Crystal Symmetry Effects on Protein Structural Vibrational Signatures

The method of terahertz anisotropic microspectroscopy (ATM) is robust method to fingerprint protein structural vibrations. Potentially ATM could address why there are evolutionarily dominant structural motifs as well as the development of allosteric drugs for protein function regulation. However, it is important to confront the possible pitfall that the structural vibrational bands measured by ATM do not reflect in vivo dynamics. In particular, it is well known that while many proteins can function in crystallo, proteins requiring large conformational change cannot, due the crystal contact forces preventing large-scale motions. Here we compare ATM measurements of hen egg white lysozyme (HEWL) crystallized in three different symmetry groups to spectra calculated using normal mode ensemble analysis (NMEA). The studies demonstrate that in fact, as the actual atomic displacements are vastly smaller than the crystal unit cell, there is little effect of the crystal contact forces on the spectra.