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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 9th 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<->OS2<->1C4 altropyranose units. Although the 3JH-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 4C1OS21C4 reaction coordinate indicates cooperative conformational transitions 1C4OS2<->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

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