V-Amylose at atomic resolution: X-ray structure of a cycloamylose with 26 glucose residues (cyclomaltohexaicosaose)

The amylose fraction of starch occurs in double-helical A- and B-amyloses and the single-helical V-amylose. The latter contains a channel-like central cavity that is able to include molecules, “iodine’s blue” being the best-known representative. Molecular models of these amylose forms have been deduced by solid state ¹³C cross-polarization/magic angle spinning NMR and by x-ray fiber and electron diffraction combined with computer-aided modeling. They remain uncertain, however, as no structure at atomic resolution is available. We report here the crystal structure of a hydrated cycloamylose containing 26 glucose residues (cyclomaltohexaicosaose, CA26), which has been determined by real/reciprocal space recycling starting from randomly positioned atoms or from an oriented diglucose fragment. This structure provides conclusive evidence for the structure of V-amylose, as the macrocycle of CA26 is folded into two short left-handed V-amylose helices in antiparallel arrangement and related by twofold rotational pseudosymmetry. In the V-helices, all glucose residues are in <i>syn</i> orientation, forming systematic interglucose O(3)_<i>n</i>⋅⋅⋅O(2)_<i>n</i>_+l and O(6)_<i>n</i>⋅⋅⋅O(2)_<i>n</i>₊₆/O(3)_<i>n</i>₊₆ hydrogen bonds; the central cavities of the V-helices are filled by disordered water molecules. The folding of the CA26 macrocycle is characterized by typical “band-flips” in which diametrically opposed glucose residues are in <i>anti</i> rather than in the common <i>syn</i> orientation, this conformation being stabilized by interglucose three-center hydrogen bonds with O(3)_<i>n</i> as donor and O(5)_<em>n</em>_+l, O(6)_<i>n</i>_+l as acceptors. The structure of CA26 permitted construction of an idealized V-amylose helix, and the band-flip motif explains why V-amylose crystallizes readily and may be packed tightly in seeds.