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. 2022 Aug 24;25(1):58–73. doi: 10.1007/s11307-022-01768-4

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Fig. 2 — Different types of quenched cathepsin-activatable fluorescent probes. A Quenched substrate-based probe. The probe is activated by enzymatic cleavage of the peptide linker by a target cathepsin. Upon cleavage, two fragments—one containing the quencher and the other the now unquenched fluorophore—are released. B Quenched activity-based probe. The probe covalently binds in the active site of the target cathepsin forming a permanent bond. Upon binding the active site, the quencher is released and the probe is activated. Abbreviations: qSPB, quenched substrate-based probe; qABP, quenched activity-based probe; Q, quencher; F, fluorophore.