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Download fileMolecular Recognition in Cyclodextrin Complexes of Amino Acid Derivatives: The Effects of Kinetic Energy on the Molecular Recognition of a Pseudopeptide in a Nonconstraining Host Environment as Revealed by a Temperature-Dependent Crystallographic Study
journal contribution
posted on 2006-12-28, 00:00 authored by Joanna L. Clark, Jessica Peinado, John J. Stezowski, Robert L. Vold, Yuanyuan Huang, Gina L. HoatsonThe crystal structure of a triclinic 2:2 inclusion complex of β-cyclodextrin with N-acetyl-l-phenylalanine
methyl ester has been determined at several temperatures between 298 and 20 K to further study molecular
recognition using solid-state supramolecular β-cyclodextrin complexes. The study reveals kinetic energy
dependent changes in guest molecule conformations, orientations, and positions in the binding pocket presented
by the crystal lattice. Accompanying these changes are observable differences in guest−guest interactions
and hydrogen-bonding interactions in the binding pocket that involve guest molecules, water of hydration
molecules, and β-cyclodextrin molecules. On the basis of the differences observed in the crystal structures,
we present a solid-state example of a system that displays the properties of both classical and quantum chemical
models. At higher temperatures, the structure conforms to a classical mechanical model with dynamic disorder.
At lower temperatures, the observations conform to examples in which there is static disorder representative
of models in which quantum states differing in conformation, position, and orientation of components in the
crystal structure are occupied. Ab initio theoretical calculations on the different guest molecule conformations
have been carried out. Superpositions of theoretical electrostatic surface potential diagrams on the observed
molecular positions in the complexes provide confidence that the deconvolution of the guest molecule disorder
is acceptable. Temperature-dependent solid-state magic angle spinning deuteron NMR measurements provide
evidence for large-amplitude, diffusive motion on a microsecond time scale in the complex.