Binding of Fe²⁺ to Polygalacturonate: Structure of Dimers and Coordination Geometry

We performed enhanced sampling molecular dynamics simulations to check to what extent the egg-box model (EBM) describes the binding of Fe²⁺ to α-D-(1→4) polygalacturonic acid (polyGalA). In agreement with the EBM, two octameric polyGalA chains in the presence of four Fe²⁺ cations (R_total = [Fe²⁺]/[GalA] = 0.25) formed dimers mostly composed of 4 GalA-Fe²⁺-GalA cross-links (R_bound ≈ 0.25), and Fe²⁺ cations were preferentially bound to polyGalA chains in a monodentate coordination mode, owing to their tight interaction with their hydration water. Yet, at variance with the EBM, dimers with irregular cross-link patterns and/or with a missing cross-link were also sampled to significant amounts, and the hydroxyl oxygen atoms from GalA units did not participate significantly to the first coordination shell of Fe²⁺. Therefore, this study suggests that the EBM does not capture the details of the local structures of Fe²⁺-cross-linked polyGalA dimers and of the first coordination shell of bound Fe²⁺ cations, as previously observed for Ca²⁺.