Does the Dynamic Stokes Shift Report on Slow Protein Hydration Dynamics?

The time-dependent fluorescence frequency shift of protein-attached probes has a much slower decay than that for the free probe. The decay times, ranging from 10 ps to several nanoseconds, have been attributed to hydration water motions several orders of magnitude slower than those in the hydration shell of small solutes. This interpretation deviates strongly from the prevailing picture of protein hydration dynamics. We argue here that the slow decay in the fluorescence shift can be explained by a ubiquitous solvent polarization mechanism, with no need to invoke slow water motions or a dynamic coupling with protein motions. This mechanism can be qualitatively understood with the aid of a dielectric continuum model. We therefore conclude that the long decay times measured with time-dependent fluorescence spectroscopy contain no information about protein hydration dynamics.