Terahertz Dielectric Response of Photoactive Yellow Protein (PYP): Influence of Conformational-Vibrational State during Photocycle and Hydration Effects

Protein conformational change alters flexibility and conformational-vibrational modes that occur on a picosecond or sub-picosecond time scale. Terahertz dielectric measurements are sensitive to protein flexibility as they directly probe the density of states of these vibrational modes. Using terahertz time-domain spectroscopy, we measured the dielectric response of PYP thin films as a function of resting and photointermediate state. The absorbance increases smoothly as a function of frequency while the index of refraction exhibits no frequency dependence. A sharp transition in the dielectric response of the ground state is observed at 86{\%} relative humidity (r.h.), corresponding to the point where the protein film has lost $\sim $50 water molecules relative to a 100{\%} r.h. environment. Similar transitions observed for hen egg white lysozyme and cytochrome c correspond to the filling point of the first hydration shell.