Variation of a Theme: Crystal Structure with Four Octakis(2,3,6-tri-<i>O</i>-methyl)-γ-cyclodextrin Molecules Hydrated Differently by a Total of 19.3 Water^,

Octakis(2,3,6-tri-O-methyl)-γ-cyclodextrin (TRIMEG) crystallized from saturated aqueous solution at 18 °C in the monoclinic space group P21 with 4 TRIMEG and a total of 19.3 water molecules in the asymmetric unit [4(C72H128O40)]·19.3H2O. The structure was solved by ab initio real/reciprocal space recycling procedure and refinement converged at an R-factor of 0.084 for 19 782 data with F2 > 2σ(F2). The structures of the four TRIMEG molecules are similar with O4 atoms nearly coplanar and pseudo-2-fold rotation axes passing through the centers of the macrocycles. The glucoses are oriented syn except for the diametrically opposed glucoses 1 and 5 which are flipped by ∼180° (anti), their C5−C6−O6−C9 chains being rotated toward the center and closing the cavity from one side yielding bowl-shaped molecules; the O6 atoms of these chains are hydrogen bonded to water molecules located on the pseudo-2-fold axes. The 19.3 water molecules are distributed over 27 sites hydrogen bonded in different patterns to the 4 TRIMEG.