Small Number Of Water Molecules Research Articles

Melting and Bubble Formation in a Double-Stranded DNA: Microscopic Aspects of Early Base-Pair Opening Events and the Role of Water.

Despite its rigid structure, DNA is a remarkably flexible molecule. Flexibility is essential for biological functions (such as transcription and gene repair), which require large-amplitude structural changes such as bubble formation. The bubbles thus formed are required to have a certain stability of their own and survive long on the time scale of molecular motions. A molecular understanding of fluctuations leading to quasi-stable structures is not available. Through extensive atomistic molecular dynamics simulations, we identify a sequence of microscopic events that culminate in local bubble formation, which is initiated by base-pair (BP) opening, resulting from the cleavage of native BP hydrogen bonds (HBs). This is followed by the formation of mismatched BPs with non-native contacts. These metastable structures can either revert to their original forms or undergo a flipping transition to form a local bubble that can span across 3-4 BPs. A substantial distortion of the DNA backbone and a disruption of BP stacking are observed because of the structural changes induced by these local perturbations. We also explored how water helps in the entire process. A small number of water molecules undergo rearrangement to stabilize the intermediate states by forming HBs with DNA bases. Water thus acts as a lubricant that counteracts the enthalpic penalty suffered from the loss of native BP contacts. Although the process of bubble formation is reversible, the sequence of steps involved poses an entropic barrier, preventing it from easily retracing the path to the native state.

Thermally Activated vs. Photochemical Hydrogen Evolution Reactions-A Tale of Three Metals.

Molecular processes behind hydrogen evolution reactions can be quite complex. In macroscopic electrochemical cells, it is extremely difficult to elucidate and understand their mechanism. Gas phase models, consisting of a metal ion and a small number of water molecules, provide unique opportunities to understand the reaction pathways in great detail. Hydrogen evolution in clusters consisting of a singly charged metal ion and one to on the order of 50 water molecules has been studied extensively for magnesium, aluminum and vanadium. Such clusters with around 10-20 water molecules are known to eliminate atomic or molecular hydrogen upon mild activation by room temperature black-body radiation. Irradiation with ultraviolet light, by contrast, enables hydrogen evolution already with a single water molecule. Here, we analyze and compare the reaction mechanisms for hydrogen evolution on the ground state as well as excited state potential energy surfaces. Five distinct mechanisms for evolution of atomic or molecular hydrogen are identified and characterized.

Molecular Dynamics Simulations of Cerium Ion Transport Phenomena in Polymer Electrolyte Membranes of Polymer Electrolyte Fuel Cells

We analyzed the transport properties of cerium ions, which are added to improve the chemical durability of a polymer electrolyte membrane (PEM), using molecular dynamics simulations. For the analysis, the diffusion coefficient of cerium ions and the radial distribution function around sulfonic acid groups were obtained. The results show that at low water content, the diffusion coefficient is small because of the direct coordination of cerium ions to the sulfonic acid groups due to the small number of water molecules around the sulfonic acid groups. At high water content, water molecules gather around the sulfonic acid groups, and cerium ions are coordinated far from the sulfonic acid groups, resulting in a weaker binding than at low water content and an increase in the diffusion coefficient of cerium ions.

Self – assembly of model surfactants as reverse micelles in nonpolar solvents and their role as interfacial tension modifiers

The self-assembly of linear surfactants into reverse micelles (RMs) in nonpolar solvents, and their efficiency in reducing the interfacial tension is studied using dissipative particle dynamic simulations. Given the importance of RMs as thickeners, among many other applications, their properties are studied here when formed in oil and in supercritical carbon dioxide (scCO2). Our simulations are found to be in agreement with experimental results of surfactant self-assembly in scCO2 that found viscosity increments of up to 90% with 10 wt % of surfactants. The role played by a small number of water molecules in RM formation is studied as well in both solvents, corroborating experiments reporting the enhancement of RM formation with the addition of a small quantity of water. The dynamics of water surfactant aggregation in nonpolar solvents is also discussed. Lastly, the performance of the model surfactant as an interfacial tension modifier is studied in detail. The results show that RMs are more easily formed in oil than in scCO2 and that the effectiveness of the surfactant in reducing the interfacial tension lies in its preference to adsorb at interfaces rather than self – association.

Evaluating Simple Ab Initio Models of the Hydrated Electron: The Role of Dynamical Fluctuations.

Despite its importance in electron transfer reactions and radiation chemistry, there has been disagreement over the fundamental nature of the hydrated electron, such as whether or not it resides in a cavity. Mixed quantum/classical simulations of the hydrated electron give different structures depending on the pseudopotential employed, and ab initio models of computational necessity use small numbers of water molecules and/or provide insufficient statistics to compare to experimental observables. A few years ago, Kumar et al. (J. Phys. Chem. A 2015, 119, 9148) proposed a minimalist ab initio model of the hydrated electron with only a small number of explicitly treated water molecules plus a polarizable continuum model (PCM). They found that the optimized geometry had four waters arranged tetrahedrally around a central cavity, and that the calculated vertical detachment energy and radius of gyration agreed well with experiment, results that were largely independent of the level of theory employed. The model, however, is based on a fixed structure at 0 K and does not explicitly incorporate entropic contributions or the thermal fluctuations that should be associated with the room-temperature hydrated electron. Thus, in this paper, we extend the model of Kumar et al. by running Born-Oppenheimer molecular dynamics (BOMD) of a small number of water molecules with an excess electron plus PCM at room temperature. We find that when thermal fluctuations are introduced, the level of theory chosen becomes critical enough when only four waters are used that one of the waters dissociates from the cluster with certain density functionals. Moreover, even with an optimally tuned range-separated hybrid functional, at room temperature the tetrahedral orientation of the 0 K first-shell waters is entirely lost and the central cavity collapses, a process driven by the fact that the explicit water molecules prefer to make H-bonds with each other more than with the excess electron. The resulting average structure is quite similar to that produced by a noncavity mixed quantum/classical model, so that the minimalist 4-water BOMD models suffer from problems similar to those of noncavity models, such as predicting the wrong sign of the hydrated electron's molar solvation volume. We also performed BOMD with 16 explicit water molecules plus an extra electron and PCM. We find that the inclusion of an entire second solvation shell of explicit water leads to little change in the outcome from when only four waters were used. In fact, the 16-water simulations behave much like those of water cluster anions, in which the electron localizes at the cluster surface, showing that PCM is not acceptable for use in minimalist models to describe the behavior of the bulk hydrated electron. For both the 4- and 16-water models, we investigate how the introduction of thermal motions alters the predicted absorption spectrum, vertical detachment energy, and resonance Raman spectrum of the simulated hydrated electron. We also present a set of structural criteria that can be used to numerically determine how cavity-like (or not) a particular hydrated electron model is. All of the results emphasize that the hydrated electron is a statistical object whose properties are inadequately captured using only a small number of explicit waters, and that a proper treatment of thermal fluctuations is critical to understanding the hydrated electron's chemical and physical behavior.

Molecular dynamics study of the removal mechanism of SiC in a fixed abrasive polishing in water lubrication

The mechanical removal mechanism of SiC substrates in the water-lubricated environment is investigated by molecular dynamics (MD) simulation. The surface quality, subsurface damage, removal efficiency and structural phase transition of SiC substrates were analyzed in a fixed abrasive polishing (FAP) under various water film thickness and external load. In water-lubricated environment, a small number of water molecules are pressed on the interface between the SiC substrates and the diamond abrasive. The results show that the largest pressing depth decreases with the increase of the water film thickness, contrary to the effect of the external load. The phase transition and the frictional heat both decrease as the pressing depth decreases, which further reduces the stress and temperature of the substrate in the FAP. The water layer has positive impact on the surface roughness and subsurface damage depth. Furthermore, the removal efficiency in the water-lubricated nano-abrading is lower than that in the dry process. Our researches benefit to understand the mechanical removal mechanism of SiC substrates under a water-lubricated environment and give theoretical guide for improving the machining technology of FAP.

A Deep Dive into DNA Base Pairing Interactions Under Water.

Base pairing plays a pivotal role in DNA functions and replication fidelity. But while the complementarity between Watson-Crick matched bases is generally believed to arise from the different number of hydrogen bonds in G|C pairs versus A|T, the energetics of these interactions are heavily renormalized by the aqueous solvent. Employing large-scale Monte Carlo simulations, we have extracted the solvent contribution to the free energy for canonical and some noncanonical and stacked base pairs. For all of them, the solvent's contribution to the base pairing free energy is exclusively destabilizing. While the direct hydrogen bonding interactions in the G|C pair is much stronger than A|T, the thermodynamic resistance produced by the solvent also pushes back much stronger against G|C compared to A|T, generating an only ∼1 kcal/mol free energy difference between them. We have profiled the density of water molecules in the solvent adjacent to the bases and observed a "freezing" behavior where waters are recruited into the gap between the bases to compensate for the unsatisfied hydrogen bonds between them. A very small number of water molecules that are associated with the Watson-Crick donor/acceptor atoms turn out to be responsible for the majority of the solvent's thermodynamic resistance to base pairing. The absence or presence of these near-field waters can be used to enhance fidelity during DNA replication.

A Deep Dive into DNA Base Pairing Interactions Under Water

Base pairing plays a pivotal role in DNA functions and replication fidelity. But while the complementarity between Watson-Crick matched bases is generally believed to arise from the different number of hydrogen bonds in G|C pairs versus A|T, the energetics of these interactions are heavily renormalized by the aqueous solvent. Employing large-scale Monte Carlo simulations, we have extracted the solvent contribution to the free energy for canonical and some noncanonical and stacked base pairs. For all of them, the solvent's contribution to the base pairing free energy is exclusively destabilizing. While the direct hydrogen bonding interactions in the G|C pair is much stronger than A|T, the thermodynamic resistance produced by the solvent also pushes back much stronger against G|C compared to A|T, generating an only ∼1 kcal/mol free energy difference between them. We have profiled the density of water molecules in the solvent adjacent to the bases and observed a freezing behavior where waters are recruited into the gap between the bases to compensate for the unsatisfied hydrogen bonds between them. A very small number of water molecules that are associated with the Watson-Crick donor/acceptor atoms turn out to be responsible for the majority of the solvent's thermodynamic resistance to base pairing. The absence or presence of these near-field waters can be used to enhance fidelity during DNA replication.

Evaluating Simple Ab Initio Models of the Hydrated Electron: The Role of Dynamical Fluctuations

Despite its importance in electron transfer reactions and radiation chemistry, there has been disagreement over the fundamental nature of the hydrated electron, such as whether or not it resides in a cavity. Mixed quantum/classical simulations of the hydrated electron give different structures depending on the pseudopotential employed, and ab initio models of computational necessity use small numbers of water molecules and/or provide insufficient statistics to compare to experimental observables. A few years ago, Kumar et al. (J. Phys. Chem. A 2015, 119, 9148) proposed a minimalist ab initio model of the hydrated electron with only a small number of explicitly treated water molecules plus a polarizable continuum model (PCM). They found that the optimized geometry had four waters arranged tetrahedrally around a central cavity, and that the calculated vertical detachment energy and radius of gyration agreed well with experiment, results that were largely independent of the level of theory employed. The model, however, is based on a fixed structure at 0 K and does not explicitly incorporate entropic contributions or the thermal fluctuations that should be associated with the room-temperature hydrated electron. Thus, in this paper, we extend the model of Kumar et al. by running Born−Oppenheimer molecular dynamics (BOMD) of a small number of water molecules with an excess electron plus PCM at room temperature. We find that when thermal fluctuations are introduced, the level of theory chosen becomes critical enough when only four waters are used that one of the waters dissociates from the cluster with certain density functionals. Moreover, even with an optimally tuned range-separated hybrid functional, at room temperature the tetrahedral orientation of the 0 K first-shell waters is entirely lost and the central cavity collapses, a process driven by the fact that the explicit water molecules prefer to make H-bonds with each other more than with the excess electron. The resulting average structure is quite similar to that produced by a noncavity mixed quantum/classical model, so that the minimalist 4-water BOMD models suffer from problems similar to those of noncavity models, such as predicting the wrong sign of the hydrated electron’s molar solvation volume. We also performed BOMD with 16 explicit water molecules plus an extra electron and PCM. We find that the inclusion of an entire second solvation shell of explicit water leads to little change in the outcome from when only four waters were used. In fact, the 16-water simulations behave much like those of water cluster anions, in which the electron localizes at the cluster surface, showing that PCM is not acceptable for use in minimalist models to describe the behavior of the bulk hydrated electron. For both the 4- and 16-water models, we investigate how the introduction of thermal motions alters the predicted absorption spectrum, vertical detachment energy, and resonance Raman spectrum of the simulated hydrated electron. We also present a set of structural criteria that can be used to numerically determine how cavity-like (or not) a particular hydrated electron model is. All of the results emphasize that the hydrated electron is a statistical object whose properties are inadequately captured using only a small number of explicit waters, and that a proper treatment of thermal fluctuations is critical to understanding the hydrated electron’s chemical and physical behavior.

1-nm-Wide Hydrated Dipole Arrays Regulate AuNW Assembly on Striped Monolayers in Nonpolar Solvent

Summary We show that striped phases of horizontally oriented phospholipids presenting 1-nm-wide orientable dipole arrays can order and straighten flexible 2-nm-diameter gold nanowires (AuNWs) with lengths (up to 1 μm) that greatly exceed the template pitch (∼7.5 nm). AuNW ordering can extend over areas > 100 μm2. Whereas wires bundle in solution, with contact between ligand shells, wires adsorbing to the template separate to much larger center-to-center distances (e.g., 12–30 nm) near multiples of the template pitch, consistent with repulsive interactions between wires emerging during interactions with dipole arrays in the template. AuNW assembly was carried out in cyclohexane, a nonpolar solvent. Interestingly, we found that wire ordering was contingent on the extent of phospholipid headgroup hydration and the presence of excess oleylamine, which assembled into hemicylindrical micelles around the hydrated headgroups. Together, these components generated a protected polar environment that enabled the phospholipid headgroups to function collectively to regulate AuNW assembly.

Structure of Hydrated Crystal (Lc), Tilted Gel (Lβ'), and Liquid Crystal (Lα) Phases of Linear Alkylbenzene Sulfonate (LAS) Studied by X-ray Diffraction and Molecular Dynamics Simulation.

Precipitation of crystals in aqueous surfactant solutions hampers their use as liquid detergents. Understanding the molecular structure that causes the crystal formation is of key importance in designing a new surfactant. In this study, we first identified the hydrated crystal (Lc) structure of linear alkylbenzene sulfonate (LAS) at ambient temperature by combining X-ray diffraction (XRD) with all-atom molecular dynamics (MD) simulation. It is known that the Lc domain involves only a stoichiometrically small number of water molecules, and the water number of existing interlamellar layers may change when the system shows the thermally induced phase transition. Therefore, we explored possible crystal structures with five different hydration levels of 0, 0.5, 1, 2, and 4 water per LAS using MD simulations with the d-spacing measured by XRD. Among them, only the diffraction pattern calculated for the monohydrate LAS well coincided with the experimental diffraction pattern at 300 K. A structural change of the monohydrate from the Lc phase to the tilted gel (Lβ') phase was also observed by heating from 300 to 360 K in the MD simulations, where a cross-sectional structure of the alkyl groups of LAS molecules changed from a face-centered rectangular lattice to a hexagonal one. Further heating to 400 K resulted in a disordered liquid crystal (Lα) phase. Thus, configurational changes of the detergent during thermally induced phase transitions were well explored and characterized by MD simulations with the aid of XRD data. This approach can be broadly applied where we explore the phase behavior of hydrated crystals of any surfactant species.

Uncovering dehydration in cytochrome <i>c</i> refolding from urea- and guanidine hydrochloride-denatured unfolded state by high pressure spectroscopy

To investigate the dehydration associated with protein folding, the partial molar volume changes for protein unfolding (ΔVu) in cytochrome c (Cyt c) were determined using high pressure absorption spectroscopy. ΔVu values for the unfolding to urea- and guanidine hydrochloride (GdnHCl)-denatured Cyt c were estimated to be 56±5 and 29±1 mL mol−1, respectively. Considering that the volume change for hydration of hydrophobic groups is positive and that Cyt c has a covalently bonded heme, a positive ΔVu reflects the primary contribution of the hydration of heme. Because of the marked tendency of guanidium ions to interact with hydrophobic groups, a smaller number of water molecules were hydrated with hydrophobic groups in GdnHCl-denatured Cyt c than in urea-denatured Cyt c, resulting in the smaller positive ΔVu. On the other hand, urea is a relatively weak denaturant and urea-denatured Cyt c is not completely hydrated, which retains the partially folded structures. To unfold such partial structures, we introduced a mutation near the heme binding site, His26, to Gln, resulting in a negatively shifted ΔVu (4±2 mL mol−1) in urea-denatured Cyt c. The formation of the more solvated and less structured state in the urea-denatured mutant enhanced hydration to the hydrophilic groups in the unfolding process. Therefore, we confirmed the hydration of amino acid residues in the protein unfolding of Cyt c by estimating ΔVu, which allows us to discuss the hydrated structures in the denatured states of proteins.

Solvent Networks Tune Thermodynamics of Oligosaccharide Complex Formation in an Extended Protein Binding Site.

The principles of protein-glycan binding are still not well understood on a molecular level. Attempts to link affinity and specificity of glycan recognition to structure suffer from the general lack of model systems for experimental studies and the difficulty to describe the influence of solvent. We have experimentally and computationally addressed energetic contributions of solvent in protein-glycan complex formation in the tailspike protein (TSP) of E.coli bacteriophage HK620. HK620TSP is a 230 kDa native trimer of right-handed, parallel beta-helices that provide extended, rigid binding sites for bacterial cell surface O-antigen polysaccharides. A set of high-affinity mutants bound hexa- or pentasaccharide O-antigen fragments with very similar affinities even though hexasaccharides introduce an additional glucose branch into an occluded protein surface cavity. Remarkably different thermodynamic binding signatures were found for different mutants; however, crystal structure analyses indicated that no major oligosaccharide or protein topology changes had occurred upon complex formation. This pointed to a solvent effect. Molecular dynamics simulations using a mobility-based approach revealed an extended network of solvent positions distributed over the entire oligosaccharide binding site. However, free energy calculations showed that a small water network inside the glucose-binding cavity had the most notable influence on the thermodynamic signature. The energy needed to displace water from the glucose binding pocket depended on the amino acid at the entrance, in agreement with the different amounts of enthalpy-entropy compensation found for introducing glucose into the pocket in the different mutants. Studies with small molecule drugs have shown before that a few active water molecules can control protein complex formation. HK620TSP oligosaccharide binding shows that similar fundamental principles also apply for glycans, where a small number of water molecules can dominate the thermodynamic signature in an extended binding site.

Ice-Liquid Oscillations in Nanoconfined Water.

Nanoscale confinement has a strong effect on the phase behavior of water. Studies in the last two decades have revealed a wealth of novel crystalline and quasicrystalline structures for water confined in nanoslits. Less is known, however, about the nature of ice-liquid coexistence in extremely nanoconfined systems. Here, we use molecular simulations to investigate the ice-liquid equilibrium for water confined between two nanoscopic disks. We find that the nature of ice-liquid phase coexistence in nanoconfined water is different from coexistence in both bulk water and extended nanoslits. In highly nanoconfined systems, liquid water and ice do not coexist in space because the two-phase states are unstable. The confined ice and liquid phases coexist in time, through oscillations between all-liquid and all-crystalline states. The avoidance of spatial coexistence of ice and liquid originates on the non-negligible cost of the interface between confined ice and liquid in a small system. It is the result of the small number of water molecules between the plates and has no analogue in bulk water.

Single Mutations Reshape the Structural Correlation Network of the DMXAA-Human STING Complex.

Subtle changes in protein sequences are able to alter ligand-protein interactions. Unraveling the mechanism of such phenomena is important for understanding ligand-protein interactions, including the DMXAA-STING interaction. DMXAA specifically binds to mouse STING instead of human STING. However, the S162A mutation and a newly discovered E260I mutation endow human STINGAQ with DMXAA sensitivity. Through molecular dynamics simulations, we revealed how these single mutations alter the DMXAA-STING interaction. Compared to mutated systems, structural correlations in the interaction of STINGAQ with DMXAA are stronger, and the correlations are cross-protomers in the dimeric protein. Analyses on correlation coefficients lead to the identification of two key interactions that mediate the strong cross-protomer correlation in the DMXAA-STINGAQ interaction network: DMXAA-267T-162S* and 238R-260E*. These two interactions are partially and totally interrupted by the S162A and E260I mutations, respectively. Moreover, a smaller number of water molecules are displaced upon DMXAA binding to STINGAQ than that on binding to its mutants, leading to a larger entropic penalty for the former. Considering the sensitivity of STINGAQ and two of its mutants to DMXAA, a strong structural correlation appears to discourage DMXAA-STING binding. Such an observation suggests that DMXAA derivatives, which are deprived of hydrogen-bond interaction with both 162S* and 267T, are potential agonists of human STING.

Dehydration of Methanediol in Aqueous Solution: An ONIOM(QM/MM) Study.

We used ONIOM(QM/MM) method to examine the dehydration of a methanediol in aqueous solution. A methanediol and a small number of water molecules in the proximity of the methanediol are calculated with quantum mechanics (QM), while a number of water molecules far from the methanediol are calculated with molecular mechanics (MM). A molecular dynamical simulation shows that 12 water molecules are located within the hydration shell of a methanediol. The energy barrier for the dehydration of a methanediol decreases when we increase the number of water molecules in the QM region and converges toward a finite value when 12 water molecules are included in the QM region. This indicates a significant effect of water molecules within the hydration shell on the dehydration process of a methanediol in aqueous solution. The dehydration rate calculated with the ONIOM(QM/MM) method agrees well with that obtained from a laboratory experiment.

Shock-wave compression of silica gel as a model material for comets

A shock-wave compression experiment using synthesized silica gel was investigated as a model for a comet impact event on the Earth’s surface. The sample shocked at 20.7 GPa showed considerable structural changes, a release of water molecules, and the dehydration of silanol (Si–OH) that led to the formation of a new Si–O–Si network structure containing larger rings (e.g., six-membered ring of SiO4 tetrahedra). The high aftershock temperature at 20.7 GPa, which could be close to 800 °C, influenced the sample structure. However, some silanols, which were presumed to be the mutually hydrogen-bonded silanol group, remained at pressures >20.7 GPa. This type of silanol along with a small number of water molecules may remain even after shock compression at 30.9 GPa, although the intermediate structure of the sample recovered was similar to that of silica glass.

Linear relationship between the 1H-chemical shift of water in a highly concentrated aqueous solution of an ionic liquid and the Jones–Dole B coefficient

In this study, we investigated the chemical shift of water in several ionic liquid-water mixtures using NMR spectroscopy. The chemical shift gradually moved to higher magnetic fields with increasing ionic liquid concentration. Remarkably, the present results indicate that the extrapolated value of the chemical shift at 0mol% water concentration was linearly related to the Jones–Dole B coefficient of the cations and anions, as determined by viscosity measurements. Therefore, the trend for the ion–water interaction strength that is exhibited in water-rich regions holds even in the water pockets confined within the nano-domain structure of ionic liquid, where a very small number of water molecules occupy the limited space.

Neutron scattering studies of the hydration structure of Li+.

New results derived from the experimental method of neutron diffraction and isotopic substitution (NDIS) are presented for the hydration structure of the lithium cation (Li(+)) in aqueous solutions of lithium chloride in heavy water (D2O) at concentrations of 6, 3, and 1 m and at 1.5 m lithium sulfate. By introducing new and more-accurate data reduction procedures than in our earlier studies (I. Howell and G. W. Neilson, J. Phys: Condens. Matter, 1996, 8, 4455-4463), we find, in the first hydration shell of Li(+), ∼4.3(2) water molecules at 6 m, 4.9(3) at 3 m, 4.8(3) at 1 m in the LiCl solutions, and 5.0(3) water molecules in the case of Li2SO4 solution. The general form of the first hydration shell is similar in all four solutions, with the correlations for Li-O and Li-D sited at 1.96 (0.02) Å and 2.58 (0.02) Å, respectively. The results resemble those presented in 1996, in terms of ion-water distances and local coordination, but the hydration number is significantly lower for the case at 1 m than the 6.5 (1.0) given at that time. Thus, experimental and theoretical results now agree that lithium is hydrated by a small number of water molecules (4-5) in the nearest coordination shell.

Hydrophobic Hydration in Water-tert-Butyl Alcohol Solutions by Extended Depolarized Light Scattering.

Molecular dynamics and structural properties of water-tert-butyl alcohol (TBA) mixtures are studied as a function of concentration by extended depolarized light scattering (EDLS) experiments. The wide frequency range, going from fraction to several thousand GHz, explored by EDLS allows distinguishing TBA rotational dynamics from structural relaxation of water and intermolecular vibrational and librational modes of the solution. Contributions to the water relaxation originating from two distinct populations, i.e. hydration and bulk water, are clearly identified. The dynamic retardation factor of hydration water with respect to the bulk, ξ ≈ 4, almost concentration independent, is one of the smallest found by EDLS among a variety of systems of different nature and complexity. This result, together with the small number of water molecules perturbed by the presence of TBA, supports the idea that hydrophobic simple molecules are less effective than hydrophilic and more complex molecules in perturbing the H-bond network of liquid water. At increasing TBA concentrations the average number of perturbed water molecules shows a pronounced decrease and the characteristic frequency of librational motions reduces significantly, both of which are results consistent with the occurrence of self-aggregation of TBA molecules.