|TUD Organische Chemie||Immel||Publications||Papers||Abstract 08||View or Print (this frame only)|
Stefan Immel, Jürgen Brickmann, and Frieder W .Lichtenthaler
Liebigs Ann. Chem. 1995, 929-942.
A detailed force-field-based evaluation of the molecular geometries of small-ring cyclodextrins 1 - 3 with three, four, and five α(1→4)-linked glucose residues and the starch-derived α-cyclodextrin (4) was performed by using molecular mechanics and high-temperature annealing. The resulting minimum-energy structures reveal that the progressive strain imposed by diminution of the cyclodextrin macrocycle from six units (α-CD, 4) to five (cycloglucopentaoside 3), four (tetraoside 2), and three (cyclotrioside 1) is reflected in a widening of the intersaccharidic torsion angle, in a complex balance of the interrelated glucose tilt angles t and the glycosidic torsions F and Y against each other, and most strikingly, in a specific unilateral distortion of the pyranoid rings, i.e. flattening at C-4 towards the E1 envelope conformation. This successive leveling at C-4, enabled by a decrease of the two related ring torsion angles, is small in the pentamer 3, pronounced in 2, and fully realized in the cyclotrioside 1. - The respective contact surfaces of the minimum-energy conformers and their molecular lipophilicity patterns (MLP), in color-coded form, were also generated, allowing an assessment of their capabilities to form inclusion complexes. Accordingly, only the cyclopentaoside 3 exhibits a hydrophobic central cavity similar to that of α-CD (4), the smaller cyclodextrins 2 and 1 are closed, yet contain a hydrophobic indentation for potential binding.
Additional Graphics: Cyclodextrins