Inclusion Complexations of Levamisole and Cocaine With Molecular Baskets. Fluorescence Displacement Assay for Detection of Cocaine and Its Adulterant Levamisole in Aqueous Media

ABSTRACT We investigated inclusion complexation of levamisole (LEV + ), a common adulterant of cocaine (COC + ), with six molecular baskets 1 3− –6 18− of increasing cavity depth and anionic charge at the portal. 1 H NMR and mass spectrometry showed that 1 3− to 6 18− bind to LEV + to form binary complexes in water, with the stability increasing across the series and highest for dendritic basket 6 18− ( K d = 10 µM). From 1 H NMR magnetic shielding and OPLS4 calculations, we deduced that LEV + inserts its benzene in the binding pocket of 1 3− – 6 18− while the protonated heterocycle remains near the anionic portal. The stability of [LEV⊂ 6 ] 17− arises from C−H … π/cation … π/π−π contacts, ion pairing, and hydrophobic effect (Δ H° < 0, ITC). Dendritic basket 6 18− was subsequently used to develop a fluorescent displacement assay (FDA) for the detection of LEV + with alizarin red S (ALZ − ) as a fluorescence reporter. LEV + displaced the dye to generate a fluorescence response suitable for its detection in water over circa 10–300 µM range. In contrast, COC + bound more strongly to basket 6 18− , forming binary ( K d = 1 µM) and ternary complexes ( 1 H NMR, calorimetry, and FDA). The binding mode of COC + differs from that of LEV + , with the drug's cationic fragment occupying the basket's cavity.