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α-Cycloaltrin: Conformation and Properties in Aqueous Solution

Stefan Immel, Guido Schmitt, and Frieder W. Lichtenthaler

9thInternational Symposium on Cyclodextrins, Santiago de Compostela, Spain, May 31-June 3, 1998, Abstract 2-O-5.

α-Cycloaltrin, readily available from -cyclodextrin via a straightforward four-step protocol[1], adopts in the solid-state[2] a C3 symmetrical conformation with nearly perfect 4C1 and 1C4 chairs in an alternating sequence, but lacks a "through-going" cavity.
CA6 4C1/1C4 conformation
CA6 all-OS2 conformation
altrose conformational equilibrium

1H and 13C NMR experiments of α-cycloaltrin show a complex conformational equilibrium 4C1<-> OS2<-> 1C4 for each of the altrose units.

CA6 snapshot geometries
CA6 snapshot geometries
CA6 MD simulation box
MD simulation box incl. 606 water molecules
4C1 / 1C4-conformation

Molecular dynamics (MD) simulations and temperature-dependent 800 MHz 1H and 13C NMR spectra (D2O at 30 and 4°C) point toward a fast dynamic equilibrium with preferred altrose conformations in the 1C4<-> OS2 range. Low-temperature broadening of the C-4 and C-5 NMR signals indicate multiple conformations of α-CA in solution which are likely to ellaborate central cavities similar to those formed by cyclodextrins.

[1] Y. Nogami, K. Fujita, K. Ohta, K. Nasu, H. Shimada, C. Shinohara, and T. Koga, J. Inclusion Phenom. Mol. Recognit. Chem. 1996, 25, 57-60.
[2] Y. Nogami, K. Nasu, T. Koga, K. Ohta, K. Fujita, S. Immel, H. J. Lindner, G. E. Schmitt, and F. W. Lichtenthaler, Angew. Chem. 1997, 109, 1987-1991; Angew. Chem. Int. Ed. Engl. 1997, 35, 1899-1902.

Additional Graphics: Cycloaltrins

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