Crystal structure of octakis(2,3,6-tri-O-methyl)-gamma-cyclodextrin x 4.5 H2O: evidence for conformational flexibility of permethylated cyclodextrins.

Octakis(2,3,6-tri-O-methyl)-gamma-CD (TRIMEG) cocrystallized at 18 degrees C with 4.5 water molecules in the orthorhombic space group P2(1)2(1)2(1), unit cell dimensions a = 10.7879(3), b = 29.0580(9), c = 32.2909(11) A. The TRIMEG macrocycle is in a 'round' form with all glucose units oriented syn, and one O-6-CH3 methoxy group points 'toward' the molecular cavity. The TRIMEG x 4.5 H2O molecules are stacked to form infinite cylinders with the central cavities aligned into channels filled for each TRIMEG by 4.5 water molecules distributed over 15 partially occupied sites. This structure differs from the two known structures of TRIMEG in which two diametrically opposed glucoses are oriented anti to yield an 'elliptical' form, and their O-6-CH3 groups are directed 'toward' the cavity and close it at this side to form a bowl-shaped molecule.