Anisotropic Terahertz Microscopy of Lysozyme in Different Crystal Lattice Systems

Abstract

Long-range vibrational modes of proteins at terahertz (THz) frequencies have been associated with protein function and allosteric control. The characterization of these motions has been challenging due to energy overlap with water absorption and a large vibrational density of states. Recently it has been demonstrated both experimentally and theoretically that vibrational bands can be isolated using stationary sample anisotropic terahertz microscopy (SSTAM) for oriented samples, typically realized using protein crystals [1, 2]. In those early measurements, inhibitor binding contrast was demonstrated for high symmetry tetragonal crystals. While high symmetry crystals are ideal for structural determinations, they can limit the types of vibrations observable in the ATM measurements. Here we show a survey of ATM measurements of triclinic, monoclinic and tetragonal crystals, demonstrating the unique signatures observable for the different symmetry groups, leading to a more complete determination of the vibrational hot spots that may contribute to enzymatic efficiency. The SSATM spectra indicate the presence of conserved vibrational modes near 40cm−1 and 55cm−1 for CEWL in triclinic, monoclinic and tetragonal lattice systems respectively. For CEWL in the monoclinic lattice system, a prominent band at 20cm−1 was consistently observed in the SSATM spectra but not in the triclinic or tetragonal systems. The conserved bands may represent vibrational modes that are unperturbed by crystal contact forces while the differences may be related to unique molecular orientation in different crystal systems. 1.Niessen, K., Y. Deng, and A.G. Markelz, Near-field THz micropolarimetry. Opt Express, 2019. 27(20): p. 28036-28047.2.Romo, T.D., A. Grossfield, and A.G. Markelz Persistent Protein Motions in a Rugged Energy Landscape Revealed by Normal Mode Ensemble Analysis. Accepted Journal of Chemical Information and Modeling, 2020. https://doi.org/10.26434/chemrxiv.12885203.v1.