Investigation of the Isotope Shift in Protein Collective Vibrations*

Recently it has been established that the collective vibrations of proteins can be characterized using anisotropic terahertz spectroscopy. These motions are necessary for biological function. Ideally the observed resonances can be identified with specific motions of the macromolecular subdomains and one could determine if and how mutations effect these motions. Current computational techniques do not accurately predict the measured spectra so isotope shift measurements can provide a means to identify which calculated features are associated with the observed resonances. Protonated and deuterated chicken egg white lysozyme (CEWL) crystals were grown concurrently in buffer solutions that were made using deionized water or heavy water. The absorption spectra of the resulting crystals are determined using anisotropic terahertz microscopy, which removes isotropic background features and identifies structural modes of the protein. Our preliminary results find a 0.3 THz red shift for the deuterated CEWL vibrations. These results are compared to calculations done on normal modes for anisotropic spectra for the two crystal compositions.