Biquinolino-Modified beta-Cyclodextrin Dimers and Their Metal Complexes as Efficient Fluorescent Sensors for the Molecular Recognition of Steroids

摘要

A series of bridged P-cyclodextrin (|3-CyD)dimers possessing functional tethers of various lengths was synthesized in moderate yield by the treatment of 2,2'-biquinoline-4,4/-dicarboxylic dichloride with beta-CyD or mono[6-oligo(ethylenediamino)-6-deoxy]-p-CyDs.The products were 2,2'-biquinoline-4,4'-dicarboxy-bridged bis(6-O-beta-CyD)(8),N,N-bis(2-amino-ethyl)-2,2'-biquinoline-4,4'-dicarbox-amide-bridged bis(6-amino-6-deoxy-beta-CyD)(9),and N,N'-bis(5-amino-3-aza-pentyl)-2,2'-biquinoline-4,4'-dicarbox-amide-bridged bis(6-amino-6-deoxy-p-CyD)(10).The reaction of 8-10 with copper perchlorate give their copper(II)complexes 11-13 in satisfactory yields of over 77%.All the bis(p-CyD)s 8-13 act as efficient fluorescent sensors and display remarkable fluorescence enhancement upon addition of optically inert steroids.The inclusion complexa-tion behaviors of 8-13 when treated with the representative steroids cholate (14),deoxycholate (15),and glycocho-late (16)in aqueous solution at 25 deg C were investigated by means of UV/Vis spectroscopy,conductivity and fluorescence measurements,circular dichroism spectroscopy,and 2D NMR spectroscopy.The tether length of bis(p-CyD)9 allows it to adopt a cooperative host-tether-guest binding mode in which the spacer and guest are co-included in the two CyD cavities.As a result of this cooperation,9 has a stability constant (K_s)about 2Xl0~2 times higher than that of monomodified p-CyD 4 for inclusion complexation with cholate.Metallooligo(p-CyD)s with four beta-CyD units have enhanced binding abilities compared with monomodified beta-CyDs.These metallo compounds have binding affinities for guest steroids that are up to 50-4.1 X 10~3 times higher than those of CyDs 2-4.The guest-induced fluorescence enhancement of bis-(CyD)s opens a new channel for the design of sensor materials.The complex stability constants of these compounds are discussed from the viewpoint of induced-fit interaction and cooperative multiple binding between host and guest.