Coupling Of Solvent And Protein Dynamics: Mossbauer And Incoherent Neutron Scattering From Dielectric Relaxation Data

A wide variety of protein dynamics are accounted for by two classes of solvent processes: the bulk-solvent viscosity and hydration-shell dynamics. In glass-forming solvents the bulk viscosity arises from the well-characterized microscopic alpha relaxation; in liquid solvents that freeze the alpha process is essentially molecular reorientation. In past work we showed that the solvent alpha relaxation determines the activation enthalpy of alpha-slaved protein motions.We have now measured the dielectric spectrum of the hydration-shell dynamics in myoglobin solutions as a function of hydration, temperature and frequency. These hydration-shell data and a minimal model of protein-solvent coupling predict the temperature- and hydration-dependence of the Mossbauer effect. Furthermore, we show agreement between incoherent neutron scattering data and our measurements of hydration dynamics.These improvements in understanding protein-solvent dynamical coupling will be discussed in terms of earlier work describing the slaving of many protein functional motions to the solvent alpha-process and the slaving of protein folding. We demonstrate that many enthalpy barriers to protein motion arise almost entirely from solvent dynamical processes.