Global and Internal Diffusive Dynamics of Proteins in Solution Studied by Neutron Spectroscopy

Abstract

Proteins are macromolecules naturally occurring in living cells and organisms, involved in a great number of processes essential for life, but they can be interesting not only from a biomedical perspective, but also for colloid physics, chemical engineering and nanotechnology, especially in the prospect of the smart production of self-assembling structures. In this thesis, the results of experiments carried out at the Institut Laue-Langevin, Grenoble, France, and at the Spallation Neutron Source at the Oak Ridge National Laboratory, Tennessee, USA, are presented. The picosecond to nanosecond (short-time) self-diffusion and internal dynamics of two model proteins in aqueous (D2O) solution is studied by neutron backscattering as a function of protein concentration, temperature and multivalent salt concentration. First, the concentration dependence of the translational diffusion of the antibody γ-globulin is rationalized in the context of colloid physics, while the protein internal dynamics is observed to slow down with increasing protein volume fraction. Second, temperature effects are studied on both the diffusion and the internal dynamics of the globular protein bovine serum albumin (BSA), below and above the denaturation temperature. A novel model is proposed to describe the dynamics of the protein side-chains, yielding a rather complete and consistent physical picture of the pico- to nanosecond dynamical changes occurring upon protein denaturation. Third, the change of the diffusion of BSA as a function of the concentration of the trivalent salt YCl3 is investigated, and a remarkably universal slowing down of the apparent diffusion coefficient of BSA molecules as a function of the number of cations per protein cs/cp in solution is found. The result is interpreted in terms of the theory of colloidal suspensions of patchy particles as a result of the semi-quantitative binding of Y3+ ions to specific sites on the protein surface leading to the formation of protein clusters with a cluster size distribution easily tunable by cs/cp.