Dynamics of Myoglobin in Light and Heavy Water Probed by Dielectric Relaxation Spectroscopy

It is largely accepted that the hydration water plays an important role in the function and dynamics of proteins. However, yet, the detailed nature of protein – hydration water interactions, is still unclear. Here, we investigated the protein-solvent interactions by measuring the dielectric response of myoglobin (Mb) in light and heavy water as a function of frequency (10^-3 Hz to 10⁷Hz), temperature (133K to 283K), and hydration ratio (100 to 2). In light water, we observed an additional peak in the dielectric spectra as compared to heavy water. We used two Havriliak- Negami functions plus conductivity to fit the relaxation processes. We determined the relaxation time for all observed processes and calculated the corresponding activation energies. We further determined the electric dipole moment of myoglobin in light and heavy water and its behavior as a function of temperature and hydration. As predicted, our data shows that the additional relaxation observed in the myoglobin-light water solutions corresponds to the hydration layer.