Physics Colloquium - Friday, March 6th, 2009, 4:00 P.M.

Martina Havenith
Department of Physical Chemistry II
Ruhr University Bochum

The focus in protein folding has been very much on the protein backbone and sidechains. Yet hydration waters make comparable contributions to the structure and energy of proteins. Although the dynamics of the hydration water occurs on the picosecond time scale, 'slaving' to fast solvent modes profoundly affects the slower but larger-scale protein motions. In return the protein influences the structure and dynamics of surrounding water molecules. Fundamental questions of biomolecule 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? Terahertz spectroscopy is shown to directly probe such solvation dynamics, and the width of the dynamic hydration layer [1]. Solvation of the five helix bundle protein λ^*_6-85 leads to an unexpected non-monotonic trend in the measured terahertz absorbance as a function of the protein:water molar ratio. The trend can be explained by overlapping solvation layers around the proteins [2]. Molecular dynamics simulations indicate water dynamics in the solvation layer around one protein to be distinct from bulk water out to about 10Å. At higher protein concentrations such that solvation layers overlap, the calculated absorption spectrum varies non-monotonically, qualitatively consistent with the experimental observations. The experimental data suggest an influence on the correlated water motion beyond 20Å, greater than the structural correlation length usually observed. Most recent results demonstrate that the long range hydration layer is protein sequence and pH dependent. Whereas the native wildtype shows the most pronounced effect on the fast water dynamics, the denaturated protein have much less influence on the fast water network motions [3].

References:
[1]U. Heugen, G. Schwaab, E. Bründermann, M. Heyden, X. Yu, D.M. Leitner, M. Havenith, Solute-induced retardation of water dynamics probed directly by THz spectroscopy, PNAS 103, 12301 (2006).
[2]S. Ebbinghaus, S.J. Kim, M. Heyden, X. Yu, U. Heugen, M. Gruebele, D.M. Leitner, and M. Havenith, An extended dynamical solvation shell around proteins PNAS 104, 20749 (2007).
[3]S. Ebbinghaus, S.J. Kim, M. Heyden, X. Yu, M. Gruebele, D.M. Leitner, M. Havenith, Protein sequence- and pH dependend hydration probed by THz spectroscopy, JACS 130 (8), 2374 (2008).