Solvent Viscosity at the Protein Surface

Biochemical activity of biological macromolecules depends on solvent's viscosity, $\eta $, at their surface. The latter might differ from the bulk solvent viscosity due to preferential hydration. In order to estimate $\eta $ at the protein surface, we studied dielectric relaxation spectra of lysozyme-water-glycerol mixtures. Additional relaxation process that appears in the presence of proteins has been assigned to their rotation. Employing Debye-Stokes-Einstein relationship [$\tau _{R}$ = (4$\pi $R$_{R}^{3}\eta $/KT)],$^{ }$and assuming that hydrodynamics radius of protein, R$_{R}$, does not change, we estimated $\eta $ at the protein surface. Analysis of the obtained results indeed reveals a significant difference between bulk solvent's viscosity and the viscosity experienced by a protein. The water concentration appears to be significantly enhanced at the protein surface, in agreement with earlier thermodynamics study. Using the viscosity data, we estimate solvent composition at the protein surface.$^{ }$We expect that the developed approach will help to unravel the role of the solvent and its viscosity in dynamics, stability and biochemical activity of proteins.