Vibrational Echo Studies of Protein Dynamics

The first picosecond infrared vibrational echo experiments on a protein, myoglobin-CO, are described. The experiments were performed at temperatures ranging from 60 to 300 K with a midinfrared free electron laser tuned to $1945{\mathrm{cm}}^{-1\mathrm{}}$. Below $\sim 185\mathrm{K}$, the pure dephasing, $T_2^{*}$, displays a power law temperature dependence, $T^{1.3}$. This behavior is reminiscent of that associated with the properties of low temperature glasses $(<5\mathrm{}\mathrm{K})$ but is observed here at much higher temperatures. Above the solvent glass transition temperature, $T_2^{*}$ is exponentially activated.