Dynamics Of Interfacial Water Research Articles

Comparison of the interfacial dynamics of water sandwiched between static and free-standing fully flexible graphene sheets.

Classical molecular dynamics simulations are used to present a detailed atomistic picture of the instantaneous local structures of water and the structural evolution of stationary and dynamically evolving graphene-water interfaces. The confinement effects are strongly coupled to the nature of the interface, which eventually governs its nanoscopic structural arrangements and interface dynamics. We show that the structure, transport properties, and vibrational densities of states of proximal water molecules are strongly correlated with the nature of the graphene-water interface. We identify and correlate features in vibrational spectra with characteristic structural features observed at the atomic scale for the confined water molecules near a stationary and dynamically evolving hydrophobic surface such as graphene. Our simulations indicate that the local orientation, ordering, and solvation dynamics of interfacial water molecules are a strong function of the graphene slit-width, which is controlled by the nature of the interface (fully flexible vs. static). A monotonic decrease in local ordering with increasing slit-width was observed for the static graphene-water interface, whereas a non-monotonic variation was seen for its fully flexible counterpart. The simulation results offer useful insights into the effect of interfacial dynamics in defining the structure and transport properties at graphene-aqueous media interfaces. Finally these simulations provide a molecular level interpretation of the differential confinement effects arising from the dynamically evolving graphene-water interface.

Direct Probing of Solvent Accessibility and Mobility at the Binding Interface of Polymerase (Dpo4)-DNA Complex

Water plays essential structural and dynamical roles in protein-DNA recognition through contributing to enthalpic or entropic stabilization of binding complex and by mediating intermolecular interactions and fluctuations for biological function. These interfacial water molecules are confined by the binding partners in nanospace, but in many cases they are highly mobile and exchange with outside bulk solution. Here, we report our studies of the interfacial water dynamics in the binary and ternary complexes of a polymerase (Dpo4) with DNA and an incoming nucleotide using a site-specific tryptophan probe with femtosecond resolution. By systematic comparison of the interfacial water motions and local side chain fluctuations in the apo, binary, and ternary states of Dpo4, we observed that the DNA binding interface and active site are dynamically solvent accessible and the interfacial water dynamics are similar to the surface hydration water fluctuations on picosecond time scales. Our molecular dynamics simulations also show the binding interface full of water molecules and nonspecific weak interactions. Such a fluid binding interface facilitates the polymerase sliding on DNA for fast translocation whereas the spacious and mobile hydrated active site contributes to the low fidelity of the lesion-bypass Y-family DNA polymerase.

Hydrogen-Bond Dynamics of Water at the Interface with InP/GaP(001) and the Implications for Photoelectrochemistry

We investigate the structure, topology, and dynamics of liquid water at the interface with natively hydroxylated (001) surfaces of InP and GaP photoelectrodes. Using ab initio molecular dynamics simulations, we show that contact with the semiconductor surface enhances the water hydrogen-bond strength at the interface. This leads to the formation of an ice-like structure, within which dynamically driven water dissociation and local proton hopping are amplified. Nevertheless, the structurally similar and isovalent InP and GaP surfaces generate qualitatively different interfacial water dynamics. This can be traced to slightly more covalent-like character in the binding of surface adsorbates to GaP, which results in a more rigid hydrogen-bond network that limits the explored topological phase space. As a consequence, local proton hopping can give rise to long-range surface proton transport on InP, whereas the process is kinetically limited on GaP. This allows for spatial separation of individual stages of hydrogen-evolving, multistep reactions on InP(001). Possible implications for the mechanisms of cathodic water splitting and photocorrosion on the two surfaces are considered in light of available experimental evidence.

Site-Specific Measurements of Lipid Membrane Interfacial Water Dynamics with Multidimensional Infrared Spectroscopy

One route to accessing site-specific dynamical information available with ultrafast multidimensional infrared spectroscopy is the development of robust and versatile vibrational probes. Here we synthesize and characterize a vibrationally labeled cholesterol derivative, (cholesteryl benzoate) chromium tricarbonyl, to probe model lipid membranes, focusing specifically on the membrane-water interface. Utilizing FTIR and polarized-ATR spectroscopies, we determine the location of the chromium tricarbonyl motif to be situated at the water-membrane interface with an orientation of 46 ± 2° relative to the vector normal to the membrane surface. We test the dynamical sensitivity of the (cholesteryl benzoate) chromium tricarbonyl label with two different nonlinear infrared spectroscopy methods, both of which show that the probe is well-suited to the study of membrane dynamics as well as the dynamics of water at the membrane interface. The metal carbonyl vibrational probe located at the surface of a bicelle exhibits spectral diffusion dynamics induced by membrane hydration water that is roughly three times slower than observed using a nearly identical vibrational probe in bulk water.

Ultrafast interfacial solvation dynamics in specific protein DNA recognition

An overwhelming number of structural and functional studies on specific protein-DNA complexes reveal the existence of water molecules at the interaction interface. What role does the interfacial water molecules play in determining the specificity of association is thus a critical question. Herein, we have explored the dynamical role of minor groove water molecules and DNA side chain flexibility in lambda repressor-operator DNA interaction using well-characterized DNA minor groove binder dye, Hoechst 33258. The most striking finding of our studies reveals that the solvation time scale corresponding to the minor groove water molecules (∼50 ps) and DNA side chain flexibility (∼10 ns) remain unaltered even in protein-DNA complex in comparison to unbound operator DNA. The temperature dependent study further reveals the slower exchange of minor grove water molecules with bulk water in DNA-protein complex in comparison to the unbound DNA. Detailed structural studies including circular dichroism (CD) and Förster resonance energy transfer (FRET) have also been performed to elucidate the interaction between protein and DNA.

Ab initio and classical molecular dynamics studies of the structural and dynamical behavior of water near a hydrophobic graphene sheet

The behavior of water near a graphene sheet is investigated by means of ab initio and classical molecular dynamics simulations. The wetting of the graphene sheet by ab initio water and the relation of such behavior to the strength of classical dispersion interaction between surface atoms and water are explored. The first principles simulations reveal a layered solvation structure around the graphene sheet with a significant water density in the interfacial region implying no drying or cavitation effect. It is found that the ab initio results of water density at interfaces can be reproduced reasonably well by classical simulations with a tuned dispersion potential between the surface and water molecules. Calculations of vibrational power spectrum from ab initio simulations reveal a shift of the intramolecular stretch modes to higher frequencies for interfacial water molecules when compared with those of the second solvation later or bulk-like water due to the presence of free OH modes near the graphene sheet. Also, a weakening of the water-water hydrogen bonds in the vicinity of the graphene surface is found in our ab initio simulations as reflected in the shift of intermolecular vibrational modes to lower frequencies for interfacial water molecules. The first principles calculations also reveal that the residence and orientational dynamics of interfacial water are somewhat slower than those of the second layer or bulk-like molecules. However, the lateral diffusion and hydrogen bond relaxation of interfacial water molecules are found to occur at a somewhat faster rate than that of the bulk-like water molecules. The classical molecular dynamics simulations with tuned Lennard-Jones surface-water interaction are found to produce dynamical results that are qualitatively similar to those of ab initio molecular dynamics simulations.

Molecular dynamics simulations of an electrified water/Pt(1 1 1) interface using point charge dissociative water

Water dissociation and structural diffusion of protons at the electrified water/Pt(111) interface are examined using molecular dynamics simulations. A combination of reactive water with a Pt(111) electrode under constant potential conditions is unique and relevant to fuel cell relevant electrochemistry. We use a modified central force model to describe reactive water and electrode charge dynamics to describe the electrode. We perform control simulations using SPC/E water, a contrast that clarifies when the influence of water ions on interfacial water structure and dynamics and electrochemical properties can no longer be neglected. Both mCF and SPC/E water have structured interfacial water layers regardless of electrode potential, but a reactive model is important when considering water structure away from the surface and interfacial dynamics. As opposed to SPC/E water, an applied potential does not induce preferred water orientation for mCF water in the middle of the electrolyte, despite the fact that interfacial OH and H3O ions cannot completely screen the electrode potential. This occurs because fast exchanges among OH/H2O/H3O relax the electric field constraint in the surface normal direction. mCF water accurately describes the timescales of hydrogen bond vibration and structural diffusion of both hydronium and hydroxyl ions. This simple reactive water model distinguishes structural diffusion between H3O and OH ions, where H3O ion transfer is three times faster than OH ion transfer. The model allows us to determine the influence of applied potential and H3O/OH ions on charge transfer effectiveness near the electrode surface, directly relevant to fuel cell electrochemistry.

Structure and Dynamics of Interfacial Water Studied by Heterodyne-Detected Vibrational Sum-Frequency Generation

Vibrational sum-frequency generation (VSFG) spectroscopy is a powerful tool to study interfaces. Recently, multiplex heterodyne-detected VSFG (HD-VSFG) has been developed, which enables the direct measurement of complex second-order nonlinear susceptibility [χ((2))]. HD-VSFG has remarkable advantages over conventional VSFG. For example, the imaginary part of χ((2)) [Imχ((2))] obtained with this interferometric technique is the direct counterpart to the infrared [Imχ((1))] and Raman [Imχ((3))] spectra in the bulk, and it is free from the spectral deformation inevitable in conventional VSFG [|χ((2))|(2)] spectra. The Imχ((2)) signal is obtained with a sign that contains unambiguous information about the up/down orientation of interfacial molecules. Furthermore, HD-VSFG can be straightforwardly extended to time-resolved measurements when combined with photoexcitation. In this review, we describe the present status of experiments and applications of multiplex HD-VSFG spectroscopy, in particular with regard to the orientation and structure of interfacial water at charged, neutral, and biorelevant water interfaces.

Simulation of Influence of Bilayer Melting on Dynamics and Thermodynamics of Interfacial Water

We investigate the effect of bilayer melting transition on thermodynamics and dynamics of interfacial water using molecular dynamics simulation with the two-phase thermodynamic model. We show that the diffusivity of interface water depicts a dynamic crossover at the chain melting transition following an Arrhenius behavior until the transition temperature. The corresponding change in the diffusion coefficient from the bulk to the interface water is comparable with experimental observations found recently for water near 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) vesicles [Phys. Chem. Chem. Phys. 13, 7732 (2011)]. The entropy and potential energy of interfacial water show distinct changes at the bilayer melting transition, indicating a strong correlation in the thermodynamic state of water and the accompanying first-order phase transition of the bilayer membrane.

The interfacial dynamics of water sandwiched between graphene sheets are governed by the slit width

Atomic scale characterization and fluxional properties of water molecules in the vicinity of the graphene interface is carried out using molecular dynamics (MD) simulations. The structural properties of proximal water molecules near the graphene interface are strongly correlated to their vibrational densities of states while being studied as a function of the slit width of the graphene sheets. Our simulations indicate that the local orientation, ordering and solvation dynamics of interfacial water molecules are a strong function of the graphene slit width. Systematic trends in libration, bending, and stretching bands are correlated with local ordering of water molecules and hydrogen-bonding network. Smaller blue shifts in the intermolecular O…O…O bending mode and larger blue shifts in the O…O intermolecular stretching modes of water molecules are observed for strongly confined water molecules in comparison to bulk water, which is attributed to the interfacial proximity effects resulting in the restricted transverse oscillations of confined water. The O―H stretching band is red-shifted for confined water in comparison to bulk water whereas the libration and bending bands for interfacial water are blue shifted with respect to bulk water. The observed frequency shifts are a consequence of the distortion of the tetrahedral order in confined water caused by lateral diffusion being reduced and also by changes in the distribution of hydrogen bonds. These simulations suggest that the extent of the shifts of confined water in comparison to bulk water are due to the proximity from the hydrophobic surface, their local confinement and hydrogen bonding status.

Ultrafast Vibrational Dynamics of a Charged Aqueous Interface by Femtosecond Time-Resolved Heterodyne-Detected Vibrational Sum Frequency Generation

Abstract Newly developed time-, frequency-, and phase-resolved heterodyne-detected vibrational sum frequency generation (TR-HD-VSFG) spectroscopy opens a new door to explore ultrafast vibrational dynamics of interfacial water. Here, we present the first observation of TR-HD-VSFG spectra of water at a charged monolayer/aqueous solution interface.

Further evidence that interfacial water is the main “driving force” of protein dynamics: a neutron scattering study on perdeuterated C-phycocyanin

The fundamental role of hydration water (also called interfacial water) is widely recognized in protein flexibility, especially in the existence of the so-called protein "dynamical transition" at around 220 K. In the present study, we take advantage of perdeuterated C-phycocyanin (CPC) and elastic incoherent neutron scattering (EINS) to distinguish between protein dynamics and interfacial water dynamics. Powders of hydrogenated (hCPC) and perdeuterated (dCPC) CPC protein have been hydrated, respectively, with D(2)O or H(2)O and measured by EINS to separately probe protein dynamics (hCPC/D(2)O) and water dynamics (dCPC/H(2)O) at different time- and length-scales. We find that "fast" (<20 ps) local mean-square displacements (MSD) of both protein and interfacial water coincide all along the temperature range, with the same dynamical transition temperature at ~220 K. On higher resolution (<400 ps), two different types of motions can be separated: (i) localized motions with the same amplitude for CPC and hydration water and two transitions at ~170 and ~240 K for both; (ii) large scale fluctuations exhibiting for both water molecules and CPC protein a single transition at ~240 K, with a significantly higher amplitude for the interfacial water than for CPC. Moreover, by comparing these motions with bulk water MSD measured under the same conditions, we show no coupling between bulk water dynamics and protein dynamics all along the temperature range. These results show that interfacial water is the main "driving force" governing both local and large scale motions in proteins.

Vibrational Spectroscopy of Water at Interfaces

Understanding liquid water's behavior at the molecular level is essential to progress in fields as disparate as biology and atmospheric sciences. Moreover, the properties of water in bulk and water at interfaces can be very different, making the study of the hydrogen-bonding networks therein very important. With recent experimental advances in vibrational spectroscopy, such as ultrafast pulses and heterodyne detection, it is now possible to probe the structure and dynamics of bulk and interfacial water in unprecedented detail. We consider here three aqueous interfaces: the water liquid-vapor interface, the interface between water and the surfactant headgroups of reverse micelles, and the interface between water and the lipid headgroups of aligned multi-bilayers. In the first case, sum-frequency spectroscopy is used to probe the interface. In the second and third cases, the confined water pools are sufficiently small that techniques of bulk spectroscopy (such as FTIR, pump-probe, two-dimensional IR, and the like) can be used to probe the interfacial water. In this Account, we discuss our attempts to model these three systems and interpret the existing experiments. For the water liquid-vapor interface, we find that three-body interactions are essential for reproducing the experimental sum-frequency spectrum, and presumably for the structure of the interface as well. The observed spectrum is interpreted as arising from overlapping and canceling positive and negative contributions from molecules in different hydrogen-bonding environments. For the reverse micelles, our theoretical models confirm that the experimentally observed blue shift of the water OD stretch (for dilute HOD in H(2)O) arises from weaker hydrogen bonding to sulfonate oxygens. We interpret the observed slow-down in water rotational dynamics as arising from curvature-induced frustration. For the water confined between lipid bilayers, our theoretical models confirm that the experimentally observed red shift of the water OD stretch arises from stronger hydrogen bonding to phosphate oxygens. We develop a model for heterogeneous vibrational lifetime distributions, and we implement the model to calculate isotropic and anisotropic pump-probe decays. We then compare these results with experimental data. Clearly, recent experimental advances in vibrational spectroscopy have led to beautiful new results, providing information about the structure and dynamics of water at interfaces. These experimental and concomitant theoretical advances (particularly the unified theoretical framework of non-linear response functions) have greatly contributed to our understanding of this unique and important substance.

Alkali Metal and H2O Dynamics at the Smectite/Water Interface

Molecular-scale dynamic processes involving ions and water at smectite–water interfaces play crucial roles in issues such as contaminant transport, reactivity of geochemical systems, and carbon sequestration, yet little is known about the specific manner in which interfacial ion and water dynamics influence one another, particularly at temperatures below 0 °C. In this work, we discuss the effects of the charge-balancing cation on the structure and dynamics of alkali metals and H2O at the mineral–water interfaces of alkali metal–smectite pastes over a broad range of temperatures. New variable-temperature 2H and 23Na NMR spectroscopic data for a Na–hectorite paste presented here in combination with the results from our previous studies of Cs– and K–hectorite pastes reveal a common anisotropic mechanism of 2H motion for 2H2O restricted by proximity to a surface or cation between −50 and −20 °C. This motion is well modeled by combined fast-limit C2 librations about the 2H2O molecular dipole moment and fast-limit octahedral-type jumps of 2H2O molecules about the C3 symmetry axis of a slightly compressed metal–H2O complex. At higher temperatures, 2H2O dynamics are dominated by diffusion and/or chemical exchange of deuterons and differ for Na- and K-exchanged samples. Comparing our collective 39K, 133Cs, and 23Na VT NMR results shows that Na+ has less affinity for the smectite surface in pastes than K+ or Cs+, that the influence of 2H2O and 2H2O content on alkali metal motion decreases from Na+ to K+ to Cs+, and that slow-to-intermediate (rate < 104–105 Hz) two-site exchange is a significant dynamic process above −80 °C only for Cs+.

Ultrafast vibrational energy transfer at the water/air interface revealed by two-dimensional surface vibrational spectroscopy

Water is very different from liquids of similar molecular weight, and one of its unique properties is the very efficient transfer of vibrational energy between molecules, which arises as a result of strong dipole-dipole interactions between the O-H oscillators. Although we have a sound understanding of such energy transfer in bulk water, we know less about how, and how quickly, transfer occurs at its interface with a hydrophobic phase, because specifically addressing the outermost monolayer is difficult. Here, we use ultrafast two-dimensional surface-specific vibrational spectroscopy to probe the interfacial energy dynamics of heavy water (D(2)O) at the water/air interface. The measurements reveal the presence of surprisingly rapid energy transfer, both between hydrogen-bonded interfacial water molecules (intermolecular), and between O-D groups sticking out from the water surface and those located on the same molecule and pointing towards the water bulk (intramolecular). Vibrational energy transfer occurs on sub-picosecond timescales, and its rates and pathways can be quantified directly.

Ultrafast Reorientation of Dangling OH Groups at the Air-Water Interface Using Femtosecond Vibrational Spectroscopy

We report the real-time measurement of the ultrafast reorientational motion of water molecules at the water-air interface, using femtosecond time- and polarization-resolved vibrational sum-frequency spectroscopy. Vibrational excitation of dangling OH bonds along a specific polarization axis induces a transient anisotropy that decays due to the reorientation of vibrationally excited OH groups. The reorientation of interfacial water is shown to occur on subpicosecond time scales, several times faster than in the bulk, which can be attributed to the lower degree of hydrogen bond coordination at the interface. Molecular dynamics simulations of interfacial water dynamics are in quantitative agreement with experimental observations and show that, unlike in bulk, the interfacial reorientation occurs in a largely diffusive manner.

Nonlinear spectroscopy of bio-interfaces

In the past five years the Max Planck Research Group for Nonlinear spectroscopy of bio-interfaces has worked at the intersection of surface science, nonlinear optics and soft and biological matter. For hard matter there is molecular level understanding of the surface region. This has enabled researchers to develop the highly complex structures that are nowadays found in (e. g.) computers, and mobile phones. Although biological and soft systems (such as liquid droplets, nanoparticles, liposomes, and viruses) are ubiquitous in our daily lives, our understanding is often limited to macroscopic theories. Consequently, the level of control of soft and biological matter is largely empirical and far behind that of hard matter. To initiate a change, we have in the past period worked on three main themes: (i) The investigation of structure and properties of biologically and medically relevant interfaces (supported lipid bilayers, biopolymer interfaces, water, protein-surface interactions). (ii) Development of nonlinear light scattering techniques as probes for molecular structure at the interfaces of small particles. (iii) The study of molecular interfacial structure, kinetics and dynamics of water, surfactants, phospholipids, sugars and peptides on small objects (nano-droplets, vesicles) in suspensions. The result of our work on these themes is summarized for this special issue article.

Effect of Electric Fields on the Ultrafast Vibrational Relaxation of Water at a Charged Solid–Liquid Interface as Probed by Vibrational Sum Frequency Generation

The effect of the surface electric field on the ultrafast vibrational dynamics of interfacial water is studied using IR pump-sum frequency generation (SFG) probe spectroscopy. At very low salt concentrations, a vibrational lifetime (T1) of ∼200 fs, similar to bulk H2O, is observed for the O–H stretch at the H2O/silica interface at pH 6, where the silica surface is negatively charged. However, T1 increases to ∼700 fs by increasing the NaCl concentration to 0.01 M. The observation of similar dynamics for a range of salt concentrations, associated with different extensions of the electric field, suggest that the surface electric field is screened faster than predicted by classical electrical double-layer theories and that the Debye length may not be the appropriate measure of the depth sampling of the SFG response. An interfacial excess of cations is hypothesized to explain the faster decay of the static electric field than predicted by the Gouy–Chapman theory.

Impact of Hydrophilic Surfaces on Interfacial Water Dynamics Probed with NMR Spectroscopy

In suspensions of Nafion beads and of cationic gel beads, NMR spectroscopy showed two water-proton resonances, one representing intimate water layers next to the polymer surface, the other corresponding to water lying beyond. Both resonances show notably shorter spin-lattice relaxation times (T1) and smaller self-diffusion coefficients (D) indicating slower dynamics than bulk water. These findings confirm the existence of highly restricted water layers adsorbed onto hydrophilic surfaces and dynamically stable water beyond the first hydration layers. Thus, aqueous regions on the order of micrometers are dynamically different from bulk water.

Orientational dynamics for an amphiphilic-solvent solution

In this work, we performed Monte Carlo simulations on a lattice model for spontaneous amphiphilic aggregation, in order to study the orientational and hydrogen-bonding dynamics of water on different regions inside the micellar solution. We employed an associating lattice gas model that mimics the aqueous solvent, which presents a rich phase diagram with first- and second-order transition lines. Even though this is a simplified model, it makes possible to investigate the orientational dynamics of water in an equilibrium solution of amphiphiles, as well as the influence of the different phases of the solvent in the interfacial and bulk water dynamics. By means of extensive simulations, we showed that, at high temperatures, the behavior of the orientational relaxation and hydrogen bonding of water molecules in the bulk, first, and second hydration shells are considerable different. We observe the appearance of a very slow component for water molecules in the first hydration shell of micelles when the system reaches a high-density phase, consistent with previous theoretical and experimental studies concerning biological water. Also, at high temperatures, we find that water molecules in the second hydration shell of micelles have an orientational decay similar to that of bulk water, but with a generally slower dynamics. Otherwise, at low temperatures, we have two components for the orientational relaxation of bulk water in the low density liquid phase, and only a single component in the high density liquid (HDL) phase, which reflect the symmetry properties of the different phases of the solvent model. In the very dense region of water molecules in the first hydration shell of micelles at low temperatures, we find two components for the orientational relaxation on both liquid phases, one of them much slower than that in the single component of bulk water in the HDL phase. This happens even though our model does not present any hindrance to the water rotational freedom caused by the presence of the amphiphiles.