Organometallic palladium reagents for cysteine bioconjugation

Abstract

Transition-metal based reactions have found wide use in organic synthesis and are used frequently to functionalize small molecules.1,2 However, there are very few reports of using transition-metal based reactions to modify complex biomolecules3,4, which is due to the need for stringent reaction conditions (for example, aqueous media, low temperature, and mild pH) and the existence of multiple, reactive functional groups found in biopolymers. Here we report that palladium(II) complexes can be used for efficient and highly selective cysteine conjugation reactions. The bioconjugation reaction is rapid and robust under a range of biocompatible reaction conditions. The straightforward synthesis of the palladium reagents from diverse and easily accessible aryl halide and trifluoromethanesulfonate precursors makes the method highly practical, providing access to a large structural space for protein modification. The resulting aryl bioconjugates are stable towards acids, bases, oxidants, and external thiol nucleophiles. The broad utility of the new bioconjugation platform was further corroborated by the synthesis of new classes of stapled peptides and antibody-drug conjugates. These palladium complexes show potential as a new set of benchtop reagents for diverse bioconjugation applications.

Transition Metal-Promoted Arylation: An Emerging Strategy for Protein Bioconjugation

“…Interestingly, elevated temperatures (658C) were also required, suggesting that the arylpalladium(II) species formed in situ during these transformations is less reactive than the isolated palladium complexes employed for the room temperature arylations reported by Buchwald and coworkers. [15] These results highlight a primary benefit of using transition metals for protein modification -judicious choice of ligands allows precise tailoring of both reactivity and selectivity. Davis and coworkers went on to demonstrate selective palladium-mediated S-arylation of MGS using a broad range of substituted aryl iodides, including isotope labels, carbohydrates, and functional handles for bioconjugation.…”

“…Modification of substituents on the aryl ring also enabled tuning of the oxidative stability of the S-aryl moiety. [15] Another powerful approach to palladium-mediated Cys arylation was recently reported in 2016 by Davis and coworkers. [16] This method exploits the endogenous metal binding motifs often displayed in the active sites of metal-dependent enzymes to direct regioselective palladium-catalyzed S-arylation in the presence of multiple Cys residues.…”

“…[15] The organopalladium species were readily formed by oxidative addition of an aryl halide or triflate and the judicious choice of a ligand (2-dicyclohexylphosphino-2 0 ,6 0 -diisopropoxybiphenyl, RuPhos), which provided a stable yet reactive palladium species that could effectively facilitate C-S reductive elimination to afford diverse S-aryl cysteine derivatives at room temperature in aqueous media (pH 5.5-8.5) and at high dilution (low micromolar concentrations). Although pi-allylpalladium complexes have been previously employed for the modification of Tyr residues, [11] the arylpalladium species employed by Buchwald and coworkers [15] are less electrophilic, which facilitates their selectivity for cysteine. With a model peptide, quantitative conversion to the Cys-labelled products was achieved with a variety of different arylpalladium reagents, including fluorescent tags, bioconjugation linkers, affinity labels, and therapeutics.…”

“…Arylpalladium(II) complexes for cysteine bioconjugation and peptide stapling. [15] To overcome this limitation, Buchwald, Pentelute, and coworkers reported in 2015 a robust umpolung approach to the selective arylation of Sec that relies on the electrophilicity of oxidized Sec in a copper-promoted reaction with nucleophilic arylboronic acids (Fig. 4).…”

Antibody-Drug Conjugates in Head and Neck Cancer

“…Coupled with selection of proper linker structure and payload, this method can potentially lead to ADCs with enhanced PK/PD and therapeutic efficacy. Recently, Buchwald and co-workers developed a rapid, highly selective cysteine conjugation using aryl palladium complexes (Vinogradova et al, 2015) (Fig. 4C).…”