An extended dynamical hydration shell around proteins

^†Lehrstuhl für Physikalische Chemie II, Ruhr-Universität Bochum 44780 Bochum, Germany;
^‡Department of Physics and
^¶Department of Chemistry and Center for Biophysics and Computational Biology, University of Illinois at Urbana–Champaign, Urbana, IL 61801; and
^§Department of Chemistry, University of Nevada, Reno, NV 89557

Communicated by R. Stephen Berry, University of Chicago, Chicago, IL, October 3, 2007 (received for review April 11, 2007)

Abstract

The focus in protein folding has been very much on the protein backbone and sidechains. However, hydration waters make comparable contributions to the structure and energy of proteins. The coupling between fast hydration dynamics and protein dynamics is considered to play an important role in protein folding. Fundamental questions of protein hydration include, how far out into the solvent does the influence of the biomolecule reach, how is the water affected, and how are the properties of the hydration water influenced by the separation between protein molecules in solution? We show here that Terahertz spectroscopy directly probes such solvation dynamics around proteins, and determines the width of the dynamical hydration layer. We also investigate the dependence of solvation dynamics on protein concentration. We observe an unexpected nonmonotonic trend in the measured terahertz absorbance of the five helix bundle protein λ_6–85* as a function of the protein: water molar ratio. The trend can be explained by overlapping solvation layers around the proteins. Molecular dynamics simulations indicate water dynamics in the solvation layer around one protein to be distinct from bulk water out to ≈10 Å. At higher protein concentrations such that solvation layers overlap, the calculated absorption spectrum varies nonmonotonically, qualitatively consistent with the experimental observations. The experimental data suggest an influence on the correlated water network motion beyond 20 Å, greater than the pure structural correlation length usually observed.

Footnotes

^‖To whom correspondence should be addressed. E-mail: martina.havenith{at}rub.de
Author contributions: S.E., S.J.K., X.Y., U.H., M.G., and D.M.L. performed research; S.E., M. Heyden, X.Y., M.G., D.M.L., and M. Havenith analyzed data; M.G., D.M.L., and M. Havenith designed research; and M.G., D.M.L., and M. Havenith wrote the paper.
The authors declare no conflict of interest.
This article contains supporting information online at www.pnas.org/cgi/content/full/0709207104/DC1.