Spectrophotometric and calorimetric titration studies on molecular recognition of camphor and borneol by nucleobase-modified beta-cyclodextrins.
A series of modified beta-cyclodextrins with nucleobase substituents, that is, mono(6-ade-6-deoxy)-beta-cyclodextrin (2) and mono(6-ura-6-deoxy)-beta-cyclodextrin (3) as well as mono(6-thy-6-deoxy)-beta-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 beta-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 beta-cyclodextrin cavity, but coexisted with guest molecule in the beta-cyclodextrin cavity upon inclusion complexation. Significantly, these nucleobase-modified beta-cyclodextrins efficiently enhanced the molecular binding ability and the chiral recognition ability of native beta-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.
Liu
Yu
Y
Department of Chemistry, State Key Laboratory of Elemento-Organic Chemistry, Nankai University, Tianjin 300071, P. R. China. yuliu@nankai.edu.cn
Zhang
Qian
Q
Chen
Yong
Y
D000225:Adenine / Q000737:chemistry*; D002151:Calorimetry; D001892:Camphanes / Q000737:chemistry*; D002164:Camphor / Q000737:chemistry*; D002942:Circular Dichroism; D009682:Magnetic Resonance Spectroscopy; D008958:Models, Molecular; D011485:Protein Binding; D013237:Stereoisomerism; D013816:Thermodynamics; D013941:Thymine / Q000737:chemistry*; D047392:beta-Cyclodextrins / Q000138:chemical synthesis / Q000737:chemistry*
D001892:Camphanes; D047392:beta-Cyclodextrins; D002164:Camphor; D000225:Adenine; C022871:isoborneol; D013941:Thymine