Fluorescence Turn-ON Displacement Assays with Cucurbit[7]uril–Thiophenylpyridinium Complexes as Host–Dye Reporter Pairs

The <i>N</i>-methyl-4-thiophenylpyridinium cation (ThioPy) is a high affinity (<i>K</i>_d ca. 5 nM), fast-exchanging fluorescent probe for cucurbit[7]uril (CB7). The CB7/ThioPy complex shows a unique fluorescence turn-ON response upon displacement by an analyte in sensing application. This enabled the development of a real-time fluorescence assay with the MRFA peptide for the protease thermolysin, which is also suitable for the cancer biomarker cathepsin B. Moreover, liposome encapsulation of CB7/ThioPy in large unilamellar vesicles (LUVs) provided mechanistic insight into intravesicular dye displacement reactions.