Rigidity versus flexibility. A review of experimental and theoretical studies pertaining to the cyclodextrin nonrigidity

For many years on the basis of X-ray studies cyclodextrins were considered to have a rigid truncated-cone structure although this concept was incompatible with the ease of their complex formation and several experimental findings (mainly obtained by NMR technique). Recent experimental results demonstrate that the complexes, held together by weak nonbonded interactions, must be flexible not only in solution but also in the solid state. The weakness of these interactions and the dynamic character of the complexes indicate that apparently conflicting results can be obtained for them using different experimental methods. (C) 2002 Elsevier Science B.V. All rights reserved.