Fundamental limitations of the time-dependent Stokes shift for investigating protein hydration dynamics

The time-dependent Stokes shift (TDSS) has attracted increasing interest for measuring hydration dynamics around biomolecules during the last decades. Its ability to report on hydration dynamics around proteins, however, was questioned recently since the experimental signal stems from both water and protein motion with an unknown ratio of contribution. Using large-scale computer simulations, we examine the ability of the TDSS to capture local hydration dynamics at nine different sites around the protein ubiquitin. By computationally constraining protein motion, it is shown that the remaining water component is meaningful and in line with the picture of a heterogeneous yet overall mobile hydration layer. However, protein contributions are excessively large and cannot be removed in an experimental context, thus obscuring the water component. Consequently, we conclude that the experimental TDSS may not be suitable for the investigation of hydration dynamics around proteins.