Evidence for Egg-Box-Compatible Interactions in Calcium−Alginate Gels from Fiber X-ray Diffraction

Abstract

The structures of guluronic-acid-rich alginate in the acid and calcium forms were investigated using fiber X-ray diffraction. Data recorded for alginate fibers in the acid form show a repeat along the chain axis of c = 0.87 nm, a value that is in agreement with the one measured by Atkins et al. (Biopolymers 1973, 12, 1865) and contradicts a repeat of 0.78 nm recently suggested by Li et al. (Biomacromolecules 2007, 8, 464). In the Ca²⁺ form, our observations indicate that the junction zone involves dimerization of polymer chains through Ca²⁺ coordination according to the egg-box model. For reasons that are not understood at present, coordination of the divalent cations reduces the ability for the lateral crystallographic packing of the dimers. A proposed model for the junction zone involves polymer chains packed on a hexagonal lattice with a lattice constant a = 0.66 nm. Random pairs of chains form dimers through coordination of Ca²⁺ cations. Further lateral interaction between dimers is mediated by disordered Na⁺ and Ca²⁺ cations, water molecules, and hydrogen bonding.