TUD Organische Chemie

Immel

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Abstract 21

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Molecular Modeling of Saccharides, Part XVIII.

α-Cycloaltrin: Conformation and Properties in Aqueous Solution

Stefan Immel, Guido E. Schmitt, and Frieder W. Lichtenthaler

Proceedings of the 9^th Internat. Symp. on Cyclodextrins (Ed.: J. J. Torres-Labandeira), Kluwer Acad. Publ., Dordrecht, NL, 1998, 41-48.

Analysis of the conformational properties of α-cycloaltrin (α-CA) in aqueous solution by 800 MHz ¹H, 200 MHz ¹³C NMR, and molecular dynamics (MD) simulations points towards a complex equilibrium of ⁴C₁^OS₂¹C₄ altropyranose units. Although the ³J_H-H coupling constants do not reveal a preference for the alternating ⁴C₁ / ¹C₄ or the all-^OS₂ conformation of α-CA, low-temperature ¹³C NMR line-broadening indicates at least two different conformations of the altrose residues. Both α-CA geometries are stable during 600 ps MD simulations without conformational transitions, but constrained MDs forcing one altropyranose unit to vary along the ⁴C₁→^OS₂→¹C₄ reaction coordinate indicates cooperative conformational transitions ¹C₄→^OS₂^3,OB of neighboring units and statistical scrambling of the pyranose geometries in the macrocycle. In particular, the all-^OS₂ conformation of α-CA features a central cavity capable to form inclusion complexes, whereas alternate forms may have surface indentations only.

Additional Graphics: Cycloaltrins