Band-flip and kink as novel structural motifs in α-(1→4)-d-glucose oligosaccharides. Crystal structures of cyclodeca- and cyclotetradecaamylose

Cycloamyloses (CAs) with 10 and 14 glucose units (cyclodeca- and cyclotetradecaamylose, or ι-cyclodextrin and ε-cyclodextrin, respectively) crystallize as 23.5 and 29.7 hydrates, respectively. Both crystals belong to the monoclinic space group C2, with half molecules in the asymmetric unit, the other halves being related to the first by crystallographic twofold axes. NMR and X-ray analyses show that in CA10 and CA14, two diametrically opposed glucoses are flipped ∼180°, leading to elliptical structures with narrow, slit-like cavities. In both molecules, the flipped glucoses are stabilized in orientation by intramolecular three-centered hydrogen bonds with O-3(n)–H as donor, namely O-3(n)–H⋯O-6(n+1)B and O-3(n)–H⋯O-5(n+1). The other glucoses are stabilized by O-2(n)⋯O-3(n−1) hydrogen bonds (as observed in the smaller α-, β- and γ-cyclodextrins) except at two kinks where these distances are long (3.98 and 3.39 Å, respectively). The geometry at the flip site, unusual for α-d-glucoses oligosaccharides, is similar to the one observed in the crystal structure of β-d-cellotetraose (structural model for cellulose II). In both molecules, conformations are unstrained with glucoses in 4C1 form. Due to their conformational pecularities, CA10 and CA14 crystal structures combine in their crystal lattice the two common packing motifs characteristic of the smaller cyclodextrins, namely parallel channels with intermolecular contacts in herringbone arrangement. Since 13C NMR chemical shifts observed with CAs and amylose in solution and solid state are similar, we anticipate the presence of band-flips and kinks in the polymer. As demonstrated by computer simulations, the band-flip’s novel structural motif occurs in (cyclo)amylose to relieve steric strain.