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The 200 K protein dynamical transition is observed for the first time in the teraherz dielectric response.The complex dielectric permittivity ε = ε' + iε" is determined in the 0.2 -2.0 THz and 80-294 K ranges.ε" has a linear temperature dependence up to 200 K then sharply increases. The low temperature linear dependence in ε" indicates anharmonicity for temperatures 80 K < T < 180 K, challenging the assumed harmonicity below 200K. The temperature dependence is consistent with beta relaxation response and shows the protein motions involved in the dynamical transition extend to subpicosecond time scales.

Previously we have shown that the terahertz dielectric response is sensitive to the oxidation state for cytochrome c (CytC) films. Here we discuss the origin of this sensitivity through hydration dependent measurements on films, solution phase measurements as a function of temperature and normal mode calculations as a function of hydration. We find that the hydration dependence of the terahertz response rapidly approaches its fully solvated value for ferri CytC, whereas for ferro CytC the effect of added water is more gradual, with the system not reaching fully hydrated values up to 0.5 gm water/gm protein. For solution phase samples below 270 K we do not see a significant difference in the terahertz response suggesting that the fully hydrated ferro cytochrome c has the same picosecond dynamics as ferri cytochrome c. These results contradict X-ray B factor measurements that suggest that ferri cytochrome c is significantly more flexible than ferro suggesting that the B-factor determination from X-ray crystal measurements may not represent in vivo values as crystal water is less than 0.5 gm water/gm protein.

Previoisly we reported the sensitivity of terahertz dielectric responseas to the heme oxidation state for cytochrome c. Here we discuss measurements and calculations determining how correlated and diffusive motions give rise to this contrast.Protein flexibility is critical to biological function. Structural motions are necessary for protein-protein and protein-ligand interactions. We can characterize two types of flexibility: local scale diffusive flexibility and large scale correlated flexibility. Local rotational motions of amino acid side chains allows the structure to diffusively sample conformationals of functional relevance. Large scale correlated motions are vibrational motions of the entire 3D structure. A number of investigators have demonstrated that many large-scale functionally relevant conformational changes can be simulated using only the first few structural vibrational modes. The time scales and vibrational frequencies have been calculated using molecular force fields. Rotational relaxation times are ~ 5 ps, whereas the structural vibrational modes for a number of proteins are in the 5-200 cm-1 range. Terahertz time domain spectroscopy can be used to access these picosecond dynamics. The dielectric response is given by 2 2 0

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