The folded structure and stability of proteins emanates from their interaction with the water solvent. Water at the protein surface is strongly impacted, resulting in a region with a perturbed hydrogen bonding network. This region, the solvation shell, has distinct properties from that of bulk water, including retarded dynamics and fewer hydrogen bonds. Changes in the structure and dynamics of solvation water can be both perturbed and reported on by Terahertz radiation. Yet, some fundamental properties of solvation water, such as energy transfer within the hydrogen bonding network, remain largely unexplored.

                   

Here, we utilize the TELBE free electron laser source to investigate the nonlinear transmission of lysozyme protein solutions. Z-scan experiments were performed at 0.5 THz, revealing a large nonlinear transmission of water. The nonlinear transmission of lysozyme solutions had a concentration dependent effect, showing that the amount of available water has a role. For the largest protein concentration measured, an inversion in the sign of the nonlinear transmission was observed. These results indicate that the nonlinear properties of protein solutions depend on the fraction of bulk and solvation water, and suggest that the mechanism of energy transfer changes at a threshold value. This work has implications for the study of nonlinear properties in biological systems.