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Molecular Modeling of Saccharides, Part XIII.

Per-O-methylated α- and β-CD: Cyclodextrins with Inverse Hydrophobicity

Stefan Immel and Frieder W. Lichtenthaler

Starch/Stärke 1996, 48, 225-232.

The investigation focuses on the computer-aided generation of the molecular geometries, contact surfaces, and lipophilicity patterns of per-O-methylated α-CD (1) and its β-CD homolog 2, and compares them with their parent non-substituted cyclodextrins. The molecular geometries, compared via statistical analysis of crystal structure data available, reveal 1 and 2 to be considerably more flexible than α- and β-CD, allowing wide variations in the tilting of the glucose units relative to the macrocyclic ring axes. The comparative evaluation of their contact surfaces not only discloses a substantial increase of the torus heights upon per-O-methylation (from 8.0 Å in α- and β-CD, to 11.1 Å in 1 and 2), but also an enlargement of their cavity areas by 40 % (+ 35 Å2 for α-CD 1) and 70 % (+ 75 Å2 for β-CD 2), respectively. The hydrophobic characteristics of 1 and 2, emerging from the molecular lipophilicity patterns (MLPs) generated and projected onto the contact surfaces in color-coded form, are inverse to those for α and β-CD: the most hydrophobic surface regions of 1 and 2 are located at the torus rims made up by the 2-OMe and 3-OMe groups at one side, and the 6-CH2OMe moieties at the other, with a hydrophobic "band" wrapping around the outside of the macrocycles; these "exo-lipophilic" topographies are opposed by pronouncedly hydrophilic central cavities. A variety of experimental findings can be rationalized on the basis of the opposite lipophilicity profiles of the CDs and their permethylated analogs, such as for example the opposite orientation of benzaldehyde, p-nitrophenol, and 3-iodopropionic acid in the cavities of α-CD and of 1. Thus, the notion is substantiated that the operation of dispersive interactions between guest and CD-host cavities play a more dominant role in inclusion complex formation than hitherto appreciated.

Additional Graphics: Cyclodextrins

© Copyright PD Dr. S. Immel