An acid-responsive rhodamine-based fluorescent probe for N-terminal protein labeling and aqueous Tsuji-Trost allylic...

The development of acid-responsive fluorescent probes with additional chemical functionality remains of considerable interest in chemical biology. Herein, we report an acid-responsive rhodamine-based fluorescent probe, 2PCA-RhB, which exhibits a pronounced fluorescence "turn-on" response under acidic aqueous conditions through spirolactam ring opening. The probe displays favorable photophysical properties, good aqueous compatibility, and enables fluorescence imaging in living cells. Beyond its fluorescence response, 2PCA-RhB incorporates a 2-pyridinecarboxaldehyde (2PCA) moiety that enables N-terminal-selective protein labeling, while its allyl carbonate functionality is compatible with palladium-catalyzed Tsuji-Trost allylic alkylation under aqueous conditions. These studies establish 2PCA-RhB as an acid-responsive fluorescent probe with both N-terminal labeling capability and compatibility with aqueous transition-metal-mediated transformation. The present work provides a versatile molecular scaffold for the future development of fluorescent chemical tools for protein modification and bioorthogonal applications in complex biological environments.