LAUSR

Abstract The dynamics of the hydration shell of the inhibitor barstar is analysed at low temperature (300–243 K), through all-atom molecular dynamics simulations, and compared with that of bulk water. The relaxation of residence times of solvent molecules in the protein hydration shell follows a stretched exponential function exp[−(t/τ)β] with β = 0.48 ± 0.01, independent of temperature, showing that the decay process is mainly dominated by long-range molecular relaxation channels (short-range for bulk water). The percentage of water molecules exhibiting 4 hydrogen bonds, xHB4, is found to be a parameter essential for understanding some room and low temperature dependent properties of the protein hydration shell, suggesting an explanation for the unfreezing of protein hydration water as temperature decrease below 273 K. Moreover the dynamical transition that proteins and their hydration water exhibit at ~225 K can be explained by the decrease of ‘hydrogen bond defects’ in the protein hydration shell as temperature goes down. If most of those water molecules would present a tetrahedral arrangement (nearly no ‘hydrogen bond defects’), the bioactivity of proteins would be negligible. Comparison with experimental results is provided all along the work. Experimental data are quantitatively reproduced.