Evidence for an egg-box-like structure in iron(ii)-polygalacturonate hydrogels: a combined EXAFS and molecular dynamics simulation study

The local structure of Fe2+ in Fe2+-polygalacturonic acid (polyGalA) hydrogels has been studied by coupling Extended X-ray Absorption Fine Structure (EXAFS) spectroscopy and molecular dynamics (MD) simulation. The EXAFS fitting results reveal an octahedral coordination geometry of Fe2+ both in aqueous solution and in the hydrogel, with similar Fe-O distances (2.09 +/- 0.01 angstrom in the hydrogel and 2.11 +/- 0.01 angstrom in aqueous solution). The MD simulations evidence that standard empirical force fields are unable to accurately reproduce the EXAFS spectra of Fe2+ in both aqueous solution and hydrogel. Based on the EXAFS distance determinations, we then performed restrained MD simulations of hypothetical octahedral coordination modes of Fe2+ with polyGalA chains. The best agreement between experimental and simulated EXAFS spectra was found when Fe2+ is monodentately coordinated to two carboxylate and two hydroxyl oxygens from a pair of polyGalA chains as well as to two water oxygens in an octahedral coordination geometry compatible with the "egg-box model".