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.