J Phys Chem B. 2022 Jun 8. doi: 10.1021/acs.jpcb.2c00970. Online ahead of print.
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.