Molecular clip 1 remains monomeric in water and engages in host-guest recognition processes with suitable guests. We report the Ka values for 32 1⋅guest complexes measured by 1 H NMR, UV/Vis, and fluorescence titrations. The cavity of 1 is shaped by aromatic surfaces of negative electrostatic potential and therefore displays high affinity and selectivity for planar and cationic aromatic guests that distinguishes it from CB[n] receptors that prefer aliphatic over aromatic guests. Electrostatic effects play a dominant role in the recognition process whereby ion-dipole interactions may occur between ammonium ions and the C=O groups of 1, between the SO3- groups of 1 and pendant cationic groups on the guest, and within the cavity of 1 by cation-π interactions. Host 1 displays a high affinity toward dicationic guests with large planar aromatic surfaces (e.g. naphthalene diimide NDI+ and perylene diimide PDI+) and cationic dyes derived from acridine (e.g. methylene blue and azure A). The critical importance of cation-π interactions was ascertained by a comparison of analogous neutral and cationic guests (e.g. methylene violet vs. methylene blue; quinoline vs. N-methylquinolinium; acridine vs. N-methylacridinium; neutral red vs. neutral red H+ ) the affinities of which differ by up to 380-fold. We demonstrate that the high affinity of 1 toward methylene blue (Ka =3.92×107 m-1 ; Kd =25 nm) allows for the selective sequestration and destaining of U87 cells stained with methylene blue.
Keywords: cation-π interactions; dyes; glycoluril; hydrophobic effects; molecular clip.
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