Effects of solvent viscosity on protein dynamics: Infrared vibrational echo experiments and theory

The influence of solvent viscosity on the surface and internal structural d ynamics of the protein myoglobin is studied using ultrafast infrared vibrat ional echo measurements of the pure dephasing of the A(1) CO stretching mod e of myoglobin-CO (Mb-CO). The dephasing reflects protein structural fluctu ations as sensed by the CO ligand bound at the protein's active site. Measu rements made as a function of solvent viscosity at 295 K show that the pure dephasing has a marked dependence on viscosity. In addition, the pure deph asing of Mb-CO in the solvents trehalose and 50:50 ethylene glycol:water ar e compared as a function of temperature T(10-295 K). The pure dephasing dat a in the two solvents have identical T-1,T-3 temperature dependences at low temperatures, where both solvents are glassy solids. At higher temperature s. the Mb-CO pure dephasing has a much steeper temperature dependence in et hylene glycol:water, which is a liquid, than in trehalose, which is a glass at all temperatures studied. The steep temperature dependence in liquid et hylene glycol: water is described as a combination of a viscosity-dependent component and a temperature-dependent component. The viscosity-dependent d ata are analyzed using a theory that connects the fluctuations of the prote in surface to the solvent's viscoelastic response. When the solvent's visco sity is lowered, the increased rate of fluctuation of the protein's surface allows more rapid internal protein dynamics, which result in more rapid de phasing. Good agreement is obtained fur physically reasonable parameters. T he experimental echo decay times are proportional to the cube root of the s olvent viscosity eta (1/3) This proportionality is characteristic of protei n structural fluctuations that give rise to CO frequency fluctuations that are in the spectral diffusion regime (relatively slow evolution).