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 ι-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 Å can all be generated from the smallest trigonal lattice having cell edge 13.7 Å. The peripheral sulfate groups on the prism shaped ι-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 ι-carrageenan.