シクロファン 人工レセプター・人工酵素としての分子認識能

Abstract

Host-guest complexation behavior of various functionalized cyclophanes has been discussed in view of their efficiency as artificial receptors and enzymes. Hydrophobic cavities of simple and monocyclic cyclophanes are too small and shallow to allow effective inclusion of hydrophobic guests of various bulkiness, even though the primary hydrophobic binding interaction is further enhanced by electrostatic and charge-transfer interactions. In order to improve molecular recognition ability of cyclophanes, modifications of macrocyclic skeletons are required to give three-dimensionally extended hydrophobic cavities. One approach to create such sizable internal cavities is to construct cage-type macrocycles, e.g., cubic cyclophanes, which are capable of incorporating guest molecules through the lock-and-key mechanism. Another is to introduce multiple hydrocarbon chains into macrocyclic skeletons, e.g., octopus cyclophanes, so that the induced-fit binding is exercised. As regards simulation of coenzyme-dependent enzymatic reactions, coenzyme factors must be introduced into cyclophanes covalently or noncovalently. Under cooperation of an effective substrate-activation process that can be performed with vanadium trichloride and molecular oxygen, a real artificial holoenzyme system has been composed with an octopus cyclophane and a hydrophobic vitamin B12 The holoenzyme model system catalyzes various carbon-skeleton rearrangement reactions in a manner as observed for the naturally occurring enzymes.