Cooperative molecular recognition of dyes by dyad and triad cyclodextrin–crown ether conjugates

Abstract

Three β-cyclodextrin (β-CyD) derivatives with crown ether units, that is N-(4′-benzo-15-crown-5)-6-imino-6-deoxy-β-CyD (2), 6,6′-[N-(4,4′-dibenzo-18-crown-6)-imino]-bridged bis(β-CyD) (3), and 2,2′-[O-(4′,5′-benzo-15-crown-5)-ethyl]-bridged bis (β-CyD) (5), were synthesized as cooperative recognition receptor models. Their molecular binding behavior with four representative fluorescent dyes, i.e., ammonium 8-anilino-1-naphthalenesulfonate (ANS), sodium-6-toluidino-2-naphthalene-sulfonate (TNS), Acridine Red (AR) and Rhodamine B (RhB), was investigated in buffer solutions (pH = 7.20) at 25 °C by means of circular dichroism, NMR and fluorescence spectroscopy. 2D-ROESY experiments showed that dyad host 2 and triad host 3 adopted a CyD–guest–crown ether binding mode, while triad host 5 adopted a CyD–guest–CyD binding mode, upon inclusion complexation with guest molecules. Therefore, hosts 2 and 3 showed high molecular recognition ability towards charged guests, giving an enhanced binding ability up to 115 times for ANS by 3 and fairly high molecular selectivity up to 1450 times for the ANS/AR pair by 2 as compared with native β-CyD in an aqueous phosphate buffer solution. On the other hand, host 5 was found to be able to effectively recognize the shape of a guest molecule, showing significantly higher binding ability towards linear guests. The binding affinities and molecular recognition abilities of these CyD–crown ether conjugates towards guest molecules are discussed from the viewpoint of electrostatic and/or hydrophobic interactions, size/shape-fit concept, and multiple recognition mechanism between host and guest.