Protein Structural Mode Separation with Modulated Orientation Sensitive Terahertz Spectroscopy

Abstract

Department of Physics, University at Buffalo, SUNY, Buffalo, NY 14260.The far infrared spectroscopy of molecular crystals reveals both intra-molecular and intermolecular vibrational modes. Such spectroscopic measurements may also be used for protein crystals to detect correlated structural motions necessary for function. However with the significant increase in complexity of protein structures compared to simple molecules such as sucrose, one finds increasing overlap in the internal modes. Here we demonstrate a new technique called MOSTS (Modulated Orientation Sensitive THz Spectroscopy). We achieve high sensitivity and mode separation by using single protein crystal and rapid modulation of the relative alignment of the terahertz polarization and the crystal axes by rotating the sample. By locking into the signal at the rotation frequency we determine the polarization sensitive signal as a function of phase and map out the optically active vibrational resonances for different orientations of the molecular crystal. The method is only sensitive to the anisotropic part of the spectroscopic response.To illustrate the technique we compare two methods of detection. First, the signal modulated by generating antenna bias, the standard Terahertz time domain spectroscopy (THz TDS) method and the second, the MOSTS method where the signal is modulated by the sample orientation. We present measurements on a wire grid linear polarizer, a sucrose crystal and a hen egg white lysozyme crystal. This work is supported by NSF MRIˆ2 grant DBI2959989.