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

Cyclomaltohexaicosaose (CA26) is folded into two <f>12/3</f> 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 <f>I3−</f> units are located in the ∼5 A˚ wide central channels of the V-helices. In the complex with NH4I3, two CA26 molecules are stacked to form <f>2×12/3</f> turns long channels harbouring 3 <f>I3−</f> at 3.66–3.85 A˚ inter <f>I3−</f> distance (shorter than van der Waals distance, 4.3 A˚), whereas in the Ba(I3)2 complex, CA26 are not stacked and only one <f>I3−</f> each fills the V-helices. Glucose⋯I contacts are formed with C5&z.sbnd;H, C3&z.sbnd;H, C6&z.sbnd;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 A˚ and exceed the van der Waals I⋯O sum of radii by about 0.5 A˚ except for one O2⋯I distance of 3.68 A˚ near the end of one V-helix. Raman spectra indicate that the complexes share the presence of <f>I3−</f> with blue amylose–iodine. [Copyright &y& Elsevier]