Unusual Dynamics of Concentration Fluctuations in Solutions of Weakly Attractive Globular Proteins

The globular protein gamma(B)-crystallin exhibits a complex phase behavior, where liquid liquid phase separation characterized by a critical volume fraction phi(c) = 0.154 and a critical temperature T-c = 291.8 K coexists with dynamical arrest on all length scales at volume fractions around phi approximate to 0.3-0.35, and an arrest line that extends well into the unstable region below the spinodal. However, although the static properties such as the osmotic compressibility and the static correlation length are in quantitative agreement with predictions for binary liquid mixtures, this is not the case for the dynamics of concentration fluctuations described by the dynamic structure factor S(q,t). Using a combination of dynamic light scattering and neutron spin echo measurements, we demonstrate that the competition between critical slowing down and dynamical arrest results in a much more complex wave vector dependence of S(q,t) than previously anticipated.