  Molecular dynamics simulations have revealed a dramatic increase, with
increasing temperature, of the amplitude of electrostatic fluctuations caused
by water at the active site of metalloprotein plastocyanin. The increased
breadth of electrostatic fluctuations, expressed in terms of the reorganization
energy of changing the redox state of the protein, is related to the formation
of the hydrophobic protein/water interface allowing large-amplitude collective
fluctuations of the water density in the protein's first solvation shell. On
the top of the monotonic increase of the reorganization energy with increasing
temperature, we have observed a spike at 220 K also accompanied by a
significant slowing of the exponential collective Stokes shift dynamics. In
contrast to the local density fluctuations of the hydration-shell waters, these
spikes might be related to the global property of the water solvent crossing
the Widom line.
