Catalyst-Free, Divergent Cysteine Modification via Indole Isocyanide Photochemistry.

Chemoselective cysteine modification is pivotal for chemical biology and drug discovery. While photochemical strategies offer spatiotemporal control, most current methods rely on exogenous catalysts, complicating purification processes. Furthermore, the structural diversity of accessible conjugates remains limited. Here, we report an additive/catalyst-free, visible-light-driven platform for divergent cysteine modification via indole isocyanide photochemistry. This strategy eliminates the need for external additives or photocatalysts, ensuring excellent biocompatibility and operational simplicity. A key practical advantage of this method is its compatibility with both solution-phase and solid-phase reaction systems, offering unmatched flexibility for diverse experimental setups. By leveraging the tunable structure of indole isocyanides, this photoreaction readily generates a diverse array of indole-fused or indole-spiro aza-cycles at cysteine residues. We demonstrate broad utility of this method, ranging from site-specific modification of peptides and proteins to the synthesis of cyclic peptides and the assembly of proteolysis-targeting chimeras (PROTACs). Moreover, a chemical proteomics profiling employing indole isocyanide-based photoprobe achieved selective modification of C77 within the therapeutic phosphatase target PPP5C. This interaction revealed a novel druggable pocket at the TPR-catalytic domain interface, uncovering a previously unknown allosteric inhibition strategy. Collectively, this work establishes a robust, versatile platform for advancing therapeutic discovery and chemical biology.