Thermally activated coupling between protein vibrations and interfacial water dynamics revealed by terahertz spectroscopy.

Proteins exhibit complex dynamics that are strongly governed by their hydration environment. At the protein-water interface, strong electrostatic fields and topological confinement generate a heterogeneous landscape where water molecules exhibit distinct orientational behaviors. Broadband terahertz dielectric spectroscopy provides direct insight into the temperature-dependent coupling between protein collective vibrations and interfacial water dynamics. Using aqueous ubiquitin as a model system, three populations of loosely bound, tightly bound, and bulk water are identified. Tightly bound water exhibits an anomalous increase in dielectric strength with increasing temperature, in contrast to the expected trend observed for bulk and loosely bound water. This behavior correlates with enhanced protein vibrations, revealing a thermally activated vibrational, orientational coupling mechanism. Arrhenius analysis quantifies distinct activation energies and attempt-frequencies of each water population, demonstrating that protein vibrations actively modulate interfacial water behavior. These findings present a mechanistic framework for hydration-mediated biomolecular function and highlight terahertz spectroscopy as a unique tool to probe protein-water interactions.