Long-range influence of carbohydrates on the solvation dynamics of water--answers from terahertz absorption measurements and molecular modeling simulations

High-precise determination of the drought and cold resistance of forage seeds using terahertz time-domain spectroscopy

Owing to the complicated geographical locations and climates, cultivation and selection of forage seeds are challenging. For the first time, we qualitatively distinguished the drought and cold resistance of forage seeds with the time domain and refractive index spectra using terahertz (THz) time-domain spectroscopy. A multilayer structure propagation (MSP) model was developed based on the effective medium and light transport theory to reveal the underlying biological mechanisms of drought and cold resistance of forage seeds. The proposed MSP model accurately explained the behavior of the THz waves transmitted through the forage seeds, with a high accuracy rate of 94.433%. The impact of THz wave transmission was influenced by the presence of various biological components in the alfalfa seeds, particularly protein and carbohydrate. More interestingly, the cold and drought resistance of forage seeds can be effectively differentiated with the ratio of the thickness-dependent argument parameter (Ψ) of protein and carbohydrate components. The obtained results offered important insights into the interaction mechanism between THz wave and forage seeds, and proposed a promising MSP model in the screening process for selecting high-quality forage seeds based on their stress resistance characteristics.

Theoretical Basis for Refractive Index Changes Resulting from Solution Phase Molecular Interaction

Refractive index (RI) is a fundamental optical property widely used to investigate the physical and chemical characteristics of materials. Here, we build on our previous work to refine the framework for RI sensing in solution-phase chemical and biochemical interactions. Starting from the Clausius-Mossotti relation, we present a first-principles derivation of a relationship for the RI signal resulting from chemical binding. We then demonstrate how the binding-induced conformational and hydration changes of interacting species relate to their estimated change in dielectric and thus the solution-phase change in refractive index (ΔRI). By varying the model parameters, such as solvation shell size and polarizability, we investigate the RI changes for two interactions: Ca2+ with the protein Recoverin and benzenesulfonamide with carbonic anhydrase 2 (CAII). These examples show that our theory predicts that even for small changes in binding-induced polarizability (relative to previous literature values), a quantifiable RI change is produced within the detectable range of RI detectors operating at ca. 10-6 RIU. Empirical observations confirm our theoretical predictions. Surprisingly, theory and experiment yield a decrease in ΔRI for the benzenesulfonamide-CAII interaction. We attribute this observation to shielding of charged residues and water molecule displacement during the binding event. Our approach is generalized, enabling it to be extended to other binding systems, as well as those undergoing nonbinding conformational changes, and facilitates the exploration of diverse biological and chemical processes by solution-phase RI sensing.

Mechanism of Water Freezing in Solutions: Solutes Affect the Formation of Critical Ice Nuclei

The microscopic mechanisms by which solutes modulate water freezing are fundamental for controlling the freezing of various environmental and cryobiotic systems. Although our understanding of the initiation mechanisms of pure water freezing is becoming clearer, the microscopic pictures regarding ice nucleation in complex systems such as solutions still rely on theory assumption and empirical formulation. Here, we experimentally demonstrate that solutes modulate water freezing through affecting critical ice nucleus formation. Upon addition of a solute, which can be quantified by the single parameter of water activity (aw), critical ice nuclei are more difficult to form because the solute increases the critical ice nucleus radius (r*) by decreasing aw. The value of r* that can be applied to solution systems depends firmly on not only the nucleation temperature but also aw. Furthermore, using molecular dynamics simulations, we give a microscopic picture of ice nucleus formation in solution and explain the underlying reasons for solute-induced changes in r*.

Critical Role of Water beyond the Media to Maintain Protein Stability and Activity in Hydrated Deep Eutectic Solvent

Hydrated deep eutectic solvents (DESs) are recognized for their potential in biocatalysis due to their tunability, biocompatibility, greenness, and ability to keep protein stable and active. However, the mechanisms governing enzyme stability and activity in DES remain poorly understood. Herein, using bromelain as the model enzyme and acetamide (0.5)/urea(0.3)/sorbitol(0.2) as the model DES, we provide experimental evidence that modulation of associated water plays a key role in dictating protein stability and activity in hydrated DES. Specifically, rigid associated water at higher DES concentrations (beyond 40% v/v) stabilizes bromelain through entropy but destabilizes it through enthalpy. On the other hand, flexible associated water dynamics at lower DES concentrations result in an opposite thermodynamic outcome. Importantly, the bulk water dynamics cannot explain the stability trend, which emphasizes the critical role of water near the protein surface. Strikingly, associated water dynamics also correlates strongly with bromelain's proteolytic activity. An increasing flexibility of the associated water dynamics leads to the enhancement of the activity. This is the first study to experimentally link associated water dynamics to enzyme behavior in hydrated DES, offering insights that could guide future developments in solvent engineering for enzyme catalysis.

Terahertz time-domain transmission spectroscopy of water and hydrogel thin films: Extraction of optical parameters and application to agarose gel characterization

Terahertz time-domain spectroscopy (THz-TDS) is an emerging optical technique that has potential applications in the characterization of (bio)materials. However, the complicated extraction of optical parameters from multi-layered and optically thin samples is a barrier towards its acceptance by applied scientists. Therefore, the aim of this work is to provide a straightforward approach for the extraction of the THz absorption coefficient and index of refraction profiles of aqueous thin films in a window-sample-window configuration, which is ubiquitous in many laboratories (i.e., sample in a cuvette). A numerical approach-based methodology that accounts for multiple layers, Fabry-Pérot effect, and sample thickness is elaborated which involves an optical interference model based on a tri-layer structure and a simple thickness estimation technique. This method was validated on water samples where a good agreement was found with the THz optical parameters of water reported in the literature, while the use of a commercial software resulted in erroneous optical parameters estimates when used without due regard to its limitations. A case study was then performed to demonstrate the ability of the proposed method to characterize agarose hydrogels with varying degree of sulfation. It was demonstrated that THz-TDS can provide insight into the hydration state of the agarose hydrogels, including the relative number of the hydrogen bonds between the hydroxyl moieties of water and the polysaccharide network which is perturbed by the presence of sulfate. The trend in the index of refraction profiles suggested microstructural differences between the agarose hydrogels, which were confirmed by visualizing the agarose network morphology using cryo-SEM imaging.

Limitations of effective medium models for tissue phantoms in the THz frequency range

The absorption of terahertz (THz) radiation by water molecules facilitates its application to several biomedical applications such as cancer detection. Therefore, it is critical for the THz technologies to be characterised with water content in a sample. In this paper, we analyse gelatine phantoms in the THz frequency range, with continuously varying hydration levels as they dry over time. Water molecules in close proximity to the protein molecule, termed 'bound water', feature properties different from the 'free water' molecules at larger distances. We find that a common model for predicting electromagnetic properties of phantoms and tissue samples, which assumes that only the free water varies with hydration while the bound water remains constant, does not agree well with measured results. To gain insight into this behaviour, we simultaneously measured the phantom in Raman spectroscopy, which shows a continuously varying concentration of bound water with hydration level. It follows from this investigation, that the permittivity contributions of neither the biomolecules nor water are expected to be linear with water density. This means that the often used, simple effective medium model will not be accurate for many biological tissues or phantoms.

Fluorescence changes in carbon nanotube sensors correlate with THz absorption of hydration

Single wall carbon nanotubes (SWCNTs) functionalized with (bio-)polymers such as DNA are soluble in water and sense analytes by analyte-specific changes of their intrinsic fluorescence. Such SWCNT-based (bio-)sensors translate the binding of a molecule (molecular recognition) into a measurable optical signal. This signal transduction is crucial for all types of molecular sensors to achieve high sensitivities. Although there is an increasing number of SWCNT-based sensors, there is yet no molecular understanding of the observed changes in the SWCNT's fluorescence. Here, we report THz experiments that map changes in the local hydration of the solvated SWCNT upon binding of analytes such as the neurotransmitter dopamine or the vitamin riboflavin. The THz amplitude signal serves as a measure of the coupling of charge fluctuations in the SWCNTs to the charge density fluctuations in the hydration layer. We find a linear (inverse) correlation between changes in THz amplitude and the intensity of the change in fluorescence induced by the analytes. Simulations show that the organic corona shapes the local water, which determines the exciton dynamics. Thus, THz signals are a quantitative predictor for signal transduction strength and can be used as a guiding chemical design principle for optimizing fluorescent biosensors.

Outstanding Properties of the Hydration Shell around β-d-Glucose: A Computational Study

Ab initio molecular dynamics (AIMD) simulations have been performed on aqueous solutions of four simple sugars, α-d-glucose, β-d-glucose, α-d-mannose, and α-d-galactose. Hydrogen-bonding (HB) properties, such as the number of donor- and acceptor-type HB-s, and the lengths and strengths of hydrogen bonds between sugar and water molecules, have been determined. Related electronic properties, such as the dipole moments of water molecules and partial charges of the sugar O atoms, have also been calculated. The hydrophilic and hydrophobic shells were characterized by means of spatial distribution functions. β-d-Glucose was found to form the highest number of hydrophilic and the smallest number of hydrophobic connections to neighboring water molecules. The average sugar-water H-bond length was the shortest for β-d-glucose, which suggests that these are the strongest such H-bonds. Furthermore, β-d-glucose appears to stand out in terms of the symmetry properties of both its hydrophilic and hydrophobic hydration shells. In summary, in all aspects considered here, there seems to be a correlation between the distinct characteristics of β-d-glucose reported here and its outstanding solubility in water. Admittedly, our findings represent only some of the important factors that influence the solubility.

Effects of Crowding and Cosolutes on Biomolecular Function at Extreme Environmental Conditions

The extent of the effect of cellular crowding and cosolutes on the functioning of proteins and cells is manifold and includes the stabilization of the biomolecular systems, the excluded volume effect, and the modulation of molecular dynamics. Simultaneously, it is becoming increasingly clear how important it is to take the environment into account if we are to shed light on biological function under various external conditions. Many biosystems thrive under extreme conditions, including the deep sea and subseafloor crust, and can take advantage of some of the effects of crowding. These relationships have been studied in recent years using various biophysical techniques, including neutron and X-ray scattering, calorimetry, FTIR, UV-vis and fluorescence spectroscopies. Combining knowledge of the structure and conformational dynamics of biomolecules under extreme conditions, such as temperature, high hydrostatic pressure, and high salinity, we highlight the importance of considering all results in the context of the environment. Here we discuss crowding and cosolute effects on proteins, nucleic acids, membranes, and live cells and explain how it is possible to experimentally separate crowding-induced effects from other influences. Such findings will contribute to a better understanding of the homeoviscous adaptation of organisms and the limits of life in general.

Osmolyte induced protein stabilization: modulation of associated water dynamics might be a key factor

The mechanism of protein stabilization by osmolytes remains one of the most important and long-standing puzzles. The traditional explanation of osmolyte-induced stability through the preferential exclusion of osmolytes from the protein surface has been seriously challenged by the observations like the concentration-dependent reversal of osmolyte-induced stabilization/destabilization. The more modern explanation of protein stabilization/destabilization by osmolytes considers an indirect effect due to osmolyte-induced distortion of the water structure. It provides a general mechanism, but there are numerous examples of protein-specific effects, i.e., a particular osmolyte might stabilize one protein, but destabilize the other, that could not be rationalized through such an explanation. Herein, we hypothesized that osmolyte-induced modulation of associated water might be a critical factor in controlling protein stability in such a medium. Taking different osmolytes and papain as a protein, we proved that our proposal could explain protein stability in osmolyte media. Stabilizing osmolytes rigidify associated water structures around the protein, whereas destabilizing osmolytes make them flexible. The strong correlation between the stability and the associated water dynamics, and the fact that such dynamics are very much protein specific, established the importance of considering the modulation of associated water structures in explaining the osmolyte-induced stabilization/destabilization of proteins. More interestingly, we took another protein, bromelain, for which a traditionally stabilizing osmolyte, sucrose, acts as a stabilizer at higher concentrations but as a destabilizer at lower concentrations. Our proposal successfully explains such observations, which is probably impossible by any known mechanisms. We believe this report will trigger much research in this area.

Terahertz spectroscopy as a method for investigation of hydration shells of biomolecules

The hydration of biomolecules is one of the fundamental processes underlying the construction of living matter. The formation of the native conformation of most biomolecules is possible only in an aqueous environment. At the same time, not only water affects the structure of biomolecules, but also biomolecules affect the structure of water, forming hydration shells. However, the study of the structure of biomolecules is given much more attention than their hydration shells. A real breakthrough in the study of hydration occurred with the development of the THz spectroscopy method, which showed that the hydration shell of biomolecules is not limited to 1-2 layers of strongly bound water, but also includes more distant areas of hydration with altered molecular dynamics. This review examines the fundamental features of the THz frequency range as a source of information about the structural and dynamic characteristics of water that change during hydration. The applied approaches to the study of hydration shells of biomolecules based on THz spectroscopy are described. The data on the hydration of biomolecules of all main types obtained from the beginning of the application of THz spectroscopy to the present are summarized. The emphasis is placed on the possible participation of extended hydration shells in the realization of the biological functions of biomolecules and at the same time on the insufficient knowledge of their structural and dynamic characteristics.

Effect of solvent viscosity on the activation barrier of hydrogen tunneling in the lipoxygenase reaction

Hydrogen tunneling in enzyme reactions has played an important role in linking protein thermal motions to the chemical steps of catalysis. Lipoxygenases (LOXs) have served as model systems for such reactions, showcasing deep hydrogen tunneling mechanisms associated with enzymatic C-H bond cleavage from polyunsaturated fatty acids. Here, we examined the effect of solvent viscosity on the protein thermal motions associated with LOX catalysis using trehalose and glucose as viscogens. Kinetic analysis of the reaction of the paradigm plant orthologue, soybean lipoxygenase (SLO), with linoleic acid revealed no effect on the first-order rate constants, kcat, or activation energy, Ea. Further studies of SLO active site mutants displaying varying Eas, which have been used to probe catalytically relevant motions, likewise provided no evidence for viscogen-dependent motions. Kinetic analyses were extended to a representative fungal LOX from M. oryzae, MoLOX, and a human LOX, 15-LOX-2. While MoLOX behaved similarly to SLO, we show that viscogens inhibit 15-LOX-2 activity. The latter implicates viscogen sensitive, conformational motions in animal LOX reactions. The data provide insight into the role of water hydration layers in facilitating hydrogen (quantum) tunneling in LOX.

Trehalose-Guanosine Glycopolymer Hydrogels Direct Adaptive Glia Responses in CNS Injury

Neural tissue damaged after central nervous system (CNS) injury does not naturally regenerate but is instead replaced by non-neural fibrotic scar tissue that serves no neurological function. Scar-free repair to create a more permissive environment for regeneration requires altering the natural injury responses of glial cells. In this work, glycopolymer-based supramolecular hydrogels are synthesized to direct adaptive glia repair after CNS injury. Combining poly(trehalose-co-guanosine) (pTreGuo) glycopolymers with free guanosine (fGuo) generates shear-thinning hydrogels through stabilized formation of long-range G-quadruplex secondary structures. Hydrogels with smooth or granular microstructures and mechanical properties spanning three orders of magnitude are produced through facile control of pTreGuo hydrogel composition. Injection of pTreGuo hydrogels into healthy mouse brains elicits minimal stromal cell infiltration and peripherally derived inflammation that is comparable to a bioinert methyl cellulose benchmarking material. pTreGuo hydrogels alter astrocyte borders and recruit microglia to infiltrate and resorb the hydrogel bulk over 7 d. Injections of pTreGuo hydrogels into ischemic stroke alter the natural responses of glial cells after injury to reduce the size of lesions and increase axon regrowth into lesion core environments. These results support the use of pTreGuo hydrogels as part of neural regeneration strategies to activate endogenous glia repair mechanisms.

Quantifying the Molecular Polarization Response of Liquid Water Interfaces at Heterogeneously Charged Surfaces

The hydration shells of proteins mediate interactions, such as small molecule binding, that are vital to their biological function or in some cases their dysfunction. However, even when the structure of a protein is known, the properties of its hydration environment cannot be easily predicted due to the complex interplay between protein surface heterogeneity and the collective structure of water's hydrogen bonding network. This manuscript presents a theoretical study of the influence of surface charge heterogeneity on the polarization response of the liquid water interface. We focus our attention on classical point charge models of water, where the polarization response is limited to molecular reorientation. We introduce a new computational method for analyzing simulation data that is capable of quantifying water's collective polarization response and determining the effective surface charge distribution of hydrated surfaces over atomistic length scales. To illustrate the utility of this method, we present the results of molecular dynamics simulations of liquid water in contact with a heterogeneous model surface and the CheY protein.

Regulating Antifreeze Activity through Water: Latent Functions of the Sugars of Antifreeze Glycoprotein Revealed by Total Chemical Synthesis

Antifreeze glycoprotein (AFGP), which inhibits the freezing of water, is highly O-glycosylated with a disaccharide, d-Galβ1-3-d-GalNAcα (GalGalNAc). To elucidate the function of the sugar residues for antifreeze activity at the molecular level, we conducted a total chemical synthesis of partially sugar deleted AFGP derivatives, and unnatural forms of AFGPs incorporating glucose (Glc)-type sugars instead of galactose (Gal)-type sugars. These elaborated AFGP derivatives demonstrated that the stereochemistry of each sugar residue on AFGPs precisely correlates with the antifreeze activity. A hydrogen-deuterium exchange experiment using synthetic AFGPs revealed a different dynamic behavior of water around sugar residues depending on the sugar structures. These results indicate that sugar residues on AFGP form a unique dynamic water phase that disturbs the absorbance of water molecules onto the ice surface, thereby inhibiting freezing.

Control strategies of ice nucleation, growth, and recrystallization for cryopreservation

The cryopreservation of biomaterials is fundamental to modern biotechnology and biomedicine, but the biggest challenge is the formation of ice, resulting in fatal cryoinjury to biomaterials. To date, abundant ice control strategies have been utilized to inhibit ice formation and thus improve cryopreservation efficiency. This review focuses on the mechanisms of existing control strategies regulating ice formation and the corresponding applications to biomaterial cryopreservation, which are of guiding significance for the development of ice control strategies. Herein, basics related to biomaterial cryopreservation are introduced first. Then, the theoretical bases of ice nucleation, growth, and recrystallization are presented, from which the key factors affecting each process are analyzed, respectively. Ice nucleation is mainly affected by melting temperature, interfacial tension, shape factor, and kinetic prefactor, and ice growth is mainly affected by solution viscosity and cooling/warming rate, while ice recrystallization is inhibited by adsorption or diffusion mechanisms. Furthermore, the corresponding research methods and specific control strategies for each process are summarized. The review ends with an outlook of the current challenges and future perspectives in cryopreservation. STATEMENT OF SIGNIFICANCE: Ice formation is the major limitation of cryopreservation, which causes fatal cryoinjury to cryopreserved biomaterials. This review focuses on the three processes related to ice formation, called nucleation, growth, and recrystallization. The theoretical models, key influencing factors, research methods and corresponding ice control strategies of each process are summarized and discussed, respectively. The systematic introduction on mechanisms and control strategies of ice formation is instructive for the cryopreservation development.

Trivalent cation-induced phase separation in proteins: ion specific contribution in hydration also counts

Multivalent (specifically trivalent) metal ions are known to induce microscopic phase separation (commonly termed as liquid-liquid phase separation (LLPS)) in negatively charged globular proteins even at ambient temperatures, the process being mostly driven by protein charge neutralization followed by aggregation. Recent simulation studies have revealed that such self-aggregation of proteins is entropy driven; however, it is associated with a solvation effect, which could as well be different from the usual notion of hydrophobic hydration. In this contribution we have experimentally probed the explicit change in hydration associated with ion-induced LLPS formation of a globular protein bovine serum albumin (BSA) at ambient temperature using FIR-THz FTIR spectroscopy (50-750 cm^-1; 1.5-22.5 THz). We have used ions of different charges: Na⁺, K⁺, Ca²⁺, Mg²⁺, La³⁺, Y³⁺, Ho³⁺ and Al³⁺. We found that all the trivalent ions induce LLPS; the formation of large aggregates has been evidenced from dynamic light scattering (DLS) measurements, but without perturbing the protein structure as confirmed from circular dichroism (CD) measurements. From the frequency dependent absorption coefficient (α(ν)) measurements in the THz frequency domain we estimate the various stretching/vibrational modes of water and we found that ions, forming LLPS, produce definite perturbation in the overall hydration, the extent of which is ion specific, invoking the definite role of hydrophilic (electrostatic) hydration of ions in the observed LLPS process.

Enhanced Mobility during Diels-Alder Reaction: Results of Molecular Simulations

Recent measurements indicate enhanced mobility of solvent molecules during Diels-Alder (DA) and other common chemical reactions. We present results of molecular dynamics simulations of the last stages of the DA cycloaddition reaction, from the transition state configuration to product, of furfurylamine and maleimide in acetonitrile at reactant concentrations studied experimentally. We find enhanced mobility of solvent and reactant molecules up to at least a nanometer from the DA product over hundreds of picoseconds. Local heating is ruled out as a factor in the enhanced mobility observed in the simulations, which is instead found to be due to solvent relaxation following the formation of the DA product.

Enhanced sensitivity of dilute aqueous adrenaline solution with an asymmetric hexagonal ring structure in the terahertz frequencies

Quantitative detection of neurotransmitters in aqueous environment is crucial for the early diagnosis of many neurological disorders. Terahertz waves, as a non-contact and non-labeling tool, have demonstrated large potentials in quantitative biosensing. Although the detection of trace-amount analyte has been achieved with terahertz metamaterials in the recent decades, most studies have been focused on dried samples. Here, a hexagonal asymmetric metamaterial sensor was designed and fabricated for aqueous solution sensing with terahertz waves in the reflection geometry. An absorption enhancement of 43 was determined from the simulation. Dilute adrenaline solutions ranging from 30 µM to 0.6 mM were measured on our sensor using a commercial terahertz time-domain spectroscopy system, and the effective absorption was found to be linearly correlated with the concentration (R²= 0.81). Furthermore, we found that as the concentration becomes higher (>0.6 mM), a non-linear relationship starts to take place, which confirmed the previous theory on the extended solvation shell that can be probed on the picosecond scale. Our sensor, without the need of high-power and stable terahertz sources, has enabled the detection of subtle absorption changes induced by the solvation dynamics.

Broadband Dielectric Spectroscopic Analysis toward Characterization of the Hydration State and Bioprotective Superiority of Trehalose

Alteration of the hydrogen-bond (H-bond) network by trehalose is acknowledged as a bioprotective agent. However, most studies exploring the hydration superiority of the trehalose structure are limited structure are limited by the computational cost or a narrow-range spectrum. In the present study, the structural and dynamical behaviors of the H-bond network of trehalose and maltose solutions were observed and compared with a broadband dielectric spectrum (100 MHz-18 THz) to investigate the influence of the trehalose structure on the bioprotective function. From the relaxation time, the reorientation cooperativity, resonant frequency, and damping constant of water-water vibration, the symmetric structure of trehalose allowed a more significant H-bond strengthening effect and homogeneous aqueous environment. In contrast, the difference in the hydration number between trehalose and maltose was negligible. Thus, the enhanced H-bond strengthening effect and homogeneous aqueous environment owing to the symmetric structure are the essential factors that contribute to the remarkable bioprotective effect of trehalose.

Transformation process of ice crystallized from a glassy dilute trehalose aqueous solution

Crystal growth of ice Isd occurring after crystallization of a glassy dilute trehalose aqueous solution is slower than that of ice Isd in a dilute glycerol solution and pure ice Isd, and ice Isd in trehalose aqueous solution survives to ∼230 K.

Conformational Consequences for Compatible Osmolytes on Thermal Denaturation

Compatible osmolytes are a broad class of small organic molecules employed by living systems to combat environmental stress by enhancing the native protein structure. The molecular features that make for a superior biopreservation remain elusive. Through the use of time-resolved and steady-state spectroscopic techniques, in combination with molecular simulation, insight into what makes one molecule a more effective compatible osmolyte can be gained. Disaccharides differing only in their glycosidic bonds can exhibit different degrees of stabilization against thermal denaturation. The degree to which each sugar is preferentially excluded may explain these differences. The present work examines the biopreservation and hydration of trehalose, maltose, and gentiobiose.

Relationships between Molecular Structure of Carbohydrates and Their Dynamic Hydration Shells Revealed by Terahertz Time-Domain Spectroscopy

Despite more than a century of research on the hydration of biomolecules, the hydration of carbohydrates is insufficiently studied. An approach to studying dynamic hydration shells of carbohydrates in aqueous solutions based on terahertz time-domain spectroscopy assay is developed in the current work. Monosaccharides (glucose, galactose, galacturonic acid) and polysaccharides (dextran, amylopectin, polygalacturonic acid) solutions were studied. The contribution of the dissolved carbohydrates was subtracted from the measured dielectric permittivities of aqueous solutions based on the corresponding effective medium models. The obtained dielectric permittivities of the water phase were used to calculate the parameters describing intermolecular relaxation and oscillatory processes in water. It is established that all of the analyzed carbohydrates lead to the increase of the binding degree of water. Hydration shells of monosaccharides are characterized by elevated numbers of hydrogen bonds and their mean energies compared to undisturbed water, as well as by elevated numbers and the lifetime of free water molecules. The axial orientation of the OH(4) group of sugar facilitates a wider distribution of hydrogen bond energies in hydration shells compared to equatorial orientation. The presence of the carboxylic group affects water structure significantly. The hydration of polysaccharides is less apparent than that of monosaccharides, and it depends on the type of glycosidic bonds.

Correlating solvation with conformational pathways of proteins in alcohol-water mixtures: a THz spectroscopic insight

Water, being an active participant in most of the biophysical processes, is important to trace how protein solvation changes as its conformation evolves in the presence of solutes or co-solvents. In this study, we investigate how the secondary structures of two diverse proteins - lysozyme and β-lactoglobulin - change in the aqueous mixtures of two alcohols - ethanol and 2,2,2-trifluoroethanol (TFE) using circular dichroism measurements. We observe that these alcohols change the secondary structures of these proteins and the changes are protein-specific. Subsequently, we measure the collective solvation dynamics of these two proteins both in the absence and in the presence of alcohols by measuring the frequency-dependent absorption coefficient (α(ν)) in the THz (0.1-1.2 THz) frequency domain. The alcohol-water mixtures exhibit a non-ideal behaviour with the highest absorption difference (Δα) obtained at X_alcohol = 0.2. The protein solvation in the presence of the alcohols shows an oscillating behaviour in which Δα_protein changes with X_alcohol. Such an oscillatory behaviour of protein solvation results from a delicate interplay between the protein-water, protein-alcohol and water-alcohol associations. We attempt to correlate the various structural conformations of the proteins with the associated solvation.

Optimizing a coarse-grained space for approximate normal-mode vibrations of molecular heterodimers

We applied the method of coarse-graining the intermolecular vibrations to molecular heterodimers assembled by double hydrogen bonding. This method is based on principal component analysis, by which the original atomic displacement vectors are projected onto a lower-dimensional space spanned by a basis set of translations, librations, and intramolecular vibrations of the constituent molecules. Compared with homodimers, the following points are particularly noted: (1) alignment of the constituent molecules in a non-symmetric atomic arrangement of the whole system and (2) the scheme of reordering the bases to construct an optimal coarse-grained space. We tested three schemes for reordering the intramolecular vibration vectors to determine that the best one is equivalent to size reduction based on the singular value decomposition. The coarse-graining analysis affords three parameters, Φ_intra, Φ_inter, and Φ_app, which are relevant to the mechanical nature of the molecular assembly. The Φ_intra values account for the internal stiffness of molecules, while the Φ_inter values are true stiffness constants of the intermolecular force and show a good correlation with the association energies of the dimers. The Φ_app values are the apparent intermolecular stiffness smaller than Φ_inter, as a result of compensation for neglecting intramolecular vibrations. All these values are consistent with each other under the coupled oscillator model, showing that the present coarse-graining analysis is valid for heterodimers as well as homodimers.

Key Differences of the Hydrate Shell Structures of ATP and Mg·ATP Revealed by Terahertz Time-Domain Spectroscopy and Dynamic Light Scattering

ATP is one of the main biological molecules. Many of its biological and physicochemical properties, such as energy capacity of the phosphate bonds, significantly depend on hydration. However, the structure of the hydration shell of the ATP molecule is still a matter of discussion. In this work, the hydration shells of ATP in water and MgCl₂ solutions were examined by terahertz time-domain spectroscopy and dynamic light scattering. Terahertz spectroscopy reveals the distorted water structure in the ATP water solution displaying tightly bound water molecules, which could be explained by the hydration of phosphate groups. Upon ATP binding to a Mg²⁺ ion, the situation is principally different: Instead of the distorted water structure, its arranged structure with increased hydrogen bond number is observed. Dynamic light scattering showed that the hydrodynamic diameter of ATP increases by 0.5 nm after Mg²⁺ binding. Meanwhile, according the characteristics of scattering, the increase of the shell size occurs via formation of a layer with a refraction coefficient similar to water. This layer can be interpreted as hydration shell differing from unaltered water by increased number of hydrogen bonds.

Hydration Shells of DPPC Liposomes from the Point of View of Terahertz Time-Domain Spectroscopy

Analysis of structural and dynamic properties of water in suspensions of liposomes composed from 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) in three phase states (gel, rippled gel, liquid crystalline phase) by means of terahertz time-domain spectroscopy in 0.3-3.3 THz range was conducted in the current work. Fraction of free water molecules in DPPC liposome suspension was shown to decrease with temperature (compared to the analogous aqueous solution without liposomes), and intermolecular water binding was enhanced. The most crucial changes occur during gel-rippled gel phase transition (pretransition): at temperatures below pretransition point, liposomes alleviate water binding degree, while at temperatures above the transition point, they enhance water binding. This study has demonstrated the high information content of the terahertz time-domain spectroscopy method for exploring the hydration properties of phospholipids in water.

Chemical Synthesis of an Erythropoietin Glycoform Having a Triantennary N-Glycan: Significant Change of Biological Activity of Glycoprotein by Addition of a Small Molecular Weight Trisaccharide

The glycosylation of proteins contributes to the modulation of the structure and biological activity of glycoproteins. Asparagine-linked glycans (N-glycans) of glycoproteins naturally exhibit diverse antennary patterns, such as bi-, tri-, and tetra-antennary forms. However, there are no chemical or biological methods to obtain homogeneous glycoproteins via the intentional alteration of the antennary form of N-glycans. Thus, the functions of the individual antennary form of N-glycan at a molecular level remain unclear. Herein, we report the chemical synthesis of an erythropoietin (EPO) glycoform having a triantennary sialylglycan at position 83, as well as two biantennary sialylglycans at both positions 24 and 38. We demonstrated efficient liquid-phase condensation reactions to prepare a sialylglycopeptide having a triantennary N-glycan prepared by the addition of a Neu5Ac-α-2,6-Gal-β-1,4-GlcNAc element to the biantennary glycan under semisynthetic conditions. The molecular weight of the newly added antennary element was ∼3% of the EPO glycoform, and the introduced position was the most distant from the bioactive protein. However, in vivo assays using mice revealed that the additional antennary element at position 83 dramatically increased the hematopoietic activity compared to a commercially available native EPO. These unprecedented data clearly indicate that the antennary pattern of N-glycans inherently plays a critical role in the modulation of protein functions.

Empirical comparison between effective medium theory models for the dielectric response of biological tissue at terahertz frequencies

We study the use of three effective medium theory models, namely Maxwell-Garnett, Bruggeman, and Landau-Lifshitz-Looyenga, for the dielectric response of biological tissue in the terahertz band of the electromagnetic spectrum. In order to accomplish our objectives, we performed measurements on water-dehydrated basil binary mixtures encompassing the entire concentration range, and we further analyze the dielectric function with the models. Our results indicate that the Landau-Lifshitz-Looyenga and Bruggeman models provide marginally better fit to the experimentally measured dielectric function in the terahertz band. We further discuss the biological relevance of the models in the context of our experimental data based on their fundamental assumptions.

Near-infrared spectroscopic study of molecular interaction in ethanol-water mixtures

Given the importance of ethanol-water mixtures in many chemical and biological processes, the molecular interaction in ethanol-water binary system was studied using near-infrared (NIR) spectroscopy. Excess spectra (in form of excess absorption coefficient) and Gaussian fitting were applied to analyze low concentration ethanol-water mixtures, ranging from 0 to 10% (v/v). With the knowledge of aquaphotomics, six kinds of water species were identified for 0-10% ethanol-water system, and it was indicated that water can be a sensitive probe for analyzing the structural changes and the interactions in the solutions. The excess spectra and two-dimensional (2D) correlation spectroscopy were introduced for high concentration mixtures (10-100%) analysis and found that the intermolecular hydrogen bonding strength between ethanol and water reaches to the maximum at 40% ethanol concentration which may be related to some abnormal properties of alcoholic solutions reported previously. In 40-100% mixtures, ethanol molecules tend to initiate the self-association which leads to the weakening of the interaction between ethanol and water. This paper not only deepens the understanding of the structure and dynamics of alcoholic solution, but also opens a new perspective in molecular interaction analysis in aqueous system by understanding the roles of water.

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.

Urea's match in the hydrogen-bond network? A high pressure THz study

We present results of the measurement of the low frequency spectrum of solvated urea. The study revealed a blue shift of the intramolecular mode of urea centered at 150 cm^-1 of Δν= 17 cm^-1 upon increasing the pressure up to 10 kbar. The blue shift scaled linearly with the increase in density and was attributed to a stiffening of the water-urea intermolecular potential. We deduced an increase in the number of affected water molecules from 1 to 2 up to 5-7, which corresponds to the sterical coordination number of urea. The increase in hydration number can be explained by an suppression of the NH₂ inversion and the hydrogen bond switching around the NH₂ group. Pressure induced sterical constraints are proposed to hinder the rapid switching of hydrogen bond partners and make the water around urea less bulk-like than under ambient conditions.

A Comprehensive Review on Food Applications of Terahertz Spectroscopy and Imaging

Food product safety is a public health concern. Most of the food safety analytical and detection methods are expensive, labor intensive, and time consuming. A safe, rapid, reliable, and nondestructive detection method is needed to assure consumers that food products are safe to consume. Terahertz (THz) radiation, which has properties of both microwave and infrared, can penetrate and interact with many commonly used materials. Owing to the technological developments in sources and detectors, THz spectroscopic imaging has transitioned from a laboratory-scale technique into a versatile imaging tool with many practical applications. In recent years, THz imaging has been shown to have great potential as an emerging nondestructive tool for food inspection. THz spectroscopy provides qualitative and quantitative information about food samples. The main applications of THz in food industries include detection of moisture, foreign bodies, inspection, and quality control. Other applications of THz technology in the food industry include detection of harmful compounds, antibiotics, and microorganisms. THz spectroscopy is a great tool for characterization of carbohydrates, amino acids, fatty acids, and vitamins. Despite its potential applications, THz technology has some limitations, such as limited penetration, scattering effect, limited sensitivity, and low limit of detection. THz technology is still expensive, and there is no available THz database library for food compounds. The scanning speed needs to be improved in the future generations of THz systems. Although many technological aspects need to be improved, THz technology has already been established in the food industry as a powerful tool with great detection and quantification ability. This paper reviews various applications of THz spectroscopy and imaging in the food industry.

Multimodal Optical Diagnostics of Glycated Biological Tissues

Diabetes mellitus is a metabolic disorder characterized by chronic hyperglycemia accompanied by the disruption of carbohydrate, lipid, and proteins metabolism and development of long-term microvascular, macrovascular, and neuropathic changes. This review presents the results of spectroscopic studies on the glycation of tissues and cell proteins in organisms with naturally developing and model diabetes and in vitro glycated samples in a wide range of electromagnetic waves, from visible light to terahertz radiation. Experiments on the refractometric measurements of glycated and oxygenated hemoglobin in broad wavelength and temperature ranges using digital holographic microscopy and diffraction tomography are discussed, as well as possible application of these methods in the diabetes diagnostics. It is shown that the development and implementation of multimodal approaches based on a combination of phase diagnostics with other methods is another promising direction in the diabetes diagnostics. The possibilities of using optical clearing agents for monitoring the diffusion of substances in the glycated tissues and blood flow dynamics in the pancreas of animals with induced diabetes have also been analyzed.

Pressure response of the THz spectrum of bulk liquid water revealed by intermolecular instantaneous normal mode analysis

The radial distribution functions of liquid water are known to change significantly their shape upon hydrostatic compression from ambient conditions deep into the kbar pressure regime. It has been shown that despite their eye-catching changes, the fundamental locally tetrahedral fourfold H-bonding pattern that characterizes ambient water is preserved up to about 10 kbar (1 GPa), which is the stability limit of liquid water at 300 K. The observed increase in coordination number comes from pushing water molecules into the first coordination sphere without establishing an H-bond, resulting in roughly two such additional interstitial molecules at 10 kbar. THz spectroscopy has been firmly established as a powerful experimental technique to analyze H-bonding in aqueous solutions given that it directly probes the far-infrared lineshape and thus the prominent H-bond network mode around 180 cm^-1. We, therefore, set out to assess pressure effects on the THz response of liquid water at 10 kbar in comparison to the 1 bar (0.1 MPa) reference, both at 300 K, with the aim to trace back the related lineshape changes to the structural level. To this end, we employ the instantaneous normal mode approximation to rigorously separate the H-bonding peak from the large background arising from the pronounced librational tail. By exactly decomposing the total molecular dynamics into hindered translations, hindered rotations, and intramolecular vibrations, we find that the H-bonding peak arises from translation-translation and translation-rotation correlations, which are successively decomposed down to the level of distinct local H-bond environments. Our utmost detailed analysis based on molecular pair classifications unveils that H-bonded double-donor water pairs contribute most to the THz response around 180 cm^-1, whereas interstitial waters are negligible. Moreover, short double-donor H-bonds have their peak maximum significantly shifted toward higher frequencies with respect to such long H-bonds. In conjunction with an increasing relative population of these short H-bonds versus the long ones (while the population of other water pair classes is essentially pressure insensitive), this explains not only the blue-shift of the H-bonding peak by about 20-30 cm^-1 in total from 1 bar to 10 kbar but also the filling of the shallow local minimum of the THz lineshape located in between the network peak and the red-wing of the librational band at 1 bar. Based on the changing populations as a function of pressure, we are also able to roughly estimate the pressure-dependence of the H-bond network mode and find that its pressure response and thus the blue-shifting are most pronounced at low kbar pressures.

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.

Bioprotection Can Be Tuned with a Proper Protein/Saccharide Ratio: The Case of Solid Amorphous Matrices

Saccharides, and in particular trehalose, are well known for their high efficiency in protecting biostructures against adverse environmental conditions. The protein dynamics is known to be highly inhibited in a low-water trehalose host medium, the inhibition being markedly dependent on the amount of residual water. Besides hydration, the protein/sugar ratio is expected to affect the properties of saccharide amorphous matrices. In this work, we report an infrared spectroscopy study in dry amorphous matrices of various sugars (the disaccharides trehalose, maltose, sucrose, and lactose, and the trisaccharide raffinose) containing myoglobin, at different protein/sugar ratios. We analyze the stretching band of the bound CO molecule and the water association band. Such bands have already been successfully exploited for the simultaneous study of thermal evolution of a matrix and embedded protein. The results show a high dependence of protein and matrix signals on the protein/sugar ratio, the system behavior evolving from situations where (i) the protein slaves the matrix to (ii) protein ↔ matrix coupling/uncoupling, then to (iii) the matrix slaving the protein, with increasing sugar concentration. This supports a mutual protein ↔ matrix structural and dynamic influence in low hydrated systems, indicating that the protein/solvent master and slave paradigm does not strictly hold, but the mutual relationship depends on the relative concentrations. Furthermore, for each sugar, an optimal protein/sugar concentration ratio can be identified, which maximizes the protein preservation; under such a condition, the water content is minimal.

Indices to evaluate the reliability of coarse-grained representations of mixed inter/intramolecular vibrations

We propose some methods for quantifying the reliability of coarse-grained representations of displacement vectors of normal mode vibrations. In the framework of our basic theory, the original displacement vectors are projected onto a lower-dimensional (i.e., a coarse-grained) space. Four types of functions denoted fidelity indices were introduced as measures of the similarity of the original to the restored displacement vectors. These indices were applied to several hydrogen-bonded homodimers, and the behavior of each index was examined. We found that a coarse-grained representation with high reliability resulted in the accurate restoration of properties such as eigenfrequency, modal mass, and modal stiffness.

Aqueous solvation from the water perspective

The response of water re-solvating a charge-transfer dye (deprotonated Coumarin 343) after photoexcitation has been measured by means of transient THz spectroscopy. Two steps of increasing THz absorption are observed, a first ∼10 ps step on the time scale of Debye relaxation of bulk water and a much slower step on a 3.9 ns time scale, the latter of which reflecting heating of the bulk solution upon electronic relaxation of the dye molecules from the S₁ back into the S₀ state. As an additional reference experiment, the hydroxyl vibration of water has been excited directly by a short IR pulse, establishing that the THz signal measures an elevated temperature within ∼1 ps. This result shows that the first step upon dye excitation (10 ps) is not limited by the response time of the THz signal; it rather reflects the reorientation of water molecules in the solvation layer. The apparent discrepancy between the relatively slow reorientation time and the general notion that water is among the fastest solvents with a solvation time in the sub-picosecond regime is discussed. Furthermore, non-equilibrium molecular dynamics simulations have been performed, revealing a close-to-quantitative agreement with experiment, which allows one to disentangle the contribution of heating to the overall THz response from that of water orientation.