Slow water dynamics in polygalacturonate hydrogels revealed by NMR relaxometry and molecular dynamics simulation

Diffusions in gels are of prime importance, but their measurements are mainly focused on the diffusion in the pores or through the mesh of the gels. In this study, we performed a deeper dynamic analysis of the water in close interaction with the fibers structuring two heterogeneous polygalacturonate (polyGalA) hydrogels formed by Ca and Zn ions (crosslinking agents). Nuclear magnetic resonance dispersion (NMRD) profiles recorded in-situ by fast-field cycling relaxometry allow to observe the very slow dynamics of water within the gels. Two distinct interpretations of the NMRD profiles are discussed, the first in regard of rotational and translational dynamics in the fibers and the second with respect to a Levy-walk on the fibers' surface. These discussions are confronted with molecular dynamics simulations on a model Ca-polyGalA fiber.