A Dynamical Transition in the Protein Myoglobin Observed by Infrared Vibrational Echo Experiments

Ultrafast infrared vibrational echo measurements of the temperature-dependent pure dephasing of the A1 CO stretching mode of myoglobin-CO (Mb-CO) were performed in the solvents trehalose and 50:50 ethylene glycol:water. The results are compared to previously reported data in 95:5 glycerol:water. The temperature dependence (11-300 K) of the pure dephasing in trehalose (a glass at all temperatures studied) is a power law, T1.3, below T = 200 K, while at higher temperature it becomes dramatically steeper. The change in functional form occurs although the solvent does not go through its glass transition. In the other two solvents, the breaks in the temperature dependences occur at lower temperatures, and the temperature dependences are even steeper above the power law region. The results are discussed in terms of a combination of a temperature and viscosity dependence of protein dynamics.