Protein/Hydration Water Dynamics in Hard Confinement: Dielectric Relaxations and Picoseconds Hydrogen Fluctuations

In this review we report on some experimental studies on the dynamics of Myoglobin in a confined geometry, obtained by encapsulation in a porous silica matrix, at low hydration levels. After formation through the solgel method, the samples were left aging/drying in order to reach a condition where only one or two water layers surround the proteins. In order to put in evidence the specific effect of confinement in the silica host, we compared this system with another one (i.e. hydrated powder) where proteins are confined by other proteins. Using elastic neutron scattering we investigate the temperature dependence of the mean square displacements of non-exchangeable hydrogen atoms of sol-gel encapsulated Myoglobin. In order to clarify the effect of hydration the study was extended to samples at 0.2, 0.3 and 0.5 [gr water]/[gr protein] fractions and comparison was made with Myoglobin powders at the same average hydration and with a dry powder sample. Comparison between the data relative to the different samples indicates that geometrical confinement within the matrix plays a crucial role in protein dynamics and conformational stability, the effect of sol-gel encapsulation being essentially a reduction of collective protein motions likely related to the slowing down of solvent confined diffusion. A dielectric spectroscopy investigation on the same systems helped us to clarify the effect of encapsulation on protein/solvent dynamics. In agreement with elastic neutron scattering, although in a much slower time scale, dielectric spectroscopy indicates a suppression of cooperative relaxation inside the gel, together with a clear dependence of relaxation rates on the hydration degree.