  Here we reveal details of the interaction between human lysozyme proteins,
both native and fibrils, and their water environment by intense terahertz time
domain spectroscopy. With the aid of a rigorous dielectric model, we determine
the amplitude and phase of the oscillating dipole induced by the THz field in
the volume containing the protein and its hydration water. At low
concentrations, the amplitude of this induced dipolar response decreases with
increasing concentration. Beyond a certain threshold, marking the onset of the
interactions between the extended hydration shells, the amplitude remains fixed
but the phase of the induced dipolar response, which is initially in phase with
the applied THz field, begins to change. The changes observed in the THz
response reveal protein-protein interactions me-diated by extended hydration
layers, which may control fibril formation and may have an important role in
chemical recognition phenomena.
