THz spectroscopy
The THz dance of the water
Water molecules in the bulk hydrogen-bond on average 3 to 4 other water molecules at any given time, as deduced from, e.g., neutron diffraction studies, but these hydrogen bonds are in a constant state of flux. Water molecules librate (hindered rotational motion of a water molecule in its hydrogen bonded environment) in solution on a subpicosecond time scale, so that within a picosecond, a hydrogen bond between any two molecules may break and reform many times. Diffusion of water molecules occurs on a picosecond time scale. Over this longer time scale, a given hydrogen bond between two water molecules may no longer hold, as reorientation and translation of a given water molecule favours new bond formation. All these motions lead to a fluctuation in the water network and thus to fluctuations of the water dipole moments on the sub-psec and psec time scale. Terahertz absorption measurements are sensitive to this dynamical reorientation of dipole moments. The rearrangement occurs on the picosecond time scale (1 THz = 1012 Hz = 1 ps-1). This makes THz spectroscopy a sensitive tool to probe solute induced changes in the fast collective water network motions.
THz spectroscopy as a new tool to probe solvation of biomolecules
The role of water in biomolecule dynamics has attracted much interest over the past decade, due in part to new probes of biomolecule-water interactions and developments in molecular simulations. THz spectroscopy, among the most recent experimental methods brought to bear on this problem, is able to detect even small solute induced changes of the collective water network dynamics in the psec range at the biomolecule-water interface. When studying the properties of water in aqueous solutions via THz absorption, proteins show a long ranged influence on water network dynamics and even small saccharides influence the dynamics of several hydration layers. The THz spectrum of water solvating a protein is sensitive to mutation and depends on the surface charge and flexibility of the protein. Native proteins are found to have the largest impact on the solvation dynamics. The influence on the solvation water decreases upon partial unfolding or mutation. THz spectra of solvated saccharides reveal that the number of water molecules coupled dynamically to a saccharide, forming a dynamical hydration shell around it, is correlated with the number of exposed oxygen atoms on the solute surface. The thickness of this layer appears correlated with the bioprotection efficiency of saccharides and antifreeze proteins. All findings support the hypothesis of a long-range dynamic coupling between biomolecule and solvent.
Kinetic THz absorption (KITA) has been developed by us as a tool to probe changes in solvation dynamics upon biological function. KITA studies of protein folding in real time revealed that changes in solvent dynamics are coupled to secondary structure formation of the protein. The solvent water network is found to be dynamically rearranged in milliseconds before the protein folds to its native state within the following seconds. THz spectroscopy gives experimental evidence that collective long-range dynamics are a key factor of biomolecular hydration.
Most relevant publications
"Solute-induced retardation of water dynamics probed directly by THz spectroscopy," U. Heugen, G.Schwaab, E. Bründermann, M. Heyden, X. Yu, D.M Leitner, and M. Havenith, PNAS 103, 12301 (2006)
"An extended dynamical solvation shell around proteins,"S. Ebbinghaus, S.J. Kim, M. Heyden, X. Yu, U. Heugen, M. Gruebele, D.M Leitner, and M. Havenith, PNAS 104, 20749 (2007)
"The Terahertz dance of water with the proteins: The effect of protein flexibility on the dynamical hydration shell of ubiquitin," B. Born, S.J. Kim, S. Ebbinghaus, M. Gruebele, and M. Havenith Letzner, G.W. Schwaab, M. H
Faraday Discussion 141, 161 (2009)
"Real-time detection of protein-water dynamics upon protein folding by Terahertz absorption," B. Born, M. Havenith, and M. Gruebele, Angewandte Chemie Intl. Edition 47 (34), 6486 (2008)
"Rattling in the cage: ions as probes of sub-ps water network dynamics," D. A. Schmidt, Ö. Birer, S. Funkner, B. Born, R. Gnanasekaran, G. Schwaab, D. M. Leitner, and M. Havenith, J. Am. Chem. Soc.,,of water with the 131 (51), 18512 (2009)
"Antifreeze glycoprotein activity correlates with long-range protein-water dynamics" S. Ebbinghaus, K. Meister, B. Born, Arthur L. DeVries, M. Gruebele, M. Havenith Communication, J. Am. Chem. Soc. August 16,2010