Dynamics of highly concentrated protein solutions around the denaturing transition

Abstract

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 〈u²〉 is recorded. Below and above the denaturing, we observe that 〈u²〉 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 〈u²〉. The latter is strongly hindered due to entanglement and cross-linking of the chains and causes the pronounced decrease of 〈u²〉. Using information from the full quasi-elastic signal, we separate the diffusion contribution from 〈u²〉 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.