Evidence for egg-box-compatible interactions in calcium-alginate gels from fiber X-ray diffraction.
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.
Sikorski
Pawel
P
Department of Physics and Norwegian Biopolymer Laboratory, Department of Biotechnology, Norwegian University of Science and Technology, Trondheim, Norway. pawel.sikorski@phys.ntnu.no
Mo
Frode
F
Skjåk-Braek
Gudmund
G
Stokke
Bjørn T
BT
D000464:Alginates / Q000737:chemistry*; D002118:Calcium / Q000737:chemistry*; D018360:Crystallography, X-Ray / Q000379:methods*; D020723:Glucuronic Acid / Q000737:chemistry; D006603:Hexuronic Acids / Q000737:chemistry
D000464:Alginates; D006603:Hexuronic Acids; D020723:Glucuronic Acid; D002118:Calcium