Spectrophotometric and Calorimetric Titration Studies on Molecular Recognition of Camphor and Borneol by Nucleobase-Modified β-Cyclodextrins

A series of modified β-cyclodextrins with nucleobase substituents, that is, mono(6-ade-6-deoxy)-β-cyclodextrin (2) and mono(6-ura-6-deoxy)-β-cyclodextrin (3) as well as mono(6-thy-6-deoxy)-β-cyclodextrin (4), were selected as molecular receptors to investigate their conformation and inclusion complexation behaviors with some chiral molecules, that is, (+)-camphor, (−)-camphor, (+)-borneol, and (−)-borneol, by spectrophotometric and microcalorimetric titrations in aqueous phosphate buffer solution (pH 7.2) at 298.15 K. Circular dichroism and NMR studies demonstrated that these nucleobase-modified β-cyclodextrins adopted a co-inclusion mode upon complexation with guest molecules; that is, the originally self-included nucleobase substituents of the host did not move out from the β-cyclodextrin cavity, but coexisted with guest molecule in the β-cyclodextrin cavity upon inclusion complexation. Significantly, these nucleobase-modified β-cyclodextrins efficiently enhanced the molecular binding ability and the chiral recognition ability of native β-cyclodextrin, displaying enantioselectivity up to 3.7 for (+)-camphor/(−)-camphor pair by 2 and 3.5 for (−)-borneol/(+)-borneol pair by 3. The enhanced molecular/chiral recognition abilities of 2−4 toward (±)-camphor were mainly attributed to the increased entropic gains due to the extensive desolvation effects, while the favorable enthalpic gains originating from the good size-fit relationship as well as the hydrogen bond interactions between host and guest result in the enhanced molecular/chiral recognition abilities of 2−4 toward (±)-borneol.