Strain-induced 'band flips' in cyclodecaamylose and higher homologues

In the well–known cyclodextrins, all glucose units are cis-oriented and connected by O3···O2´ hydrogen bonds forming cyclically closed bands. In larger cyclodextrins (cycloamyloses) with 10 and more glucoses, strain induced in the macrocycle is relieved by two 180º flips to trans orientation between diametrically opposed glucose units. These two “band flips“ are stabilized by hydrogen bonds (O3···O5´ and O3···O6´), and lead to butterflylike structures. In the figures, cyclodextrins with 7 and 14 glucoses are shown; atoms O2, O3 are red, O6 blue, C6 grey, arrows mark chain direction, and dashed lines indicate hydrogen bonds.