PD Dr. Stefan Immel - Flexible Non-glucose Cyclooligosaccharides

TUD Organische Chemie

Immel

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Flexible Non-glucose Cyclooligosaccharides

S. Immel

Proceedings of the 10^th Internat. Symp. on Cyclodextrins (Ed.: J. Szejtli), Mia Digital Publ., Ann Arbor, Michigan, 2000, pp. 24-31.

In contrast to the static "lock-and-key" type complexation behavior of the cyclodextrins, cyclooligosaccharides in which one or all glucose units are replaced against α-D-altrose residues are thoroughly flexible, and the pyranose units can adopt various conformations along the ⁴C₁ ^OS₂ / ^3,OB ¹C₄ pathway (Fig. 1). Thus, α-cycloaltrin (α-CA, 1)^[1] and mono-altro-β-CD (2)^[2] are the first prominent examples of cyclooligosaccharide hosts able to realistically mimic an "induced-fit" type mechanism of inclusion complex formation.

Conformational equilibrium of alpha-D-altrose Fig. 1. Calculated free energy profile ("potential of mean force", CHARMM) of α-D-altrose in aqueous solution as a function of the pyranose ring conformation; energies are given in kcal/mol. Both the ¹C₄ (left side of the plot) and ⁴C₁ (right side) ring geometries have almost equal energies, whilst the intermediate ^OS₂ / ^3,OB forms are slightly less stable.

Conformational equilibrium of alpha-cycloaltrin
Inclusion complex of mono-altro-beta-cyclodextrin

Fig. 2. α-CA (1) adopts a multitude of conformations along the ⁴C₁ ^OS₂ / ^3,OB ¹C₄ pathway. Fig. 3. Induced-fit type complexation of mono-altro-β-cyclodextrin (2) with adamantane-1-carboxylate.

References

S. Immel, K. Fujita, and F. W. Lichtenthaler, Chem. Eur. J. 1999, 5, 3185-3192.
K. Fujita, W.-H. Chen, D.-Q. Yuan, Y. Nogami, T. Koga, T. Fujioka, K. Mihashi, S. Immel, and F. W. Lichtenthaler, Tetrahedron: Asymmetry 1999, 10, 1689-1696.

		Fig. 1. Calculated free energy profile ("potential of mean force", CHARMM) of α-D-altrose in aqueous solution as a function of the pyranose ring conformation; energies are given in kcal/mol. Both the ¹C₄ (left side of the plot) and ⁴C₁ (right side) ring geometries have almost equal energies, whilst the intermediate ^OS₂ / ^3,OB forms are slightly less stable.


Fig. 2. α-CA (1) adopts a multitude of conformations along the ⁴C₁ ^OS₂ / ^3,OB ¹C₄ pathway.		Fig. 3. Induced-fit type complexation of mono-altro-β-cyclodextrin (2) with adamantane-1-carboxylate.