THz spectroscopy

The THz dance of water


Water molecules in bulk hydrogen-bond, on average, 3 to 4 other water molecules at any given time, as deduced from neutron diffraction studies, for example. However, these hydrogen bonds are in a constant state of flux. The hydrogen-bonded environment hinders the free rotation of water molecules in solution, forcing water molecules to undergo a librational motion on a sub-picosecond timescale. This means that within a picosecond, a hydrogen bond between any two molecules may break and reform many times. The diffusion of water molecules occurs on a picosecond timescale. Over this longer timescale, a given hydrogen bond between two water molecules may no longer exist, as reorientation and translation of a specific water molecule could favour 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 timescales. Terahertz absorption measurements are sensitive to this dynamical reorientation of dipole moments. The rearrangement occurs on the picosecond timescale (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 is a new tool able to probe the solvation of biomolecules


The role of water in the dynamics of biomolecules has attracted much interest over the past decade, due to the development of new experimental methods to investigate biomolecule-water interactions and to advancements in molecular simulations. THz spectroscopy, one of the most recent experimental methods able to tackle this problem, can 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. This influence decreases upon partial unfolding or mutation of the protein.

THz spectra of solvated saccharides reveal that the number of water molecules coupled dynamically to a saccharide, forming its hydration shell, correlates with the number of exposed oxygen atoms on the solute surface. All findings support the hypothesis of a long-range dynamic coupling between biomolecule and solvent.

Kinetic THz absorption (KITA) has been developed in our group 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 dynamically rearrange milliseconds before the protein folds to its native state. THz spectroscopy delivers experimental evidence that collective long-range dynamics are a key factor of biomolecular hydration.


Most relevant publications

 

Gruenerpfeil"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).

Gruenerpfeil"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).

Gruenerpfeil"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).
 
Gruenerpfeil
"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. 131 (51), 18512 (2009).

Gruenerpfeil"Understanding THz spectra of aqueous solutions: Glycine in light and heavy water", J. Sun, G. Niehues, H. Forbert, D. Decka, G. Schwaab, D. Marx, and M. Havenith, J. Am. Chem. Soc. 136 (13), 5031 (2014).

Gruenerpfeil"Hydrophobic collapse induces changes in the collective protein and hydration low frequency modes", T.Q. Luong, Y. Xu, E. Bründermann, D.M. Leitner, and M. Havenith, Chem. Phys. Lett. 651, 1-7 (2016).

Gruenerpfeil"Water dynamics from THz spectroscopy reveal the locus of a liquid-liquid binodal limit in aqueous CaCO3 solutions", F. Sebastiani, S.L.P. Wolf, B. Born, T.Q. Luong, H. Colfen, D. Gebauer, and M. Havenith, Angewandte Chemie Intl. Edition 56, 490 (2017).

Gruenerpfeil"Hydration water mapping around alcohol chains by THz-calorimetry reveals local changes in heat capacity and free energy upon solvation", F. Böhm, G. Schwaab, and M. Havenith, Angewandte Chemie 56, 9981 (2017).