Fluorescent-tagged sp2-iminosugars with potent β-glucosidase inhibitory activity.

New fluorescently-labelled sp(2)-iminosugars based on the 5N,6S-[N'-(4-aminobutyl)iminomethylidene]-6-thionojirimycin skeleton have been synthesized as photoprobes to monitor glycosidase binding. Dansyl, dapoxyl and coumarin fluorophores were appended to the terminal amino group at the N'-substituent by either sulfonamide or amide bridging reaction. All the conjugates behaved as strong (low micromolar to nanomolar) and selective inhibitors of β-glucosidases (almonds and bovine liver) and naringinase, in agreement with the inhibition pattern previously encountered for related iso(thio)urea-type bicyclic sp(2)-iminosugars. The presence of the fluorescent probe allows real-time and continuous monitoring of β-glucosidase inhibition by fluorescence resonance energy transfer (FRET), taking advantage of the intrinsic tryptophan-associated fluorescence of the protein.