New evidence for conformational flexibility in cyclodextrins from vibrational Raman optical activity

Conformational flexibility in cyclodextrins (CDs) as a function of methylation, solvent interaction and the extent of inclusion complex formation has been studied by using vibrational Raman optical activity (ROA). The work exploited the sensitivity of ROA to skeletal mobility by comparing the intensity of the glycosidic ROA couplet between about 850 and 970 cm(-1) in maltoheptose (MH), beta-cyclodextrin (beta-CD), heptakis(2,6-di-O-methyl)-beta-cyclodextrin (DM-beta-CD) and heptakis(2,3,6-tri-O-methyl)-beta-cyclodextrin (TM-beta-CD) in buffered aqueous solution, in DMSO and with sodium benzoate and benzoic acid as guests in buffered aqueous solution. Increases in couplet signal strength were interpreted in terms of a reduction in conformational flexibility of the CD ring. In buffered aqueous solution the ROA intensity order MH < TM-beta-CD < beta-CD < DM-beta-CD was observed. The linear molecule MH is expected to be the most flexible of the four oligosaccharides studied, while the changes registered for the three CD macrocycles may be related to the degree of intramolecular hydrogen bond formation and its influence on conformational flexibility. In DMSO, the same ROA intensity order is observed, but with an approximately constant increase relative to the values obtained in aqueous solution, This call be explained by the tighter binding of DMSO in the CD cavities compared with H2O. For the inclusion complexes, our results indicate that the tighter the guest is bound, the larger is the reduction in the conformational flexibility of the CD macrocycle. The residual mobility sensed by ROA in CDs is similar to that sensed in proteins; this provides further insight into their analogous ligand-binding and catalytic properties.