Impact of protein crystal contact forces on intramolecular picosecond dynamics*

While many protein dynamics studies depend on crystal phase measurements, it is not known how the intermolecular interactions effect these dynamics. We examine the dynamical impact of intermolecular interactions by measuring the collective structural vibrations using anisotropic terahertz microspectroscopy and tuning interactions through controlled dehydration of the crystal water content.  For tetragonal and triclinic crystals no frequency shifts are observed.  Instead, the bands initially narrow then broaden with dehydration, consistent with increased and then decreased ordering as found in X-ray crystallography.  For monoclinic crystals on the other hand, we find strong anisotropic bands at 14.8 and 21.6 cm^-1, which blue shift by 5.0 and 3.1 cm^-1 respectively. This blue shift in the vibrations suggests a stiffening of the potential due to either a reduction in the solvent, an increase in the interaction with the neighboring molecules or both. Monoclinic lysozyme crystals are known to reversibly transform to another monoclinic form due to dehydration, thus it is possible that the spectral shifts are associated with this new ordering and not changes in the interactions. We explore these different possible mechanisms by comparison with Normal Mode Ensemble Analysis calculations.