometallic reactions that have shown great potential in the field of organic synthesis, like the metal-catalyzed transfer of carbenes. While, at first sight, performing metal carbene chemistry in aqueous solvents, and especially in biologically relevant mixtures, does not seem obvious, in recent years there has been a growing number of reports demonstrating the feasibility of the task. Either using small molecule metal catalysts or artificial metalloenzymes, a number of carbene transfer reactions that tolerate aqueous and biorelevant media are being developed. This review intends to summarize the most relevant contributions, and establish the state of the art in this emerging research field.

Synthetic prodrug design enables biocatalytic activation in mice to elicit tumor growth suppression

Considering the intrinsic toxicities of transition metals, their incorporation into drug therapies must operate at minimal amounts while ensuring adequate catalytic activity within complex biological systems. As a way to address this issue, this study investigates the design of synthetic prodrugs that are not only tuned to be harmless, but can be robustly transformed in vivo to reach therapeutically relevant levels. To accomplish this, retrosynthetic prodrug design highlights the potential of naphthylcombretastatin-based prodrugs, which form highly active cytostatic agents via sequential ring-closing metathesis and aromatization. Structural adjustments will also be done to improve aspects related to catalytic reactivity, intrinsic bioactivity, and hydrolytic stability. The developed prodrug therapy is found to possess excellent anticancer activities in cell-based assays. Furthermore, in vivo activation by intravenously administered glycosylated artificial metalloenzymes can also induce significant reduction of implanted tumor growth in mice.

Highlights from the 55th Bürgenstock Conference on Stereochemistry 2022

In May 2022, the 55th Bürgenstock Conference on Stereochemistry happened in person once again. This summary provides insight into the scientific themes discussed during the most recent meeting of this historic and multi-disciplinary conference.

Metal substrate catalysis in the confined space for platinum drug delivery

Catalysis-based approaches for the activation of anticancer agents hold considerable promise. These principally rely on the use of metal catalysts capable of deprotecting inactive precursors of organic drugs or transforming key biomolecules available in the cellular environment. Nevertheless, the efficiency of most of the schemes described so far is rather low, limiting the benefits of catalytic amplification as strategy for controlling the therapeutic effects of anticancer compounds. In the work presented here, we show that flavin reactivity within a hydrogel matrix provides a viable solution for the efficient catalytic activation and delivery of cisplatin, a worldwide clinically-approved inorganic chemotherapy agent. This is achieved by ionically adsorbing a flavin catalyst and a Pt(iv) prodrug as substrate into porous amino-functionalized agarose beads. The hydrogel chassis supplies high local concentrations of electron donating groups/molecules in the surrounding of the catalyst, ultimately boosting substrate conversion rates (TOF >200 min-1) and enabling controlled liberation of the drug by light or chemical stimuli. Overall, this approach can afford platforms for the efficient delivery of platinum drugs as demonstrated herein by using a transdermal diffusion model simulating the human skin.

Plasmonic-Assisted Thermocyclizations in Living Cells Using Metal-Organic Framework Based Nanoreactors

We describe a microporous plasmonic nanoreactor to carry out designed near-infrared (NIR)-driven photothermal cyclizations inside living cells. As a proof of concept, we chose an intramolecular cyclization that is based on the nucleophilic attack of a pyridine onto an electrophilic carbon, a process that requires high activation energies and is typically achieved in bulk solution by heating at ∼90 °C. The core-shell nanoreactor (NR) has been designed to include a gold nanostar core, which is embedded within a metal-organic framework (MOF) based on a polymer-stabilized zeolitic imidazole framework-8 (ZIF-8). Once accumulated inside living cells, the MOF-based cloak of NRs allows an efficient diffusion of reactants into the plasmonic chamber, where they undergo the transformation upon near-IR illumination. The photothermal-driven reaction enables the intracellular generation of cyclic fluorescent products that can be tracked using fluorescence microscopy. The strategy may find different type of applications, such as for the spatio-temporal activation of prodrugs.

Inside a Shell-Organometallic Catalysis Inside Encapsulin Nanoreactors

Compartmentalization of chemical reactions inside cells are a fundamental requirement for life. Encapsulins are self-assembling protein-based nanocompartments from the prokaryotic repertoire that present a highly attractive platform for intracellular compartmentalization of chemical reactions by design. Using single-molecule Förster resonance energy transfer and 3D-MINFLUX analysis, we analyze fluorescently labeled encapsulins on a single-molecule basis. Furthermore, by equipping these capsules with a synthetic ruthenium catalyst via covalent attachment to a non-native host protein, we are able to perform in vitro catalysis and go on to show that engineered encapsulins can be used as hosts for transition metal catalysis inside living cells in confined space.

Exporting Metal-Carbene Chemistry to Live Mammalian Cells: Copper-Catalyzed Intracellular Synthesis of Quinoxalines Enabled by N-H Carbene Insertions

Implementing catalytic organometallic transformations in living settings can offer unprecedented opportunities in chemical biology and medicine. Unfortunately, the number of biocompatible reactions so far discovered is very limited, and essentially restricted to uncaging processes. Here, we demonstrate the viability of performing metal carbene transfer reactions in live mammalian cells. In particular, we show that copper (II) catalysts can promote the intracellular annulation of alpha-keto diazocarbenes with ortho-amino arylamines, in a process that is initiated by an N-H carbene insertion. The potential of this transformation is underscored by the in cellulo synthesis of a product that alters mitochondrial functions, and by demonstrating cell selective biological responses using targeted copper catalysts. Considering the wide reactivity spectrum of metal carbenes, this work opens the door to significantly expanding the repertoire of life-compatible abiotic reactions.

Directed Evolution of a Surface-Displayed Artificial Allylic Deallylase Relying on a GFP Reporter Protein

Artificial metalloenzymes (ArMs) combine characteristics of both homogeneous catalysts and enzymes. Merging abiotic and biotic features allows for the implementation of new-to-nature reactions in living organisms. Here, we present the directed evolution of an artificial metalloenzyme based on Escherichia coli surface-displayed streptavidin (SavSD hereafter). Through the binding of a ruthenium-pianostool cofactor to SavSD, an artificial allylic deallylase (ADAse hereafter) is assembled, which displays catalytic activity toward the deprotection of alloc-protected 3-hydroxyaniline. The uncaged aminophenol acts as a gene switch and triggers the overexpression of a fluorescent green fluorescent protein (GFP) reporter protein. This straightforward readout of ADAse activity allowed the simultaneous saturation mutagenesis of two amino acid residues in Sav near the ruthenium cofactor, expediting the screening of 2762 individual clones. A 1.7-fold increase of in vivo activity was observed for SavSD S112T-K121G compared to the wild-type SavSD (wt-SavSD). Finally, the best performing Sav isoforms were purified and tested in vitro (SavPP hereafter). For SavPP S112M-K121A, a total turnover number of 372 was achieved, corresponding to a 5.9-fold increase vs wt-SavPP. To analyze the marked difference in activity observed between the surface-displayed and purified ArMs, the oligomeric state of SavSD was determined. For this purpose, crosslinking experiments of E. coli cells overexpressing SavSD were carried out, followed by sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) and Western blot. The data suggest that SavSD is most likely displayed as a monomer on the surface of E. coli. We hypothesize that the difference between the in vivo and in vitro screening results may reflect the difference in the oligomeric state of SavSD vs soluble SavPP (monomeric vs tetrameric). Accordingly, care should be applied when evolving oligomeric proteins using E. coli surface display.

In situ activation of therapeutics through bioorthogonal catalysis

Bioorthogonal chemistry refers to any chemical reactions that can occur inside of living systems without interfering with native biochemical processes, which has become a promising strategy for modulating biological processes. The development of synthetic metal-based catalysts to perform bioorthogonal reactions has significantly expanded the toolkit of bioorthogonal chemistry for medicinal chemistry and synthetic biology. A wide range of homogeneous and heterogeneous transition metal catalysts (TMCs) have been reported, mediating different transformations such as cycloaddition reactions, as well as bond forming and cleaving reactions. However, the direct application of 'naked' TMCs in complex biological media poses numerous challenges, including poor water solubility, toxicity and catalyst deactivation. Incorporating TMCs into nanomaterials to create bioorthogonal nanocatalysts can solubilize and stabilize catalyst molecules, with the decoration of the nanocatalysts used to provide spatiotemporal control of catalysis. This review presents an overview of the advances in the creation of bioorthogonal nanocatalysts, highlighting different choice of nano-scaffolds, and the therapeutic and diagnostic applications.

Iridium-Triggered Allylcarbamate Uncaging in Living Cells

Designing a metal catalyst that addresses the major issues of solubility, stability, toxicity, cell uptake, and reactivity within complex biological milieu for bioorthogonal controlled transformation reactions is a highly formidable challenge. Herein, we report an organoiridium complex that is nontoxic and capable of the uncaging of allyloxycarbonyl-protected amines under biologically relevant conditions and within living cells. The potential applications of this uncaging chemistry have been demonstrated by the generation of diagnostic and therapeutic agents upon the activation of profluorophore and prodrug in a controlled fashion within HeLa cells, providing a valuable tool for numerous potential biological and therapeutic applications.

Bioorthogonal catalytic patch

Bioorthogonal catalysis mediated by transition metals has inspired a new subfield of artificial chemistry complementary to enzymatic reactions, enabling the selective labelling of biomolecules or in situ synthesis of bioactive agents via non-natural processes. However, the effective deployment of bioorthogonal catalysis in vivo remains challenging, mired by the safety concerns of metal toxicity or complicated procedures to administer catalysts. Here, we describe a bioorthogonal catalytic device comprising a microneedle array patch integrated with Pd nanoparticles deposited on TiO₂ nanosheets. This device is robust and removable, and can mediate the local conversion of caged substrates into their active states in high-level living systems. In particular, we show that such a patch can promote the activation of a prodrug at subcutaneous tumour sites, restoring its parent drug's therapeutic anticancer properties. This in situ applied device potentiates local treatment efficacy and eliminates off-target prodrug activation and dose-dependent side effects in healthy organs or distant tissues.

Bioorthogonal strategies for the in vivo synthesis or release of drugs

The site-specific delivery of antitumor agents is a rapidly developing field that relies on prodrug activation and uncaging strategies. For this purpose, a wide range of homogeneous and heterogeneous biocompatible activators/catalysts have been developed to convert caged drugs with low toxicity and high stability in physiological settings into active substances in a bioorthogonal manner. The current methods allow for the site-specific delivery of activators and prodrugs to organelles, target cells, or tumors in living organisms. Here, we present an overview of the latest advances in catalytic drugs, highlighting the expanding toolbox of bioorthogonal activation strategies made possible by transition metals acting as activators or catalysts.

Bioorthogonal Azide-Thioalkyne Cycloaddition Catalyzed by Photoactivatable Ruthenium(II) Complexes

Tailored ruthenium sandwich complexes bearing photoresponsive arene ligands can efficiently promote azide–thioalkyne cycloaddition (RuAtAC) when irradiated with UV light. The reactions can be performed in a bioorthogonal manner in aqueous mixtures containing biological components. The strategy can also be applied for the selective modification of biopolymers, such as DNA or peptides. Importantly, this ruthenium‐based technology and the standard copper‐catalyzed azide–alkyne cycloaddition (CuAAC) proved to be compatible and mutually orthogonal.

Artificial Organelles: Towards Adding or Restoring Intracellular Activity

Compartmentalization is one of the main characteristics that define living systems. Creating a physically separated microenvironment allows nature a better control over biological processes, as is clearly specified by the role of organelles in living cells. Inspired by this phenomenon, researchers have developed a range of different approaches to create artificial organelles: compartments with catalytic activity that add new function to living cells. In this review we will discuss three complementary lines of investigation. First, orthogonal chemistry approaches are discussed, which are based on the incorporation of catalytically active transition metal-containing nanoparticles in living cells. The second approach involves the use of premade hybrid nanoreactors, which show transient function when taken up by living cells. The third approach utilizes mostly genetic engineering methods to create bio-based structures that can be ultimately integrated with the cell's genome to make them constitutively active. The current state of the art and the scope and limitations of the field will be highlighted with selected examples from the three approaches.

Tools and Methods for Investigating Synthetic Metal-Catalyzed Reactions in Living Cells

Although abiotic catalysts are capable of promoting numerous new-to-nature reactions, only a small subset has so far been successfully integrated into living systems. Research in intracellular catalysis requires an interdisciplinary approach that takes advantage of both chemical and biological tools as well as state-of-the-art instrumentations. In this perspective, we will focus on the techniques that have made studying metal-catalyzed reactions in cells possible using representative examples from the literature. Although the lack of quantitative data in vitro and in vivo has somewhat limited progress in the catalyst development process, recent advances in characterization methods should help overcome some of these deficiencies. Given its tremendous potential, we believe that intracellular catalysis will play a more prominent role in the development of future biotechnologies and therapeutics.

Gold Nanoparticles in Cancer Theranostics

Conventional cancer treatments, such as surgical resection, radiotherapy, and chemotherapy, have achieved significant progress in cancer therapy. Nevertheless, some limitations (such as toxic side effects) are still existing for conventional therapies, which motivate efforts toward developing novel theranostic avenues. Owning many merits such as easy surface modification, unique optical properties, and high biocompatibility, gold nanoparticles (AuNPs and GNPs) have been engineered to serve as targeted delivery vehicles, molecular probes, sensors, and so on. Their small size and surface characteristics enable them to extravasate and access the tumor microenvironment (TME), which is a promising solution to realize highly effective treatments. Moreover, stimuli-responsive properties (respond to hypoxia and acidic pH) of nanoparticles to TME enable GNPs’ unrivaled control for effective transport of therapeutic cargos. In this review article, we primarily introduce the basic properties of GNPs, further discuss the recent progress in gold nanoparticles for cancer theranostics, with an additional concern about TME stimuli-responsive studies.