TUD Organische ChemieImmelPublicationsPosterAbstract 11View or Print (this frame only)View or Print (this frame only)

α-Cycloaltrin: Molecular Shape and Hydration Properties

Stefan Immel, Guido Schmitt, and Frieder W. Lichtenthaler

XIXth 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.
CA6 13C NMR (30C)
CA6 13C NMR (4C)

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
CA6 (4C1/1C4) formula
CA6 (4C1/1C4) cross-section cut
CA6 (4C1/1C4) hydration shell
1b
CA6 (all-OS2) formula
CA6 (all-OS2) cross-section cut
CA6 (all-OS2) hydration shell
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

© Copyright PD Dr. S. Immel