Anisotropic Ring Current Effect of p-Nitrophenolate Ion Inclusion on the 1H NMR Signals of the Pyridinio Derivatives of (alpha)-Cyclodextrin

The p-nitrophenolate ion (pNP^-) was strongly bound to the mono- and dipyridinio derivatives of (alpha)-cyclodextrin ((alpha)-CD) in an aqueous 0.1 mol dm^-3 Na2CO3 solution, and shifted the 1H NMR signals due to the (alpha)-CD C5-H’s to different directions, depending on the position of (alpha)-D-glucopyranose (anisotropic ring-current effect). In contrast, the iodide ion and p-hydroxybenzoate ion showed no such effect upon complexation. Furthermore, the C5-H’s of a dipyridinio derivative of (beta)-CD was not subject to such an anisotropic ring-current effect of pNP^-. These results indicate that the molecular rotation of pNP- is significantly retarded within the cavities of the pyridinio derivatives of (alpha)-CD by electrostatic and van der Waals interactions. Thus, electrostatic interactions are available for controlling the molecular motion or orientation of an ionic guest compound in a CD cavity and for the design of an enzyme-mimic modified CD.