A dynamical transition in the protein myoglobin observed by infrared vibrational echo experiments

Ultrafast infrared vibrational echo measurements of the temperature-depende nt pure dephasing of the A(1) CO stretching mode of myoglobin-CO (Mb-CO) we re performed in the solvents trehalose and 50:50 ethylene glycol:water. The results are compared to previously reported data in 95:5 glycerol:water. T he temperature dependence (11-300 K) of the pure dephasing in trehalose (a glass at all temperatures studied) is a power law, T-1.3, below T congruent to 200 K, while at higher temperature it becomes dramatically steeper. The change in functional form occurs although the solvent does not go through its grass transition. In the other two solvents, the breaks in the temperat ure dependences occur at lower temperatures, and the temperature dependence s are even steeper above the power law region. The results are discussed in terms of a combination of a temperature and viscosity dependence of protei n dynamics.