The JG β-relaxation in water and impact on the dynamics of aqueous mixtures and hydrated biomolecules

Origin of ργ/T scaling of primary and secondary conductivity relaxation times in mixture of water with protic ionic liquid

Murali et al. [J. Phys. Chem. Lett., 2024, 15, 3376-3382] made ambient and high pressure dielectric measurements of a supercooled aqueous mixture of an acidic ionic liquid to find the presence of the primary (σ) conductivity relaxation together with the secondary (ν) conductivity relaxation originating from the water clusters confined by the cations and anions with relaxation times τσ and τν respectively. From the isothermal and isobaric conductivity relaxation data found on varying thermodynamic conditions (i.e. T and P) at constant τσ are the invariance of (i) the frequency dispersion or the Kohlrausch function exponent (1 - n) of the primary conductivity relaxation, and (ii) the ratio of the primary and secondary conductivity times, τσ/τν. This co-invariance of τσ, τν, and (1 - n) at constant τσ was observed before in non-aqueous ionic liquids, but it is found for the first time in aqueous ionic liquids. The new data together with PVT measurements enable Murali et al. to show additionally that both τσ and τν are functions of ργ/T with a single exponent γ = 0.58. The Coupling model is the only theory predicting the co-invariance of τσ, τν, and (1 - n) as well as the ργ/T scaling of both τσ and τν. It is applied herein to address and explain the data of the ionic liquid-water mixture.

Complex dynamics of partially freezable confined water revealed by combined experimental and computational studies

We investigate water dynamics in mesoporous silica across partial crystallization by combining broadband dielectric spectroscopy (BDS), nuclear magnetic resonance (NMR), and molecular dynamics simulations (MDS). Exploiting the fact that not only BDS but also NMR field-cycling relaxometry and stimulated-echo experiments provide access to dynamical susceptibilities in broad frequency and temperature ranges, we study both the fully liquid state above the melting point Tm and the dynamics of coexisting water and ice phases below this temperature. It is found that partial crystallization leads to a change in the temperature dependence of rotational correlation times τ, which occurs in addition to previously reported dynamical crossovers of confined water and depends on the pore diameter. Furthermore, we observe that dynamical susceptibilities of water are strongly asymmetric in the fully liquid state, whereas they are much broader and nearly symmetric in the partially frozen state. Finally, water in the nonfreezable interfacial layer below Tm does not exhibit a much debated dynamical crossover at ∼220 K. We argue that its dynamics is governed by a static energy landscape, which results from the interaction with the bordering silica and ice surfaces and features a Gaussian-like barrier distribution. Consistently, our MDS analysis of the motional mechanism reveals a hopping motion of water in thin interfacial layers. The rotational correlation times of the confined ice phases follow Arrhenius laws. While the values of τ depend on the pore diameter, freezable water in various types of confinements and mixtures shows similar activation energies of Ea ≈ 0.43 eV.

Dynamics of Water Clusters Confined in Ionic Liquid at an Elevated Pressure

Over the years, numerous experimental and theoretical efforts have been dedicated to investigating the mysteries of water and determining its new unexplored physical properties. Despite this, high-pressure studies of water and aqueous mixtures close to the glass transition still represent an unknown area of research. Herein, we address a fundamental issue: the validity of the density scaling concept for fast water dynamics. For this purpose, we performed ambient and high-pressure dielectric measurements of a supercooled equimolar aqueous mixture of an acidic ionic liquid. All isothermal and isobaric relaxation data describing the time scale of charge transport (τσ) and fast dynamics within the water clusters (τν) reveal visual evidence of a liquid-glass transition. Furthermore, both relaxation processes satisfy the ργ/T scaling concept with a single exponent γ = 0.58. Thus, the scaling exponent is a state-point-independent parameter for the dynamics of water clusters confined in ionic liquid investigated in the pressure range up to 300 MPa.

Glass transition phenomena and dielectric relaxations in supercooled d-lyxose aqueous solutions

Differential scanning calorimeter and broadband dielectric spectroscopy in a broad range of temperatures (150-300 K) were employed to study the d-lyxose aqueous mixture at different hydration levels. Two relaxation processes were observed in all investigated d-lyxose aqueous mixtures. A relaxation process (process-I) usually known as the primary relaxation mode which is accountable for the collective motion of d-lyxose aqueous solution, was observed above the glass transition temperature (Tg). Below Tg, another process designated as process-II was found which is mainly related to the water molecule relaxation inside the d-lyxose matrix. The average relaxation times as a function of temperature and dielectric strengths of both observed relaxation processes (I & II) were analyzed for all hydration levels in d-lyxose. It was identified that the relaxation amplitude of process-II in the d-lyxose aqueous mixture was increased drastically and their activation energies were found to be approximately independent of the content of water above critical concentration, xc = 0.28. This suggests that the dynamical process observed above xc was dominated by the presence of water clusters. In the current aqueous mixture, the critical content of water (xc) is slightly higher as compared to previously reported aqueous mixtures, indicating a more cooperative nature of water molecules with a d-lyxose matrix. Additionally, the Tg of pure water was estimated at 128 ± 5.8 K from the extrapolation of DSC Tg data of the d-lyxose aqueous solution by using the well-known Gordon-Taylor equation. Our current result gives further support to the well-accepted glass transition (Tg) of pure water.

Dynamical Susceptibilities of Confined Water from Room Temperature to the Glass Transition

We confine water to narrow silica pores, where crystallization is suppressed, and determine the dynamical susceptibilities of the liquid from room temperature down to the glass transition by combining broadband dielectric spectroscopy (BDS) with ¹H and ²H nuclear magnetic resonance (NMR), in particular, by establishing NMR field-cycling relaxometry. For the correlation times, derivative analysis reveals Vogel-Fulcher-Tammann and Arrhenius regimes at T ≥ 215 K and T ≤ 160 K, respectively, which are separated by a broad crossover region. The continuous transition in the temperature dependence is accompanied by a gradual change from asymmetric high-temperature shapes of the dynamical susceptibilities to symmetric low-temperature ones and by a steady decrease of the dielectric relaxation strength. In the Arrhenius regime (E_a = 0.48 eV) at T ≤ 160 K, 2D ²H NMR spectra reveal quasi-isotropic water reorientation. We rationalize these results in terms of a crossover to an interface-affected, noncooperative relaxation involving both rotational and translational motions.

Temperature-Dependent Dynamics at Protein-Solvent Interfaces

We perform differential scanning calorimetry, broadband dielectric spectroscopy (BDS), and nuclear magnetic resonance (NMR) studies to ascertain the molecular dynamics in mixtures of ethylene glycol with elastin or lysozyme over broad temperature ranges. To focus on the protein-solvent interface, we use mixtures with about equal numbers of amino acids and solvent molecules. The elastin and lysozyme mixtures show similar glass transition steps, which extend over a broad temperature range of 157-185K. The BDS and NMR studies yield fully consistent results for the fastest process P1, which is caused by the structural relaxation of ethylene glycol between the protein molecules and follows an Arrhenius law with an activation energy of Ea=0.63eV. It involves quasi-isotropic reorientation and is very similar in the elastin and lysozyme matrices but different from the alpha and beta relaxations of bulk ethylene glycol. Two slower BDS processes P2 and P3 have protein-dependent time scales, but exhibit a similar Arrhenius-like temperature dependence with an activation energy of Ea~0.81eV. However, P2 and P3 do not have a clear NMR signature. In particular, the NMR results for the lysozyme mixture reveal that the protein backbone does not show isotropic alpha-like motion on the P2 and P3 time scales but only restricted beta-like reorientation. The different activation energies of the P1 and P2/P3 processes do not support an intimate coupling of protein and ethylene glycol dynamics. The present results are compared with previous findings for mixtures of proteins with water or glycerol, implying qualitatively different dynamical couplings at various protein-solvent interfaces.

Local and diffusive dynamics of LiCl aqueous solutions in pristine and modified silica nanopores

We use 1H, 2H, and 7Li NMR to investigate local and diffusive dynamics of LiCl-7H2O and LiCl-7D2O solutions in pristine and functionalized silica nanopores in a component-selective manner. Recently, we showed that the solution dynamics become slower when the diameter of the pristine pores is reduced. Here, we determine the effects of (aminopropyl)triethoxysilane and dye surface functionalizations on the motions of the water molecules and lithium ions from ambient temperatures down to the glass transition. The local and diffusive solution dynamics are similar in both functionalized pores but, on average, slower than in pristine pores with comparable diameters. When the exchange between different confinement regions is sufficiently slow at reduced temperatures, bimodal water and lithium dynamics may be observed. We attribute this bimodality to bulk-like motion in the pore centers and slowed-down motion at the pore walls. For the lithium ions, a bimodality observed in the pristine pores is absent in the functionalized ones. We conjecture that the steric hindrance and electrostatic interactions associated with the grafted functional groups interfere with the formation of a defined electric double layer, while the enhanced surface roughness and unequal charge distribution result in overall slower dynamics. Thus, the nature of the walls is an important parameter for the solution dynamics. Thereby, in-situ measurements of the pH value inside the silica pores using the grafted dye molecules reveal that observed changes in the pH value in response to the surface functionalization are of limited relevance for the water reorientation.

Probing Adaptation of Hydration and Protein Dynamics to Temperature

Protein dynamics is strongly influenced by the surrounding environment and physiological conditions. Here we employ broadband megahertz-to-terahertz spectroscopy to explore the dynamics of water and myoglobin protein on an extended time scale from femto- to nanosecond. The dielectric spectra reveal several relaxations corresponding to the orientational polarization mechanism, including the dynamics of loosely bound, tightly bound, and bulk water, as well as collective vibrational modes of protein in an aqueous environment. The dynamics of loosely bound and bulk water follow non-Arrhenius behavior; however, the dynamics of water molecules in the tightly bound layer obeys the Arrhenius-type relation. Combining molecular simulations and effective-medium approximation, we have determined the number of water molecules in the tightly bound hydration layer and studied the dynamics of protein as a function of temperature. The results provide the important impact of water on the biochemical functions of proteins.

Confinement effects on glass-forming mixtures: Insights from a combined experimental approach to aqueous ethylene glycol solutions in silica pores

We perform nuclear magnetic resonance, broadband dielectric spectroscopy, and differential scanning calorimetry studies to ascertain the dynamical behaviors of aqueous ethylene glycol (EG) solutions in silica pores over broad temperature ranges. Both translational and rotational motions are analyzed, and the pore diameter (2.4–9.2 nm) and the EG concentration (12–57 mol. %) are varied, leading to fully liquid or partially crystalline systems. It is found that the translational diffusion coefficient strongly decreases when the diameter is reduced, resulting in a slowdown of nearly three orders of magnitude in the narrowest pores, while the confinement effects on the rotational correlation times are moderate. For the fully liquid solutions, we attribute bulk-like and slowed down reorientation processes to the central and interfacial pore regions, respectively. This coexistence is found in all the studied pores, and, hence, the range of the wall effects on the solution dynamics does not exceed ∼1 nm. Compared to the situation in the bulk, the concentration dependence is reduced in confinements, implying that the specific interactions of the molecular species with the silica walls lead to preferential adsorption. On the other hand, bulk-like structural relaxation is not observed in the partially frozen samples, where the liquid is sandwiched between the silica walls and the ice crystallites. Under such circumstances, there is another relaxation process with a weaker temperature dependence, which is observed in various kinds of partially frozen aqueous systems and denoted as the x process.

Effects of partial crystallization on the glassy slowdown of aqueous ethylene glycol solutions

Combining differential scanning calorimetry, nuclear magnetic resonance, and broadband dielectric spectroscopy studies, we ascertain the glass transition of aqueous ethylene glycol (EG) solutions, in particular the effects of partial crystallization on their glassy slowdown. For the completely liquid solutions in the weakly supercooled regime, it is found that the dynamics of the components occur on very similar time scales, rotational and translational motions are coupled, and the structural (α) relaxation monotonously slows down with increasing EG concentration. Upon cooling, partial crystallization strongly alters the glassy dynamics of EG-poor solutions; in particular, it strongly retards the α relaxation of the remaining liquid fraction, causing a non-monotonous concentration dependence, and it results in a crossover from non-Arrhenius to Arrhenius temperature dependence. In the deeply supercooled regime, a recrossing of the respective α-relaxation times results from the Arrhenius behaviors of the partially frozen EG-poor solutions together with the non-Arrhenius behavior of the fully liquid EG-rich solutions. Exploiting the isotope selectivity of nuclear magnetic resonance, we observe different rotational dynamics of the components in this low-temperature range and determine the respective contributions to the ν relaxation decoupling from the α relaxation when the glass transition is approached. The results suggest that the ν process, which is usually regarded as a water process, actually also involves the EG molecules. In addition, we show that various kinds of partially crystalline aqueous systems share a common relaxation process, which is associated with the frozen fraction and differs from that of bulk hexagonal ice.

2H NMR study on temperature-dependent water dynamics in amino-acid functionalized silica nanopores

We prepare various amino-acid functionalized silica pores with diameters of ∼6 nm and study the temperature-dependent reorientation dynamics of water in these confinements. Specifically, we link basic Lys, neutral Ala, and acidic Glu to the inner surfaces and combine ²H nuclear magnetic resonance spin-lattice relaxation and line shape analyses to disentangle the rotational motions of the surfaces groups and the crystalline and liquid water fractions coexisting below partial freezing. Unlike the crystalline phase, the liquid phase shows reorientation dynamics, which strongly depends on the chemistry of the inner surfaces. The water reorientation is slowest for the Lys functionalization, followed by Ala and Glu and, finally, the native silica pores. In total, the rotational correlation times of water at the different surfaces vary by about two orders of magnitude, where this span is largely independent of the temperature in the range ∼200-250 K.

The Dynamics of Hydrated Proteins Are the Same as Those of Highly Asymmetric Mixtures of Two Glass-Formers

Customarily, the studies of dynamics of hydrated proteins are focused on the fast hydration water ν-relaxation, the slow structural α-relaxation responsible for a single glass transition, and the protein dynamic transition (PDT). Guided by the analogy with the dynamics of highly asymmetric mixtures of molecular glass-formers, we explore the possibility that the dynamics of hydrated proteins are richer than presently known. By providing neutron scattering, dielectric relaxation, calorimetry, and deuteron NMR data in two hydrated globular proteins, myoglobin and BSA, and the fibrous elastin, we show the presence of two structural α-relaxations, α1 and α2, and the hydration water ν-relaxation, all coupled together with interconnecting properties. There are two glass transition temperatures T _g ^α1and T _g ^α2 corresponding to vitrification of the α1 and α2 processes. Relaxation time τ_α2(T) of the α2-relaxation changes its Arrhenius temperature dependence to super-Arrhenius on crossing T _g ^α1 from below. The ν-relaxation responds to the two vitrifications by changing the T-dependence of its relaxation time τ_ν(T) on crossing consecutively T _g ^α2 and T _g ^α1. It generates the PDT at T _d where τ_ν(T _d) matches about five times the experimental instrument timescale τ_exp, provided that T _d > T _g ^α1. This condition is satisfied by the hydrated globular proteins considered in this paper, and the ν-relaxation is in the liquid state with τ_ν(T) having the super-Arrhenius temperature dependence. However, if T _d < T _g ^α1, the ν-relaxation fails to generate the PDT because it is in the glassy state and τ_ν(T) has Arrhenius T-dependence, as in the case of hydrated elastin. Overall, the dynamics of hydrated proteins are the same as the dynamics of highly asymmetric mixtures of glass-formers. The results from this study have expanded the knowledge of the dynamic processes and their properties in hydrated proteins, and impact on research in this area is expected.

NMR studies on the influence of silica confinements on local and diffusive dynamics in LiCl aqueous solutions approaching their glass transitions

We use ¹H, ²H, and ⁷Li NMR to investigate the molecular dynamics of glass-forming LiCl-7H₂O and LiCl-7D₂O solutions confined to MCM-41 or SBA-15 silica pores with diameters in the range of d = 2.8 nm-5.4 nm. Specifically, it is exploited that NMR experiments in homogeneous and gradient magnetic fields provide access to local and diffusive motions, respectively, and that the isotope selectivity of the method allows us to characterize the dynamics of the water molecules and the lithium ions separately. We find that the silica confinements cause a slowdown of the dynamics on all length scales, which is stronger at lower temperatures and in narrower pores and is more prominent for the lithium ions than the water molecules. However, we do not observe a temperature-dependent decoupling of short-range and long-range dynamics inside the pores. ⁷Li NMR correlation functions show bimodal decays when the pores are sufficiently wide (d > 3 nm) so that bulk-like ion dynamics in the pore centers can be distinguished from significantly retarded ion dynamics at the pore walls, possibly in a Stern layer. However, we do not find evidence for truly immobile fractions of water molecules or lithium ions and, hence, for the existence of a static Stern layer in any of the studied silica pores.

Accounts of the changes in dynamics of hydrogen-bonded materials by pressure, nanoconfinement, and hyperquenching

A hydrogen-bonding network or hydrogen-bonded cluster is formed in many hydrogen-bonded glass formers. It determines the dynamics of structural α relaxation and the Johari-Goldstein (JG) β relaxation because breaking of hydrogen bonds is the prerequisite. However, the networks and clusters can be substantially reduced or totally removed in the liquid state by high temperature accompanying the applied high pressure in experiments, and in the glassy state by hyperquenching the liquid under pressure. By confining the glass former in nanometer spaces, the extended network cannot form, and in addition the finite size effect limits the growth of the length scale of the α relaxation on lowering temperature. Any of these actions will modify the structure of the original hydrogen-bonded glass former, and also the intermolecular interaction governing the relaxation processes. Consequently the dynamics of the structural α relaxation and the JG β relaxation, as well as the relation between the two processes, are expected to change. An important advance in the study of the dynamics of glass-forming materials is the existence of the strong connection between the α relaxation and the JG β relaxation. In particular, the ratio of their relaxation times, t_{α}(T)/t_{β}(T), is quantitatively determined by the exponent of the Kohlrausch relaxation function of the α relaxation. This property is valid in hydrogen-bonded glass formers as well as in non-hydrogen-bonded glass formers. The interesting question is whether this property continues to hold after the hydrogen-bonded glass former has been modified by high temperature under high pressure, nanoconfinement, and hyperquenching under pressure. Remarkably, the answer is positive as concluded from the analyses of the data in several hydrogen-bonded glass formers reported in this paper. So far the main theoretical explanation of this property has been the coupling model.

Confinement Effects on Glass-Forming Aqueous Dimethyl Sulfoxide Solutions

Combining broadband dielectric spectroscopy and nuclear magnetic resonance studies, we analyze the reorientation dynamics and the translational diffusion associated with the glassy slowdown of the eutectic aqueous dimethyl sulfoxide solution in nano-sized confinements, explicitly, in silica pores with different diameters and in ficoll and lysozyme matrices at different concentrations. We observe that both rotational and diffusive dynamics are slower and more heterogeneous in the confinements than in the bulk but the degree of these effects depends on the properties of the confinement and differs for the components of the solution. For the hard and the soft matrices, the slowdown and the heterogeneity become more prominent when the size of the confinement is reduced. In addition, the dynamics are more retarded for dimethyl sulfoxide than for water, implying specific guest-host interactions. Moreover, we find that the temperature dependence of the reorientation dynamics and of the translational diffusion differs in severe confinements, indicating a breakdown of the Stokes-Einstein-Debye relation. It is discussed to what extent these confinement effects can be rationalized in the framework of core-shell models, which assume bulk-like and slowed-down motions in central and interfacial confinement regions, respectively.

Dynamics and Glass Transition of Supercooled Water Confined in Amphiphilic Polymer Films

The glass transition of supercooled water is not well understood yet. We have observed a clear glass transition of the supercooled water confined in channel of amphiphilic polymer films at 145 K. Using NMR, we probe two types of relaxations occurred in the glass former, e.g., a rapid local β-process and a slow α-process (most likely). It is found that slow α-relaxation follows the Arrhenius relationship, indicating the glass former is a strong liquid. We also find a dynamic crossover from low-temperature Arrhenius α-process to high-temperature VFT process at 198-208 K, accompanying with simultaneous disappearing of local β-relaxation.

Quasielastic neutron scattering studies on couplings of protein and water dynamics in hydrated elastin

Performing quasielastic neutron scattering measurements and analyzing both elastic and quasielasic contributions, we study protein and water dynamics of hydrated elastin. At low temperatures, hydration-independent methyl group rotation dominates the findings. It is characterized by a Gaussian distribution of activation energies centered at about E_m = 0.17 eV. At ∼195 K, coupled protein-water motion sets in. The hydration water shows diffusive motion, which is described by a Gaussian distribution of activation energies with E_m = 0.57 eV. This Arrhenius behavior of water diffusion is consistent with previous results for water reorientation, but at variance with a fragile-to-strong crossover at ∼225 K. The hydration-related elastin backbone motion is localized and can be attributed to the cage rattling motion. We speculate that its onset at ∼195 K is related to a secondary glass transition, which occurs when a β relaxation of the protein has a correlation time of τ_β ∼ 100 s. Moreover, we show that its temperature-dependent amplitude has a crossover at the regular glass transition T_g = 320 K of hydrated elastin, where the α relaxation of the protein obeys τ_α ∼ 100 s. By contrast, we do not observe a protein dynamical transition when water dynamics enters the experimental time window at ∼240 K.