Simplifying Protein Collective Vibrational Assignment Through Symmetry

Anisotropic terahertz microspectroscopy (ATM) isolates protein collective vibrations from a highly congested spectrum based on the direction of the vibrational transition dipole.  ATM changes with inhibitor binding and photo excitation reveal that the vibrational bath evolves with functional state.  Assignment of the spectral bands to specific structural motions is needed to understand the impact of these dynamical changes.  The complexity of the anisotropic spectral structure is dependent on the alignment of the molecules.  This alignment is achieved by crystallization, which results in biomacromolecular arrays along with 30-70% water by volume.  We examine crystal symmetry effects on the anisotropic spectra for hen egg white lysozyme crystals (HEWL).  ATM measurements of triclinic, monoclinic and tetragonal HEWL crystals identify the conserved versus unique spectral features. Peaks near 40 cm⁻¹ and 55 cm⁻¹ were common among three lattice systems while a peak near 20 cm⁻¹ was observed only in triclinic CEWL. Further we find that the spectral structure decreases with increasing symmetry, an expected result from dipole cancellation.  The spectral symmetry shows good agreement with normal mode ensemble analysis calculations providing initial assignment of the bands.