Picosecond Dynamics Evolution During Function For Photoactive Yellow Protein

Abstract

Picosecond protein dynamics refer to both diffusive motion at the protein surface and adjacent solvent as well as possible underdamped structural vibrational modes. Functional protein structure changes result in possible changes in both these diffusive and collective dynamics which can lead to either an increase or decrease in flexibility in the active state. In the case of photoactive yellow protein (PYP), a large conformational change occurs as one proceeds from the resting pG state to the active pB state with partial molten globule formation. Previous terahertz dielectric response has been used to monitor changes in picosecond dynamics for the photoactive protein PYP with opposing results [1, 2]. While in one set of measurements, hydrated films were used and the pG/pB relative state population was monitored, in another set of measurements low conc PYP solution was used without monitoring of the conversion. In this paper we present THz dielectric response as a function of photocycle state for fully solvated PYP with in situ monitoring of the conversion using UV/Vis absorbance, both at room temperature and below freezing. Freezing reduces the background relaxational absorption of bulk water, and increases conversion to pB by slowing the photocycling time.