Organometallic Gold(III) Reagents for Cysteine Arylation

Visible-Light-Promoted C(sp3 )-H Alkylation by Intermolecular Charge Transfer: Preparation of Unnatural α-Amino Acids and Late-Stage Modification of Peptides

Disclosed herein is the visible-light-promoted deaminative C(sp³ )-H alkylation of glycine and peptides using Katritzky salts as electrophiles. Simple reaction conditions and excellent functional-group tolerance provide a general strategy for the efficient preparation of unnatural α-amino acids and precise modification of peptides with unnatural α-amino-acid residues. Mechanistic studies suggest that visible-light-promoted intermolecular charge transfer within a glycine-Katritzky salt electron donor-acceptor (EDA) complex induces a single-electron transfer process without the assistance of photocatalyst.

Oxidative Addition of Alkenyl and Alkynyl Iodides to a AuI Complex

The first isolated examples of intermolecular oxidative addition of alkenyl and alkynyl iodides to Au^I are reported. Using a 5,5'-difluoro-2,2'-bipyridyl ligated complex, oxidative addition of geometrically defined alkenyl iodides occurs readily, reversibly and stereospecifically to give alkenyl-Au^III complexes. Conversely, reversible alkynyl iodide oxidative addition generates bimetallic complexes containing both Au^III and Au^I centers. Stoichiometric studies show that both new initiation modes can form the basis for the development of C-C bond forming cross-couplings.

An Organometallic Strategy for Assembling Atomically Precise Hybrid Nanomaterials

For decades, chemists have strived to mimic the intricate design and diverse functions of naturally occurring systems through the bioinspired synthesis of programmable inorganic nanomaterials. The development of thiol-capped gold nanoparticles (AuNPs) has driven advancement in this area; however, although versatile and readily accessible, hybrid AuNPs are rarely atomically precise, which limits control over their surface topology and therefore the study of complex structure-function relationships. Here, we present a bottom-up approach to the systematic assembly of atomically precise hybrid nanoclusters employing a strategy that mimics the synthetic ease with which thiol-capped AuNPs are normally constructed, while producing well-defined covalent nanoscale assemblies with diverse surface topologies. For the first time, using a structurally characterized cluster-based organometallic building block, we demonstrate the systematic synthesis of nanoclusters with multivalent binding capabilities to complex protein targets.

Preparation of Biomolecule-Polymer Conjugates by Grafting-From Using ATRP, RAFT, or ROMP

Biomolecule-polymer conjugates are constructs that take advantage of the functional or otherwise beneficial traits inherent to biomolecules and combine them with synthetic polymers possessing specially tailored properties. The rapid development of novel biomolecule-polymer conjugates based on proteins, peptides, or nucleic acids has ushered in a variety of unique materials, which exhibit functional attributes including thermo-responsiveness, exceptional stability, and specialized specificity. Key to the synthesis of new biomolecule-polymer hybrids is the use of controlled polymerization techniques coupled with either grafting-from, grafting-to, or grafting-through methodology, each of which exhibit distinct advantages and/or disadvantages. In this review, we present recent progress in the development of biomolecule-polymer conjugates with a focus on works that have detailed the use of grafting-from methods employing ATRP, RAFT, or ROMP.

Photo-induced preparation of unnatural α-amino acids: synthesis and characterization of novel Leu5-enkephalin analogues

2.6-Fold more long-lasting compared to Leu⁵-enkephalin.

Au(i)/Au(iii)-Catalyzed C–N coupling

Gold-catalyzed coupling of aryl iodides and amines via a ligand-enabled 2e redox cycle.

Gold-Catalyzed Cross-Coupling Reactions: An Overview of Design Strategies, Mechanistic Studies, and Applications

Transition-metal-catalyzed cross-coupling reactions are central to many organic synthesis methodologies. Traditionally, Pd, Ni, Cu, and Fe catalysts are used to promote these reactions. Recently, many studies have showed that both homogeneous and heterogeneous Au catalysts can be used for activating selective cross-coupling reactions. Here, an overview of the past studies, current trends, and future directions in the field of gold-catalyzed coupling reactions is presented. Design strategies to accomplish selective homocoupling and cross-coupling reactions under both homogeneous and heterogeneous conditions, computational and experimental mechanistic studies, and their applications in diverse fields are critically reviewed. Specific topics covered are: oxidant-assisted and oxidant-free reactions; strain-assisted reactions; dual Au and photoredox catalysis; bimetallic synergistic reactions; mechanisms of reductive elimination processes; enzyme-mimicking Au chemistry; cluster and surface reactions; and plasmonic catalysis. In the relevant sections, theoretical and computational studies of Au^I /Au^III chemistry are discussed and the predictions from the calculations are compared with the experimental observations to derive useful design strategies.

Labeling and Natural Post-Translational Modification of Peptides and Proteins via Chemoselective Pd-Catalyzed Prenylation of Cysteine

The prenylation of peptides and proteins is an important post-translational modification observed in vivo. We report that the Pd-catalyzed Tsuji-Trost allylation with a Pd/BIPHEPHOS catalyst system allows the allylation of Cys-containing peptides and proteins with complete chemoselectivity and high n/i regioselectivity. In contrast to recently established methods, which use non-native connections, the Pd-catalyzed prenylation produces the natural n-prenylthioether bond. In addition, a variety of biophysical probes such as affinity handles and fluorescent tags can be introduced into Cys-containing peptides and proteins. Furthermore, peptides containing two cysteine residues can be stapled or cyclized using homobifunctional allylic carbonate reagents.

Catalytic Au(i)/Au(iii) arylation with the hemilabile MeDalphos ligand: unusual selectivity for electron-rich iodoarenes and efficient application to indoles

The ability of the hemilabile (P,N) MeDalphos ligand to trigger oxidative addition of iodoarenes to gold has been thoroughly studied. Competition experiments and Hammett correlations substantiate a clear preference of gold for electron-enriched substrates both in stoichiometric oxidative addition reactions and in catalytic C-C cross-coupling with 1,3,5-trimethoxybenzene. This feature markedly contrasts with the higher reactivity of electron-deprived substrates typically encountered with palladium. Based on DFT calculations and detailed analysis of the key transition states (using NBO, CDA and ETS-NOCV methods in particular), the different behavior of the two metals is proposed to result from inverse electron flow between the substrate and metal. Indeed, oxidative addition of iodobenzene is associated with a charge transfer from the substrate to the metal at the transition state for gold, but opposite for palladium. The higher electrophilicity of the gold center favors electron-rich substrates while important back-donation from palladium favors electron-poor substrates. Facile oxidative addition of iodoarenes combined with the propensity of gold(iii) complexes to readily react with electron-rich (hetero)arenes prompted us to apply the (MeDalphos)AuCl complex in the catalytic arylation of indoles, a challenging but very important transformation. The gold complex proved to be very efficient, general and robust. It displays complete regioselectivity for C3 arylation, it tolerates a variety of functional groups at both the iodoarene and indole partners (NO2, CO2Me, Br, OTf, Bpin, OMe…) and it proceeds under mild conditions (75 °C, 2 h).

Palladium-Catalyzed Suzuki-Miyaura Reactions of Aspartic Acid Derived Phenyl Esters

Transition-metal-catalyzed transformations of amino acids and peptides could provide a powerful method for their site-selective modification. Here, we report non-decarbonylative Pd-catalyzed Suzuki-Miyaura reactions of phenyl ester derivatives of aspartic acid to form aryl-amino ketones. These products are potentially important in the synthesis of pharmaceuticals, and our methodology represents a new route to access molecules of this type.

Cyclometalated AuIII Complexes for Cysteine Arylation in Zinc Finger Protein Domains: towards Controlled Reductive Elimination

With the aim of exploiting the use of organometallic species for the efficient modification of proteins through C-atom transfer, the gold-mediated cysteine arylation through a reductive elimination process occurring from the reaction of cyclometalated AuIII C^N complexes with a zinc finger peptide (Cys2 His2 type) is here reported. Among the four selected AuIII cyclometalated compounds, the [Au(CCO N)Cl2 ] complex featuring the 2-benzoylpyridine (CCO N) scaffold was identified as the most prone to reductive elimination and Cys arylation in buffered aqueous solution (pH 7.4) at 37 °C by high-resolution LC electrospray ionization mass spectrometry. DFT and quantum mechanics/molecular mechanics (QM/MM) studies permitted to propose a mechanism for the title reaction that is in line with the experimental results. Overall, the results provide new insights into the reactivity of cytotoxic organogold compounds with biologically important zinc finger domains and identify initial structure-activity relationships to enable AuIII -catalyzed reductive elimination in aqueous media.

Bioinspired Thiophosphorodichloridate Reagents for Chemoselective Histidine Bioconjugation

Site-selective bioconjugation to native protein residues is a powerful tool for protein functionalization, with cysteine and lysine side chains being the most common points for attachment owing to their high nucleophilicity. We now report a strategy for histidine modification using thiophosphorodichloridate reagents that mimic post-translational histidine phosphorylation, enabling fast and selective labeling of protein histidines under mild conditions where various payloads can be introduced via copper-assisted alkyne-azide cycloaddition (CuAAC) chemistry. We establish that these reagents are particularly effective at covalent modification of His-tags, which are common motifs to facilitate protein purification, as illustrated by selective attachment of polyarginine cargoes to enhance the uptake of proteins into living cells. This work provides a starting point for probing and enhancing protein function using histidine-directed chemistry.

Arylation Chemistry for Bioconjugation

Bioconjugation chemistry has been used to prepare modified biomolecules with functions beyond what nature intended. Central to these techniques is the development of highly efficient and selective bioconjugation reactions that operate under mild, biomolecule compatible conditions. Methods that form a nucleophile-sp2 carbon bond show promise for creating bioconjugates with new modifications, sometimes resulting in molecules with unparalleled functions. Here we outline and review sulfur, nitrogen, selenium, oxygen, and carbon arylative bioconjugation strategies and their applications to modify peptides, proteins, sugars, and nucleic acids.

Protein Substrates for Reaction Discovery: Site-Selective Modification with Boronic Acid Reagents

Chemical modification of natural proteins must navigate difficult selectivity questions in a complex polyfunctional aqueous environment, within a narrow window of acceptable conditions. Limits on solvent mixtures, pH, and temperature create challenges for most synthetic methods. While a protein's complex polyfunctional environment undoubtedly creates challenges for traditional reactions, we wondered if it also might create opportunities for pursuing new bioconjugation reactivity directly on protein substrates. This Account describes our efforts to date to discover and develop new and useful reactivity for protein modification by starting from an open-ended screen of potential transition-metal catalysts for boronic acid reactivity with a model protein substrate. By starting from a broad screen, we were hoping to take advantage of the very many potential reactive sites on even a small model protein. And perhaps more importantly, whole proteins as reaction screening substrates might exhibit uniquely reactive local environments, the results of a dense combination of functional groups that would be nearly impossible to mimic in a small-molecule context. This effort has resulted in the discovery of four new protein modification reactions with boronic acid reagents, including a remarkable modification of specific backbone N-H bonds. This histidine-directed Chan-Lam coupling, based on specific proximity of an imidazole and two amide groups, is one important example of powerful reactivity that depends on a combination of functional groups that proteins make possible. Other bioconjugation reactions uncovered include a three-component tyrosine metalation with rhodium(III), a nickel-catalyzed cysteine arylation, and an unusual ascorbate-mediated oxidative process for N-terminal modification. The remarkably broad scope of reactivity types encountered in this work is a testament to the breadth of boronic acid reactivity. It is also a demonstration of the diverse reactivities that are possible by the combined alteration of boronic acid structure and metal promoter. The discovery of specific backbone modification chemistry has been a broadly empowering reactivity. Pyroglutamate, a naturally occurring posttranslational modification, exhibits remarkably high reactivity in histidine-directed backbone modification, which allows us to treat pyroglutamate as a reactive bioorthogonal handle that is readily incorporated into proteins of interest by natural machinery. In another research direction, the development of a vinylogous photocleavage system has allowed us to view backbone modification as a photocaging modification which is released by exposure to light.

Mechanism and chemoselectivity origins of bioconjugation of cysteine with Au(iii)-aryl reagents

A detailed computational study is presented on the reaction mechanism of selective cysteine S-arylation by cationic Au(iii)-aryl reagents. The chemoselectivity origins have been elucidated through comparison with potential N- and O-arylation, showing that the acidity and nucleophilicity of the residue are two inherent controlling factors.

Modifications of amino acids using arenediazonium salts

Aryl transfer reactions from arenediazonium salts have started to make their impact in chemical biology with initial forays in the arena of arylative modifications and bio-conjugations of amino acids, peptides and proteins.

Rapid nickel(ii)-promoted cysteine S-arylation with arylboronic acids

Nickel salts catalyze fast cysteine arylation with 2-nitroarylboronic acids. The process uses cheap, readily-available reagents and allows introduction of diverse chemical handles.

Aldehyde-mediated bioconjugation via in situ generated ylides

We report a technically simple approach for rapid, high-yielding and site-selective aldehyde-mediated bioconjugation for protein labelling and cellular applications.

A guide to maximizing the therapeutic potential of protein-polymer conjugates by rational design

Proteins are an important class of therapeutics that have advantages including high target specificity, but challenges to their use include rapid clearance and low physical stability. Conjugation of synthetic polymers is an effective approach to address the drawbacks and enhance other properties such as solubility. In this review, we present various considerations in synthesizing protein-polymer conjugates for therapeutic applications with a focus on the choice of polymer, protein, and conjugation method, as well as characterization and evaluation of the resulting conjugate in order to maximize the therapeutic potential of the protein drug.

Oxidant-free oxidative gold catalysis: the new paradigm in cross-coupling reactions

The construction of C–C and C–X (X = hetero atom) bonds is the core aspect for the assembly of molecules. This feature article critically presents an overview of all the redox neutral cross-coupling reactions enabled by gold catalysis, which we believe would stimulate further research activities in this promising area.

Selective modification of natural nucleophilic residues in peptides and proteins using arylpalladium complexes

The present review outlines the recent methodologies for selective arylation of natural nucleophilic residues within unprotected peptides and proteins promoted by arylpalladium complexes, which demonstrate the advantages and potential of organometallic palladium complexes in bioconjugation.