We examine temperature-dependent picosecond dynamics of two benchmarking proteins lysozyme and cytochrome using temperature-dependent terahertz permittivity measurements. We find that a double Arrhenius temperature dependence with activation energies ∼ 0.1 kJ/mol and ∼ 10 kJ/mol fits the folded and ligand-free state response. The higher activation energy is consistent with the so-called protein dynamical transition associated with beta relaxations at the solvent-protein interface. The lower activation energy is consistent with correlated structural motions. When the structure is removed by denaturing, the lower-activation-energy process is no longer present. Additionally, the lower-activation-energy process is diminished with ligand binding but not for changes in the internal oxidation state. We suggest that the lower-energy activation process is associated with collective structural motions that are no longer accessible with denaturing or binding.
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