Inelastic incoherent neutron scattering studies of water interacting with biological macromolecules

What to feed or what not to feed-that is still the question

INTRODUCTION

This review addresses metabolic diversities after grain feeding of cattle using artificial total mixed ration (TMR), in place of pasture-based feeding.

OBJECTIVES

To determine how grain feeding impairs the deuterium-depleting functions of the anaplerotic mitochondrial matrix during milk and meat production.

METHODS

Based on published data we herein evaluate how grain-fed animals essentially follow a branched-chain amino acid and odd-chain fatty acid-based reductive carboxylation-dependent feedstock, which is also one of the mitochondrial deuterium-accumulating dysfunctions in human cancer.

RESULTS

It is now evident that food-based intracellular deuterium exchange reactions, especially that of glycogenic substrate oxidation, are significant sources of deuterium-enriched (²H; D) metabolic water with a significant impact on animal and human health. The burning of high deuterium nutritional dairy products into metabolic water upon oxidation in the human body may contribute to similar metabolic conditions and diseases as described in state-of-the-art articles for cows. Grain feeding also limits oxygen delivery to mitochondria for efficient deuterium-depleted metabolic water production by glyphosate herbicide exposure used in genetically modified crops of TMR constituents.

CONCLUSION

Developments in medical metabolomics, biochemistry and deutenomics, which is the science of biological deuterium fractionation and discrimination warrant urgent critical reviews in order to control the epidemiological scale of population diseases such as diabetes, obesity and cancer by a thorough understanding of how the compromised metabolic health of grain-fed dairy cows impacts human consumers.

Interfacial water morphology in hydrated melanin

The importance of electrically functional biomaterials is increasing as researchers explore ways to utilise them in novel sensing capacities. It has been recognised that for many of these materials the state of hydration is a key parameter that can heavily affect the conductivity, particularly those that rely upon ionic or proton transport as a key mechanism. However, thus far little attention has been paid to the nature of the water morphology in the hydrated state and the concomitant ionic conductivity. Presented here is an inelastic neutron scattering (INS) experiment on hydrated eumelanin, a model bioelectronic material, in order to investigate its 'water morphology'. We develop a rigorous new methodology for performing hydration dependent INS experiments. We also model the eumelanin dry spectra with a minimalist approach whereas for higher hydration levels we are able to obtain difference spectra to extract out the water scattering signal. A key result is that the physi-sorbed water structure within eumelanin is dominated by interfacial water with the number of water layers between 3-5, and no bulk water. We also detect for the first time, the potential signatures for proton cations, most likely the Zundel ion, within a biopolymer/water system. These new signatures may be general for soft proton ionomer systems, if the systems are comprised of only interfacial water within their structure. The nature of the water morphology opens up new questions about the potential ionic charge transport mechanisms within hydrated bioelectronics materials.

Probing Dynamics of Water Mass Transfer in Organic Porous Photocatalyst Water-Splitting Materials by Neutron Spectroscopy

The quest for efficient and economically accessible cleaner methods to develop sustainable carbon-free energy sources induced a keen interest in the production of hydrogen fuel. This can be achieved via the water-splitting process and by exploiting solar energy. However, the use of adequate photocatalysts is required to reach this goal. Covalent triazine-based frameworks (CTFs) are potential target photocatalysts for water splitting. Both electronic and structural characteristics of CTFs, particularly energy levels, optical band gaps, and porosities are directly relevant to water splitting and can be engineered through chemical design. Porosity can, in principle, be beneficial to water splitting by providing a larger surface area for the catalytic reactions to take place. However, porosity can also affect both charge transport within the photocatalyst and mass transfer of both reactants and products, thus impacting the overall kinetics of the reaction. Here, we focus on the link between chemical design and water (reactant) mass transfer, which plays a key role in the water uptake process and the subsequent hydrogen generation in practice. We use neutron spectroscopy to study the mass transfer of water in two porous CTFs, CTF-CN and CTF-2, that differ in the polarity of their struts. Quasi-elastic neutron scattering is used to quantify the amount of bound water and the translational diffusion of water. Inelastic neutron scattering measurements complement the quasi-elastic neutron scattering study and provide insights into the softness of the CTF structures and the changes in librational degrees of freedom of water in the porous CTFs. We show that two different types of interaction between water and CTFs take place in CTF-CN and CTF-2. CTF-CN exhibits a smaller surface area and lower water uptake due to its softer structure than CTF-2. However, the polar cyano group interacts locally with water leading to a large amount of bound water and a strong rearrangement of the water hydration monolayer, while water diffusion in CTF-2 is principally impacted by microporosity. The current study leads to new insights into the structure-dynamics-property relationship of CTF photocatalysts that pave the road for a better understanding of the guest-host interaction on the basis of water-splitting applications.

Impact of Chemical Structure on the Dynamics of Mass Transfer of Water in Conjugated Microporous Polymers: A Neutron Spectroscopy Study

Hydrogen fuel can contribute as a masterpiece in conceiving a robust carbon-free economic puzzle if cleaner methods to produce hydrogen become technically efficient and economically viable. Organic photocatalytic materials such as conjugated microporous materials (CMPs) are potential attractive candidates for water splitting as their energy levels and optical band gap as well as porosity are tunable through chemical synthesis. The performances of CMPs depend also on the mass transfer of reactants, intermediates, and products. Here, we study the mass transfer of water (H2O and D2O) and of triethylamine, which is used as a hole scavenger for hydrogen evolution, by means of neutron spectroscopy. We find that the stiffness of the nodes of the CMPs is correlated with an increase in trapped water, reflected by motions too slow to be quantified by quasi-elastic neutron scattering (QENS). Our study highlights that the addition of the polar sulfone group results in additional interactions between water and the CMP, as evidenced by inelastic neutron scattering (INS), leading to changes in the translational diffusion of water, as determined from the QENS measurements. No changes in triethylamine motions could be observed within the CMPs from the present investigations.

A New Look into the Mode of Action of Metal-Based Anticancer Drugs

The mode of action of Pt- and Pd-based anticancer agents (cisplatin and Pd₂Spm) was studied by characterising their impact on DNA. Changes in conformation and mobility at the molecular level in hydrated DNA were analysed by quasi-elastic and inelastic neutron scattering techniques (QENS and INS), coupled to Fourier transform infrared (FTIR) and microRaman spectroscopies. Although INS, FTIR and Raman revealed drug-triggered changes in the phosphate groups and the double helix base pairing, QENS allowed access to the nanosecond motions of the biomolecule’s backbone and confined hydration water within the minor groove. Distinct effects were observed for cisplatin and Pd₂Spm, the former having a predominant effect on DNA’s spine of hydration, whereas the latter had a higher influence on the backbone dynamics. This is an innovative way of tackling a drug’s mode of action, mediated by the hydration waters within its pharmacological target (DNA).

Water dynamics in MCF-7 breast cancer cells: a neutron scattering descriptive study

Water mobility in cancer cells could be a powerful parameter to predict the progression or remission of tumors. In the present descriptive work, new insight into this concept was achieved by combining neutron scattering and thermal analyses. The results provide the first step to untangle the role played by water dynamics in breast cancer cells (MCF-7) after treatment with a chemotherapy drug. By thermal analyses, the cells were probed as micrometric reservoirs of bulk-like and confined water populations. Under this perspective we showed that the drug clearly alters the properties of the confined water. We have independently validated this idea by accessing the cellular water dynamics using inelastic neutron scattering. Finally, analysis of the quasi-elastic neutron scattering data allows us to hypothesize that, in this particular cell line, diffusion increases in the intracellular water in response to the action of the drug on the nanosecond timescale.

Reversible Hydrophobicity-Hydrophilicity Transition Modulated by Surface Curvature

Wettability (hydrophobicity and hydrophilicity) is of fundamental importance in physical, chemical, and biological behaviors, resulting in widespread interest. Herein, by modulating surface curvature, we observed a reversible hydrophobic-hydrophilic transition on a model referred to a platinum surface. The underlying mechanism is revealed to be the competition between strong water-solid attraction and interfacial water orderliness. On the basis of the competition, we further propose an equation of wetting transition in the presence of an ordered interfacial liquid. It quantitatively reveals the relation of solid wettability with interfacial water orderliness and solid surface curvature, which can be used for predicting the critical point of the wetting transition. Our findings thus provide an innovative perspective on the design of a functional device demonstrating a reversible wettability transition and even a molecular-level understanding of biological functions.

Intracellular water – an overlooked drug target? Cisplatin impact in cancer cells probed by neutrons

Intracellular water as a secondary pharmacological target?

Computational study of inelastic neutron scattering vibrational spectra of water clusters and their relevance to hydration water in proteins

BACKGROUND

Inelastic neutron scattering (INS) vibrational spectra for hydration water in proteins can be obtained from spectral differences, but their interpretation has mainly been limited to comparisons with various forms of ice at high hydration levels without making use of available structural information from neutron protein crystallography.

METHODS

The INS vibrational spectra of free and partially constrained water clusters (up to n=17) were calculated with DFT methods using published energy-minimized structures.

RESULTS

Reference is made to neutron diffraction studies of hydrated proteins, which contain a wealth of structural information both on individual water molecules and small clusters in the inner "shell" in order to select representative clusters to serve as models for bound, rather than free clusters as they would occur in a protein.

CONCLUSIONS

INS spectra of the water librational region calculated for a combination of model bound clusters provide a qualitative account of the essentially featureless experimental spectra on water in proteins at very low hydration levels, but do indicate that the well-known rise in intensity near 500cm^-1 is connected to increasing numbers of four-coordinate water molecules in larger clusters.

GENERAL SIGNIFICANCE

The combination of structural information of hydration water from neutron protein crystallography with much more sophisticated computational methods than used herein should lead to a much more detailed picture of the hydration of proteins. This article is part of a Special Issue entitled "Science for Life" Guest Editor: Dr. Austen Angell, Dr. Salvatore Magazù and Dr. Federica Migliardo.

Probing Interfacial Water on Nanodiamonds in Colloidal Dispersion

The structure of interfacial water layers around nanoparticles dispersed in an aqueous environment may have a significant impact on their reactivity and on their interaction with biological species. Using transmission soft X-ray absorption spectroscopy in liquid, we demonstrate that the unoccupied electronic states of oxygen atoms from water molecules in aqueous colloidal dispersions of nanodiamonds have a different signature than bulk water. X-ray absorption spectroscopy can thus probe interfacial water molecules in colloidal dispersions. The impacts of nanodiamond surface chemistry and concentration on interfacial water electronic signature are discussed.

Collective density fluctuations of DNA hydration water in the time-window below 1 ps

The coherent density fluctuations propagating through DNA hydration water were studied by neutron scattering spectroscopy. Two collective modes were found to be sustained by the aqueous solvent: a propagating excitation, characterised by a speed of about 3500 m/s, and another one placed at about 6 meV. These results globally agree with those previously found for the coherent excitations in bulk water, although in DNA hydration water the speed of propagating modes is definitely higher than that of the pure solvent. The short-wavelength collective excitations of DNA hydration water are reminiscent of those observed in protein hydration water and in the amorphous forms of ice.

H-bond network in amino acid cocrystals with H2O or H2O2. The DFT study of serine-H2O and serine-H2O2

The structure, IR spectrum, and H-bond network in the serine-H(2)O and serine-H(2)O(2) crystals were studied using DFT computations with periodic boundary conditions. Two different basis sets were used: the all-electron Gaussian-type orbital basis set and the plane wave basis set. Computed frequencies of the IR-active vibrations of the titled crystals are quite different in the range of 10-100 cm(-1). Harmonic approximation fails to reproduce IR active bands in the 2500-2800 frequency region of serine-H(2)O and serine-H(2)O(2). The bands around 2500 and 2700 cm(-1) do exist in the anharmonic IR spectra and are caused by the first overtone of the OH bending vibrations of H(2)O and a combination vibration of the symmetric and asymmetric bendings of H(2)O(2). The quantum-topological analysis of the crystalline electron density enables us to describe quantitatively the H-bond network. It is much more complex in the title crystals than in a serine crystal. Appearance of water leads to an increase of the energy of the amino acid-amino acid interactions, up to ~50 kJ/mol. The energy of the amino acid-water H-bonds is ~30 kJ/mol. The H(2)O/H(2)O(2) substitution does not change the H-bond network; however, the energy of the amino acid-H(2)O(2) contacts increases up to 60 kJ/mol. This is caused by the fact that H(2)O(2) is a much better proton donor than H(2)O in the title crystals.

Low-frequency vibrational spectrum of water around cyclodextrin and its methyl-substituted derivatives

Atomistic molecular dynamics (MD) simulations have been carried out to study the low-frequency intermolecular vibrational spectrum of water present in the surrounding hydration layers and inside the cavities of beta-cyclodextrin (BCD) and its di- and trimethyl substituted derivatives, namely, heptakis(2,6-di-O-methyl)-beta-cyclodextrin (DIMEB) and heptakis(2,3,6-tri-O-methyl)-beta-cyclodextrin (TRIMEB) in aqueous solutions. Attempts have been made to explore the effects of confinement in and around these cyclic macromolecules and the formation of hydrogen bonds between water and the glucopyranose rings on the distribution of the intermolecular vibrational density of states of water. The calculations revealed that compared to bulk water these bands are blue-shifted for water in proximity to these molecules, the extents of the shifts being more pronounced for the cavity water molecules. It is further noticed that the relatively more restricted local motions of water bound to the cyclodextrins by hydrogen bonds result in larger blue shifts of these bands. These results can be verified by suitable experiments.

C-phycocyanin hydration water dynamics in the presence of trehalose: an incoherent elastic neutron scattering study at different energy resolutions

We present a study of C-phycocyanin hydration water dynamics in the presence of trehalose by incoherent elastic neutron scattering. By combining data from two backscattering spectrometers with a 10-fold difference in energy resolution we extract a scattering law S(Q,omega) from the Q-dependence of the elastic intensities without sampling the quasielastic range. The hydration water is described by two dynamically different populations--one diffusing inside a sphere and the other diffusing quasifreely--with a population ratio that depends on temperature. The scattering law derived describes the experimental data from both instruments excellently over a large temperature range (235-320 K). The effective diffusion coefficient extracted is reduced by a factor of 10-15 with respect to bulk water at corresponding temperatures. Our approach demonstrates the benefits and the efficiency of using different energy resolutions in incoherent elastic neutron scattering over a large angular range for the study of biological macromolecules and hydration water.

The Effects of Hydrogen-Bonding Environment on the Polarization and Electronic Properties of Water Molecules

Adequate representation of the interactions that take place between water molecules has long been a goal of force field design. A full understanding of how the molecular charge distribution of water is altered by adjacent water molecules and by the hydrogen-bonding environment is a vital step toward achieving this task. For this purpose we generated ab initio electron densities of pure water clusters and hydrated serine and tyrosine. Quantum chemical topology enabled the study of a well-defined water molecule inside these clusters, by means of its volume, energy, and multipole moments. Intra- and intermolecular charge transfer was monitored and related to the polarization of water in hydrogen-bonded networks. Our analysis affords a way to define different types of water molecules in clusters.

Exploring DNA groove water dynamics through hydrogen bond lifetime and orientational relaxation

Dynamics of water molecules in the grooves of DNA are of great interest both for practical (functionality of DNA) and fundamental (as examples of confined systems) interest. Here the authors employ atomistic molecular dynamics simulations to understand varying water dynamics at the minor and the major grooves of a 38 base-pair long DNA duplex in water. In order to understand and quantify the diversity in the nature of hydrogen bond due to many hydrogen bond donors and acceptors present in the four bases, they have undertaken study of hydrogen bond lifetime (HBLT) correlation functions of all the specific hydrogen bonds between the base atoms and water molecules. They find that the HBLT correlation functions are in general multiexponential, with the average lifetime depending significantly on the specificity and may thus be biologically relevant. The average hydrogen bond lifetime is longer in the minor groove than that in the major groove by almost a factor of 2. Analysis further shows that water hydrogen bonds with phosphate oxygen have substantially shorter lifetimes than those with the groove atoms. They also compute two different orientational time correlation functions (OTCFs) of the water molecules present at the major and the minor grooves and attempt to correlate OTCF with HBLT correlation function. The OTCFs in the minor groove exhibit three time scales, with the time constant of the slowest component one to two orders of magnitude longer than what is observed for bulk water. A slow component is also present for the major groove water but with shorter time constant. Interestingly, correlation between reformations allowed HBLT correlation function [C(HB)(t)] and the OTCF markedly deviates from each other in the grooves, indicating enhanced rigidity of water molecules in the grooves.

Do we underestimate the importance of water in cell biology?

Liquid water is a highly versatile material. Although it is formed from the tiniest of molecules, it can shape and control biomolecules. The hydrogen-bonding properties of water are crucial to this versatility, as they allow water to execute an intricate three-dimensional 'ballet', exchanging partners while retaining complex order and enduring effects. Water can generate small active clusters and macroscopic assemblies, which can both transmit information on different scales.

Vibrational Spectroscopic Studies of the Interaction of Water with Serine

The vibrational dynamics of water around serine was investigated by using Raman spectroscopy and inelastic incoherent neutron scattering. Experiments with serine in deuterium oxide were performed to assist the assignment. The study shows that for the serine, the exchange of protons-deuterons on the active -NH3+ and -OH groups were relatively easy, whereas there were hardly any exchanged on the -CH or -CH2- groups. The main features of the spectra for hydrated samples (versus the dry samples) were altered considerably; new sharp peaks in the measured spectra appeared, indicating that the hydrogen bonding between water and serine had disturbed the structure of the serine molecule.

Thermodynamic and dynamic factors involved in the stability of native protein structure in amorphous solids in relation to levels of hydration

The internal, dynamical fluctuations of protein molecules exhibit many of the features typical of polymeric and bulk small molecule glass forming systems. The response of a protein's internal molecular mobility to temperature changes is similar to that of other amorphous systems, in that different types of motions freeze out at different temperatures, suggesting they exhibit the alpha-beta-modes of motion typical of polymeric glass formers. These modes of motion are attributed to the dynamic regimes that afford proteins the flexibility for function but that also develop into the large-scale collective motions that lead to unfolding. The protein dynamical transition, T(d), which has the same meaning as the T(g) value of other amorphous systems, is attributed to the temperature where protein activity is lost and the unfolding process is inhibited. This review describes how modulation of T(d) by hydration and lyoprotectants can determine the stability of protein molecules that have been processed as bulk, amorphous materials. It also examines the thermodynamic, dynamic, and molecular factors involved in stabilizing folded proteins, and the effects typical pharmaceutical processes can have on native protein structure in going from the solution state to the solid state.

Hydration Layer of a Cationic Micelle, C10TAB: Structure, Rigidity, Slow Reorientation, Hydrogen Bond Lifetime, and Solvation Dynamics

We report a theoretical study of the structure and dynamics of the water layer (the hydration layer) present at the surface of the cationic micelle decyltrimethylammonium bromide (DeTAB) by using atomistic molecular dynamics simulations. The simulated micelle consisted of 47 surfactant molecules (and an equal number of bromide ions), in good agreement with the pioneering light scattering experiments by Debye which found an aggregation number of 50. In this micelle, three partially positively charged methyl groups of each surfactant headgroup face the surrounding water. The nature of the cationic micellar surface is found to play an important role in determining the arrangement of water which is quite different from that in the bulk or on the surface of an anionic micelle, like cesium perfluorooctanoate. Water molecules present in the hydration layer are found to be preferentially distributed in the region between the three partially charged methyl headgroups. It is found that both the translational and rotational motions of water exhibit appreciably slower dynamics in the layer than those in the bulk. The solvation time correlation function (TCF) of bromide ions exhibits a long time component which is found to originate primarily from the interaction of the probe with the micellar headgroups. Thus, the decay of the solvation TCF is controlled largely by the residence time of the probe in the surface. The residence time distribution of the water molecules also exhibits a slow time component. We also calculate the collective number density fluctuation in the layer and find a prominent slow component compared to the similar quantity in the bulk. This slow component demonstrates that water structure in the hydration layer is more rigid than that in the bulk. These results demonstrate that the slow dynamics of hydration layer water is generic to macromolecular surfaces of either polarity.

Dynamics and Thermodynamics of Water in PAMAM Dendrimers at Subnanosecond Time Scales

Atomistic molecular dynamics simulations are used to study generation 5 polyamidoamine (PAMAM) dendrimers immersed in a bath of water. We interpret the results in terms of three classes of water: buried water well inside of the dendrimer surface, surface water associated with the dendrimer-water interface, and bulk water well outside of the dendrimer. We studied the dynamic and thermodynamic properties of the water at three pH values: high pH with none of the primary or tertiary amines protonated, intermediate pH with only the primary amines protonated, and low pH with all amines protonated. For all pH values we find that both buried and surface water exhibit two relaxation times: a fast relaxation ( approximately 1 ps) corresponding to the libration motion of the water and a slow ( approximately 20 ps) diffusional component related to the escaping of water from one domain to another. In contrast for bulk water the fast relaxation is approximately 0.4 ps while the slow relaxation is approximately 14 ps. These results are similar to those found in biological systems, where the fast relaxation is found to be approximately 1 ps while the slow relaxation ranges from 20 to 1000 ps. We used the 2PT MD method to extract the vibrational (power) spectrum and found substantial differences for the three classes of water. The translational diffusion coefficient for buried water is 11-33% (depending on pH) of the bulk value while the surface water is about 80%. The change in rotational diffusion is quite similar: 21-45% of the bulk value for buried water and 80% for surface water. This shows that translational and rotational dynamics of water are affected by the PAMAM-water interactions as well as due to the confinement in the interior of the dendrimer. We find that the reduction of translational or rotational diffusion is accompanied by a blue shift of the corresponding libration motions ( approximately 10 cm(-1) for translation, approximately 35 cm(-1) for rotation), indicating higher local force constants for these motions. These effects are most pronounced for the lowest pH, probably because of the increased rigidity caused by the internal charges. From the vibrational density of states we also calculate the enthalpies and entropies of the various waters. We find that water molecules are enthalpically favored near the PAMAM dendrimer: energy for surface water is approximately 0.1 kcal/mol lower to that in the bulk, and approximately 0.5-0.9 kcal/mol lower for buried water. In contrast, we find that both the buried and surface water are entropically unfavored: buried water is 0.9-2.2 kcal/mol lower than the bulk while the surface water is 0.1-0.2 kcal/mol lower. The net result is a thermodynamically unfavored state of the water surrounding the PAMAM dendrimer: 0.4-1.3 kcal/mol higher for buried water and 0.1-0.2 kcal/mol for surface water. This excess free energy of the surface and buried waters is released when the PAMAM dendrimer binds to DNA or metal ions, providing an extra driving force.

Supercooled Water in PVA Matrixes. II. A Molecular Dynamics Simulation Study and Comparison with QENS Results

Molecular dynamics (MD) simulations were carried out to elucidate the dynamic behavior of water confined in poly(vinyl alcohol), PVA, hydrogels. Model topology is supported by experimental network parameters, and simulation results are compared to an incoherent quasielastic neutron scattering (QENS) investigation carried out on PVA hydrogels. From the QENS dynamic scattering law (part I), a random jump model was adopted for the description of water diffusion to extract a microscopic diffusion coefficient and a residence time between two "jumps". In the present work, consistently with this framework, water diffusion parameters as diffusion coefficients and residence times have been evaluated using the mean square displacement of water in a time window of 10 ps and the time autocorrelation function of water hydrogen bonds. The calculated parameters are in good agreement with the experimental ones, giving confidence to this approach. Further developments are in progress to take into account a more realistic description of hydrogel structure in the molecular dynamics simulations.

The extended interface: measuring non-local effects in biomolecular interactions

Improvements in the sensitivity and availability of biophysical techniques for the detection of the formation of complexes in solution are revealing that the effects of binding are not restricted to the direct contacts between the biomolecules or even to a localised site. Rather, information about the binding event is transmitted throughout the biomolecules and the surrounding solution through changes in the hydrogen bonding, hydration and electrostatic field as the complex is formed. Calorimetric, volumetric and NMR methods are beginning to provide a quantitative view of the nature and thermodynamic consequences of this extended interface, and the resulting data pose a major challenge for computational models of binding.

Inelastic Incoherent Neutron Scattering Measurements of Intact Cells and Tissues and Detection of Interfacial Water

We have previously used inelastic incoherent neutron scattering spectroscopy to investigate the properties of aqueous suspensions of biomolecules as a function of hydration. These experiments led to the identification of signals corresponding to interfacial (hydration) water at low water content. A prediction from these studies was that in the crowded environment inside living cells, a significant proportion of the water would be interfacial, with profound implications for biological function. Here we describe the first inelastic incoherent neutron scattering spectroscopy studies of living cells and tissues. We find that the interfacial water signal is similar to that observed for water interacting with purified biomolecules and other solutes, i.e., it is strongly perturbed in the librational and translational intermolecular optical regions of the spectrum at 20-150 meV. The ratio of interfacial water compared to total water in cells (approximately 30%) is in line with previous experimental data for hydration water and calculations based on simple assumptions.

Dynamics of bound and free water in an aqueous micellar solution: analysis of the lifetime and vibrational frequencies of hydrogen bonds at a complex interface

In order to understand the nature and dynamics of interfacial water molecules on the surface of complex systems, large scale, fully atomistic molecular dynamics simulations of an aqueous micelle of cesium perfluorooctanoate (CsPFO) surfactant molecules have been carried out. The lifetime and the intermolecular vibrational frequencies of the hydrogen bonds that the water molecules form with the hydrophilic, polar head groups (PHG) of the surfactants are calculated. Our earlier classification [S. Balasubramanian et al., Curr. Sci. 84, 428 (2003); e-print cond-mat/0212097] of the interfacial water molecules, based on structural and energetic considerations, into bound and free types is further validated by their dynamics. Lifetime correlation functions of the water-surfactant hydrogen bonds show the long-lived nature of the bound water species. Surprisingly, the water molecules that are singly hydrogen bonded to the surfactants have a longer lifetime than those that form two such hydrogen bonds. The free water molecules that do not form any such hydrogen bonds behave similarly to bulk water in their reorientational dynamics. A few water molecules that form two such hydrogen bonds are orientationally locked in for durations of the order of a few hundreds of picoseconds; that is, much longer than their average lifetime. The intermolecular vibrational frequencies of these interfacial water molecules have been studied from the power spectra of their velocity autocorrelation function. We find a significant blue shift in the librational band of the interfacial water molecules, apart from a similar shift in the near neighbor bending modes, relative to water molecules in bulk. These blue shifts suggest an increase in rigidity in the structure around interfacial water molecules. This is in good agreement with recent incoherent, inelastic neutron scattering data on macromolecular solutions [S. Ruffle et al., J. Am. Chem. Soc. 124, 565 (2002)]. The results of the present simulations appear to be rather general and should be relevant to the understanding of the dynamics of water near any hydrophilic surface.

Bridging implicit and explicit solvent approaches for membrane electrostatics

Conformations of a zwitterionic bilayer were sampled from a molecular dynamics simulation and their electrostatic properties analyzed by solution of the Poisson equation. These traditionally implicit electrostatic calculations were performed in the presence of varying amounts of explicit solvent to assess the magnitude of error introduced by a uniform dielectric description of water surrounding the bilayer. It was observed that membrane dipole potential calculations in the presence of explicit water were significantly different than wholly implicit solvent calculations with the calculated dipole potential converging to a reasonable value when four or more hydration layers were included explicitly.