Dynamics of Water Molecules in Glucose Solutions

Insights into Sorption and Molecular Transport of Aqueous Glucose into Zeolite Nanopores

Liquid-phase heterogeneous catalysis using zeolites is important for biomass conversion to fuels and chemicals. There is a substantial body of work on gas-phase sorption in zeolites with different topologies; however, studies investigating the diffusion of complex molecules in liquid medium into zeolitic nanopores are scarce. Here, we present a molecular dynamics study to understand the sorption and diffusion of aqueous β-d-glucose into β-zeolite silicate at T = 395 K and P = 1 bar. Through 2-μs-long molecular dynamics trajectories, we reveal the role of the solvent, the kinetics of the pore filling, and the effect of the water model on these properties. We find that the glucose and water loading is a function of the initial glucose concentration. Although the glucose concentration increases monotonically with the initial glucose concentration, the water loading exhibits a nonmonotonic behavior. At the highest initial concentration (∼20 wt %), we find that the equilibrium loading of glucose is approximately five molecules per unit cell and displays a weak dependence on the water model. Glucose molecules follow a single-file diffusion in the nanopores due to confinement. The dynamics of glucose and water molecules slows significantly at the interface. The average residence time for glucose molecules is an order of magnitude larger than that in the bulk solution, while it is about twice as large for the water molecules. Our simulations reveal critical molecular details of the glucose molecule's local environment inside the zeolite pore relevant to catalytic conversion of biomass to valuable chemicals.

Optimizing Nonbonded Interactions of the OPLS Force Field for Aqueous Solutions of Carbohydrates: How to Capture Both Thermodynamics and Dynamics

Knowledge on thermodynamic and transport properties of aqueous solutions of carbohydrates is of great interest for process and product design in the food, pharmaceutical, and biotechnological industries. Molecular simulation is a powerful tool to calculate these properties, but current classical force fields cannot provide accurate estimates for all properties of interest. The poor performance of the force fields is mainly observed for concentrated solutions, where solute–solute interactions are overestimated. In this study, we propose a method to refine force fields, such that solute–solute interactions are more accurately described. The OPLS force field combined with the SPC/Fw water model is used as a basis. We scale the nonbonded interaction parameters of sucrose, a disaccharide. The scaling factors are chosen in such a way that experimental thermodynamic and transport properties of aqueous solutions of sucrose are accurately reproduced. Using a scaling factor of 0.8 for Lennard-Jones energy parameters (ϵ) and a scaling factor of 0.95 for partial atomic charges (q), we find excellent agreement between experiments and computed liquid densities, thermodynamic factors, shear viscosities, self-diffusion coefficients, and Fick (mutual) diffusion coefficients. The transferability of these optimum scaling factors to other carbohydrates is verified by computing thermodynamic and transport properties of aqueous solutions of d-glucose, a monosaccharide. The good agreement between computed properties and experiments suggests that the scaled interaction parameters are transferable to other carbohydrates, especially for concentrated solutions.

Characterization of the hydrogen-bond network of water around sucrose and trehalose: Microwave and terahertz spectroscopic study

Modification of the water hydrogen bond network imposed by disaccharides is known to serve as a bioprotective agent in living organisms, though its comprehensive understanding is still yet to be reached. In this study, aiming to characterize the dynamical slowing down and destructuring effect of disaccharides, we performed broadband dielectric spectroscopy, ranging from 0.5 GHz to 12 THz, of sucrose and trehalose aqueous solutions. The destructuring effect was examined in two ways (the hydrogen bond fragmentation and disordering) and our result showed that both sucrose and trehalose exhibit an obvious destructuring effect with a similar strength, by fragmenting hydrogen bonds and distorting the tetrahedral-like structure of water. This observation strongly supports a chaotropic (structure-breaking) aspect of disaccharides on the water structure. At the same time, hydration water was found to exhibit slower dynamics and a greater reorientational cooperativity than bulk water because of the strengthened hydrogen bonds. These results lead to the conclusion that strong disaccharide-water hydrogen bonds structurally incompatible with native water-water bonds lead to the rigid but destructured hydrogen bond network around disaccharides. Another important finding in this study is that the greater dynamical slowing down of trehalose was found compared with that of sucrose, at variance with the destructuring effect where no solute dependent difference was observed. This discovery suggests that the exceptionally greater bioprotective impact especially of trehalose among disaccharides is mainly associated with the dynamical slowing down (rather than the destructuring effect).

Proper balance of solvent-solute and solute-solute interactions in the treatment of the diffusion of glucose using the Drude polarizable force field

Motivated by underestimation of the diffusion constant of glucose in aqueous solution at high glucose concentrations we performed additional optimization of the Drude polarizable hexopyranose monosaccharide force field. This indicated aggregation of the glucose at higher concentrations, which is a concern for studies of complex glycan systems such as the HIV Envelope where high effective concentrations of sugars are present. High-level quantum mechanical calculations were undertaken on water monohydrate-glucose interactions, on water cluster-glucose interactions and on glucose-glucose dimers in stacked (parallel) and perpendicular orientations. Optimization of the nonbond and dihedral parameters targeting these data yielded a revised model that showed improved agreement with experimental aqueous diffusion data. However, limitations in the diffusion constants were still present. These were due to the SWM4-NDP inherently overestimating the diffusion constant of water, a problem that was validated by calculation of the aqueous diffusion constants using the SWM6-NDP water model. In addition, results show the water diffusion constant to be significantly overestimated at high glucose concentrations though the glucose diffusion is in satisfactory agreement with experiment. These results indicate the subtle balance of water-sugar, water-water and sugar-sugar interactions that needs to be properly modeled to account for the full range of aqueous behavior of sugars in aqueous solution.

Investigation of the dynamics of aqueous proline solutions using neutron scattering and molecular dynamics simulations

We applied quasielastic neutron scattering (QENS) techniques to samples with two different contrasts (deuterated solute/hydrogenated solvent and the opposite label) to selectively study the component dynamics of proline/water solutions. Results on diluted and concentrated solutions (31 and 6 water molecules/proline molecule, respectively) were analyzed in terms of the susceptibility and considering a recently proposed model for water dynamics [Arbe et al., Phys. Rev. Lett., 2016, 117, 185501] which includes vibrations and the convolution of localized motions and diffusion. We found that proline molecules not only reduce the average diffusion coefficient of water but also extend the time/frequency range of the crossover region ('cage') between the vibrations and purely diffusive behavior. For the high proline concentration we also found experimental evidence of water heterogeneous dynamics and a distribution of diffusion coefficients. Complementary molecular dynamics simulations show that water molecules start to perform rotational diffusion when they escape the cage regime but before the purely diffusive behavior is established. The rotational diffusion regime is also retarded by the presence of proline molecules. On the other hand, a strong coupling between proline and water diffusive dynamics which persists with decreasing temperature is directly observed using QENS. Not only are the temperature dependences of the diffusion coefficients of both components the same, but their absolute values also approach each other with increasing proline concentration. We compared our results with those reported using other techniques, in particular using dielectric spectroscopy (DS). A simple approach based on molecular hydrodynamics and a molecular treatment of DS allows rationalizing the a priori puzzling inconsistency between QENS and dielectric results regarding the dynamic coupling of the two components. The interpretation proposed is based on general grounds and therefore should be applicable to other biomolecular solutions.

Nanoconfinement's Dramatic Impact on Proton Exchange between Glucose and Water

Glucose nanoconfined by solubilization in water-containing AOT (sodium bis(2-ethylhexyl) sulfosuccinate) reverse micelles has been investigated using ¹H NMR. NMR spectra reveal well-defined signals for the glucose hydroxyl groups that suggest slow chemical exchange between them and the water hydroxyl groups. Using the EXSY (ZZ-exchange) method, the chemical exchange rate from water to glucose hydroxyl groups was measured for glucose in reverse micelles as a function of size (water pool diameter of ∼1-5 nm) at 25 °C. The chemical exchange rates observed in the nanoconfined interior are dramatically slower (5-20 times) than those observed for glucose in bulk aqueous solution at the same concentration as the micelle interior. Exchange rate constants are calculated via a mechanism that accounts for these observations, and implications of these results are presented and discussed.

Bio-protective effects of homologous disaccharides on biological macromolecules

In this contribution the effects of the homologous disaccharides trehalose and sucrose on both water and hydrated lysozyme dynamics are considered by determining the mean square displacement (MSD) from elastic incoherent neutron scattering (EINS) experiments. The self-distribution function (SDF) procedure is applied to the data collected, by use of IN13 and IN10 spectrometers (Institute Laue Langevin, France), on trehalose and sucrose aqueous mixtures (at a concentration corresponding to 19 water molecules per disaccharide molecule), and on dry and hydrated (H(2)O and D(2)O) lysozyme also in the presence of the disaccharides. As a result, above the glass transition temperature of water, the MSD of the water-trehalose system is lower than that of the water-sucrose system. This result suggests that the hydrogen-bond network of the water-trehalose system is stronger than that of the water-sucrose system. Furthermore, by taking into account instrumental resolution effects it was found that the system relaxation time of the water-trehalose system is longer than that of the water-sucrose system, and the system relaxation time of the protein in a hydrated environment in the presence of disaccharides increases sensitively. These results explain the higher bioprotectant effectiveness of trehalose. Finally, the partial MSDs of sucrose/water and trehalose/water have been evaluated. It clearly emerges from the analysis that these are almost equivalent in the low-Q domain (0-1.7 Å(-1)) but differ substantially in the high-Q range (1.7-4 Å(-1)). These findings reveal that the lower structural sensitivity of trehalose to thermal changes is connected with the local spatial scale.

Molecular dynamics and neutron scattering study of glucose solutions confined in MCM-41

Glucose aqueous solutions confined in MCM-41 cylindrical pores of diameter 3.2 nm have been studied by molecular dynamics (MD) simulations and quasielastic neutron scattering (QENS). MD simulations reveal a strong preferential interaction of glucose molecules with the silica walls, which induces significant concentration gradients within the pore. The influence of glucose on the structural and dynamical properties of water strongly depends on the region of the pore considered. The distortion of the hydrogen bond network (HBN) and of the tetrahedral organization of interfacial water molecules induced by silica is much stronger than that induced by glucose molecules. The interfacial glucose molecules diffuse about 1 order of magnitude slower than those in the core region. Differences in affinities for silica of the different species in confined hydrogen-bonded mixtures induce significant structural and dynamical heterogeneities not present in bulk solutions.

Translational and rotational dynamics of monosaccharide solutions

Molecular dynamics computer simulations have been carried out on aqueous solutions of glucose at concentrations bracketing those previously measured with quasi-elastic neutron scattering (QENS), in order to investigate the motions and interactions of the sugar and water molecules. In addition, QENS measurements have been carried out on fructose solutions to determine whether the effects previously observed for glucose apply to monosaccharide solutions. The simulations indicate a dynamical analogy between higher solute concentration and lower temperature that could provide a key explanation of the bioprotective phenomena observed in many living organisms. The experimental results on fructose solutions show qualitatively similar behavior to the glucose solutions. The dynamics of the water molecules are essentially the same, while the translational diffusion of the sugar molecules is slightly faster in the fructose solutions.

1H NMR relaxation studies of protein-polysaccharide mixtures

NMR water proton relaxation was used to characterize the structure of plant proteins and plant protein-polysaccharide mixtures in aqueous solutions. The method is based on the mobility determination of the water molecules in the biopolymer environment in solutions through relaxation time measurements. Differences of conformation between pea globulin and alpha gliadin seem to control the water molecules mobility in their environment. As deduced from the study of complexes, the electrostatic interactions may also play a major role in the water molecule motions. The phase separation induced under specific conditions seems to promote the translational diffusion of structured water molecules whereas the rotational motion was more restricted.

Dynamics of trehalose molecules in confined solutions

The dynamics of trehalose molecules in aqueous solutions confined in silica gel have been studied by quasielastic neutron scattering (QENS). Small-angle neutron scattering measurements confirmed the absence of both sugar clustering and matrix deformation of the gels, indicating that the results obtained are representative of homogeneous trehalose solutions confined in a uniform matrix. The pore size in the gel is estimated to be 18 nm, comparable to the distances in cell membranes. For the QENS measurements, the gel was prepared from D2O in order to accentuate the scattering from the trehalose. Values for the translational diffusion constant and effective jump distance were derived from model fits to the scattering function. Comparison with QENS and NMR results in the literature for bulk trehalose shows that confinement on a length scale of 18 nm has no significant effect on the translational diffusion of trehalose molecules.

Energetics of xylose decomposition as determined using quantum mechanics modeling

The decomposition of xylose has been studied using quantum mechanical calculations supported by NMR data. Proposed mechanisms for the decomposition of xylose have been investigated by obtaining the structures and energies of transition states and products. The intent of this study was to understand the experimentally observed formation of furfural and formic acid that occurs during the decomposition of xylose in mildly hot acidic solutions. A mechanism of furfural formation involving the opening of the pyranose ring and subsequent dehydration of the aldose was compared to a direct intramolecular rearrangement of the protonated pyranose. Energies were determined using CBS-QB3, and it was shown that the barriers for dehydration of the aldose were high compared to intramolecular rearrangement. This result suggests that the latter mechanism is a more likely mechanism for furfural formation. The intramolecular rearrangement step results from protonation of xylose at the O2 hydroxyl group. In addition, it has been shown that formic acid formation is a likely result of the protonation of xylose at the O3 hydroxyl group. Finally, solvation of xylose decomposition was studied by calculating energy barriers for xylose in selected water clusters. The mechanisms proposed here were supported in part by 13C-labeling studies using NMR.

Hydrogen bond dynamics and water structure in glucose-water solutions by depolarized Rayleigh scattering and low-frequency Raman spectroscopy

The effect of glucose on the relaxation process of water at picosecond time scales has been investigated by depolarized Rayleigh scattering (DRS) experiments. The process is assigned to the fast hydrogen bonding dynamics of the water network. In DRS spectra this contribution can be safely separated from the slower relaxation process due to the sugar. The detected relaxation time is studied at different glucose concentrations and modeled considering bulk and hydrating water contributions. As a result, it is found that in diluted conditions the hydrogen bond lifetime of proximal water molecules becomes about three times slower than that of the bulk. The effect of the sugar on the hydrogen bond water structure is investigated by analyzing the low-frequency Raman (LFR) spectrum sensitive to intermolecular modes. The addition of glucose strongly reduces the intensity of the band at 170 cm(-1) assigned to a collective stretching mode of water molecules arranged in cooperative tetrahedral domains. These findings indicate that proximal water molecules partially lose the tetrahedral ordering typical of the bulk leading to the formation of high density environments around the sugar. Thus the glucose imposes a new local order among water molecules localized in its hydration shell in which the hydrogen bond breaking dynamics is sensitively retarded. This work provides new experimental evidences that support recent molecular dynamics simulation and thermodynamics results.

Dynamics of Fresh and Freeze-Dried Strawberry and Red Onion by Quasielastic Neutron Scattering

The microscopic behavior of fresh and freeze-dried strawberry and red onion at different water contents (45 and 20 wt % water) has been investigated by quasielastic neutron scattering (QENS). To distinguish between the dynamics of the water and the biological material isotopic (H/D) substitution was used. The results show that all samples exhibit an onset of anharmonic motions on the experimental time scale (3-100 ps) at about 230-240 K. Above 250 K the dynamics is mainly of translational character and strongly dependent on the hydration level. The diffusion constant increases rapidly with increasing water content and at 280 K it is approximately 20% higher for the hydration water in freeze-dried strawberry than in freeze-dried red onion and around 2 orders of magnitude faster for the hydration water than for the biological material. Moreover, the diffusion constant of the biological part is about 50% faster in freeze-dried strawberry than in freeze-dried red onion. It was also found that the average relaxation time is slightly faster in fresh strawberry than in freeze-dried strawberry. From the results we can conclude that the water dynamics is not only promoting motions in the biological material, it is also affected by the structure (and possibly also the dynamics) of the biological material. Thus, the microscopic properties of the biological materials are interrelated with the properties of their hydration water.

Tetrahedral order in homologous disaccharide-water mixtures

The present work aims at evidencing the "kosmotrope" nature of trehalose through the analysis of inelastic neutron scattering measurements on trehalose and sucrose water solutions at different temperatures. Neutron spectra were collected by using the spectrometer MARI at the ISIS pulsed neutron source of the Rutherford Appleton Laboratory (Chilton, UK). To study the structural modifications induced on the tetrahedral hydrogen-bond network of water by homologous disaccharides, as a first step, the vibrational properties of pure water at different temperatures have been investigated. In particular, the temperature behavior of the intramolecular OH stretching mode has been analyzed. Successively, the vibrational properties for pure water have been compared with those of the sugar water solutions focusing the attention on the tetrahedral network-forming tendency. Finally, the obtained findings have been compared with previous Raman scattering evidences, and the results interpreted in the frame of recent molecular dynamics simulation works.

Molecular dynamics of confined glucose solutions

Silica gels containing solutions of glucose in heavy water at different concentrations have been prepared by a sol-gel method. Dynamical studies with quasielastic neutron scattering, compared with previous results on bulk solutions, show that the dynamics of the glucose molecules are not appreciably affected by the confinement, even though the gels behave macroscopically as solid materials. Small-angle neutron-scattering spectra on the same systems, fitted with a fractal model, yield a correlation length that decreases from 20 to 2.5 nm with increasing glucose concentration, suggesting a clustering of glucose molecules in concentrated solutions that is consistent with the dynamical measurements. These two sets of results imply that 20 nm is an upper limit for the scale at which the dynamics of glucose molecules in solution are affected by confinement.