Mapping Hydration Water around Alcohol Chains by THz Calorimetry

Real-time measure of solvation free energy changes upon liquid-liquid phase separation of α-elastin

Biological condensates are known to retain a large fraction of water to remain in a liquid and reversible state. Local solvation contributions from water hydrating hydrophilic and hydrophobic protein surfaces were proposed to play a prominent role for the formation of condensates through liquid-liquid phase separation (LLPS). However, although the total free energy is accessible by calorimetry, the partial solvent contributions to the free energy changes upon LLPS remained experimentally inaccessible so far. Here, we show that the recently developed THz calorimetry approach allows to quantify local hydration enthalpy and entropy changes upon LLPS of α-elastin in real time, directly from experimental THz spectroscopy data. We find that hydrophobic solvation dominates the entropic solvation term, whereas hydrophilic solvation mainly contributes to the enthalpy. Both terms are in the order of hundreds of kJ/mol, which is more than one order of magnitude larger than the total free energy changes at play during LLPS. However, since we show that entropy/enthalpy mostly compensates, a small entropy/enthalpy imbalance is sufficient to tune LLPS. Theoretically, a balance was proposed before. Here we present experimental evidence based on our spectroscopic approach. We finally show that LLPS can be steered by inducing small changes of solvation entropy/enthalpy compensation via concentration or temperature in α-elastin.

Computation of Overhauser dynamic nuclear polarization processes reveals fundamental correlation between water dynamics, structure, and solvent restructuring entropy

Hydration water dynamics, structure, and thermodynamics are crucially important to understand and predict water-mediated properties at molecular interfaces. Yet experimentally and directly quantifying water behavior locally near interfaces at the sub-nanometer scale is challenging, especially at interfaces submerged in biological solutions. Overhauser dynamic nuclear polarization (ODNP) experiments measure equilibrium hydration water dynamics within 8-15 angstroms of a nitroxide spin probe on instantaneous timescales (10 picoseconds to nanoseconds), making ODNP a powerful tool for probing local water dynamics in the vicinity of the spin probe. As with other spectroscopic techniques, concurrent computational analysis is necessary to gain access to detailed molecular level information about the dynamic, structural, and thermodynamic properties of water from experimental ODNP data. We chose a model system that can systematically tune the dynamics of water, a water-glycerol mixture with compositions ranging from 0 to 0.3 mole fraction glycerol. We demonstrate the ability of molecular dynamics (MD) simulations to compute ODNP spectroscopic quantities, and show that translational, rotational, and hydrogen bonding dynamics of hydration water align strongly with spectroscopic ODNP parameters. Moreover, MD simulations show tight correlations between the dynamic properties of water that ODNP captures and the structural and thermodynamic behavior of water. Hence, experimental ODNP readouts of varying water dynamics suggest changes in local structural and thermodynamic hydration water properties.

Protein adsorption on blood-contacting surfaces: A thermodynamic perspective to guide the design of antithrombogenic polymer coatings

Blood-contacting medical devices often succumb to thrombosis, limiting their durability and safety in clinical applications. Thrombosis is fundamentally initiated by the nonspecific adsorption of proteins to the material surface, which is strongly governed by thermodynamic factors established by the nature of the interaction between the material surface, surrounding water molecules, and the protein itself. Along these lines, different surface materials (such as polymeric, metallic, ceramic, or composite) induce different entropic and enthalpic changes at the surface-protein interface, with material wettability significantly impacting this behavior. Consequently, protein adsorption on medical devices can be modulated by altering their wettability and surface energy. A plethora of polymeric coating modifications have been utilized for this purpose; hydrophobic modifications may promote or inhibit protein adsorption determined by van der Waals forces, while hydrophilic materials achieve this by mainly relying on hydrogen bonding, or unbalanced/balanced electrostatic interactions. This review offers a cohesive understanding of the thermodynamics governing these phenomena, to specifically aid in the design and selection of hemocompatible polymeric coatings for biomedical applications. STATEMENT OF SIGNIFICANCE: Blood-contacting medical devices often succumb to thrombosis, limiting their durability and safety in clinical applications. A plethora of polymeric coating modifications have been utilized for addressing this issue. This review offers a cohesive understanding of the thermodynamics governing these phenomena, to specifically aid in the design and selection of hemocompatible polymeric coatings for biomedical applications.

Entropy Tug-of-War Determines Solvent Effects in the Liquid-Liquid Phase Separation of a Globular Protein

Liquid-liquid phase separation (LLPS) plays a key role in the compartmentalization of cells via the formation of biomolecular condensates. Here, we combined atomistic molecular dynamics (MD) simulations and terahertz (THz) spectroscopy to determine the solvent entropy contribution to the formation of condensates of the human eye lens protein γD-Crystallin. The MD simulations reveal an entropy tug-of-war between water molecules that are released from the protein droplets and those that are retained within the condensates, two categories of water molecules that were also assigned spectroscopically. A recently developed THz-calorimetry method enables quantitative comparison of the experimental and computational entropy changes of the released water molecules. The strong correlation mutually validates the two approaches and opens the way to a detailed atomic-level understanding of the different driving forces underlying the LLPS.

Bridging the Gap in Cryopreservation Mechanism: Unraveling the Interplay between Structure, Dynamics, and Thermodynamics in Cryoprotectant Aqueous Solutions

Cryoprotectants play a crucial role in preserving biological material, ensuring their viability during storage and facilitating crucial applications such as the conservation of medical compounds, tissues, and organs for transplantation. However, the precise mechanism by which cryoprotectants modulate the thermodynamic properties of water to impede the formation and growth of ice crystals, thus preventing long-term damage, remains elusive. This is evident in the use of empirically optimized recipes for mixtures that typically contain DMSO, glycerol, and various sugar constituents. Here, we use terahertz calorimetry, Overhauser nuclear polarization, and molecular dynamics simulations to show that DMSO exhibits a robust structuring effect on water around its methyl groups, reaching a maximum at a DMSO mole fraction of XDMSO = 0.33. In contrast, glycerol exerts a smaller water-structuring effect, even at higher concentrations (Scheme 1). These results potentially suggest that the wrapped water around DMSO's methyl group, which can be evicted upon ligand binding, may render DMSO a more surface-active cryoprotectant than glycerol, while glycerol may participate more as a viscogen that acts on the entire sample. These findings shed light on the molecular intricacies of cryoprotectant solvation behavior and have potentially significant implications for optimizing cryopreservation protocols.

Emerging experimental methods to study the thermodynamics of biomolecular condensate formation

The formation of biomolecular condensates in vivo is increasingly recognized to underlie a multitude of crucial cellular functions. Furthermore, the evolution of highly dynamic protein condensates into progressively less reversible assemblies is thought to be involved in a variety of disorders, from cancer over neurodegeneration to rare genetic disorders. There is an increasing need for efficient experimental methods to characterize the thermodynamics of condensate formation and that can be used in screening campaigns to identify and rationally design condensate modifying compounds. Theoretical advances in the field are also identifying the key parameters that need to be measured in order to obtain a comprehensive understanding of the underlying interactions and driving forces. Here, we review recent progress in the development of efficient and quantitative experimental methods to study the driving forces behind and the temporal evolution of biomolecular condensates.

Nanoscale water-polymer interactions tune macroscopic diffusivity of water in aqueous poly(ethylene oxide) solutions

The separation and anti-fouling performance of water purification membranes is governed by both macroscopic and molecular-scale water properties near polymer surfaces. However, even for poly(ethylene oxide) (PEO) - ubiquitously used in membrane materials - there is little understanding of whether or how the molecular structure of water near PEO surfaces affects macroscopic water diffusion. Here, we probe both time-averaged bulk and local water dynamics in dilute and concentrated PEO solutions using a unique combination of experimental and simulation tools. Pulsed-Field Gradient NMR and Overhauser Dynamic Nuclear Polarization (ODNP) capture water dynamics across micrometer length scales in sub-seconds to sub-nanometers in tens of picoseconds, respectively. We find that classical models, such as the Stokes-Einstein and Mackie-Meares relations, cannot capture water diffusion across a wide range of PEO concentrations, but that free volume theory can. Our study shows that PEO concentration affects macroscopic water diffusion by enhancing the water structure and altering free volume. ODNP experiments reveal that water diffusivity near PEO is slower than in the bulk in dilute solutions, previously not recognized by macroscopic transport measurements, but the two populations converge above the polymer overlap concentration. Molecular dynamics simulations reveal that the reduction in water diffusivity occurs with enhanced tetrahedral structuring near PEO. Broadly, we find that PEO does not simply behave like a physical obstruction but directly modifies water's structural and dynamic properties. Thus, even in simple PEO solutions, molecular scale structuring and the impact of polymer interfaces is essential to capturing water diffusion, an observation with important implications for water transport through structurally complex membrane materials.

Energy Dissipation into the Solvent during Proton Transfer Occurs via Acoustic Phonons

In photochemistry, rapid energy dissipation into the solvent is mandatory to prevent radiation damages. By optical pump THz spectroscopy, we are able to follow the details of the energy dissipation mechanism upon photoexcitation of the photoacid to the hydrogen-bonded network of water: Based on the frequency maps subsequent to photoexcitation, we propose that energy transfer takes place via propagation of an acoustic phonon. The dissipation into the solvent can be rationalized by an initial first hydration shell response followed by energy dissipation via an acoustic phonon. Surprisingly, for the first 10 ps, the propagation in the water network can be described by a wave packet with a constant group velocity, indicating a long-range correlation. After 300 ps, thermalization in the liquid jet is reached and the THz spectrum reflects a Boltzmann population, corresponding a temperature increase of ΔT = 0.5 °C.

From Local Hydration Motifs in Aqueous Acetic Acid Solutions to Macroscopic Mixing Thermodynamics: A Quantitative Connection from THz-Calorimetry

We report the results of THz measurements (30-440 cm-1) of aqueous acetic acid solutions over the full mixing range (XAcAc = 0-1). We recorded spectroscopic observables as a function of the acetic acid concentration in the frequency range of the intermolecular stretch at 150 cm-1 and of the librational modes at 350-440 cm-1. This allowed us to unravel changes in hydrophobic and hydrophilic hydration motifs, respectively. By means of a novel THz-calorimetry approach, we quantitatively correlated these changes in local hydration motifs to excess mixing entropy and enthalpy. We find that ΔHmix is determined by both hydrophobic and hydrophilic solvation contributions. In contrast, ΔSmix is governed by hydrophobic cavity formation. Our results further suggest that acetic acid-water mixtures are systems at the edge of phase separation due to endothermic contributions from both hydrophilic and hydrophobic solvation in a large portion of the miscibility range. Our work establishes a quantitative relationship between the balance of local hydrophobic and hydrophilic solvation motifs and the macroscopic mixing thermodynamic properties.

Hydration makes a difference! How to tune protein complexes between liquid-liquid and liquid-solid phase separation

Understanding how protein rich condensates formed upon liquid-liquid phase separation (LLPS) evolve into solid aggregates is of fundamental importance for several medical applications, since these are suspected to be hot-spots for many neurotoxic diseases. This requires developing experimental approaches to observe in real-time both LLPS and liquid-solid phase separation (LSPS), and to unravel the delicate balance of protein and water interactions dictating the free energy differences between the two. We present a vibrational THz spectroscopy approach that allows doing so from the point of view of hydration water. We focus on a cellular prion protein of high medical relevance, which we can drive to undergo either LLPS or LSPS with few mutations. We find that it is a subtle balance of hydrophobic and hydrophilic solvation contributions that allows tuning between LLPS and LSPS. Hydrophobic hydration provides an entropic driving force to phase separation, through the release of hydration water into the bulk. Water hydrating hydrophilic groups provides an enthalpic driving force to keep the condensates in a liquid state. As a result, when we modify the protein by a few mutations to be less hydrophilic, we shift from LLPS to LSPS. This molecular understanding paves the way for a rational design of proteins.

Impact of hydrophobicity on local solvation structures and its connection with the global solubilization thermodynamics of amphiphilic molecules

The relationship between the local solvation structures and global thermodynamics, specifically in the case of amphiphilic molecules, is a complex phenomenon and is not yet fully understood. With the prior knowledge that local solvation structures can impose a significant impact on the overall solvation process, we here combine THz spectroscopic analysis with MD simulations to investigate the impact of the altered hydrophobicity and polarity of amphiphilic solute molecules on the local solvation configurations. We use two water soluble alcohols: ethanol (EtOH) and its fluorinated counterpart, 2,2,2-trifluoroethanol (TFE), as model solutes. Our study is aimed to determine the relative abundance of different hydrogen bonded conformers and to establish a correlation between the spectral signatures (as obtained from THz spectroscopic measurements) and microscopic solute-solvent interactions associated with the local solvation structures (as obtained from MD simulations). Finally, we estimate the possible energetic parameters associated with the alcohol solubilization process. We found that while both the alcohols are completely water soluble, they receive a contrasting solvation energy share in terms of entropy and enthalpy. We understand that these findings are not limited to the specific system studied here but can be broadly extrapolated to other amphiphilic aqueous solutions.

Solvation Plays a Key Role in Antioxidant-Mediated Attenuation of Elevated Creatinine Level: An In Vitro Spectroscopic Investigation

An elevated level of creatinine (CRN) is a mark of kidney ailment, and prolonged retention of such condition could lead to renal failure, associated with severe ischemia. Antioxidants are clinically known to excrete CRN from the body through urine, thereby reducing its level in blood. The molecular mechanism of such an exclusion process is still illusive. As the excretion channel is urine, solvation of the solute is expected to play a pivotal role. Here, we report a detailed time-domain and frequency-domain terahertz (THz) spectroscopic investigation to understand the solvation of CRN in the presence of two model antioxidants, mostly used to treat elevated CRN level: N-Acetyl-l-cysteine (NAC) and ascorbic acid (ASC). FTIR spectroscopy in the mid-infrared region and UV absorption spectroscopy measurements coupled with quantum chemical calculations [at the B3LYP/6-311G++(d,p) level] reveal that both NAC and ASC form HBonded complexes with CRN and rapidly undergo a barrier-less proton transfer process to form creatinium ions. THz measurements provide explicit evidence of the formation of highly solvated complexes compared with bare CRN, which eventually enables its excretion through urine. These observations could provide a foundation for designing more beneficial drugs to resolve kidney diseases..

Local solvation structures govern the mixing thermodynamics of glycerol-water solutions

Glycerol is a major cryoprotective agent and is widely used to promote protein stabilization. By a combined experimental and theoretical study, we show that global thermodynamic mixing properties of glycerol and water are dictated by local solvation motifs. We identify three hydration water populations, i.e., bulk water, bound water (water hydrogen bonded to the hydrophilic groups of glycerol) and cavity wrap water (water hydrating the hydrophobic moieties). Here, we show that for glycerol experimental observables in the THz regime allow quantification of the abundance of bound water and its partial contribution to the mixing thermodynamics. Specifically, we uncover a 1 : 1 connection between the population of bound waters and the mixing enthalpy, which is further corroborated by the simulation results. Therefore, the changes in global thermodynamic quantity - mixing enthalpy - are rationalized at the molecular level in terms of changes in the local hydrophilic hydration population as a function of glycerol mole fraction in the full miscibility range. This offers opportunities to rationally design polyol water, as well as other aqueous mixtures to optimize technological applications by tuning mixing enthalpy and entropy based on spectroscopic screening.

Water structure modification by d-(+)-glucose at different concentrations and temperatures-effect of mutarotation

Water structure modification by carbohydrates is essential both in chemistry and life processes and in particular, molecular level interaction of glucose with water is very important. With a view to developing a fundamental knowledge base, thermodynamic parameters derived from measurements of density, viscosity, and refractive index have been analyzed to investigate how d-(+)-glucose alters the structure of water at various concentrations and temperatures. The nature and extent of the interactions have been investigated using apparent molar volume, Jones-Dole constants, changes in free energy (ΔG), changes in entropy (ΔS), and changes in enthalpy (ΔH) for viscous flow. Using measurements from dynamic light scattering (DLS), the sizes of the aggregates were studied. The kinetics of mutarotation have been investigated using polarimetry and the structural effect on water during mutarotation between α-d-glucose and β-d-glucose with time has been explored by near-infrared (NIR) spectroscopy. The spectroscopic results were examined using difference spectroscopy and two-dimensional correlation spectroscopy (2DCOS). The absorption bands of water shift to a higher wavenumber irrespective of the concentration of the solution with time due to the enhancement of the cleavage of hydrogen bonding in water. At high temperatures, three bands in the region 7100-7350 cm^-1 are attributed to the first overtones of the hydrogen-bonded -O-H stretching vibration. Refractive index values indicate an increase in the density of the anomer solutions with time, suggesting an increase in free water concentration. These results provide evidence for more than one water molecule being involved in the mechanism of mutarotation and propose a concerted mechanism for proton transfer.

The birth and evolution of solvated electrons in the water

The photo-induced radiolysis of water is an elementary reaction in biology and chemistry, forming solvated electrons, OH radicals, and hydronium cations on fast time scales. Here, we use an optical-pump terahertz-probe spectroscopy setup to trigger the photoionization of water molecules with optical laser pulses at ~400 nm and then time-resolve the transient solvent response with broadband terahertz (THz) fields with a ~90 fs time resolution. We observe three distinct spectral responses. The first is a positive broadband mode that can be attributed to an initial diffuse, delocalized electron with a radius of (22 ± 1) Å, which is short lived (<200 fs) because the absorption is blue-shifting outside of the THz range. The second emerging spectroscopic signature with a lifetime of about 150 ps is attributed to an intermolecular mode associated with a mass rearrangement of solvent molecules due to charge separation of radicals and hydronium cations. After 0.2 ps, we observe a long-lasting THz signature with depleted intensity at 110 cm^-1 that is well reproduced by ab initio molecular dynamics. We interpret this negative band at 110 cm^-1 as the solvent cage characterized by a weakening of the hydrogen bond network in the first and second hydration shells of the cavity occupied by the localized electron.

Liquid-Liquid Phase Separation? Ask the Water!

Water is more than an inert spectator during liquid-liquid phase separation (LLPS), the reversible compartmentalization of protein solutions into a protein-rich and a dilute phase. We show that LLPS is driven by changes in hydration entropy and enthalpy. Tuning LLPS by adjusting experimental parameters, e.g., addition of co-solutes, is a major goal for biological and medical applications. This requires a general model to quantify thermodynamic driving forces. Here, we develop such a model based on the measured amplitudes of characteristic THz-features of two hydration populations: "Cavity-wrap" water hydrating hydrophobic patches is released during LLPS leading to an increase in entropy. "Bound" water hydrating hydrophilic patches is retained since it is enthalpically favorable. We introduce a THz-phase diagram mapping these spectroscopic/thermodynamic changes. This provides not only a precise understanding of hydrophobic and hydrophilic hydration driving forces as a function of temperature and concentration but also a rational means to tune LLPS.

Connecting Non-Gaussian Water Density Fluctuations to the Lengthscale Dependent Crossover in Hydrophobic Hydration

We connect density fluctuations in liquid water to lengthscale dependent crossover in hydrophobic hydration. Specifically, we employ indirect umbrella sampling (INDUS) simulations to characterize density fluctuations in observation volumes of various sizes and shapes in water and as a function of temperature and salt concentration. Consistent with previous observations, density fluctuations are Gaussian in small molecular scale volumes, but they display non-Gaussian "low-density fat tails" in larger volumes. These non-Gaussian tails are indicative of the proximity of water to its liquid to vapor phase transition and have implications on biomolecular interactions and function. We show that the onset of non-Gaussian fluctuations in large volumes is accompanied by the formation of a cavity in the observation volume. We develop a model that uses the physics of cavity-water interface formation as a key ingredient and show that it captures the nature of non-Gaussian density fluctuations over a broad region in water and in salt solutions. We discuss the limitations of this model in the very low density region of the distribution. Our calculations provide new insights into the origins of non-Gaussian density fluctuations in water and their connections to lengthscale dependent crossover in hydrophobic hydration.

Spectroscopic Fingerprints of Cavity Formation and Solute Insertion as a Measure of Hydration Entropic Loss and Enthalpic Gain

Hydration free energies are dictated by a subtle balance of hydrophobic and hydrophilic interactions. We present here a spectroscopic approach, which gives direct access to the two main contributions: Using THz-spectroscopy to probe the frequency range of the intermolecular stretch (150-200 cm-1 ) and the hindered rotations (450-600 cm-1 ), the local contributions due to cavity formation and hydrophilic interactions can be traced back. We show that via THz calorimetry these fingerprints can be correlated 1 : 1 with the group specific solvation entropy and enthalpy. This allows to deduce separately the hydrophobic (i.e. cavity formation) and hydrophilic contributions to thermodynamics, as shown for hydrated alcohols as a case study. Accompanying molecular dynamics simulations quantitatively support our experimental results. In the future our approach will allow to dissect hydration contributions in inhomogeneous mixtures and under non-equilibrium conditions.

Size-Dependent Order-Disorder Crossover in Hydrophobic Hydration: Comparison between Spherical Solutes and Linear Alcohols

Theory and computer simulation studies have predicted that water molecules around hydrophobic molecules should undergo an order-disorder transition with increasing solute size around a 1 nm length scale. Some theories predict the formation of a clathrate-like ordered structure around smaller hydrophobic solutes (<1 nm) and the formation of disordered vapor-liquid interfaces around larger solutes (>1 nm) and surfaces. Experimental validation of these predictions has often been elusive and contradictory. High-resolution Raman spectroscopy has detected that water around small hydrophobic solutes shows a signature similar to that of bulk water at lower temperature (increased ordering and a stronger hydrogen-bonded network). Similarly, water around larger solutes shows an increasing population of dangling OH bonds very similar to higher temperature bulk water. Thus, the solute size dependence of the structure and dynamics of water around hydrophobic molecules seems to have an analogy with the temperature dependence in bulk water. In this work, using atomistic classical molecular dynamics (MD) simulations, we have systematically investigated this aspect and characterized this interesting analogy. Structural order parameters including the tetrahedral order parameter (Q), hydrogen bond distribution, and vibrational power spectrum highlight this similarity. However, in contrast to the experimental observations, we do not observe any length-dependent crossover for linear hydrophobic alcohols (n-alkanols) using classical MD simulations. This is in agreement with earlier findings that linear alkane chains do not demonstrate the length-dependent order-disorder transition due to the presence of a sub-nanometer length scale along the cross section of the chain. Moreover, the collapsed state of linear hydrocarbon chains is not significantly populated for smaller chains (number of carbons below 20). In the context of our computational results, we raise several pertinent questions related to the sensitivity of various structural and dynamical parameters toward capturing these complex phenomena of hydrophobic hydration.

Stripping away ion hydration shells in electrical double-layer formation: Water networks matter

For centuries the double layer at the solid/electrolyte interface has been a central concept in electrochemistry. Today, it is still crucial for virtually all renewable energy storage and conversion technologies. Here, the double-layer formation is probed by THz spectroscopy with ultrabright synchrotron light as a source. Our results capture the molecular details of double-layer formation at positively/negatively charged Au electrodes for an NaCl electrolyte. We reveal a contrasting response applying positive versus negative bias, which is dictated by the interfacial water network and rationalized by accompanying molecular dynamics simulations and electronic-structure calculations. While Na⁺ is directly attracted toward the negatively charged electrode, stripping of the Cl⁻ hydration shell is observed only at larger potential values.

The double layer at the solid/electrolyte interface is a key concept in electrochemistry. Here, we present an experimental study combined with simulations, which provides a molecular picture of the double-layer formation under applied voltage. By THz spectroscopy we are able to follow the stripping away of the cation/anion hydration shells for an NaCl electrolyte at the Au surface when decreasing/increasing the bias potential. While Na⁺ is attracted toward the electrode at the smallest applied negative potentials, stripping of the Cl⁻ hydration shell is observed only at higher potential values. These phenomena are directly measured by THz spectroscopy with ultrabright synchrotron light as a source and rationalized by accompanying molecular dynamics simulations and electronic-structure calculations.

Thermally Driven Transformation of Water Clustering Structures at Self-Assembled Monolayers

Water clustering structures are considered to play key roles in various temperature-dependent life activities. However, our fundamental understanding of the temperature-dependent water structures remains murky because of the limits of the real-time and real-condition monitoring techniques at the molecular level. We propose an in situ ATR-IR approach combining Gaussian fitting to qualitatively and quantitatively explore the temperature-dependent structural stability and transformation of the three water components, multimer water (MW), intermediate water (IW), and network water (NW), on interfaces with different wettabilities. Our results show that the transformation between NW and IW/MW will occur with a change in temperature. This conversion process shows reversibility on hydrophilic Au NPs film/ZnSe, while it is irreversible on a hydrophobic mercaptohexane self-assembled monolayer due to the irreversibility of the monolayer structure and the hydrophobic confinement effect. The established approach enables us to explore the change in the water properties at any interfaces upon external stimuli.

Local Mutations Can Serve as a Game Changer for Global Protein Solvent Interaction

Although it is well-known that limited local mutations of enzymes, such as matrix metalloproteinases (MMPs), may change enzyme activity by orders of magnitude as well as its stability, the completely rational design of proteins is still challenging. These local changes alter the electrostatic potential and thus local electrostatic fields, which impacts the dynamics of water molecules close the protein surface. Here we show by a combined computational design, experimental, and molecular dynamics (MD) study that local mutations have not only a local but also a global effect on the solvent: In the specific case of the matrix metalloprotease MMP14, we found that the nature of local mutations, coupled with surface morphology, have the ability to influence large patches of the water hydrogen-bonding network at the protein surface, which is correlated with stability. The solvent contribution can be experimentally probed via terahertz (THz) spectroscopy, thus opening the door to the exciting perspective of rational protein design in which a systematic tuning of hydration water properties allows manipulation of protein stability and enzymatic activity.

Aqueous TMAO solution under high hydrostatic pressure

Trimethylamine N-oxide (TMAO) is a well known osmolyte in nature, which is used by deep sea fish to stabilize proteins against High Hydrostatic Pressure (HHP). We present a combined ab initio molecular dynamics, force field molecular dynamics, and THz absorption study of TMAO in water up to 12 kbar to decipher its solvation properties upon extreme compression. On the hydrophilic oxygen side of TMAO, AIMD simulations at 1 bar and 10 kbar predict a change of the coordination number from a dominating TMAO·(H₂O)₃ complex at ambient conditions towards an increased population of a TMAO·(H₂O)₄ complex at HHP conditions. This increase of the TMAO-oxygen coordination number goes in line with a weakening of the local hydrogen bond network, spectroscopic shifts and intensity changes of the corresponding intermolecular THz bands. Using a pressure-dependent HHP force field, FFMD simulations predict a significant increase of hydrophobic hydration from 1 bar up to 4-5 kbar, which levels off at higher pressures up to 10 kbar. THz spectroscopic data reveal two important pressure regimes with spectroscopic inflection points of the dominant intermolecular modes: The first regime (1.5-2 kbar) is barely recognizable in the simulation data. However, it relates well with the observation that the apparent molar volume of solvated TMAO is nearly constant in the biologically relevant pressure range up to 1 kbar as found in the deepest habitats on Earth in the ocean. The second inflection point around 4-5 kbar is related to the amount of hydrophobic hydration as predicted by the FFMD simulations. In particular, the blueshift of the intramolecular CNC bending mode of TMAO at about 390 cm^-1 is the spectroscopic signature of increasingly pronounced pressure-induced changes in the solvation shell of TMAO. Thus, the CNC bend can serve as local pressure sensor in the multi-kbar pressure regime.

Molecular Fingerprints of Hydrophobicity at Aqueous Interfaces from Theory and Vibrational Spectroscopies

Hydrophobicity/hydrophilicity of aqueous interfaces at the molecular level results from a subtle balance in the water-water and water-surface interactions. This is characterized here via density functional theory-molecular dynamics (DFT-MD) coupled with vibrational sum frequency generation (SFG) and THz-IR absorption spectroscopies. We show that water at the interface with a series of weakly interacting materials is organized into a two-dimensional hydrogen-bonded network (2D-HB-network), which is also found above some macroscopically hydrophilic silica and alumina surfaces. These results are rationalized through a descriptor that measures the number of "vertical" and "horizontal" hydrogen bonds formed by interfacial water, quantifying the competition between water-surface and water-water interactions. The 2D-HB-network is directly revealed by THz-IR absorption spectroscopy, while the competition of water-water and water-surface interactions is quantified from SFG markers. The combination of SFG and THz-IR spectroscopies is thus found to be a compelling tool to characterize the finest details of molecular hydrophobicity at aqueous interfaces.

The key role of solvent in condensation: Mapping water in liquid-liquid phase-separated FUS

Formation of biomolecular condensates through liquid-liquid phase separation (LLPS) has emerged as a pervasive principle in cell biology, allowing compartmentalization and spatiotemporal regulation of dynamic cellular processes. Proteins that form condensates under physiological conditions often contain intrinsically disordered regions with low-complexity domains. Among them, the RNA-binding proteins FUS and TDP-43 have been a focus of intense investigation because aberrant condensation and aggregation of these proteins is linked to neurodegenerative diseases such as amyotrophic lateral sclerosis and frontotemporal dementia. LLPS occurs when protein-rich condensates form surrounded by a dilute aqueous solution. LLPS is per se entropically unfavorable. Energetically favorable multivalent protein-protein interactions are one important aspect to offset entropic costs. Another proposed aspect is the release of entropically unfavorable preordered hydration water into the bulk. We used attenuated total reflection spectroscopy in the terahertz frequency range to characterize the changes in the hydrogen bonding network accompanying the FUS enrichment in liquid-liquid phase-separated droplets to provide experimental evidence for the key role of the solvent as a thermodynamic driving force. The FUS concentration inside LLPS droplets was determined to be increased to 2.0 mM independent of the initial protein concentration (5 or 10 μM solutions) by fluorescence measurements. With terahertz spectroscopy, we revealed a dewetting of hydrophobic side chains in phase-separated FUS. Thus, the release of entropically unfavorable water populations into the bulk goes hand in hand with enthalpically favorable protein-protein interaction. Both changes are energetically favorable, and our study shows that both contribute to the thermodynamic driving force in phase separation.

Interactions of Glycerol, Diglycerol, and Water Studied Using Attenuated Total Reflection Infrared Spectroscopy

In order to examine the mixing properties of glycerol-water and diglycerol-water solutions, these solutions were measured using attenuated total reflection infrared spectroscopy. The absorbance spectra corrected for 1 µm thickness were subtracted by pure polyols for obtaining water spectra, and by pure water for polyol spectra. Both asymmetric and symmetric CH₂ stretching vibration bands (around 2940, 2885 cm^-1) shifted about 10 cm^-1 to lower wavenumber side (redshifts) with increasing polyol concentrations, especially at higher concentrations. Redshifts of C-O-H rocking bands (around 1335 cm^-1) with increasing polyol concentrations are slightly larger for diglycerol-water (10 > 6 cm^-1) than glycerol-water solutions. C-O stretching bands of CHOH groups (1125 and 1112 cm^-1) shift slightly but in opposite sides for glycerol and diglycerol at highest polyol concentrations (90-100 wt%). These shifts of CH₂ stretching, COH rocking, and CO stretching of CHOH at higher polyol concentrations suggest interactions of outer CH₂ with inner CHOH groups of surrounding polyols. The normalized band area changes with polyol concentrations could be fitted by quadratic polynomials possibly due to mixtures of different interactions between water-water, polyol-water, and polyol-polyol molecules. The OH stretching band for diglycerol 90 wt% shows three humps indicating at least three OH components: long, medium, and short H bond water molecules. Short H bond water molecules are the major component possibly between inner CHOH and outer side CH₂OH groups, while the long H component might loosely bind to outer CH₂OH groups.

Wrapping Up Hydrophobic Hydration: Locality Matters

Water, being the universal solvent, acts as a competing agent in fundamental processes, such as folding, aggregation or biomolecular recognition. A molecular understanding of hydrophobic hydration is of central importance to understanding the subtle free energy differences, which dictate function. Ab initio and classical molecular dynamics simulations yield two distinct hydration water populations in the hydration shell of solvated tert-butanol noted as "HB-wrap" and "HB-hydration2bulk". The experimentally observed hydration water spectrum can be dissected into two modes, centered at 164 and 195 cm-1. By comparison to the simulations, these two bands are attributed to the "HB-wrap" and "HB-hydration2bulk" populations, respectively. We derive a quantitative correlation between the population in each of these two local water coordination motifs and the temperature dependence of the solvation entropy. The crossover from entropy to enthalpy dominated solvation at elevated temperatures, as predicted by theory and observed experimentally, can be rationalized in terms of the distinct temperature stability and thermodynamic signatures of "HB-wrap" and "HB-hydration2bulk".

Influence of crowding on hydrophobic hydration-shell structure

The influence of molecular crowding on water structure, and the associated crossover behavior, is quantified using Raman multivariate curve resolution (Raman-MCR) hydration-shell vibrational spectroscopy of aqueous tert-butyl alcohol, 2-butyl alcohol and 2-butoxyethanol solutions of variable concentration and temperature. Changes in the hydration-shell OH stretch band shape and mean frequency are used to identify the temperature at which the hydration-shell crosses over from a more ordered to less ordered structure, relative to pure water. The influence of crowding on the crossover is found to depend on solute size and shape in a way that is correlated with the corresponding infinitely dilute hydration-shell structure (and the corresponding first hydration-shell spectra are invariably very similar to pure water). Analysis of the results using a Muller-like two-state equilibrium between more ordered and less ordered hydration-shell structures implies that crossover temperature changes are dictated primarily by enthalpic stabilization of the more ordered hydration-shell structures.

Water Structure and Properties at Hydrophilic and Hydrophobic Surfaces

The properties of water on both molecular and macroscopic surfaces critically influence a wide range of physical behaviors, with applications spanning from membrane science to catalysis to protein engineering. Yet, our current understanding of water interfacing molecular and material surfaces is incomplete, in part because measurement of water structure and molecular-scale properties challenges even the most advanced experimental characterization techniques and computational approaches. This review highlights progress in the ongoing development of tools working to answer fundamental questions on the principles that govern the interactions between water and surfaces. One outstanding and critical question is what universal molecular signatures capture the hydrophobicity of different surfaces in an operationally meaningful way, since traditional macroscopic hydrophobicity measures like contact angles fail to capture even basic properties of molecular or extended surfaces with any heterogeneity at the nanometer length scale. Resolving this grand challenge will require close interactions between state-of-the-art experiments, simulations, and theory, spanning research groups and using agreed-upon model systems, to synthesize an integrated knowledge of solvation water structure, dynamics, and thermodynamics.

Changes in IR band areas and band shifts during water adsorption to lecithin and ceramide

Adsorption of water to a phospholipid (lecithin) and a ceramide were studied by IR microspectroscopy equipped with a humidity control system and quartz crystal microbalance (QCM). The water weight ratios increase up to 12.2 wt% for lecithin and 1.2 wt% for ceramide at RH ~80%, with linear correlations with infrared OH (+NH) band areas. For lecithin, the 1230 cm^-1 band (PO₂^-) and the 1735 cm^-1 band (CO) shift to lower wavenumbers, while the 1060 cm^-1 band (PO₂^-, POC) shift to higher wavenumber with RH. Band areas of phosphates (1230 and 1060 cm^-1) increase with RH showing positive relations with the band area of bound water. Bound water molecules with shorter H bonds might be bound to these phosphate groups. Band areas of aliphatic CHs are negatively correlated with the increasing adsorption of free water. Free water molecules with longer H bonds might interact loosely with aliphatic chains of lecithin. For ceramide, only the 1045 cm^-1 band (CO) shows a small red shift at higher RHs than 60%, indicating adsorption of bound water to CO bonds. Amounts of water molecules adsorbed to ceramide are very limited due to few adsorption of free water.

Hydrogen-Bonding in Liquid Water at Multikilobar Pressures

High-precision THz (30 to 360 cm^-1) spectra of bulk liquid water are presented from ambient conditions up to hydrostatic pressures of 10 kbar. In concert with ab initio simulations, this allows us to characterize the molecular-level changes of the H-bond network under solvent stress conditions. Both the experimental and theoretical THz spectra reveal a blue shift in the intermolecular translational mode at 180 cm^-1 by 40 cm^-1 at 10 kbar and a blue shift together with an intensity increase in the relaxation mode. These changes can be traced back to a pressure-induced increase of the population of so-called short H-bond double donor configurations at the expense of those with longer such intermolecular bonds. Distinct electronic polarization effects are critical to capture the characteristic intensity changes of the THz line shape function. These advances in high-pressure THz spectroscopy open the door to investigate the pressure response of solvation shells and solute-solvent couplings.

Hydration-Shell Vibrational Spectroscopy

Hydration-shell vibrational spectroscopy provides an experimental window into solute-induced water structure changes that mediate aqueous folding, binding, and self-assembly. Decomposition of measured Raman and infrared (IR) spectra of aqueous solutions using multivariate curve resolution (MCR) and related methods may be used to obtain solute-correlated spectra revealing solute-induced perturbations of water structure, such as changes in water hydrogen-bond strength, tetrahedral order, and the presence of dangling (non-hydrogen-bonded) OH groups. More generally, vibrational-MCR may be applied to both aqueous and nonaqueous solutions, including multicomponent mixtures, to quantify solvent-mediated interactions between oily, polar, and ionic solutes, in both dilute and crowded fluids. Combining vibrational-MCR with emerging theoretical modeling strategies promises synergetic advances in the predictive understanding of multiscale self-assembly processes of both biological and technological interest.

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.

Local chemistry of the surfactant's head groups determines protein stability in reverse micelles

Protein stability in reverse micelles is determined by local chemical interactions between the surfactant molecules and the protein groups.

Signatures of Solvation Thermodynamics in Spectra of Intermolecular Vibrations

This study explores the thermodynamic and vibrational properties of water in the three-dimensional environment of solvated ions and small molecules using molecular simulations. The spectrum of intermolecular vibrations in liquid solvents provides detailed information on the shape of the local potential energy surface, which in turn determines local thermodynamic properties such as the entropy. Here, we extract this information using a spatially resolved extension of the two-phase thermodynamics method to estimate hydration water entropies based on the local vibrational density of states (3D-2PT). Combined with an analysis of solute–water and water–water interaction energies, this allows us to resolve local contributions to the solvation enthalpy, entropy, and free energy. We use this approach to study effects of ions on their surrounding water hydrogen bond network, its spectrum of intermolecular vibrations, and resulting thermodynamic properties. In the three-dimensional environment of polar and nonpolar functional groups of molecular solutes, we identify distinct hydration water species and classify them by their characteristic vibrational density of states and molecular entropies. In each case, we are able to assign variations in local hydration water entropies to specific changes in the spectrum of intermolecular vibrations. This provides an important link for the thermodynamic interpretation of vibrational spectra that are accessible to far-infrared absorption and Raman spectroscopy experiments. Our analysis provides unique microscopic details regarding the hydration of hydrophobic and hydrophilic functional groups, which enable us to identify interactions and molecular degrees of freedom that determine relevant contributions to the solvation entropy and consequently the free energy.

Mapping Hydration Water around Alcohol Chains by THz Calorimetry

THz spectroscopy was used to probe changes that occur in the dynamics of the hydrogen bond network upon solvation of alcohol chains. The THz spectra can be decomposed into the spectrum of bulk water, tetrahedral hydration water, and more disordered (or interstitial) hydration water. The tetrahedrally ordered hydration water exhibits a band at 195 cm⁻¹ and is localized around the hydrophobic moiety of the alcohol. The interstitial component yields a band at 164 cm⁻¹ which is associated with hydration water in the first hydration shell. These temperature‐dependent changes in the low‐frequency spectrum of solvated alcohol chains can be correlated with changes of heat capacity, entropy, and free energy upon solvation. Surprisingly, not the tetrahedrally ordered component but the interstitial hydration water is found to be mainly responsible for the temperature‐dependent change in ΔC_p and ΔG. The solute‐specific offset in free energy is attributed to void formation and scales linearly with the chain length.