Dynamical properties of myoglobin in an ultraviscous water-glycerol solvent investigated with elastic neutron scattering and FTIR spectroscopy

Proteins have distinctive dynamical properties, characterized by the fluctuations of protein molecules among the different minima of their energy landscape. These fluctuations, progressively activated for temperature values larger than ~180 K, lead to a steep increase in the temperature dependence of all measurable dynamical properties. This phenomenon is known as Protein Dynamical Transition and, in spite of the intense studies due to its importance in protein function and to the relation with the fascinating fundamental thermodynamics of complex systems, many aspects of it are not yet clearly understood. Among these, the relationship with the properties of the external solvent and the molecular details of the involved protein motions still need further investigations. We report here a comparative study of the Dynamical Transition in a Protein-Glycerol-Water system, from two different points of view: i) Elastic Neutron Scattering (ENS), which gives the Mean Square Displacements of the hydrogen atoms of the protein and is particularly sensitive to side chain motions; ii) Fourier Transform Infrared Spectroscopy (FTIR) in the Amide regions, which is sensitive mainly to the properties of the backbone atoms of the protein. The obtained results show an almost superimposable thermal behavior of protein backbone (FTIR data) and side chains (ENS data). Thus, in our experimental conditions, the Protein Dynamical Transition emerges as a unique thermodynamic process related to the properties of the external Glycerol/Water medium and implying a general softening of the whole protein molecule (backbone and side chains), which is a prerequisite for protein function.