Poster abstracts

Poster number 40 submitted by Tanping Li

Hydration Dynamics As Revealed By The Fluorescence Stokes Shift: The Origin Of Slow Hydration Dynamics And Breakdown Of Linear Response

Tanping Li (Biophysics), Ali A. Hassanali (Biophysics), Dongping Zhong (Physics), Sherwin J. Singer (Chemistry)

Abstract:
Hydration dynamics in the immediate vicinity of a protein, probed by time-dependent fluorescence Stokes shift
experiments, is critical to understand its biological function. Protein Stokes shifts typically exhibit biphasic
relaxation following photo-excitation: fast relaxation occurs on a time scale of several picoseconds while slower
components indicate additional hydration dynamics on a time scale of tens of picoseconds, or longer. Theoretical
studies using both linear response and non-equilibrium molecular dynamics (MD) calculation qualitatively
reproduce the observed biphasic behavior of time dependent Stokes shift for Trp-7 (W7) in myoglobin.
Comparison with constrained MD simulations with protein frozen at the instant of photo-excitation reveals the
molecular mechanism of slow hydration process and establishes the critical role of protein flexibility. Coupled
protein-water motion is shown to be necessary for the observation of the slow component of hydration dynamics.
Qualitatively similar results are found for a series of additional cases, such as monellin and staph. nuclease. We
illustrate why tracking the separate contributions to the Stokes shift without constrained MD studies may not yield
an accurate interpretation of protein hydration dynamics. Additionally, we examine the extent to which protein
fluctuations obey Gaussian statistics and the linear response approximation to the Stokes shift is valid.
Equilibrium fluctuations of the ground-excited energy difference, which control the absorption and fluorescence
line shapes, in the ground and excited electronic states are not independent of each other. We illustrate how
small differences from Gaussian statistics in one electronic state can be a signature of very significant deviations
from linear response theory, such as isomerization, in the other electronic state.

Keywords: Hydration dynamics, fluorescence Stokes shift, slow component