Thermal stability and internal dynamics of lysozyme as affected by hydration

A differential scanning micro-calorimetric (DSC) investigation has been performed to study the thermal stability of lysozyme solvated in glycerol as a function of the water content h (grams of water/grams of lysozyme). The unfolding process is strongly dependent on h, as it is witnessed by the behavior of the melting temperature Tm which shows a significant drop in the hydration range from 0 h to 0.4/0.5 h. The specific heat difference ΔCp between denatured and native state also displays an important variation for water amounts lower than ca. 0.5 h. For higher water amounts, both Tm and ΔCp seem to attain constant values. Such hydration dependent behavior is reminiscent of the trend exhibited by some quantities characterizing the protein internal dynamics on the pico- and nano-second timescale, as estimated by elastic neutron scattering. Also the protein mobility, which is measured through mean square displacements (MSD), increases mainly for water amounts lower than ca. 0.5 h, in the same range where the protein molecular rigidity rapidly drops down. This behaviour emphasizes the crucial role played by hydration water in determining both the protein thermodynamic and dynamic quantities.