PD Dr. Stefan Immel - Alpha-Cycloaltrin

TUD Organische Chemie

Immel

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Abstract 11

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α-Cycloaltrin: Molecular Shape and Hydration Properties

Stefan Immel, Guido Schmitt, and Frieder W. Lichtenthaler

XIX^th International Carbohydrate Symposium, San Diego, California, August 9-14, 1998, Abstract AP124.

α-Cycloaltrin (1)^[1] adopts in the solid-state^[2] a unique C3 symmetrical conformation with nearly perfect ⁴C₁ and ¹C₄ chairs in an alternating sequence (1a). ¹H and ¹³C NMR data in D₂O at 30°C and 4°C (cf. Fig. 1) indicate a dynamic conformational equilibrium of at least two different altrose geometries with almost equal energies in the macrocycle.

Fig. 1: ¹³C NMR (75 MHz) spectra of (α-CA (1) in D2O at 30°C (left) and 4°C (right) indicate a pronounced low-temperature broadening of the C-4 and C-5 signals, and hence, a dynamic equilibrium between at least two different altrose conformations.

Unlike the conformation 1a (Fig. 2) which features side-on indentations only, the intermediate all-^OS₂ structure 1b in the ⁴C₁ ^OS₂ ¹C₄ equilibrium features a "through-going" central cavity (Fig. 2). We have investigated the dynamic processes in 1 by means of molecular dynamics (MD) simulations in aqueous solution, focusing in particular on the hydration properties and water exchange phenomena of 1a and 1b, defining the first hydration shells of both geometries (Fig. 2).

1a
1b

Fig. 2: Superimposed surface cross section cuts (center) of the alternating ⁴C₁ / ¹C₄ (1a) and all-^OS₂ conformation (1b) of α-cycloaltrin with approximate molecular dimensions in Å. On the right, typical MD derived snap-shot geometries of the first hydration shell of both geometries with 31 and 41 water molecules hydrogen bonded to 1a and 1b are displayed.

[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

[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.