X-ray structure of the cyclomaltohexaicosaose triiodide inclusion complex provides a model for amylose–iodine at atomic resolution

Cyclomaltohexaicosaose (CA26) is folded into two 12/3 turns long V-helices that are oriented antiparallel. Crystals of complexes of CA26 with NH4I3 and Ba(I3)2 are brown and X-ray analyses show that I3 − units are located in the ∼5 Å wide central channels of the V-helices. In the complex with NH4I3, two CA26 molecules are stacked to form 2×12/3 turns long channels harbouring 3 I3 − at 3.66–3.85 Å inter I3 − distance (shorter than van der Waals distance, 4.3 Å), whereas in the Ba(I3)2 complex, CA26 are not stacked and only one I3 − each fills the V-helices. Glucose⋯I contacts are formed with C5H, C3H, C6H and (at the ends of the V-helices) with O6 in (+) gauche orientation. By contrast, O2, O3, O4 and O6 in the preferred (−) gauche orientation do not interact with I because these distances are ≥4.01 Å and exceed the van der Waals I⋯O sum of radii by about 0.5 Å except for one O2⋯I distance of 3.68 Å near the end of one V-helix. Raman spectra indicate that the complexes share the presence of I3 − with blue amylose–iodine.