Water
dynamics is essential to biochemical processes by mediating
all such reactions, including biomolecular degeneration in solutions.
To disentangle the molecular-scale distribution of water dynamics
around a solute biomolecule, we investigated here the rotational dynamics
of water around lysozyme by combining molecular dynamics (MD) simulations
and broadband dielectric spectroscopy (BDS). A statistical analysis
using the relaxation times and trajectories of every single water
molecule was proposed, and the two-dimensional probability distribution
of water at a distance from the lysozyme surface with a rotational
relaxation time was given. For the observed lysozyme solutions of
34–284 mg/mL, we discovered that the dielectric relaxation
time obtained from this distribution agrees well with the measured
γ relaxation time, which suggests that rotational self-correlation
of water molecules underlies the gigahertz domain of the dielectric
spectra. Regardless of protein concentration, water rotational relaxation
time versus the distance from the lysozyme surface revealed that the
water rotation is severely retarded within 3 Å from the lysozyme
surface and is nearly comparable to pure water when farther than 10
Å. The dimension of the first hydration layer was subsequently
identified in terms of the relationship between the acceleration of
water rotation and the distance from the protein surface.