Evidence for double helical kappa-carrageenan in aqueous LiI-solution and model for iodide binding.
NMR experiments with a 21412 Redfield observation pulse were carried out on a kappa-carrageenan aqueous solution containing 0.5M LiI. A detected hydrogen-bonded proton resonance at approximately 17.6 ppm, is in accordance with an ordered carrageenan structure in the solution similar to the double, parallel-stranded helix with interstrand hydrogen-bonds found in the condensed state by R.P. Millane, R. Chandrasekaran, S. Arnott, and I.C.M. Dea (Carbohydr. Res. 198, 1 1988). Starting with the published iota-carrageenan coordinates obtained from the Brookhaven Databank, we modelled this kappa-carrageenan structure by the molecular modelling software INSIGHTH-II. The kappa-carrageenan double-helix has a wide interior that can accommodate iodide. Conversely, the iota-carrageenan double-helix has a narrow interior not suited for a large ion like iodide. We propose that the detected selective iodide binding sites in the ordered kappa-carrageenan structure (H. Grasdalen, and O. Smidsrød, Macromolecules 14, 1842 1981) could very well be in the interior of the double-helix.
Nerdal
W
W
Department of Chemistry, University of Bergen, Norway.
Haugen
F
F
Knutsen
S
S
Grasdalen
H
H
D001665:Binding Sites; D002236:Carbohydrate Conformation; D002351:Carrageenan / Q000737:chemistry* / Q000378:metabolism; D003196:Computer Graphics; D006860:Hydrogen Bonding; D007454:Iodides / Q000378:metabolism*; D008094:Lithium; D009682:Magnetic Resonance Spectroscopy; D008958:Models, Molecular; D012984:Software; D014961:X-Ray Diffraction
D007454:Iodides; D002351:Carrageenan; D008094:Lithium