Cluster-driven dynamical arrest in concentrated lysozyme solutions.

We present a detailed experimental and numerical study of the structural and dynamical properties of salt-free lysozyme solutions. In particular, by combining small-angle X-ray scattering (SAXS) data with neutron spin echo (NSE) and rheology experiments, we are able to identify that an arrest transition takes place at intermediate densities, driven by the slowing down of the cluster motion. Using an effective pair potential among proteins, based on the combination of short-range attraction and long-range repulsion, we account remarkably well for the peculiar volume fraction dependence of the effective structure factor measured by SAXS. We show that a transition from a monomer to a cluster-dominated fluid happens at volume fractions larger than ϕ ≳ 0.05 where the close agreement between NSE measurements and Brownian dynamics simulations confirms the transient nature of the clusters. Clusters even stay transient above the geometric percolation found in simulation at ϕ > 0.15, though NSE reveals a cluster lifetime that becomes increasingly large and indicates a divergence of the diffusivity at ϕ ≃ 0.26. Macroscopic measurements of the viscosity confirm this transition where the long-lived-nature of the clusters is at the origin of the simultaneous dynamical arrest at all length scales.