Using terahertz spectroscopy as a protein binding assay

The vibrational modes corresponding to protein tertiary structural motion lay in the far infrared or terahertz frequency range. These collective large scale motions depend on global structure and thus will necessarily be perturbed by ligand binding events. We discuss the use of terahertz dielectric spectroscopy to measure these vibrational modes and the sensitivity of the technique to changes in protein conformation, oxidation state and environment. A challenge of applying this sensitivity as a spectroscopic assay for ligand binding is the sensitivity of the technique to both bulk water and water bound to the protein. This sensitivity can entirely obscure the signal from the protein or protein-ligand complex itself, thus necessitating sophisticated sample preparation making the technique impractical for industrial applications. We discuss methods to overcome this background and demonstrate how terahertz spectroscopy can be used to quickly assay protein binding for proteomics and pharmaceutical research.