Tuning Cyclometalated Gold(III) for Cysteine Arylation and Ligand-Directed Bioconjugation

Organometallic Oxidative Addition Complexes for S-Arylation of Free Cysteines

Development of bioconjugation strategies to efficiently modify biomolecules is of key importance for fundamental and translational scientific studies. Cysteine S-arylation is an approach which is becoming more popular due to generally rapid kinetics and high chemoselectivity, as well as the strong covalently bonded S-aryl linkage created in these processes. Organometallic approaches to cysteine S-arylation have been explored that feature many advantages compared to their more traditional organic counterparts. In this Viewpoint, progress in the use of Au(III) and Pd(II) oxidative addition (OA) complexes for stoichiometric cysteine S-arylation is presented and discussed. A focus is placed on understanding the rapid kinetics of these reactions under mild conditions, as well as the ability to generate biomolecular heterostructures. Potential avenues for further exploration are addressed and usefulness of these methods to the practitioner are emphasized in the discussion.

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.

Gold-Promoted Biocompatible Selenium Arylation of Small Molecules, Peptides and Proteins

A low pKa (5.2), high polarizable volume (3.8 Å), and proneness to oxidation under ambient conditions make selenocysteine (Sec, U) a unique, natural reactive handle present in most organisms across all domains of life. Sec modification still has untapped potential for site-selective protein modification and probing. Herein we demonstrate the use of a cyclometalated gold(III) compound, [Au(bnpy)Cl2 ], in the arylation of diselenides of biological significance, with a scope covering small molecule models, peptides, and proteins using a combination of multinuclear NMR (including 77 Se NMR), and LC-MS. Diphenyl diselenide (Ph-Se)2 and selenocystine, (Sec)2 , were used for reaction optimization. This approach allowed us to demonstrate that an excess of diselenide (Au/Se-Se) and an increasing water percentage in the reaction media enhance both the conversion and kinetics of the C-Se coupling reaction, a combination that makes the reaction biocompatible. The C-Se coupling reaction was also shown to happen for the diselenide analogue of the cyclic peptide vasopressin ((Se-Se)-AVP), and the Bos taurus glutathione peroxidase (GPx1) enzyme in ammonium acetate (2 mM, pH=7.0). The reaction mechanism, studied by DFT revealed a redox-based mechanism where the C-Se coupling is enabled by the reductive elimination of the cyclometalated Au(III) species into Au(I).

Metal-Mediated Ligand Affinity Chemistry (MLAC)

Metal-mediated ligand affinity chemistry (MLAC) enables site-specific protein modification and represents a powerful bioorthogonal strategy. Conventional bioorthogonal methods often involve two steps: (i) incorporation of the bioorthogonal handle (e.g., non-canonical amino acid, enzyme domain, peptide sequences) and (ii) the binding of functional molecules such as drugs, affinity tags, and fluorophores. This two-step protocol often involves genetic manipulation, which makes it impossible to chemically modify endogenous proteins in living systems. Thus, we propose the development of a transition metal-based chemical strategy that is ligand-directed to the endogenous protein of interest in a single step, which we refer to as metal-mediated ligand affinity chemistry (MLAC).

A Conformationally Restricted Gold(III) Complex Elicits Antiproliferative Activity in Cancer Cells

Diamine ligands are effective structural scaffolds for tuning the reactivity of transition-metal complexes for catalytic, materials, and phosphorescent applications and have been leveraged for biological use. In this work, we report the synthesis and characterization of a novel class of cyclometalated [C^N] Au(III) complexes bearing secondary diamines including a norbornane backbone, (2R,3S)-N2,N3-dibenzylbicyclo[2.2.1]heptane-2,3-diamine, or a cyclohexane backbone, (1R,2R)-N1,N2-dibenzylcyclohexane-1,2-diamine. X-ray crystallography confirms the square-planar geometry and chirality at nitrogen. The electronic character of the conformationally restricted norbornane backbone influences the electrochemical behavior with redox potentials of -0.8 to -1.1 V, atypical for Au(III) complexes. These compounds demonstrate promising anticancer activity, particularly, complex 1, which bears a benzylpyridine organogold framework, and supported by the bicyclic conformationally restricted diaminonorbornane, shows good potency in A2780 cells. We further show that a cellular response to 1 evokes reactive oxygen species (ROS) production and does not induce mitochondrial dysfunction. This class of complexes provides significant stability and reactivity for different applications in protein modification, catalysis, and therapeutics.

Au(III) Cyclometallated Compounds with 2-Arylpyridines and Their Derivatives or Analogues: 34 Years (1989–2022) of NMR and Single Crystal X-ray Studies

A review paper on Au(III) cyclometallated compounds with 2-arylpyridines (2-phenylpyridine, 2-benzylpyridine, 2-benzoylpyridine, 2-phenoxypyridine, 2-phenylsulfanylpyridine, 2-anilinopyridine, 2-(naphth-2-yl)pyridine, 2-(9,9-dialkylfluoren-2-yl)pyridines, 2-(dibenzofuran-4-yl)pyridine, and their derivatives) and their analogues (2-arylquinolines, 1- and 3-arylisoquinolines, 7,8-benzoquinoline), with 113 references. A total of 554 species, containing κ2-N(1),C(6′)*-Au(III), or analogous moiety (i.e., chelated by nitrogen of the pyridine-like ring and the deprotonated ortho- carbon of the phenyl-like ring) and, thus, possessing a character intermediate between metal complexes and organometallics, studied in the years 1989–2022 by NMR spectroscopy and/or single crystal X-ray diffraction (207 X-ray structures), are described. The compounds for which biological or catalytic activity and the luminescence properties were studied are also quoted.

Dual-Boronic Acid Reagents That Combine Dynamic and Covalent Bioconjugation

Boronic acids and boronate esters find appreciable use in chemical biology. Molecules containing orthogonal boronic acid pairs can be utilized for sequential metal-catalyzed cross-couplings for facile preparation of complex bioconjugates including protein-protein conjugates. In this paper, we expand bis-boronic acid reagents for tandem covalent and dynamic bioconjugation. Sequential cross-coupling of 2-nitroarylboronic acid with cysteine residues and condensation of phenylboronic acid with salicylhydroxamic acids (SHA) readily afforded bioconjugates under physiological conditions with dual covalent and dynamic linkages. Both small molecule- and macromolecule-protein conjugates were amenable with this approach and reversible upon addition of excess unfunctionalized SHA or reactive oxygen species. These investigations provide new insights into the kinetic stability of SHA adducts.

Organometallic S-arylation Reagents for Rapid PEGylation of Biomolecules

Bioconjugation techniques for biomolecule-polymer conjugation are numerous; however, slow kinetics and steric challenges generally necessitate excess reagents or long reaction times. Organometallic transformations are known to circumvent these issues; yet, harsh reaction conditions, incompatibility in aqueous media, and substrate promiscuity often limit their use in a biological context. The work reported herein demonstrates a facile and benign organometallic Au(III) S-arylation approach that enables the synthesis of poly(ethylene glycol) monomethyl ether (mPEG)-protein conjugates with high efficiency. Isolable and bench-stable 2, 5, and 10 kDa mPEG-Au(III) reagents were synthesized via oxidative addition into terminal aryl iodide substituents installed on mPEG substrates with a (Me-DalPhos)Au(I)Cl precursor. Reaction of the isolable mPEG-Au(III) oxidative addition complexes with a cysteine thiol on a biomolecule resulted in facile and selective cysteine arylation chemistry, forging covalent S-aryl linkages and affording the mPEG-biomolecule conjugates. Notably, low polymer reagent loadings were used to achieve near quantitative conversion at room temperature in 1 min due to the rapid kinetics and high chemoselectivity of this Au-based bioconjugation approach. Therefore, this work represents an important addition to the protein-polymer conjugation chemical toolbox.

Gold(I)-Mediated Rapid Cyclization of Propargylated Peptides via Imine Formation

In fundamental research and drug discovery, there is still a need for effective and straightforward chemical approaches for generating cyclic peptides. The divergent synthesis of cyclic peptides remains a challenge, in particular when cyclization is carried out in the presence of unprotected side chains and a nonpeptidic component within the cycle is needed. Herein, we describe a novel and efficient strategy based on Au(I)-mediated cyclization of unprotected peptides through rapid (30-60 min) amine addition on a propargyl group to generate an imine linkage. Mechanistic insights reveal that the reaction proceeds via regioselective Markovnikov's addition of the amine on the Au(I)-activated propargyl. This strategy was successfully applied to prepare efficiently (56-94%) over 35 diverse cyclic peptides having different sequences and lengths. We have also achieved stereoselective reduction of cyclic imines employing chiral ligands. The practicality of our method was extended for the synthesis of cyclic peptides that bind Lys48-linked di-ubiquitin chains with high affinity, leading to apoptosis of cancer cells.

Competitive Profiling of Ligandable Cysteines in Staphylococcus aureus with an Organogold Compound

With the idea of exploiting metal templated C–S bond forming reactions to achieve modification of cysteines in bacterial proteins, a cyclometalated Au(III) compound was explored in a competitive chemoproteomic approach...

Highly Efficient Cyclization Approach of Propargylated Peptides via Gold(I)-Mediated Sequential C-N, C-O, and C-C Bond Formation

A rapid and efficient cyclization of unprotected N-propargylated peptides using the Au(I) organometallic complex is reported. The method relies on the activation of the propargyl functionality using gold(I) to produce a new linkage with the N-terminus amine at the cyclization site. The presented method features a fast reaction rate (within 20 min), mild conditions, chemoselectivity, wide sequence scope, and high yields (up to 87%). The strategy was successfully tested on a wide variety of 30 unprotected peptides having various sequences and lengths, thus providing access to structurally distinct cyclic peptides. The practical usefulness of this method was demonstrated in producing peptides that bind efficiently to Lys48-linked di- and tetra-ubiquitin chains. The new cyclic peptide modulators exhibited high permeability to living cells and promoted apoptosis via binding with the endogenous Lys48-linked ubiquitin chains.