Biotinylated Cyclophane: Synthesis, Cyclophane-Avidin Conjugates, and Their Enhanced Guest-Binding Affinity

摘要

Cationic and anionic cyclophanes bearing a biotin moiety were synthesized as a water-soluble host (la and 1b, respectively). Both hosts la and 1b were found to strongly bind avidin with binding constants of 1.3 x 10(8)M(-1), as confirmed by surface plasmon resonance measurements. The present conjugate of la with avidin (la-avidin) showed an enhanced guest binding affinity toward fluorescence guests such as TNS and 2,6-ANS. The K values of la-avidin conjugate with TNS and 2,6-ANS were similar to 9-fold larger than those of monocyclic cyclophane la with the identical guests. Favorable hydrophobic and electrostatic interactions between la-avidin and TNS were suggested by computer-aided molecular modeling calculations. Moreover, addition of excess biotin to the complexes of la-avidin with the guests resulted in dissociation of la-avidin to avidin and la having less guest-binding affinity. Conversely, such enhancements in the guest-binding affinity were not obviously observed for the conjugate of anionic 1b with avidin (1b-avidin) due to electrostatic repulsion between anionic 1b and anionic guests.