Heterogeneity in iota-carrageenan molecular structure: insights for polymorph II→III transition in the presence of calcium ions

Iota-carrageenan is used in pharmaceutical and food applications due to its ability to complex with other hydrocolloids and proteins. Six distinct cation dependent allomorphs, consistent with its versatile functionality, have so far been observed in the solid state. In this contribution, X-ray structural details of calcium iota-carrageenan (form III) are reported. The polysaccharide retains the half-staggered, parallel, 3-fold, right-handed double helix stabilized by interchain hydrogen bonds from O–2H and O–6H in the Galp units. Results show that there are four helices, rather than one in I or three in II, organized in a larger pseudotrigonal unit cell of dimensions a = 27.44, c = 13.01 Å, and γ = 120°. The four helices have similar core structures, but their sulfate group orientations are quite different. Fifteen calcium ions and 64 water molecules hold the helices together and promote helix–helix interactions. The results portray how the helices would shuffle around in an orchestrated manner to yield calcium iota-carrageenan III from II.

The structural details underscore unexpected flexibility in double helix polymer morphology of polymorph III, and packing arrangement offers an elegant transition mechanism from II.