Protein-Labeling Reagents Selectively Activated by Copper(I)

Copper is an essential trace element that participates in many biological processes through its unique redox cycling between cuprous (Cu⁺) and cupric (Cu²⁺) oxidation states. To elucidate the biological functions of copper, chemical biology tools that enable selective visualization and detection of copper ions and proteins in copper-rich environments are required. Herein, we describe the design of Cu⁺-responsive reagents based on a conditional protein labeling strategy. Upon binding Cu⁺, the probes generated quinone methide via oxidative bond cleavage, which allowed covalent labeling of surrounding proteins with high Cu⁺ selectivity. Using gel- and imaging-based analyses, the best-performing probe successfully detected changes in the concentration of labile Cu⁺ in living cells. Moreover, conditional proteomics analysis suggested intramitochondrial Cu⁺ accumulation in cells undergoing cuproptosis. Our results highlight the power of Cu⁺-responsive protein labeling in providing insights into the molecular mechanisms of Cu⁺ metabolism and homeostasis.