Three-dimensional structure of the sodium salt of iota-carrageenan.

The three-dimensional structure of the sodium salt of iota-carrageenan has been determined by using X-ray diffraction data collected from its polycrystalline and oriented fibers. The molecule forms a half-staggered, parallel, threefold, right-handed double helix that is stabilized by interchain hydrogen bonds from 2- and 6-hydroxyl groups in the galactosyl units. Three helices are organized in a trigonal unit cell, of dimensions a=24.02 and c=12.93 A, with a lateral separation of 13.9 A for each pair. Both 2- and 4-sulfate groups are essential in helix-helix interactions that are mediated only by sodium ions and water molecules.