A chemoselective strategy for late-stage functionalization of complex small molecules with polypeptides and proteins

Ultrafast Au(III)-Mediated Arylation of Cysteine

Through mechanistic work and rational design, we have developed the fastest organometallic abiotic Cys bioconjugation. As a result, the developed organometallic Au(III) bioconjugation reagents enable selective labeling of Cys moieties down to picomolar concentrations and allow for the rapid construction of complex heterostructures from peptides, proteins, and oligonucleotides. This work showcases how organometallic chemistry can be interfaced with biomolecules and lead to a range of reactivities that are largely unmatched by classical organic chemistry tools.

Bioinspired Selenium-Nitrogen Exchange (SeNEx) Click Chemistry Suitable for Nanomole-Scale Medicinal Chemistry and Bioconjugation

Click chemistry is a powerful molecular assembly strategy for rapid functional discovery. The development of click reactions with new connecting linkage is of great importance for expanding the click chemistry toolbox. We report the first selenium-nitrogen exchange (SeNEx) click reaction between benzoselenazolones and terminal alkynes (Se-N to Se-C), which is inspired by the biochemical SeNEx between Ebselen and cysteine (Cys) residue (Se-N to Se-S). The formed selenoalkyne connection is readily elaborated, thus endowing this chemistry with multidimensional molecular diversity. Besides, this reaction is modular, predictable, and high-yielding, features fast kinetics (k2≥14.43 M-1 s-1), excellent functional group compatibility, and works well at miniaturization (nanomole-scale), opening up many interesting opportunities for organo-Se synthesis and bioconjugation, as exemplified by sequential click chemistry (coupled with ruthenium-catalyzed azide-alkyne cycloaddition (RuAAC) and sulfur-fluoride exchange (SuFEx)), selenomacrocycle synthesis, nanomole-scale synthesis of Se-containing natural product library and DNA-encoded library (DEL), late-stage peptide modification and ligation, and multiple functionalization of proteins. These results indicated that SeNEx is a useful strategy for new click chemistry developments, and the established SeNEx chemistry will serve as a transformative platform in multidisciplinary fields such as synthetic chemistry, material science, chemical biology, medical chemistry, and drug discovery.

Selenium chemistry for spatio-selective peptide and protein functionalization

The ability to construct a peptide or protein in a spatio-specific manner is of great interest for therapeutic and biochemical research. However, the various functional groups present in peptide sequences and the need to perform chemistry under mild and aqueous conditions make selective protein functionalization one of the greatest synthetic challenges. The fascinating paradox of selenium (Se) - being found in both toxic compounds and also harnessed by nature for essential biochemical processes - has inspired the recent exploration of selenium chemistry for site-selective functionalization of peptides and proteins. In this Review, we discuss such approaches, including metal-free and metal-catalysed transformations, as well as traceless chemical modifications. We report their advantages, limitations and applications, as well as future research avenues.

Electrochemical Nickel-Catalyzed Selective Inter- and Intramolecular Arylations of Cysteine-Containing Peptides

Here we report a simple electrochemical route towards the synthesis of S-arylated peptides by a site selective coupling of peptides with aryl halides under base free conditions. This approach demonstrates the power of electrochemistry to access both highly complex peptide conjugates and cyclic peptides.

Electrochemical Modification of Polypeptides at Selenocysteine

Mild strategies for the selective modification of peptides and proteins are in demand for applications in therapeutic peptide and protein discovery, and in the study of fundamental biomolecular processes. Herein, we describe the development of an electrochemical selenoetherification (e-SE) platform for the efficient site-selective functionalization of polypeptides. This methodology utilizes the unique reactivity of the 21st amino acid, selenocysteine, to effect formation of valuable bioconjugates through stable selenoether linkages under mild electrochemical conditions. The power of e-SE is highlighted through late-stage C-terminal modification of the FDA-approved cancer drug leuprolide and assembly of a library of anti-HER2 affibody conjugates bearing complex cargoes. Following assembly by e-SE, the utility of functionalized affibodies for in vitro imaging and targeting of HER2 positive breast and lung cancer cell lines is also demonstrated.

Pd-Catalyzed Picolinamide-Directed Late-Stage Chalcogenation of Tryptophan-Containing Peptides

This report describes the Pd-catalyzed late-stage chalcogenation of tryptophan-containing peptides with disulfides/diselenides in moderate to good yields. It comprises broad substrate scope, functional group diversity, late-stage modification of drug molecules, and various valuable synthetic transformations, including room temperature easy removal of the picolinamide auxiliary, which could be applicable to tune the structure and function of peptides.

Electrochemical Late-Stage Functionalization

Late-stage functionalization (LSF) constitutes a powerful strategy for the assembly or diversification of novel molecular entities with improved physicochemical or biological activities. LSF can thus greatly accelerate the development of medicinally relevant compounds, crop protecting agents, and functional materials. Electrochemical molecular synthesis has emerged as an environmentally friendly platform for the transformation of organic compounds. Over the past decade, electrochemical late-stage functionalization (eLSF) has gained major momentum, which is summarized herein up to February 2023.

Late-Stage Formal Double C-H Oxidation of Prenylated Molecules to Alkylidene Oxetanes and Azetidines by Strain-Enabled Cross-Metathesis

Prenylation is a ubiquitous late-stage modification in nature that often confers significantly improved bioactivity for secondary metabolites. While this lipophilic modification renders enhanced potency, the lipophilic tag(s) can diminish bioavailability and adversely alter drug transportation and metabolism. Thus, a functional-group-tolerant, mild, and selective late-stage C-H functionalization of prenyl tags would present a great potential in drug discovery programs but could also impact other fields, such as agrochemistry and chemical biology. Herein we report an exocyclic-strain-driven cross-metathesis reaction of prenyl tags, a formal double C-H oxidation protocol, that can be used for the selective late-stage derivatization of prenylated compounds and natural products. This methodology avoids the need for prefunctionalization of target molecules and affords ready access to an unprecedented library of oxo- and aza-prenylated complex molecules. Thus, in a broader context, this methodology extends late-stage functionalization beyond that available to nature.

Antibiotics and Bacterial Resistance-A Short Story of an Endless Arms Race

Despite the undisputed development of medicine, antibiotics still serve as first-choice drugs for patients with infectious disorders. The widespread use of antibiotics results from a wide spectrum of their actions encompassing mechanisms responsible for: the inhibition of bacterial cell wall biosynthesis, the disruption of cell membrane integrity, the suppression of nucleic acids and/or proteins synthesis, as well as disturbances of metabolic processes. However, the widespread availability of antibiotics, accompanied by their overprescription, acts as a double-edged sword, since the overuse and/or misuse of antibiotics leads to a growing number of multidrug-resistant microbes. This, in turn, has recently emerged as a global public health challenge facing both clinicians and their patients. In addition to intrinsic resistance, bacteria can acquire resistance to particular antimicrobial agents through the transfer of genetic material conferring resistance. Amongst the most common bacterial resistance strategies are: drug target site changes, increased cell wall permeability to antibiotics, antibiotic inactivation, and efflux pumps. A better understanding of the interplay between the mechanisms of antibiotic actions and bacterial defense strategies against particular antimicrobial agents is crucial for developing new drugs or drug combinations. Herein, we provide a brief overview of the current nanomedicine-based strategies that aim to improve the efficacy of antibiotics.

The Chemistry of Creating Chemically Programmed Antibodies (cPAbs): Site-Specific Bioconjugation of Small Molecules

Small-molecule drugs have been employed for years as therapeutics in the pharmaceutical industry. However, small-molecule drugs typically have short in vivo half-lives which is one of the largest impediments to the success of many potentially valuable pharmacologically active small molecules. The undesirable pharmacokinetics and pharmacology associated with some small molecules have led to the development of a new class of bioconjugates known as chemically programmed antibodies (cPAbs). cPAbs are bioconjugates in which antibodies are used to augment small molecules with effector functions and prolonged pharmacokinetic profiles, where the pharmacophore of the small molecule is harnessed for target binding and therefore biological targeting. Many different small molecules can be conjugated to large proteins such as full monoclonal antibodies (IgG), fragment crystallizable regions (Fc), or fragment antigen binding regions (Fab). In order to successfully and site-specifically conjugate small molecules to any class of antibodies (IgG, Fc, or Fab), the molecules must be derivatized with a functional group for ease of conjugation without altering the pharmacology of the small molecules. In this Review, we summarize the different synthetic or biological methods that have been employed to produce cPAbs. These unique chemistries have potential to be applied to other fields of antibody modification such as antibody drug conjugates, radioimmunoconjugates, and fluorophore-tagged antibodies.

The thiol-sulfoxonium ylide photo-click reaction for bioconjugation

Visible-light-mediated methods were heavily studied as a useful tool for cysteine-selective bio-conjugation; however, many current methods suffer from bio-incompatible reaction conditions and slow kinetics. To address these challenges, herein, we report a transition metal-free thiol-sulfoxonium ylide photo-click reaction that enables bioconjugation under bio-compatible conditions. The reaction is highly cysteine-selective and generally finished within minutes with naturally occurring riboflavin derivatives as organic photocatalysts. The catalysts and substrates are readily accessible and bench stable and have satisfactory water solubility. As a proof-of-concept study, the reaction was smoothly applied in chemo-proteomic analysis, which provides efficient tools to explore the druggable content of the human proteome.

Antibody-Antimicrobial Conjugates for Combating Antibiotic Resistance

As the development of new antibiotics lags far behind the emergence of drug-resistant bacteria, alternative strategies to resolve this dilemma are urgently required. Antibody-drug conjugate is a promising therapeutic platform to delivering cytotoxic payloads precisely to target cells for efficient disease treatment. Antibody-antimicrobial conjugates (AACs) have recently attracted considerable interest from researchers as they can target bacteria in the target sites and improve the effectiveness of drugs (i.e., reduced drug dosage and adverse effects), abating the upsurge of antimicrobial resistance. In this review, the selection and progress of three essential blocks that compose the AACs: antibodies, antimicrobial payloads, and linkers are discussed. The commonly used conjugation strategies and the latest applications of AACs in recent years are also summarized. The challenges and opportunities of this booming technology are also discussed at the end of this review.

Site-selective photocatalytic functionalization of peptides and proteins at selenocysteine

The importance of modified peptides and proteins for applications in drug discovery, and for illuminating biological processes at the molecular level, is fueling a demand for efficient methods that facilitate the precise modification of these biomolecules. Herein, we describe the development of a photocatalytic method for the rapid and efficient dimerization and site-specific functionalization of peptide and protein diselenides. This methodology, dubbed the photocatalytic diselenide contraction, involves irradiation at 450 nm in the presence of an iridium photocatalyst and a phosphine and results in rapid and clean conversion of diselenides to reductively stable selenoethers. A mechanism for this photocatalytic transformation is proposed, which is supported by photoluminescence spectroscopy and density functional theory calculations. The utility of the photocatalytic diselenide contraction transformation is highlighted through the dimerization of selenopeptides, and by the generation of two families of protein conjugates via the site-selective modification of calmodulin containing the 21^st amino acid selenocysteine, and the C-terminal modification of a ubiquitin diselenide.

Strategies to prevent, curb and eliminate biofilm formation based on the characteristics of various periods in one biofilm life cycle

Biofilms are colonies of bacteria embedded inside a complicated self-generating intercellular. The formation and scatter of a biofilm is an extremely complex and progressive process in constant cycles. Once formed, it can protect the inside bacteria to exist and reproduce under hostile conditions by establishing tolerance and resistance to antibiotics as well as immunological responses. In this article, we reviewed a series of innovative studies focused on inhibiting the development of biofilm and summarized a range of corresponding therapeutic methods for biological evolving stages of biofilm. Traditionally, there are four stages in the biofilm formation, while we systematize the therapeutic strategies into three main periods precisely:(i) period of preventing biofilm formation: interfering the colony effect, mass transport, chemical bonds and signaling pathway of plankton in the initial adhesion stage; (ii) period of curbing biofilm formation:targeting several pivotal molecules, for instance, polysaccharides, proteins, and extracellular DNA (eDNA) via polysaccharide hydrolases, proteases, and DNases respectively in the second stage before developing into irreversible biofilm; (iii) period of eliminating biofilm formation: applying novel multifunctional composite drugs or nanoparticle materials cooperated with ultrasonic (US), photodynamic, photothermal and even immune therapy, such as adaptive immune activated by stimulated dendritic cells (DCs), neutrophils and even immunological memory aroused by plasmocytes. The multitargeted or combinational therapies aim to prevent it from developing to the stage of maturation and dispersion and eliminate biofilms and planktonic bacteria simultaneously.

All-in-one disulfide bridging enables the generation of antibody conjugates with modular cargo loading

Antibody-drug conjugates (ADCs) are valuable therapeutic entities which leverage the specificity of antibodies to selectively deliver cytotoxins to antigen-expressing targets such as cancer cells. However, current methods for their construction still suffer from a number of shortcomings. For instance, using a single modification technology to modulate the drug-to-antibody ratio (DAR) in integer increments while maintaining homogeneity and stability remains exceptionally challenging. Herein, we report a novel method for the generation of antibody conjugates with modular cargo loading from native antibodies. Our approach relies on a new class of disulfide rebridging linkers, which can react with eight cysteine residues, thereby effecting all-in-one bridging of all four interchain disulfides in an IgG1 antibody with a single linker molecule. Modification of the antibody with the linker in a 1 : 1 ratio enabled the modulation of cargo loading in a quick and selective manner through derivatization of the linker with varying numbers of payload attachment handles to allow for attachment of either 1, 2, 3 or 4 payloads (fluorescent dyes or cytotoxins). Assessment of the biological activity of these conjugates demonstrated their exceptional stability in human plasma and utility for cell-selective cytotoxin delivery or imaging/diagnostic applications.

Selenopeptide Nanomedicine Activates Natural Killer Cells for Enhanced Tumor Chemoimmunotherapy

Chemoimmunotherapy using nanotechnology has shown great potential for cancer therapy in the clinic. However, uncontrolled transportation and synergistic responses remain challenges. Here, a self-assembled selenopeptide nanoparticle that strengthens tumor chemoimmunotherapy through the activation of natural killer (NK) cells by the oxidative metabolite of the selenopeptide is developed. With the advantages of the enzyme-induced size-reduction and the reactive-oxygen-species-driven deselenization, this selenopeptide is able to deliver therapeutics, e.g., doxorubicin (DOX), to solid tumors and further activate the NK cells in a programmed manner. Importantly, in vitro and in vivo results prove the mutual promotion between the DOX-induced chemotherapy and the selenopeptide-induced immunotherapy, which synergistically contribute to the improved antitumor efficacy. It is anticipated that the selenopeptide may provide a type of promising stimuli-responsive immune modulator for versatile biomedical applications.

Umpolung strategies for the functionalization of peptides and proteins

Umpolung strategies, defined as synthetic approaches which reverse commonly accepted reactivity patterns, are broadly recognized as enabling tools for small molecule synthesis and catalysis. However, methods which exploit this logic for peptide and protein functionalizations are comparatively rare, with the overwhelming majority of existing bioconjugation approaches relying on the well-established reactivity profiles of a handful of amino acids. This perspective serves to highlight a small but growing body of recent work that masterfully capitalizes on the concept of polarity reversal for the selective modification of proteinogenic functionalities. Current applications of umpolung chemistry in organic synthesis and chemical biology as well as the vast potential for further innovations in peptide and protein modification will be discussed.

A Tale of Two Bioconjugations: pH Controlled Divergent Reactivity of Protein α-oxo-Aldehydes in Competing α-oxo-Mannich and Catalyst-Free Aldol Ligations

Site-selective chemical methods for protein bioconjugation have revolutionized the fields of cell and chemical biology through the development of novel protein/enzyme probes bearing fluorescent, spectroscopic, or even toxic cargos. Herein, we report two new methods for the bioconjugation of α-oxo aldehyde handles within proteins using small molecule aniline and/or phenol probes. The "α-oxo-Mannich" and "catalyst-free aldol" ligations both compete for the electrophilic α-oxo aldehyde, which displays pH divergent reactivity proceeding through the "Mannich" pathway at acidic pH to afford bifunctionalized bioconjugates, and the "catalyst-free aldol" pathway at neutral pH to afford monofunctionalized bioconjugates. We explore the substrate scope and utility of both of these bioconjugations in the construction of neoglycoproteins, in the process formulating a mechanistic rationale for how both pathways intersect with each other at different reaction pH's.

Late-Stage Amination of Drug-Like Benzoic Acids: Access to Anilines and Drug Conjugates through Directed Iridium-Catalyzed C-H Activation

The functionalization of C−H bonds, ubiquitous in drugs and drug‐like molecules, represents an important synthetic strategy with the potential to streamline the drug‐discovery process. Late‐stage aromatic C−N bond–forming reactions are highly desirable, but despite their significance, accessing aminated analogues through direct and selective amination of C−H bonds remains a challenging goal. The method presented herein enables the amination of a wide array of benzoic acids with high selectivity. The robustness of the system is manifested by the large number of functional groups tolerated, which allowed the amination of a diverse array of marketed drugs and drug‐like molecules. Furthermore, the introduction of a synthetic handle enabled expeditious access to targeted drug‐delivery conjugates, PROTACs, and probes for chemical biology. This rapid access to valuable analogues, combined with operational simplicity and applicability to high‐throughput experimentation has the potential to aid and considerably accelerate drug discovery.

Copper(II)-mediated C-H sulphenylation or selenylation of tryptophan enabling macrocyclization of peptides

Cu(II)-mediated C-H sulphenylation or selenylation of Trp indole by a derivative of cysteine or selenocysteine enables access to the tryptathionine unit or its selenium congener. The mechanism of these protocols, which allow macrocyclization of Trp-containing peptides, has been studied.

Organometallic AlaM Reagents for Umpolung Peptide Diversification

Selective modifications of peptides and proteins have emerged as a promising strategy to develop novel mechanistic probes and prepare compounds with translational potentials. Here, we report alanine carbastannatranes AlaSn as a universal synthon in various C-C and C-heteroatom bond-forming reactions. These reagents are compatible with peptide manipulation techniques and can undergo chemoselective conjugation in minutes when promoted by Pd(0). Despite their increased nucleophilicity and propensity to transfer the alkyl group, C(sp3)-C(sp2) coupling with AlaSn can be accomplished at room temperature under buffered conditions (pH 6.5-8.5). We also show that AlaSn can be easily transformed into several canonical L- and D-amino acids in arylation, acylation, and etherification reactions. Furthermore, AlaSn can partake in macrocyclizations exemplified by the synthesis of medium size cyclic peptides with various topologies. Taken together, metalated alanine AlaSn demonstrates unparalleled scope and represents a new type of umpolung reagents suitable for structure-activity relationship studies and peptide diversification.

Chemoselective Copper-Mediated Modification of Selenocysteines in Peptides and Proteins

Highly valuable bioconjugated molecules must be synthesized through efficient, chemoselective chemical modifications of peptides and proteins. Herein, we report the chemoselective modification of peptides and proteins via a reaction between selenocysteine residues and aryl/alkyl radicals. In situ radical generation from hydrazine substrates and copper ions proceeds rapidly in an aqueous buffer at near neutral pH (5-8), providing a variety of Se-modified linear and cyclic peptides and proteins conjugated to aryl and alkyl molecules, and to affinity label tag (biotin). This chemistry opens a new avenue for chemical protein modifications.

Antibiotic-cell-penetrating peptide conjugates targeting challenging drug-resistant and intracellular pathogenic bacteria

The failure to treat everyday bacterial infections is a current threat as pathogens are finding new ways to thwart antibiotics through mechanisms of resistance and intracellular refuge, thus rendering current antibiotic strategies ineffective. Cell-penetrating peptides (CPPs) are providing a means to improve antibiotics that are already approved for use. Through coadministration and conjugation of antibiotics with CPPs, improved accumulation and selectivity with alternative and/or additional modes of action against infections have been observed. Herein, we review the recent progress of this antibiotic-cell-penetrating peptide strategy in combatting sensitive and drug-resistant pathogens. We take a closer look into the specific antibiotics that have been enhanced, and in some cases repurposed as broad-spectrum drugs. Through the addition and conjugation of cell-penetrating peptides to antibiotics, increased permeation across mammalian and/or bacterial membranes and a broader range in bacterial selectivity have been achieved.

The multifaceted nature of antimicrobial peptides: current synthetic chemistry approaches and future directions

Bacterial infections caused by 'superbugs' are increasing globally, and conventional antibiotics are becoming less effective against these bacteria, such that we risk entering a post-antibiotic era. In recent years, antimicrobial peptides (AMPs) have gained significant attention for their clinical potential as a new class of antibiotics to combat antimicrobial resistance. In this review, we discuss several facets of AMPs including their diversity, physicochemical properties, mechanisms of action, and effects of environmental factors on these features. This review outlines various chemical synthetic strategies that have been applied to develop novel AMPs, including chemical modifications of existing peptides, semi-synthesis, and computer-aided design. We will also highlight novel AMP structures, including hybrids, antimicrobial dendrimers and polypeptides, peptidomimetics, and AMP-drug conjugates and consider recent developments in their chemical synthesis.

Paving the way to conformationally unravel complex glycopeptide antibiotics by means of Raman optical activity

It is crucial for fundamental physical chemistry techniques to find their application in tackling real-world challenges. Hitherto, Raman optical activity (ROA) spectroscopy is one of the examples where a promising future within the pharmaceutical sector is foreseen, but has not yet been established. Namely, the technique is believed to be able to contribute in investigating the conformational behaviour of drug candidates. We, herein, strive towards the alignment of the ROA analysis outcome and the pharmaceutical expectations by proposing a fresh strategy that ensures a more complete, reliable, and transferable ROA study. The strategy consists of the treatment of the conformational space by means of a principal component analysis (PCA) and a clustering algorithm, succeeded by a thorough ROA spectral analysis and a novel way of estimating the contributions of the different chemical fragments to the total ROA spectral intensities. Here, vancomycin, an antibiotic glycopeptide, has been treated; it is the first antibiotic glycopeptide studied by means of ROA and is a challenging compound in ROA terms. By applying our approach we discover that ROA is capable of independently identifying the correct conformation of vancomycin in aqueous solution. In addition, we have a clear idea of what ROA can and cannot tell us regarding glycopeptides. Finally, the glycopeptide class turns out to be a spectroscopically curious case, as its spectral responses are unlike the typical ROA spectral responses of peptides and carbohydrates. This preludes future ROA studies of this intriguing molecular class.

Raman optical activity tackles the complex conformational space of glycopeptide antibiotics.

PdCl2/DMSO-Catalyzed Thiol-Disulfide Exchange: Synthesis of Unsymmetrical Disulfide

Unsymmetrical disulfides have been effectively prepared through thiol exchange with symmetrical disulfides employing a simple PdCl₂/DMSO catalytic system. The given method features excellent functional group tolerance, a broad substrate scope, and operational simplicity. This reaction is especially useful for late-stage functionalization of bioactive scaffolds such as peptides and pharmaceuticals. Disulfide-containing organic dyes have also been prepared. This transformation could be extended to thiol-diselenide or thiol-ditelluride exchange affording RS-SeR' or RS-TeR'.

Genetically Encoded Fragment-Based Discovery from Phage-Displayed Macrocyclic Libraries with Genetically Encoded Unnatural Pharmacophores

Genetically encoded macrocyclic peptide libraries with unnatural pharmacophores are valuable sources for the discovery of ligands for many targets of interest. Traditionally, generation of such libraries employs "early stage" incorporation of unnatural building blocks into the chemically or translationally produced macrocycles. Here, we describe a divergent late-stage approach to such libraries starting from readily available starting material: genetically encoded libraries of peptides. A diketone linchpin 1,5-dichloropentane-2,4-dione converts peptide libraries displayed on phage to 1,3-diketone bearing macrocyclic peptides (DKMP): shelf-stable precursors for Knorr pyrazole synthesis. Ligation of diverse hydrazine derivatives onto DKMP libraries displayed on phage that carries silent DNA-barcodes yields macrocyclic libraries in which the amino acid sequence and the pharmacophore are encoded by DNA. Selection of this library against carbonic anhydrase enriched macrocycles with benzenesulfonamide pharmacophore and nanomolar K_d. The methodology described in this manuscript can graft diverse pharmacophores into many existing genetically encoded phage libraries and significantly increase the value of such libraries in molecular discoveries.

Small peptide diversification through photoredox-catalyzed oxidative C-terminal modification

A photoredox-catalyzed oxidative decarboxylative coupling of small peptides is reported, giving access to a variety of N,O-acetals. They were used as intermediates for the addition of phenols and indoles, leading to novel peptide scaffolds and bioconjugates. Amino acids with nucleophilic side chains, such as serine, threonine, tyrosine and tryptophan, could also be used as partners to access tri- and tetrapeptide derivatives with non-natural cross-linking.

Single electron transfer-based peptide/protein bioconjugations driven by biocompatible energy input

Bioconjugation reactions play a central facilitating role in engendering modified peptides and proteins. Early progress in this area was inhibited by challenges such as the limited range of substrates and the relatively poor biocompatibility of bioconjugation reagents. However, the recent developments in visible-light induced photoredox catalysis and electrochemical catalysis reactions have permitted significant novel reactivities to be developed in the field of synthetic and bioconjugation chemistry. This perspective describes recent advances in the use of biocompatible energy input for the modification of peptides and proteins mainly, via the single electron transfer (SET) process, as well as key future developments in this area.

On-Resin Preparation of Allenamidyl Peptides: A Versatile Chemoselective Conjugation and Intramolecular Cyclisation Tool

The ability to modify peptides and proteins chemoselectively is of continued interest in medicinal chemistry, with peptide conjugation, lipidation, stapling, and disulfide engineering at the forefront of modern peptide chemistry. Herein we report a robust method for the on-resin preparation of allenamide-modified peptides, an unexplored functionality for peptides that provides a versatile chemical tool for chemoselective inter- or intramolecular bridging reactions with thiols. The bridging reaction is biocompatible, occurring spontaneously at pH 7.4 in catalyst-free aqueous media. By this "click" approach, a model peptide was successfully modified with a diverse range of alkyl and aryl thiols. Furthermore, this technique was demonstrated as a valuable tool to induce spontaneous intramolecular cyclisation by preparation of an oxytocin analogue, in which the native disulfide bridge was replaced with a vinyl sulfide moiety formed by thia-Michael addition of a cysteine thiol to the allenamide handle.

Conjugates of Ciprofloxacin and Levofloxacin with Cell-Penetrating Peptide Exhibit Antifungal Activity and Mammalian Cytotoxicity

Seven conjugates composed of well-known fluoroquinolone antibacterial agents, ciprofloxacin (CIP) or levofloxacin (LVX), and a cell-penetrating peptide transportan 10 (TP10-NH₂) were synthesised. The drugs were covalently bound to the peptide via an amide bond, methylenecarbonyl moiety, or a disulfide bridge. Conjugation of fluoroquinolones to TP10-NH₂ resulted in congeners demonstrating antifungal in vitro activity against human pathogenic yeasts of the Candida genus (MICs in the 6.25–100 µM range), whereas the components were poorly active. The antibacterial in vitro activity of most of the conjugates was lower than the activity of CIP or LVX, but the antibacterial effect of CIP-S-S-TP10-NH₂ was similar to the mother fluoroquinolone. Additionally, for two representative CIP and LVX conjugates, a rapid bactericidal effect was shown. Compared to fluoroquinolones, TP10-NH₂ and the majority of its conjugates generated a relatively low level of reactive oxygen species (ROS) in human embryonic kidney cells (HEK293) and human myeloid leukemia cells (HL-60). The conjugates exhibited cytotoxicity against three cell lines, HEK293, HepG2 (human liver cancer cell line), and LLC-PK1 (old male pig kidney cells), with IC₅₀ values in the 10–100 µM range and hemolytic activity. The mammalian toxicity was due to the intrinsic cytoplasmic membrane disruption activity of TP10-NH₂ since fluoroquinolones themselves were not cytotoxic. Nevertheless, the selectivity index values of the conjugates, both for the bacteria and human pathogenic yeasts, remained favourable.

A Unified and Practical Method for Carbon-Heteroatom Cross-Coupling using Nickel/Photo Dual Catalysis

While carbon-heteroatom cross-coupling reactions have been extensively studied, many methods are specific and limited to a particular set of substrates or functional groups. Reported here is a general method that allows for C-O, C-N and C-S cross-coupling reactions under one general set of conditions. We propose that an energy transfer pathway, in which an iridium photosensitizer produces an excited nickel(II) complex, is responsible for the key reductive elimination step that couples aryl bromides, iodides, and chlorides to 1° and 2° alcohols, amines, thiols, carbamates, and sulfonamides, and is amenable to scale up via a flow apparatus.

Trends in Therapeutic Conjugates: Bench to Clinic

In recent years, therapeutic conjugates have attracted considerable attention as a new class of drug due to their unique pharmacological properties, especially from the pharmaceutical community. Their molecular structure tunability, improved targeting specificity, and therapeutic efficacy have been demonstrated in a wide range of research and clinical applications. In this topical review, we summarize selected recent advances in bioconjugation strategies for the development of therapeutic conjugates, their emerging application in clinical settings, as well as perspectives on the direction of future research.

Probing for Thiol-Mediated Uptake into Bacteria

Cellular uptake mediated by cyclic oligochalcogenides (COCs) is emerging as a conceptually innovative method to penetrate mammalian cells. Their mode of action is based on dynamic covalent oligochalcogenide exchange with cellular thiols. To test thiol-mediated uptake in bacteria, five antibiotics have been equipped with up to three different COCs: One diselenolane and two dithiolanes. We found that the COCs do not activate antibiotics in Gram-negative bacteria. In Gram-positive bacteria, the COCs inactivate antibiotics that act in the cytoplasm and reduce the activity of antibiotics that act on the cell surface. These results indicate that thiol-mediated uptake operates in neither of the membranes of bacteria. COCs are likely to exchange with thiols on the inner, maybe also on the outer membrane, but do not move on. Concerning mammalian cells, the absence of a COC-mediated uptake into bacteria observed in this study disfavors trivial mechanisms, such as passive diffusion, and supports the existence of more sophisticated, so far poorly understood uptake pathways.

Intramolecular methionine alkylation constructs sulfonium tethered peptides for protein conjugation

Continuous efforts have been invested in the selective modification of proteins.