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 * 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 Ϸ10 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.solvation dynamics ͉ THz spectroscopy ͉ lambda repressor ͉ molecular modeling W ater molecules interact with proteins on many length and time scales. 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 (1). In return the protein influences the structure and dynamics of surrounding water molecules (2). X-ray crystallography has revealed ordered water structure around polar and charged sidechains (3), as well as cooperative insertion of water into hydrophobic cavities (4). Dielectric spectroscopy extends the time scale from microseconds down to 0.1 ns (5). Experiments have been extended to the THz range in films and crystals, probing motions on the picosecond time scale (6, 7). Hydrated protein powders probed by inelastic neutron scattering (0.1-100 ps) or solid-state NMR (nanoseconds) reveal that slower protein time scales and faster solvent time scales indeed show correlated dynamics (8). On the fastest time scales, 2D infrared spectroscopy and fluorescence of surface residues provide local probes of the dynamics in the femtosecond to picosecond range (9, 10). Coupling of modeling with experiments has revealed complex solvation structure around small biomolecules (11, 12), bridging our microscopic structural and thermodynamic understanding of biosolvation.Terahertz absorption spectroscopy of biomolecules fully solvated in water yields direct informa...

Hydrogen bond rearrangement in water occurs on the picosecond time scale, so relevant experiments must access these times. Here, we show that terahertz spectroscopy can directly investigate hydration layers. By a precise measurement of absorption coefficients between 2.3 THz and 2.9 THz we could determine the size and the characteristics of the hydration shell. The hydration layer around a carbohydrate (lactose) is determined to extend to 5.13 ؎ 0.24 Å from the surface corresponding to Ϸ123 water molecules beyond the first solvation shell. Accompanying molecular modeling calculations support this result and provide a microscopic visualization. Terahertz spectroscopy is shown to probe the collective modes in the water network. The observed increase of the terahertz absorption of the water in the hydration layer is explained in terms of coherent oscillations of the hydration water and solute. Simulations also reveal a slowing down of the hydrogen bond rearrangement dynamics for water molecules near lactose, which occur on the picosecond time scale. The present study demonstrates that terahertz spectroscopy is a sensitive tool to detect solute-induced changes in the water network.hydration water dynamics ͉ molecular dynamics simulations of biomolecules ͉ solvated lactose T he many unusual properties of water combined with its importance as the solvent of life account for its continued study by numerous researchers. Indeed, the properties of water are essential in the behavior of all biological systems. For example, water plays a central role in the folding and function of proteins, and the function of carbohydrates. The dynamics of water surrounding a solute is of fundamental importance in a wide range of processes in solution. In particular, the properties of water molecules near the surface of a biomolecule have been the subject of numerous and sometimes controversial experimental and theoretical studies (1-5). The characteristics and role of this ''biological'' water, with properties that differ considerably from those of bulk water, are still not fully understood (6).The importance of the hydration layer or the biological water is obvious in specific protein processes, such as opening and closing of channels, the kinetics of which has been found to be coupled with the solvent fluctuations (1). Important questions that remain so far unanswered include the following: How does solvation water differ from bulk water, and how large is the solvation layer that can be attributed to solvent water? Simulations suggest the existence of rather rigid water structures around proteins (5) and carbohydrates (7), but experimental studies that characterize the hydration layer are limited. Hydrogen bond rearrangements in water occur on the picosecond time scale (8), so that a detailed understanding of the relevant processes at a molecular level requires experimental techniques that are able to probe the hydration layers on this time scale. However, experimental investigations of fast dynamics of hydration water still remain a ch...

Solvation free energy changes induced by protein folding and function are comparable to the corresponding overall free energy changes. Yet the structure and dynamics of the protein itself have received more attention than the solvent because they are easier to probe. Here we use terahertz (far-infrared) spectroscopy to probe directly the effect of mutations and solvent pH on the solvent shellprotein interaction. We study absorption spectra of the 80 residue five helix bundle λ 6-85