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 C-13 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, ail glucose residues are in syn orientation, forming systematic interglucose O(3)(n)... O(2)(n+1) and O(6)(n)... O(2)(n+6)/O(3)(n+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 anti rather than in the common syn orientation, this conformation being stabilized by interglucose three-center hydrogen bonds with O(3)(n) as donor and O(5)(n+1), O(6)(n+1) as accepters, The structure of CA26 permitted construction of an idealized V-amylose helix, and the band-flip moth explains why V-amylose crystallizes readily and may be packed tightly in seeds.