Hydration dynamics and collection motions in aqueous Cytochrome c

Cytochrome c is a water-soluble and peripheral membrane protein that has been employed as a model protein in studies on protein hydration and function. The earlier studies encompass a broad range of experimental as well as simulation methods. The earlier studies based on the dielectric relaxation are limited to a narrow frequency region and only partially envelop the hydration water response. We have extensively probed the dielectric response from aqueous cytochrome c at 100 MHz–1.12 THz frequency range. The water molecules within the hydration shells are heterogeneous over a few hydration layers away from the protein surface. Solvation of protein in water significantly alters the relaxation dynamics of water molecules in the immediate neighborhood. Two slower relaxation processes in addition to the bulk water relaxation are observed with the characteristics relaxation times of 35 ± 5 and 330 ± 36 ps. Employing effective medium approximation at terahertz frequencies in combination with molecular dynamics simulations, dielectric response of hydrated cytochrome c could be estimated. The analysis suggests that 310 ± 40 water molecules are tightly bound to cytochrome c and are intermolecularly coupled with protein collective vibrations.