Crystal Symmetry Effects on Protein Anisotropic Absorption*

Long-range protein structural vibrations provide dynamical access to intermediate conformations during biological function.  Anisotropic Terahertz Microspectroscopy provides a method of measuring these vibrations.  To identify the measured resonances with structural motions we compare the spectra to normal mode ensemble analysis calculations. The anisotropic absorbance depends on the orientation of the molecules relative to the light polarization. This orientation is set by the crystal symmetry group and crystal facet perpendicular to the light propagation.  We determine this polarization plane using X-ray crystal face indexing for monoclinic and triclinic hen egg white lysozyme crystals. For monoclinic crystals, unit cell parameters are in agreement with fully hydrated monoclinic symmetry seen in 2D4K.pdb, and with a polarization plane given by the [101] face. Using this, the calculated monoclinic spectra show agreement with the measured spectra. For triclinic crystals, face indexing of the polarization plane did not correspond to a simple index plane. The polarization plane was determined by combining indexing of the other facets with the measured dimensions. The resulting calculated triclinic spectra agreed with observed asymmetry in the spectra and spectral resonances.