Photoclickable HaloTag Ligands for Spatiotemporal Multiplexed Protein Labeling on Living Cells

Precise spatiotemporal control over fluorescence labeling is a powerful approach for selective marking and tracking of proteins of interest within living systems. Here, we report a photoclickable labeling platform based on the 2,3-diaryl-indanone epoxide (DIO) photoswitch scaffold and the self-labeling protein HaloTag. Upon illumination, the protein-bound DIO undergoes reversible photoisomerization to form a metastable oxidopyrylium ylide (PY) that reacts with ring-strained dipolarophiles via [5+2] cycloaddition, enabling covalent spatiotemporal labeling. We synthesize and characterize a library of DIO-HaloTag and DIO-SNAP- tag ligands, systematically examining the effects of linker architecture and scaffold substitution on the photoswitching and photoclick reactivity in vitro and on living cells. We identify a naphthyl-substituted DIO ligand exhibiting superior photoswitching and photoclick efficiency, allowing robust and selective labeling of HaloTag on the surface of living cells using visible light activation. Using this system, we achieve two- and three-color labeling of defined cell surface regions with excellent spatial and temporal precision, additionally allowing combinatorial labeling. Together, this work establishes a versatile framework for multiplexed, light- directed protein labeling compatible with living systems, with promising future applications in long-term tracking and cellular barcoding.