Copper-Free Sonogashira Cross-Coupling for Functionalization of Alkyne-Encoded Proteins in Aqueous Medium and in Bacterial Cells

Abstract

Bioorthogonal reactions suitable for functionalization of genetically or metabolically encoded alkynes, e.g., copper-catalyzed azide-alkyne cycloaddition reaction (“click chemistry”), have provided chemical tools to study biomolecular dynamics and function in living systems. Despite its prominence in organic synthesis, copper-free Sonogashira cross-coupling reaction suitable for biological applications has not been reported. In this work, we report the discovery of a robust aminopyrimidine-palladium(II) complex for copper-free Sonogashira cross-coupling that enables selective functionalization of a homopropargylglycine (HPG)-encoded ubiquitin protein in aqueous medium. A wide range of aromatic groups including fluorophores and fluorinated aromatic compounds can be readily introduced into the HPG-containing ubiquitin under mild conditions with good to excellent yields. The suitability of this reaction for functionalization of HPG-encoded ubiquitin in E. coli was also demonstrated. The high efficiency of this new catalytic system should greatly enhance the utility of Sonogashira cross-coupling in bioorthogonal chemistry.

Selective chemical protein modification

“…Li et al 127 have since shown that by simple methylation of the ligand, the same catalytic system could be used to promote Sonogashira reactions (L2 in Fig. 5a), while the minimal motif guanidine-based ligands (L3 and L4 in Fig.…”

Palladium-mediated chemistry in living cells

“…Lin developed a palladium-based catalyst for copper free Sonogashira cross-coupling between a peptide/protein carrying a terminal alkyne and a wide range of aryl iodides in aqueous medium [26 ], avoiding the need to use toxic copper(I) [27].…”

Biochemical analysis with the expanded genetic lexicon

“…2 (23-25)) can also be used for bioorthogonal conjugation reactions [43,44,[87][88][89]. Aryl halides can undergo multiple organometalcatalyzed reactions [90,91]. In particular, aryl halides and boronic acids are reactants of the Suzuki reaction [91].…”