Protein function is tied to the macromolecule's ability to change its conformation. Examples include large scale structural change in photoactive proteins, induced fit antibody-antigen binding and opening and closing of membrane pores. Methods of measuring the flexibility of the protein include average atomic fluctuations determined by X-ray crystallographic B-factors or NMR structural measurements, ESR measurements, and neutron inelastic scattering. These methods of measurement can require difficult to access facilities or multiple mutations and tagging preparations. In addition the relationship between the measured atomic fluctuations and the pathways of concerted structural motion is not transparent. Related to these large scale motions are the large-scale vibrational modes where entire subunits move relative to each other. These modes are known as conformational vibrational modes and lie in the far infrared or terahertz (THz) frequency range.
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