Cation-induced polymorphism in iota-carrageenan

Recent X-ray fiber diffraction patterns from polycrystalline and well-oriented fibers of calcium, rubidium and potassium salt forms of L-carrageenan correspond to large trigonal unit cells. In contrast to the previously reported sodium and calcium forms that contain three half-staggered double helices, these new unit cells accommodate 4, 16 and 27 double helices, respectively. Interestingly, their basal net dimensions ranging from 23.6 to 68.2 angstrom can all be generated from the smallest trigonal lattice having cell edge 13.7 angstrom. The peripheral sulfate groups on the prism shaped l-carrageenan helix, along with cations and water molecules, are mainly responsible for the association of helices. The existence of several cation-dependent stable crystalline states is compatible with the observed versatility in the physical properties of l-carrageenan. (c) 2005 Elsevier Ltd. All rights reserved.