Dynamics of highly concentrated protein solutions around the denaturing transition†

Using both quasi-elastic and fixed-window neutron spectroscopy, we study the dynamics of highly concentrated aqueous protein solutions of bovine serum albumin around the denaturing transition. For the temperature range 280 K < T < 370 K, the total mean-squared displacement hu2i is recorded. Below and above the denaturing, we observe that hu2i increases monotonically with T, but at the denaturing transition it decreases strongly. This observation can be rationalized and quantitatively modeled as a transition from a liquid protein solution to a gel-like state. Atomic vibrations, molecular subunit diffusion and, most importantly, diffusion of the entire protein determine hu2i. The latter is strongly hindered due to entanglement and cross-linking of the chains and causes the pronounced decrease of hu2i. Using information from the full quasi-elastic signal, we separate the diffusion contribution from hu2i and reveal the transition temperature. For the analysis of this separation, we introduce a general concept, which is applicable to other colloid systems exhibiting both center-of-mass and internal dynamics.