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meso-Carboxyl-BODIPY responds to small electronic changes resulting from acyl substitution reactions with a marked change in fluorescence. Herein, the minute changes that accompany the thioester to amide conversion encountered in native chemical ligation (NCL) are exploited in the construction of fluorescent "turn-on" probes. Two fluorogenic probes, 1 a and 4, derived from a mesothioester-BODIPY scaffold, were designed for the selective detection of cysteine (1 a) and aminopeptidase N (4), respectively. The aromatic (1 a) and aliphatic (4) thioesters of meso-carboxyl-BODIPY are nonfluorescent. However, specific analyte-induced conversion to the meso-amide derivative caused significant spectral changes and a dramatic fluorescence enhancement. Probe 1 a exhibited a large fluorescence "turn-on" response with high selectivity toward cysteine via a tandem NCL reaction. Probe 4 was successfully applied to the monitoring and imaging of endogenous aminopeptidase N in live cancer cells.

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Urokinase-type plasminogen activator receptor (uPAR) is a glycolipid-anchored protein located on the cell surface that is implicated in the promotion of metastasis. New fluorescent probes for the detection of uPAR expression that feature a rapid "turn-on" response are reported here. They consist of a donor-π-acceptor-based fluorophore conjugated with a uPAR-binding AE105 peptide. The resulting AE105coupled uPAR-targeting probes are weakly emissive in aqueous buffer solutions; however, a fluorescence "turn-on" signal is instantly triggered upon specific binding to uPAR (K D = 63.2 nM for P1 and 49.5 nM for P2), which restricts the rotational deactivation of the fluorophore. Applications of the probes were demonstrated in the imaging of uPAR overexpressed on the membrane of cancer cell and in a cell-based uPAR inhibitor assay.