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

摘要

Cyclomaltohexaicosaose (CA26) is folded into two 1(2/3) turns long V-helices that are oriented antiparalle. Crystals of complexes of CA26 with NH_4I_3 and Ba(I_3)_2 are brown and X-ray analyses show that I_3~- units are located in the approx 5 A wide central channels of the V-helices. In the complex with NH_4I_3, two CA26 molecules are stacked to form 2 X 1(2/3) turns long channels harbouring 3 I_3~- at 3.66-3.85 A inter I_3~- distance (shorter than van der Waals distance, 4.3 A), whereas in the Ba(I_3)_2 complex, CA26 are not stacked and only one I_3~- each fills the V-helices. Glucose…I constants are found 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 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 I_3~- with blue amylose-iodine.