Selective recognition of alkanoates by a β-cyclodextrin flexibly capped with a chromophore

The association between 6-deoxy-6-(p-hydroxy-m-nitrophenacylthio)-β-cyclodextrin 1 and sodium n-alkanoate, sodium 2,2-dimethylpropionate, or sodium 3,3-dimethylbutyrate was investigated. Host-guest association was measured by means of electronic spectroscopy and circular dichroism spectroscopy and ascertained to be 1:1. The inclusion of 2,2-dimethylpropionate or butanoate marginally affected the circular dichroism spectrum of 1. The inclusion of 3,3-dimethylbutyrate, hexanoate, octanoate, decanoate, or dodecanoate, however, dramatically affected the spectrum of 1. The plots of molecular ellipticities of the inclusion complexes against the carbon number of the n-alkanoates were sigmoid. From these observations, the effective size of the guest molecule to push the chromophore in the cavity to the capping position was estimated. From the similarity of the spectrum of the octanoate-1 complex to that of 1-adamantanecarboxylate-1 complex, the carbon chain of octanoate appears to be folded in the cavity of the cyclodextrin.