Bridging Dynamic Regimes of Segmental Relaxation and Center-of-Mass Diffusion in Associative Protein Hydrogels

Knowledge of molecular transport in crowded media is of central importance to understanding biophysical processes and designing soft materials with novel properties. Here, neutron spin echo (NSE) and forced Rayleigh scattering (FRS) are used in tandem to study the interplay between segmental and center-of-mass chain dynamics in a model protein hydrogel with strong binding interactions. The results provide evidence for several regimes of gel relaxation behavior with varying length scale, including a caging regime bridging submolecular relaxation and center-of-mass diffusion due to transient binding. On mesoscopic length scales, chains undergo two distinct regimes of apparent superdiffusion before terminal Fickian scaling. The combined NSE and FRS data are interpreted in the context of prior simulations of associating star polymers, revealing insight into structural length scales and binding kinetics governing the transition from segmental relaxation to self-diffusion in the protein gel. Finally, single-sticker tracer diffusion was measured to directly probe sticker association kinetics within the gel, the results suggesting that cooperative cluster motion may play a role in gel relaxation on larger length scales.