X-ray structure of the cyclomaltohexaicosaose triiodide inclusion complex provides a model for amylose-iodine at atomic resolution.
Cyclomaltohexaicosaose (CA26) is folded into two 1(2)/(3) turns long V-helices that are oriented antiparallel. Crystals of complexes of CA26 with NH(4)I(3) and Ba(I(3))(2) are brown and X-ray analyses show that I(3)(-) units are located in the approximately 5 A wide central channels of the V-helices. In the complex with NH(4)I(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 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 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.
Nimz
O
O
Institut für Kristallographie, Freie Universität Berlin, Takustrasse 6, D-14195 Berlin, Germany.
Gessler
K
K
Usón
I
I
Laettig
S
S
Welfle
H
H
Sheldrick
G M
GM
Saenger
W
W
D000688:Amylose / Q000737:chemistry*; D002236:Carbohydrate Conformation; D002240:Carbohydrate Sequence; D018360:Crystallography, X-Ray; D003505:Cyclodextrins / Q000737:chemistry*; D017613:Iodine Compounds / Q000737:chemistry*; D008958:Models, Molecular; D008969:Molecular Sequence Data; D010100:Oxygen / Q000737:chemistry
D003505:Cyclodextrins; D017613:Iodine Compounds; C480090:cyclomaltohexaicosaose; D000688:Amylose; D010100:Oxygen