Crystal structure of octakis(2,3,6-tri-O-methyl)-gamma-cyclodextrin centerdot 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) Angstr om. The TRIMEG macrocycle is in a 'round' form with all glucose units orien ted syn, and one O-6-CH3 methoxy group points 'toward' the molecular cavity . The TRIMEG 4.5 H2O molecules are stack-ed to form infinite cylinders with the central cavities aligned into channels filled for each TRIMEG by 4.5 w ater 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 thei r O-6-CH3 groups are directed 'toward' the cavity and close it at this side to form a bowl-shaped molecule. (C) 2000 Elsevier Science I,td. All rights reserved.