Design of polygalacturonate hydrogels using iron(II) as cross-linkers: A promising route to protect bioavailable iron against oxidation

Abstract

We designed stable and highly reproducible hydrogels by external unidirectional diffusion of Fe²⁺ ions into aqueous solutions of polygalacturonate (polyGal) chains. The Fe²⁺ ions act as cross-linkers between the Gal units in such a way that both the molar ratio R ([Fe²⁺]/[Gal units] = 0.25) and the mesh size of the polyGal network at the local scale (ξ = 75 ± 5 Å) have constant values within the whole gel, as respectively determined by titration and Small Angle Neutron Scattering. From macroscopic point of view, there is a progressive decrease of polyGal concentration from the part of the gel formed in the early stages of the gelation process, which is homogeneous, transparent and whose Young modulus has a high value of ∼10⁵ Pa, up to the part of the gel formed in the late stages, which is heterogeneous, highly turbid and has a much lower Young modulus of ∼10³ Pa. Since the local organization of the polyGal chains remains identical all along the hydrogels, this macroscopic concentration gradient originates from the formation of heterogeneities at a mesoscopic length scale during the gelation process. In addition, X-ray Absorption Spectroscopy measurements remarkably reveal that Fe²⁺ ions keep their +II oxidation state in the whole gels once they have cross-linked the Gal units. These polyGal hydrogels thus protect iron against oxidation and could be used for iron fortification.

Keywords: Gelation mechanism; Hydrogels; Iron(II); Mechanical properties; Polygalacturonate.