V-Amylose at atomic resolution: X-ray structure of a cycloamylose with 26 glucose residues (cyclomaltohexaicosaose)
Gessler, K and Uson, I and Takaha, T and Krauss, N and Smith, SM and Okada, S and Sheldrick, GM and Saenger, W, V-Amylose at atomic resolution: X-ray structure of a cycloamylose with 26 glucose residues (cyclomaltohexaicosaose), Proceedings of the National Academy of Sciences of The United States of America, 96, (8) pp. 4246-4251. ISSN 0027-8424 (1999) [Refereed Article]
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
syn orientation, forming systematic interglucose O(3)n⋅⋅⋅O(2)n+l 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+l, O(6)n+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.