Rohit Singh

Improved mode assignment for molecular crystals through anisotropic terahertz spectroscopy.

We report the anisotropic terahertz response of oxalic acid and sucrose crystals in the 0.2-3.0 THz range using terahertz time domain spectroscopy on large, single crystals. We compare the observed anisotropic response with the response calculated using solid-state density functional theory and find good agreement in the orientation dependence and relative intensities of the crystal phonons. It was found that oxalic dihydrate can be reversibly converted to anhydrous by controlled relative humidity. In addition, oxalic acid was found to have a large birefringence with Δn = 0.3, suggesting the material may be useful for terahertz polarization manipulation. Sucrose has a smaller birefringence of Δn = 0.05, similar to that of x-cut quartz. The anisotropic measurements provide both mode separation and symmetry determination to more readily achieve mode assignment for the more complex sucrose spectrum.

Modulated orientation-sensitive terahertz spectroscopy

Intramolecular vibrations of large macromolecules reside in the terahertz range. In particular, protein vibrations are closely spaced in frequency, resulting in a nearly continuous vibrational density of states. This density of vibrations interferes with the identification of specific absorption lines and their subsequent association with specific functional motions. This challenge is compounded with the absorption being dominated by the solvent and local relaxational motions. A strategy for removing the isotropic relaxational loss and isolating specific vibrations is to use aligned samples and polarization-sensitive measurements. Here, we demonstrate a technique to rapidly attain the anisotropic resonant absorbance using terahertz time domain spectroscopy and a spinning sample. The technique, modulated orientation-sensitive terahertz spectroscopy (MOSTS), has a nonzero signal only for anisotropic samples, as demonstrated by a comparison between a silicon wafer and a wire grid polarizer. For sucrose and oxalic acid molecular crystals, the MOSTS response is in agreement with modeled results for the intermolecular vibrations. Further, we demonstrate that, even in the presence of a large relaxational background, MOSTS isolates underlying vibrational resonances.

Dynamical Alignment of Solution Phase Proteins for Structural Measurements

For a variety of structural measurements, aligned molecular systems are needed. However, crystallization conditions cannot always be found, or the required crystal size is out of reach for standard growth procedures. In this project, we have devised a method to create partially aligned solution phase protein systems utilizing the large dipole moment for proteins. We have developed a solution cell capable of applying very large DC and AC electric field to protein solution in order to align them dynamically to the electric field of the probing beam. Narrow channels 100 µm wide and 50 µm in thickness are fabricated on an IR grade quartz substrate with SU-8 photo resist using photo lithographic methods. Two separate sets of electrodes are formed on the side walls on these channels by angular metal deposition. A second quartz substrate UV bonded to the top surface of the SU-8 walls forms the lid. Our cell has high (>85%) transmission in the far infrared and visible region and at the same time the electrodes on the side walls provide an electric field of the order of 10s of kV/cm. We first demonstrate the cells ability to modulate alignment using visible transmission of liquid crystals. Further, by dynamically aligning the protein molecules in the solution to an alternating electric field and by locking in to that frequency we can enhance structurally related signals by several orders of magnitude and also distinguish between the different structural modes. This work is supported by NSF MRIˆ2 grant DBI2959989.

Protein Structural Mode Separation with Modulated Orientation Sensitive Terahertz Spectroscopy

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.

Characterization of Phonons in Molecular Crystals

We demonstrate a new technique for characterizing the phonons in molecular crystals, Modulated Orientation Sensitive Terahertz Spectroscopy (MOSTS). The technique suppresses crystal defects and solvent contributions, and enhances contributions due to molecular structure and anisotropy.