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The crucial role of water in biological functions is well-recognized,
numerous questions concerning the physical mechanisms behind the
importance of water for life remains unanswered. Biofunctions are possible
when the network of hydration water spans the surface of a biomolecule.
Such network may be broken upon dehydration or upon heating and this
process may be described by percolation theory. To clarify the physical
mechanisms of the crucial role of a spanning water network for
biofunctions, various properties should be studied when crossing the
percolation threshold. Dynamic coupling of protein and water dynamics is
maximal at hydration levels, corresponding to the formation of a spanning
network (monolayer) of hydration water. Water molecules strongly bounded
to the biosurface due to Coulombic or H-bond interaction show stretched
exponential rotational dynamics with large relaxation times, whereas
weakly-bound biological water shows faster bulk-like rotational dynamics.
Appearance of weakly-bound biological water upon hydration results in
drastic increase of the dielectric constant and screening of the Coulombic
interactions. This facilitates both conformational dynamic of biomolecules
and dissociation of counterions adjusted to the charged groups of
biomolecules. Break of the spanning network of biological water upon
heating occurs in the temperature range, which is surprisingly close to
the typical temperatures of heat denaturation or unfolding of
biomolecules.
University of Dortmund