Molecular properties of aqueous solutions: a focus on the collective dynamics of hydration water

Femtosecond to Microsecond Observation of Photochemical Pathways in Nitroaromatic Phototriggers Using Transient Absorption Spectroscopy

The synthetic accessibility and tolerance to structural modification of phototriggered compounds (PTs) based on the ortho- nitrobenzene (ONB) protecting group have encouraged a myriad of applications including optimization of biological activity, and supramolecular polymerization. Here, a combination of ultrafast transient absorption spectroscopy techniques is used to study the multistep photochemistry of two nitroaromatic phototriggers based on the ONB chromophore, O-(4,5-dimethoxy-2-nitrobenzyl)-l-serine (DMNB-Ser) and O-[(2-nitrophenyl)methyl]-l-tyrosine hydrochloride (NB-Tyr), in DMSO solutions on femtosecond to microsecond time scales following the absorption of UV light. From a common nitro-S1 excited state, the PTs can either undergo excited state intramolecular hydrogen transfer (ESIHT) to an aci-S1 isomer within the singlet state manifold, leading to direct S1 → S0 internal conversion through a conical intersection, or competitive intersystem crossing (ISC) to access the triplet state manifold on time scales of (1.93 ± 0.03) ps and (13.9 ± 1.2) ps for DMNB-Ser and NB-Tyr, respectively. Deprotonation of aci-T1 species to yield triplet anions is proposed to occur in both PTs, with an illustrative time constant of (9.4 ± 0.7) ns for DMNB-Ser. More than 75% of the photoexcited molecules return to their electronic ground states within 8 μs, either by direct S1 → S0 relaxation or anion reprotonation. Hence, upper limits to the quantum yields of photoproduct formation are estimated to be in the range of 13-25%. Mixed DMSO/H2O solvents show the influence of the environment on the observed photochemistry, for example, revealing two nitro-S1 lifetimes for DMNB-Ser in a 10:1 DMSO/H2O mixture of 1.95 ps and (10.1 ± 1.2) ps, which are attributed to different microsolvation environments.

Temperature dependence of the viscoelastic properties of a natural gastropod mucus by Brillouin light scattering spectroscopy

Brillouin spectroscopy was used to probe the viscoelastic properties of a natural gastropod mucus at GHz frequencies over the range -11 °C ≤ T ≤ 52 °C. Anomalies in the temperature dependence of mucus longitudinal acoustic mode peak parameters and associated viscoelastic properties at T = -2.5 °C, together with the appearance of a peak due to ice at this temperature, suggest that the mucus undergoes a phase transition from a viscous liquid state to one in which liquid mucus and solid ice phases coexist. Failure of this transition to proceed to completion even at -11 °C is attributed to glycoprotein-water interaction. The temperature dependence of the viscoelastic properties and the phase behaviour suggest that water molecules bind to glycoprotein at a temperature above the onset of freezing and that the reduced ability of this bound water to take on a configuration that facilitates freezing is responsible for the observed freezing point depression and gradual nature of the liquid-solid transition.

Molecular Dynamics Simulation of the Influence of Temperature and Salt on the Dynamic Hydration Layer in a Model Polyzwitterionic Polymer PAEDAPS

We investigate the hydration of poly(3-[2-(acrylamido) ethyldimethylammonio] propanesulfonate) over a range of temperatures in pure water and with the inclusion of 0.1 mol/L NaCl using atomistic molecular dynamics simulation. Drawing on concepts drawn from the field of glass-forming liquids, we use the Debye-Waller parameter () for describing the water mobility gradient around the polybetaine backbone extending to an overall distance ≈18 Å. The water mobility in this layer is defined through the mean-square water molecule displacement at a time on the order of water's β-relaxation time. The brushlike topology of polybetaines leads to two regions in the dynamic hydration layer. The inner region of ≈10.5 Å is explored by pendant group conformational motions, and the outer region of ≈7.5 Å represents an extended layer of reduced water mobility relative to bulk water. The dynamic hydration layer extends far beyond the static hydration layer, adjacent to the polymer.

Swelling of Thermo-Responsive Gels in Aqueous Solutions of Salts: A Predictive Model

The equilibrium degree of swelling of thermo-responsive (TR) gels is strongly affected by the presence of ions in an aqueous solution. This phenomenon plays an important role in (i) the synthesis of multi-stimuli-responsive gels for soft robotics, where extraordinary strength and toughness are reached by soaking of a gel in solutions of multivalent ions, and (ii) the preparation of hybrid gels with interpenetrating networks formed by covalently cross-linked synthetic chains and ionically cross-linked biopolymer chains. A model is developed for equilibrium swelling of a TR gel in aqueous solutions of salts at various temperatures T below and above the critical temperature at which collapse of the gel occurs. An advantage of the model is that it involves a a small (compared with conventional relations) number of material constants and allows the critical temperature to be determined explicitly. Its ability (i) to describe equilibrium swelling diagrams on poly(N-isopropylacrylamide) gels in aqueous solutions of mono- and multivalent salts and (ii) to predict the influence of volume fraction of salt on the critical temperature is confirmed by comparison of observations with results of numerical simulation.

Water does not dance as ions sing: A new approach in elucidation of ion-invariant water fluctuations

Aqueous solutions of salts composed from monovalent ions are explored using temperature-dependent FT-IR spectroscopy in transmission. Water combination band, being extremely sensitive to the network of hydrogen bonds due to the contribution of water librations (ρ_LH₂O), is analyzed in uni- and multivariate fashion. Univariate analysis of the combination band maximum (ν_max) reveals that perturbation of water hydrogen bond network by ions is primary driven by electrostatic interactions between water and ions. Using multivariate curve resolution with alternating least squares and evolving factor analysis this band is separated into two components that represent low- and high-density water. The observed asymmetry in their behavior is interpreted in terms of fluctuations of a hydrogen bond network of two water components. The significance of the found phenomenon is unambiguously confirmed by performing analogous analysis in the spectral range that contains partial signature of water linear bending (δHOH) and is free from ρ_LH₂O, in which the asymmetry is absent. Additionally, we show that this phenomenon, namely ion-invariant behavior of water fluctuations, persists even in the regime of high ionic strengths. Although ions indeed participate in shaping of water hydrogen bond network, this straightforward approach shows that its temperature-dependent fluctuations are ion-independent.

Hydration Dynamics of Model Peptides with Different Hydrophobic Character

The multi-scale dynamics of aqueous solutions of the hydrophilic peptide N-acetyl-glycine-methylamide (NAGMA) have been investigated through extended frequency-range depolarized light scattering (EDLS), which enables the broad-band detection of collective polarizability anisotropy fluctuations. The results have been compared to those obtained for N-acetyl-leucinemethylamide (NALMA), an amphiphilic peptide which shares with NAGMA the same polar backbone, but also contains an apolar group. Our study indicates that the two model peptides induce similar effects on the fast translational dynamics of surrounding water. Both systems slow down the mobility of solvating water molecules by a factor 6–8, with respect to the bulk. Moreover, the two peptides cause a comparable far-reaching spatial perturbation extending to more than two hydration layers in diluted conditions. The observed concentration dependence of the hydration number is explained considering the random superposition of different hydration shells, while no indication of solute aggregation phenomena has been found. The results indicate that the effect on the dynamics of water solvating the amphiphilic peptide is dominated by the hydrophilic backbone. The minor impact of the hydrophobic moiety on hydration features is consistent with structural findings derived by Fourier transform infrared (FTIR) measurements, performed in attenuated total reflectance (ATR) configuration. Additionally, we give evidence that, for both systems, the relaxation mode in the GHz frequency range probed by EDLS is related to solute rotational dynamics. The rotation of NALMA occurs at higher timescales, with respect to the rotation of NAGMA; both processes are significantly slower than the structural dynamics of hydration water, suggesting that solute and solvent motions are uncoupled. Finally, our results do not indicate the presence of super-slow water (relaxation times in the order of tens of picoseconds) around the peptides investigated.

Protein Hydration in a Bioprotecting Mixture

We combined broad-band depolarized light scattering and infrared spectroscopies to study the properties of hydration water in a lysozyme-trehalose aqueous solution, where trehalose is present above the concentration threshold (30% in weight) relevant for biopreservation. The joint use of the two different techniques, which were sensitive to inter-and intra-molecular degrees of freedom, shed new light on the molecular mechanism underlying the interaction between the three species in the mixture. Thanks to the comparison with the binary solution cases, we were able to show that, under the investigated conditions, the protein, through preferential hydration, remains strongly hydrated even in the ternary mixture. This supported the water entrapment scenario, for which a certain amount of water between protein and sugar protects the biomolecule from damage caused by external agents.

Effects of molecular shape on alcohol aggregation and water hydrogen bond network behavior in butanol isomer solutions

Despite butanol isomers such as n-butanol, sec-butanol, isobutanol and tert-butanol having the same chemical formula, their liquid-liquid phase diagrams are distinct. That is, tert-butanol is miscible in water at all concentrations, while the other three butanol isomers are partially miscible under ambient conditions. The molecular shape of tert-butanol is close to globular and differs from the other three butanol molecules with a relatively long carbon chain. By performing molecular dynamics simulations and graph theoretical analysis of the four water-butanol isomer mixtures at varying concentrations, we show how distinct butanol aggregates are formed which depend upon the molecular shape and affect the water H-bond network structure and phase diagram in the binary liquid. The three butanol isomers of n-butanol, sec-butanol and isobutanol at concentrated solutions form chain-like alcohol aggregates, but tert-butanol forms small aggregates due to the distinct packing behavior caused by its globular molecular shape. By employing the graph theoretical analysis such as the degree distribution and the eigenvalue spectrum from the adjacency matrix in the graphical representation of the alcohol H-bond network, we show that the tert-butanol aggregates have a different morphological structure from that of the other three butanol isomers in aqueous solution. The graph theoretically distinct butanol aggregates are categorized into two groups, water-compatible and water-incompatible, depending upon the interaction between the alcohol and water molecules. Based upon our observations, we propose that the water-incompatible networks of n-butanol, sec-butanol and isobutanol aggregates do not change the water structure significantly, forming two separate liquid phases that are alcohol-rich and water-rich. However, the water-compatible network of tert-butanol aggregates has a considerable interaction with the water molecules and causes significant disruption of the water H-bond network, forming a homogeneous solution. Understanding the alcohol aggregation behavior and water structure in butanol-water mixtures provides a critical clue in appreciating fundamental issues such as miscibility and phase separation in aqueous solution systems.

Hydration/Dehydration Behavior of Hydroxyethyl Cellulose Ether in Aqueous Solution

Hydroxyethyl cellulose (HeC) maintains high water solubility over a wide temperature range even in a high temperature region where other nonionic chemically modified cellulose ethers, such as methyl cellulose (MC) and hydroxypropylmethyl cellulose (HpMC), demonstrate cloud points. In order to clarify the reason for the high solubility of HeC, the temperature dependence of the hydration number per glucopyranose unit, n_H, for the HeC samples was examined by using extremely high frequency dielectric spectrum measuring techniques up to 50 GHz over a temperature range from 10 to 70 °C. HeC samples with a molar substitution number (MS) per glucopyranose unit by hydroxyethyl groups ranging from 1.3 to 3.6 were examined in this study. All HeC samples dissolve into water over the examined temperature range and did not show their cloud points. The value of n_H for the HeC sample possessing the MS of 1.3 was 14 at 20 °C and decreased gently with increasing temperature and declined to 10 at 70 °C. The n_H values of the HeC samples are substantially larger than the minimum critical n_H value of ca. 5 necessary to be dissolved into water for cellulose ethers such as MC and HpMC, even in a high temperature range. Then, the HeC molecules possess water solubility over the wide temperature range. The temperature dependence of n_H for the HeC samples and triethyleneglycol, which is a model compound for substitution groups of HeC, is gentle and they are similar to each other. This observation strongly suggests that the hydration/dehydration behavior of the HeC samples was essentially controlled by that of their substitution groups.

Protein-Water and Water-Water Long-Time Relaxations in Protein Hydration Water upon Cooling-A Close Look through Density Correlation Functions

We report results on the translational dynamics of the hydration water of the lysozyme protein upon cooling obtained by means of molecular dynamics simulations. The self van Hove functions and the mean square displacements of hydration water show two different temperature activated relaxation mechanisms, determining two dynamic regimes where transient trapping of the molecules is followed by hopping phenomena to allow to the structural relaxations. The two caging and hopping regimes are different in their nature. The low-temperature hopping regime has a time scale of tenths of nanoseconds and a length scale on the order of 2–3 water shells. This is connected to the nearest-neighbours cage effect and restricted to the supercooling, it is absent at high temperature and it is the mechanism to escape from the cage also present in bulk water. The second hopping regime is active at high temperatures, on the nanoseconds time scale and over distances of nanometers. This regime is connected to water displacements driven by the protein motion and it is observed very clearly at high temperatures and for temperatures higher than the protein dynamical transition. Below this temperature, the suppression of protein fluctuations largely increases the time-scale of the protein-related hopping phenomena at least over 100 ns. These protein-related hopping phenomena permit the detection of translational motions of hydration water molecules longly persistent in the hydration shell of the protein.

The effect of saccharides on equilibrium swelling of thermo-responsive gels

Mechanical and optical properties of thermo-responsive (TR) gels change drastically at their volume phase transition temperature. As the critical temperature is strongly affected by the presence of small amounts of additives in aqueous solutions, TR gels can be employed as sensors for detection and recognition of multiple analytes (from specific ions to hazardous biochemicals to pathogenic proteins) and actuators for biomedical applications. A simplified mean-field model is developed for equilibrium swelling of TR gels in aqueous solutions of additives. Its advantage is that the model involves a relatively small (compared with the conventional approaches) number of material constants and accounts for changes in the thermo-mechanical response at transition from the swollen to collapsed state. The ability of the model to describe experimental swelling diagrams and to predict the influence of additives on the equilibrium degree of swelling and the volume phase transition temperature of TR gels is confirmed by comparison of observations on poly(N-isopropylacrylamide) gel in aqueous solutions of saccharides (glucose, sucrose and galactose) with results of numerical analysis.

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.

Mobility of water and of protein atoms at the protein-water interface, monitored by anisotropic atomic displacement parameters, are largely uncorrelated

A non-redundant set of 231 protein crystal structures refined at a resolution better than (or equal to) 1 Å was extracted from the Protein Data Bank and the degree of conformational rigidity at the protein-water interface was examined by means of the Hirshfeld test and by comparing the orientations of the anisotropic Us for contacting protein and water atoms. Contacts between protein and water atoms are more rigid that contacts between water atoms and the degree of rigidity increases for shorter contacts and for more hydrogen-bonded atoms. Nevertheless, water and protein atoms are not rigidly held together. On the contrary, they seem to have little influence on their mobility to such an extent that hydration water, different from the protein atoms, cannot be considered to be properly in the solid state.

Hydration interactions beyond the first solvation shell in aqueous phenolate solution

We investigate the orientational dynamics of water molecules solvating phenolate ions using ultrafast vibrational spectroscopy and density functional theory-based molecular dynamics simulations.

Hydration Dynamics in Solutions of Cyclic Polyhydroxyl Osmolytes

Simple sugars are remarkably effective at preserving protein and enzymatic structures against thermal and hydrostatic stress. Here, we investigate the hydrodynamic and biopreservative properties of three small cyclic molecules: glucose, myo-inositol, and methyl-α-d-glucopyranoside using circular dichroism spectroscopy and isothermal calorimetry. Using ultrafast fluorescence frequency upconversion spectroscopy, we measure the dynamical retardation of hydration dynamics in cosolute solutions. We find that all three molecules are effective modifiers of hydration dynamics in solution and all are also effective at protecting model protein systems against thermal denaturation. Methyl-α-d-glucopyranoside is found to be the most effective dynamic reducer displaying an approximately 30% increase in solvation relaxation time as compared to water in a cosolute free solution. myo-Inositol and glucose both exhibit a smaller reduction in dynamics with similar magnitudes of concentration dependence. Using these cosolute models, we demonstrate that the thermal enhancement of protein structure does not correlate strongly with either the dynamical reduction of the bulk solution nor with the number of hydrogen bonds a cosolute makes with the solvent. Furthermore, solutions of glucose at twice the concentration of trehalose are shown to have similar magnitudes of dynamical impact. This implies that regulation of hydration dynamics is not a distinguishing characteristic of successful osmolytes. This work highlights the need for further studies and computational analysis to understand the phenomena of preferential exclusion and the contribution of hydration dynamics to protein structural stability.

Mainstream Ammonium Recovery to Advance Sustainable Urban Wastewater Management

Throughout the 20th century, the prevailing approach toward nitrogen management in municipal wastewater treatment was to remove ammonium by transforming it into dinitrogen (N₂) using biological processes such as conventional activated sludge. While this has been a very successful strategy for safeguarding human health and protecting aquatic ecosystems, the conversion of ammonium into its elemental form is incompatible with the developing circular economy of the 21st century. Equally important, the activated sludge process and other emerging ammonium removal pathways have several environmental and technological limitations. Here, we assess that the theoretical energy embedded in ammonium in domestic wastewater represents roughly 38-48% of the embedded chemical energy available in the whole of the discharged bodily waste. The current routes for ammonium removal not only neglect the energy embedded in ammonium, but they can also produce N₂O, a very strong greenhouse gas, with such emissions comprising the equivalent of 14-26% of the overall carbon footprint of wastewater treatment plants. N₂O emissions often exceed the carbon emissions related to the electricity consumption for the process requirements of WWTPs. Considering these limitations, there is a need to develop alternative ammonium management approaches that center around recovery of ammonium from domestic wastewater rather than deal with its "destruction" into elemental dinitrogen. Current ammonium recovery techniques are applicable only at orders of magnitude above domestic wastewater strength, and so new techniques based on physicochemical adsorption are of particular interest. A new pathway is proposed that allows for mainstream ammonium recovery from wastewater based on physicochemical adsorption through development of polymer-based adsorbents. Provided adequate adsorbents corresponding to characteristics outlined in this paper are designed and brought to industrial production, this adsorption-based approach opens perspectives for mainstream continuous adsorption coupled with side-stream recovery of ammonium with minimal chemical requirements. This proposed pathway can bring forward an effective resource-oriented approach to upgrade the fate of ammonium in urban water management without generating hidden externalized environmental costs.

Brillouin Light Scattering: Applications in Biomedical Sciences

Brillouin spectroscopy and imaging are emerging techniques in analytical science, biophotonics, and biomedicine. They are based on Brillouin light scattering from acoustic waves or phonons in the GHz range, providing a nondestructive contactless probe of the mechanics on a microscale. Novel approaches and applications of these techniques to the field of biomedical sciences are discussed, highlighting the theoretical foundations and experimental methods that have been developed to date. Acknowledging that this is a fast moving field, a comprehensive account of the relevant literature is critically assessed here.

Probing the Solute-Solvent Interaction of an Azo-Bonded Prodrug in Neat and Binary Media: Combined Experimental and Computational Study

Preferential solvation has significant importance in interpreting the molecular physicochemical properties of wide spectrum of materials in solution. In this work, the solute-solvent interaction of pro-drug Sulfasalazine (SSZ) in neat and binary media was investigated experimentally and computationally. The solute-solvent interactions of interest were spectrophotometrically probed and computationally investigated for providing insights concerning the molecular aspects of SSZ:media interaction. Experimentally, the obtained results in 1,4-dioxane:water binary mixture demonstrated a dramatic non-linear changes in the spectral behavior of SSZ indicative of the dependency of its molecular behaviors on the compositions of the molecular microenvironment in the essence of solute-solvent interaction. Computationally, geometry optimization and simulation of the absorption spectra of SSZ in media of interest were performed employing DFT and TD-DFT methods, respectively, where the solvent effects on the absorption were examined implicitly using IEFPCM method. Obtained results revealed a nonpolar nature of the molecular orbitals that are directly involved in the SSZ:medium interaction. As in good correspondence with the experimental results, these simulations demonstrated that these orbitals are of non-polar nature and hence minimally affected by polarity of the media and in turn favoring the non-polar molecular environments. On the other hand, the molecular origin of SSZ:media interaction was demonstrated explicitly through complexation of SSZ with water molecules revealing a cooperative hydrogen bonding stabilization with an average length of 1.90 Å. The findings of this work demonstrate the significance of the preferential solvation and composition of the molecular microenvironment on the physicochemical properties of molecules of pharmaceutical importance.

Toward theoretical terahertz spectroscopy of glassy aqueous solutions: partially frozen solute-solvent couplings of glycine in water

The molecular-level understanding of THz spectra of aqueous solutions under ambient conditions has been greatly advanced in recent years. Here, we go beyond previous analyses by performing ab initio molecular dynamics simulations of glycine in water with artificially frozen solute or solvent molecules, respectively, while computing the total THz response as well as its decomposition into mode-specific resonances based on the "supermolecular solvation complex" technique. Clamping the water molecules and keeping glycine moving breaks the coupling of glycine to the structural dynamics of the solvent, however, the polarization and dielectric solvation effects in the static solvation cage are still at work since the full electronic structure of the quenched solvent is taken into account. The complementary approach of fixing glycine reveals both the dynamical and electronic response of the solvation cage at the level of its THz response. Moreover, to quantitatively account for the electronic contribution solely due to solvent embedding, the solute species is "vertically desolvated", thus preserving the fully coupled solute-solvent motion in terms of the solute's structural dynamics in solution, while its electronic structure is no longer subject to solute-solvent polarization and charge transfer effects. When referenced to the free simulation of Gly(aq), this three-fold approach allows us to decompose the THz spectral contributions due to the correlated solute-solvent dynamics into entirely structural and purely electronic effects. Beyond providing hitherto unknown insights, the observed systematic changes of THz spectra in terms of peak shifts and lineshape modulations due to conformational freezing and frozen solvation cages might be useful to investigate the solvation of molecules in highly viscous H-bonding solvents such as ionic liquids and even in cryogenic ices as relevant to polar stratospheric and dark interstellar clouds.

Functional Hydration Behavior: Interrelation between Hydration and Molecular Properties at Lipid Membrane Interfaces

Water is an abundant commodity and has various important functions. It stabilizes the structure of biological macromolecules, controls biochemical activities, and regulates interfacial/intermolecular interactions. Common aspects of interfacial water can be obtained by overviewing fundamental functions and properties at different temporal and spatial scales. It is important to understand the hydrogen bonding and structural properties of water and to evaluate the individual molecular species having different hydration properties. Water molecules form hydrogen bonds with biomolecules and contribute to the adjustment of their properties, such as surface charge, hydrophilicity, and structural flexibility. In this review, the fundamental properties of water molecules and the methods used for the analyses of water dynamics are summarized. In particular, the interrelation between the hydration properties, determined by molecules, and the properties of molecules, determined by their hydration properties, are discussed using the lipid membrane as an example. Accordingly, interesting water functions are introduced that provide beneficial information in the fields of biochemistry, medicine, and food chemistry.

Molecular motions, structure and hydration behaviour of glucose oligomers in aqueous solution

The presence of helical fragment configurations is confirmed for glucose oligomers in aqueous solution at temperatures lower than 40 °C.

Structure, molecular dynamics, and interactions in aqueous xylitol solutions

Broad-band dielectric relaxation studies of xylitol-water mixture show distinctly different dynamics for distal and central –OH of xylitol molecules and indicates the presence of loose xylitol aggregates.

Spectroscopic methods for assessing the molecular origins of macroscopic solution properties of highly concentrated liquid protein solutions

In cases of subcutaneous injection of therapeutic monoclonal antibodies, high protein concentrations (>50 mg/ml) are often required. During the development of these high concentration liquid formulations (HCLF), challenges such as aggregation, gelation, opalescence, phase separation, and high solution viscosities are more prone compared to low concentrated protein formulations. These properties can impair manufacturing processes, as well as protein stability and shelf life. To avoid such unfavourable solution properties, a detailed understanding about the nature of these properties and their driving forces are required. However, the fundamental mechanisms that lead to macroscopic solution properties, as above mentioned, are complex and not fully understood, yet. Established analytical methods for assessing the colloidal stability, i.e. the ability of a native protein to remain dispersed in solution, are restricted to dilute conditions and provide parameters such as the second osmotic virial coefficient, B₂₂, and the diffusion interaction coefficient, k_D. These parameters are routinely applied for qualitative estimations and identifications of proteins with challenging solution behaviours, such as high viscosities and aggregation, although the assays are prepared for low protein concentration conditions, typically between 0.1 and 20 mg/ml ("ideal" solution conditions). Quantitative analysis of samples of high protein concentration is difficult and it is hard to obtain information about the driving forces of such solution properties and corresponding protein-protein self-interactions. An advantage of using specific spectroscopic methods is the potential of directly analysing highly concentrated protein solutions at different solution conditions. This allows for collecting/gaining valuable information about the fundamental mechanisms of solution properties of the high protein concentration regime. In addition, the derived parameters might be more predictive as compared to the parameters originating from assays which are optimized for the low protein concentration range. The provided information includes structural data, molecular dynamics at various timescales and protein-solvent interactions, which can be obtained at molecular resolution. Herein, we provide an overview about spectroscopic techniques for analysing the origins of macroscopic solution behaviours in general, with a specific focus on pharmaceutically relevant high protein concentration and formulation conditions.

High-Precision Megahertz-to-Terahertz Dielectric Spectroscopy of Protein Collective Motions and Hydration Dynamics

The low-frequency collective vibrational modes in proteins as well as the protein-water interface have been suggested as dominant factors controlling the efficiency of biochemical reactions and biological energy transport. It is thus crucial to uncover the mystery of the hydration structure and dynamics as well as their coupling to collective motions of proteins in aqueous solutions. Here, we report dielectric properties of aqueous bovine serum albumin protein solutions as a model system using an extremely sensitive dielectric spectrometer with frequencies spanning from megahertz to terahertz. The dielectric relaxation spectra reveal several polarization mechanisms at the molecular level with different time constants and dielectric strengths, reflecting the complexity of protein-water interactions. Combining the effective-medium approximation and molecular dynamics simulations, we have determined collective vibrational modes at terahertz frequencies and the number of water molecules in the tightly bound and loosely bound hydration layers. High-precision measurements of the number of hydration water molecules indicate that the dynamical influence of proteins extends beyond the first solvation layer, to around 7 Å distance from the protein surface, with the largest slowdown arising from water molecules directly hydrogen-bonded to the protein. Our results reveal critical information of protein dynamics and protein-water interfaces, which determine biochemical functions and reactivity of proteins.

Physico-chemical properties of aqueous drug solutions: From the basic thermodynamics to the advanced experimental and simulation results

The physical chemical properties of aqueous solutions of model compounds are illustrated in relation to hydration and solubility issues by using three perspectives: thermodynamic, spectroscopic and molecular dynamics simulations. The thermodynamic survey of the fundamental backgrounds of concentration dependence and experimental solubility results show some peculiar behavior of aqueous solutions with several types of similar solutes. Secondly, the use of a variety of experimental spectroscopic devices, operating under different experimental conditions of dimension and frequency, has produced a large amount of structural and dynamic data on aqueous solutions showing the richness of the information produced, depending on where and how the experiment is carried out. Finally, the use of molecular dynamics computational work is presented to highlight how the different types of solute functional groups and surface topologies organize adjacent water molecules differently. The highly valuable contribution of computer simulation studies in providing molecular explanations for experimental deductions, either of a thermodynamic or spectroscopic nature, is shown to have changed the current knowledge of many aqueous solution processes. While this paper is intended to provide a collective view on the latest literature results, still the presentation aims at a tutorial explanation of the potentials of the three methodologies in the field of aqueous solutions of pharmaceutical molecules.

Calcium ion cyclotron resonance in dissipative water structures

Weak magnetic and electromagnetic fields affect physiological processes in animals, plants, and microorganisms. Ion cyclotron resonance (ICR) is discussed as one of the sensitive mechanisms, which enable perception of the geomagnetic field and its orientation. Numerous biological effects are observed involving several small ions, showing windows of predicted frequencies and intensities. The pioneering work of Guiliano Preparata and Emilio Del Giudice using quantum electrodynamics showed that spontaneously originating coherent regions in water facilitate ICR effects at incoherent water phase boundaries. Here we examine the ICR response of the calcium ion (Ca²⁺), crucial for many life processes. We use an aqueous solution containing the biologically ubiquitous membrane lipid L-α-phosphatidylcholine that serves as a biomimetic proxy for dynamic light scattering (DLS) and nonlinear dielectric spectroscopy (NLDS) measurements. One notable result is that this system approaches a new equilibrium upon addition of calcium by means of the oscillatory Belousov-Zhabotinsky chemical reaction, oscillations are significantly reduced under Ca²⁺ ICR application. Secondly an "oscillator" of calcium ions appears to be able to itself couple coherently and predictably to large-scale coherent regions in water. This system appears able to regulate ion fluxes in response to very weak environmental electromagnetic fields.

Hydration water dynamics around a protein surface: a first passage time approach

A stochastic noise-driven dynamic model is proposed to study the diffusion of water molecules around a protein surface, under the effect of thermal fluctuations that arise due to the collision of water molecules with the surrounding environment. The underlying dynamics of such a system may be described in the framework of the generalized Langevin equation, where the thermal fluctuations are assumed to be algebraically correlated in time, which governs the non-Markovian behavior of the system. Results of the calculations of mean-square displacement and the velocity autocorrelation function reveal that the hydration water around the protein surface follows subdiffusive dynamics at long times. Analytical expressions for the first passage time distribution, survival probability, mean residence time and mean first passage time of water molecules are derived for different boundary conditions, to analyze hydration water dynamics under the effect of thermally correlated noise. The results depict a unimodal distribution of the first passage time unlike Brownian motion. The survival probability of hydration water follows a stretched exponential decay for both boundary conditions. The mean residence time of the hydration water molecule for different initial positions increases with increase in the complexity/heterogeneity of the surrounding environment for both boundary conditions. The mean first passage time of the water molecule to reach the absorbing/reflecting boundary follows an asymptotic power law with respect to the thickness of the hydration layer, and increases with increase in the complexity/heterogeneity of the environment.

Water-like Behavior of Formamide: Jump Reorientation Probed by Extended Depolarized Light Scattering

Water is a strong self-associated liquid with peculiar properties that crucially depend on H-bonding. As regards its molecular dynamics, only recently has water reorientation been successfully described based on a jump mechanism, which is responsible for the overall H-bonding exchange. Here, using high-resolution broad-band depolarized light scattering, we have investigated the reorientational dynamics of formamide (FA) as a function of concentration from the neat liquid to diluted aqueous solutions. Our main findings indicate that in the diluted regime the water rearrangement can trigger the motion of FA solute molecules, which are forced to reorient at the same rate as water. This highlights an exceptional behavior of FA, which perfectly substitutes water within its network. Besides other fundamental implications connected with the relevance of FA, its water-like behavior provides rare experimental evidence of a solute whose dynamics is completely slaved to the solvent.

Water Dynamics in the Hydration Shells of Biomolecules

The structure and function of biomolecules are strongly influenced by their hydration shells. Structural fluctuations and molecular excitations of hydrating water molecules cover a broad range in space and time, from individual water molecules to larger pools and from femtosecond to microsecond time scales. Recent progress in theory and molecular dynamics simulations as well as in ultrafast vibrational spectroscopy has led to new and detailed insight into fluctuations of water structure, elementary water motions, electric fields at hydrated biointerfaces, and processes of vibrational relaxation and energy dissipation. Here, we review recent advances in both theory and experiment, focusing on hydrated DNA, proteins, and phospholipids, and compare dynamics in the hydration shells to bulk water.

Correlations in the Solute-Solvent Dynamics Reach Beyond the First Hydration Shell of Ions

While the real-space structure of solvation shells has been explored for decades, a dynamical perspective that directly relies on changes in the H-bond network became accessible more recently mainly via far-infrared (THz) spectroscopies. A remaining key question is how many hydration shells are affected by ion-induced network perturbations. We disclose that theoretical THz difference spectra of aqueous salt solutions can be deciphered in terms of only a handful of dipolar auto- and cross-correlations, including the second solvation shell. This emphasizes the importance of cross-correlations being often neglected in multicomponent models. Analogously, experimental THz responses of simple ions can be deciphered in a similar way. Dramatic intensity cancellations due to large positive and negative contributions are found to effectively shift intensity maxima. Thus, THz spectroscopy provides an unprecedented view on the details of hydration dynamics, which can be understood by a combination of experiment and theory.

Thermal transport phenomena in nanoparticle suspensions

Nanoparticle suspensions in liquids have received great attention, as they may offer an approach to enhance thermophysical properties of base fluids. A good variety of applications in engineering and biomedicine has been investigated with the aim of exploiting the above potential. However, the multiscale nature of nanosuspensions raises several issues in defining a comprehensive modelling framework, incorporating relevant molecular details and much larger scale phenomena, such as particle aggregation and their dynamics. The objectives of the present topical review is to report and discuss the main heat and mass transport phenomena ruling macroscopic behaviour of nanosuspensions, arising from molecular details. Relevant experimental results are included and properly put in the context of recent observations and theoretical studies, which solved long-standing debates about thermophysical properties enhancement. Major transport phenomena are discussed and in-depth analysis is carried out for highlighting the role of geometrical (nanoparticle shape, size, aggregation, concentration), chemical (pH, surfactants, functionalization) and physical parameters (temperature, density). We finally overview several computational techniques available at different scales with the aim of drawing the attention on the need for truly multiscale predictive models. This may help the development of next-generation nanoparticle suspensions and their rational use in thermal applications.

Description of Hydration Water in Protein (Green Fluorescent Protein) Solution

The structurally and dynamically perturbed hydration shells that surround proteins and biomolecules have a substantial influence upon their function and stability. This makes the extent and degree of water perturbation of practical interest for general biological study and industrial formulation. We present an experimental description of the dynamical perturbation of hydration water around green fluorescent protein in solution. Less than two shells (∼5.5 Å) were perturbed, with dynamics a factor of 2-10 times slower than bulk water, depending on their distance from the protein surface and the probe length of the measurement. This dependence on probe length demonstrates that hydration water undergoes subdiffusive motions (τ ∝ q^-2.5 for the first hydration shell, τ ∝ q^-2.3 for perturbed water in the second shell), an important difference with neat water, which demonstrates diffusive behavior (τ ∝ q^-2). These results help clarify the seemingly conflicting range of values reported for hydration water retardation as a logical consequence of the different length scales probed by the analytical techniques used.