Origin of Terahertz Sensitivity to Heme Oxidation State

Abstract

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