Mechanisms of Nonexponential Relaxation in Supercooled Glucose Solutions: the Role of Water Facilitation

Recent advances in carbohydrate phase behavior and rheology

The past decades have seen major advances in the understanding of the role of phase and state transitions of food carbohydrates on the behavior during processing and on product characteristics. Specifically, the awareness of the importance of the glass transition temperature and the plasticization by water and its study for a variety of food system is having major impact on the formulation and processing of foods, and in defining shelf-life conditions. This has led to the use of phase and state diagrams in the analysis and prediction of the behavior of food systems during processing and storage. This review first summarizes the current understanding of the food carbohydrate phase behavior and rheology, with emphasis on the concentrated states close to the glass transition and in the glassy state. Several pertinent topics, including the modeling of the rheological properties close to the glass transition, the strongly non-linear diffusion of water in the rubbery and glassy states, the aging and antiplasticization of glassy carbohydrate matrices, and consequences of amorphous-amorphous phase separation for the behavior of carbohydrate blends in concentrated states are discussed. Applications in food processing and product development are discussed, including the spray drying and freeze drying, powder agglomeration of food powders, powder caking, encapsulation, baked goods, crystallization and extrusion.

Poly(l-alanine-co-l-lysine)-g-Trehalose as a Biomimetic Cryoprotectant for Stem Cells

Poly(l-alanine-co-l-lysine)-graft-trehalose (PAK_T) was synthesized as a natural antifreezing glycopolypeptide (AFGP)-mimicking cryoprotectant for cryopreservation of mesenchymal stem cells (MSCs). FTIR and circular dichroism spectra indicated that the content of the α-helical structure of PAK decreased after conjugation with trehalose. Two protocols were investigated in cryopreservation of MSCs to prove the significance of the intracellularly delivered PAK_T. In protocol I, MSCs were cryopreserved at -196 °C for 7 days by a slow-cooling procedure in the presence of both PAK_T and free trehalose. In protocol II, MSCs were preincubated at 37 °C in a PAK_T solution, followed by cryopreservation at -196 °C in the presence of free trehalose for 7 days by the slow-cooling procedure. Polymer and trehalose concentrations were varied by 0.0-1.0 and 0.0-15.0 wt %, respectively. Cell recovery was significantly improved by protocol II with preincubation of the cells in the PAK_T solution. The recovered cells from protocol II exhibited excellent proliferation and maintained multilineage potentials into osteogenic, chondrogenic, and adipogenic differentiation, similar to MSCs recovered from cryopreservation in the traditional 10% dimethyl sulfoxide system. Ice recrystallization inhibition (IRI) activity of the polymers/trehalose contributed to cell recovery; however, intracellularly delivered PEG-PAK_T was the major contributor to the enhanced cell recovery in protocol II. Inhibitor studies suggested that macropinocytosis and caveolin-dependent endocytosis are the main mechanisms for the intracellular delivery of PEG-PAK_T. ¹H NMR and FTIR spectra suggested that the intracellular PEG-PAK_Ts interact with water and stabilize the cells during cryopreservation.

A Review of Cellulose Coarse-Grained Models and Their Applications

Cellulose is the most common biopolymer and widely used in our daily life. Due to its unique properties and biodegradability, it has been attracting increased attention in the recent years and various new applications of cellulose and its derivatives are constantly being found. The development of new materials with improved properties, however, is not always an easy task, and theoretical models and computer simulations can often help in this process. In this review, we give an overview of different coarse-grained models of cellulose and their applications to various systems. Various coarse-grained models with different mapping schemes are presented, which can efficiently simulate systems from the single cellulose fibril/crystal to the assembly of many fibrils/crystals. We also discuss relevant applications of these models with a focus on the mechanical properties, self-assembly, chiral nematic phases, conversion between cellulose allomorphs, composite materials and interactions with other molecules.

Anomalous Dynamics of Water in Polyamide Matrix

Water in polymer matrixes is likely to show anomalous dynamics, a problem that has not been well understood yet. Here, we performed atomistic molecular dynamics simulations to study the water dynamics in a polyamide (PA) matrix, the bulk phase of well-known reverse osmosis membranes. For time-dependent ensemble average, water molecules experienced ballistic diffusion at a shorter time scale, followed by a crossover from subdiffusion to Brownian diffusion at a time scale ∼10 ns, and non-Gaussian diffusion, an indication of anomalous dynamics, sticks on even in the Brownian diffusion region. The anomalous dynamics mainly originates from two distinct motions including small-step continuous diffusion and jumping diffusion. The jumping motion has a mean length of 3.08 ± 0.31 Å and characteristic relaxation time of 0.218 ± 0.040 ns, which dominates the water diffusion in a fully hydrated PA matrix. It comprised low- and high-frequency jumps; the former is almost unchanged, and the latter remarkably increases with the increase of the hydration level. Surrounding neighbors of water strongly affect the jumping frequency, which exponentially or linearly decays with the increase in the number of atoms from the PA matrix. Although the PA matrix is flexible, associated with the water dynamics, the translocation of water is mainly through either tracing the position of neighboring water or jumping into the adjacent accommodation space.

Freezing of Aqueous Solutions and Chemical Stability of Amorphous Pharmaceuticals: Water Clusters Hypothesis

Molecular mobility has been traditionally invoked to explain physical and chemical stability of diverse pharmaceutical systems. Although the molecular mobility concept has been credited with creating a scientific basis for stabilization of amorphous pharmaceuticals and biopharmaceuticals, it has become increasingly clear that this approach represents only a partial description of the underlying fundamental principles. An additional mechanism is proposed herein to address 2 key questions: (1) the existence of unfrozen water (i.e., partial or complete freezing inhibition) in aqueous solutions at subzero temperatures and (2) the role of water in the chemical stability of amorphous pharmaceuticals. These apparently distant phenomena are linked via the concept of water clusters. In particular, freezing inhibition is associated with the confinement of water clusters in a solidified matrix of an amorphous solute, with nanoscaled water clusters being observed in aqueous glasses using wide-angle neutron scattering. The chemical instability is suggested to be directly related to the catalysis of proton transfer by water clusters, considering that proton transfer is the key elementary reaction in many chemical processes, including such common reactions as hydrolysis and deamidation.

Water in a Soft Confinement: Structure of Water in Amorphous Sorbitol

The structure of water in 70 wt % sorbitol-30 wt % water mixture is investigated by wide-angle neutron scattering (WANS) as a function of temperature. WANS data are analyzed using empirical potential structure refinement to obtain the site-site radial distribution functions (RDFs). Orientational structure of water is represented using OW-OW-OW triangles distributions and a tetrahedrality parameter, q, while water-water correlation function is used to estimate size of water clusters. Water structure in the sorbitol matrix is compared with that of water confined in nanopores of MCM41. The results indicate the existence of voids in the sorbitol matrix with the length scale of approximately 5 Å, which are filled by water. At 298 K, positional water structure in these voids is similar to that of water in MCM41, whereas there is a difference in the tetrahedral (orientational) arrangement. Cooling to 213 K strengthens tetrahedrality, with the orientational order of water in sorbitol becoming similar to that of confined water in MCM41 at 210 K, whereas further cooling to 100 K does not introduce any additional changes in the tetrahedrality. The results obtained allow us to propose, for the first time, that such confinement of water in a sorbitol matrix is the main reason for the lack of ice formation in this system.

Water clusters in amorphous pharmaceuticals

Amorphous materials, although lacking the long-range translational and rotational order of crystalline and liquid crystalline materials, possess certain local (short-range) structure. This paper reviews the distribution of one particular component present in all amorphous pharmaceuticals, that is, water. Based on the current understanding of the structure of water, water molecules can exist in either unclustered form or as aggregates (clusters) of different sizes and geometries. Water clusters are reported in a range of amorphous systems including carbohydrates and their aqueous solutions, synthetic polymers, and proteins. Evidence of water clustering is obtained by various methods that include neutron and X-ray scattering, molecular dynamics simulation, water sorption isotherm, concentration dependence of the calorimetric Tg , dielectric relaxation, and nuclear magnetic resonance. A review of the published data suggests that clustering depends on water concentration, with unclustered water molecules existing at low water contents, whereas clusters form at intermediate water contents. The transition from water clusters to unclustered water molecules can be expected to change water dependence of pharmaceutical properties, such as rates of degradation. We conclude that a mechanistic understanding of the impact of water on the stability of amorphous pharmaceuticals would require systematic studies of water distribution and clustering, while such investigations are lacking.

Thermodynamics of liquids: standard molar entropies and heat capacities of common solvents from 2PT molecular dynamics

We validate here the Two-Phase Thermodynamics (2PT) method for calculating the standard molar entropies and heat capacities of common liquids. In 2PT, the thermodynamics of the system is related to the total density of states (DoS), obtained from the Fourier Transform of the velocity autocorrelation function. For liquids this DoS is partitioned into a diffusional component modeled as diffusion of a hard sphere gas plus a solid component for which the DoS(υ) → 0 as υ→ 0 as for a Debye solid. Thermodynamic observables are obtained by integrating the DoS with the appropriate weighting functions. In the 2PT method, two parameters are extracted from the DoS self-consistently to describe diffusional contributions: the fraction of diffusional modes, f, and DoS(0). This allows 2PT to be applied consistently and without re-parameterization to simulations of arbitrary liquids. We find that the absolute entropy of the liquid can be determined accurately from a single short MD trajectory (20 ps) after the system is equilibrated, making it orders of magnitude more efficient than commonly used perturbation and umbrella sampling methods. Here, we present the predicted standard molar entropies for fifteen common solvents evaluated from molecular dynamics simulations using the AMBER, GAFF, OPLS AA/L and Dreiding II forcefields. Overall, we find that all forcefields lead to good agreement with experimental and previous theoretical values for the entropy and very good agreement in the heat capacities. These results validate 2PT as a robust and efficient method for evaluating the thermodynamics of liquid phase systems. Indeed 2PT might provide a practical scheme to improve the intermolecular terms in forcefields by comparing directly to thermodynamic properties.

Empirical and theoretical models of equilibrium and non-equilibrium transition temperatures of supplemented phase diagrams in aqueous systems (IUPAC Technical Report)

This paper describes the main thermodynamic concepts related to the construction of supplemented phase (or state) diagrams (SPDs) for aqueous solutions containing vitrifying agents used in the cryo- and dehydro-preservation of natural (foods, seeds, etc.) and synthetic (pharmaceuticals) products. It also reviews the empirical and theoretical equations employed to predict equilibrium transitions (ice freezing, solute solubility) and non-equilibrium transitions (glass transition and the extrapolated freezing curve). The comparison with experimental results is restricted to carbohydrate aqueous solutions, because these are the most widely used cryoprotectant agents. The paper identifies the best standard procedure to determine the glass transition curve over the entire water-content scale, and how to determine the temperature and concentration of the maximally freeze-concentrated solution.

Structure of polyamidoamide dendrimers up to limiting generations: a mesoscale description

The polyamidoamide (PAMAM) class of dendrimers was one of the first dendrimers synthesized by Tomalia and co-workers at Dow. Since its discovery the PAMAMs have stimulated many discussions on the structure and dynamics of such hyperbranched polymers. Many questions remain open because the huge conformation disorder combined with very similar local symmetries have made it difficult to characterize experimentally at the atomistic level the structure and dynamics of PAMAM dendrimers. The higher generation dendrimers have also been difficult to characterize computationally because of the large size (294,852 atoms for generation 11) and the huge number of conformations. To help provide a practical means of atomistic computational studies, we have developed an atomistically informed coarse-grained description for the PAMAM dendrimer. We find that a two-bead per monomer representation retains the accuracy of atomistic simulations for predicting size and conformational complexity, while reducing the degrees of freedom by tenfold. This mesoscale description has allowed us to study the structural properties of PAMAM dendrimer up to generation 11 for time scale of up to several nanoseconds. The gross properties such as the radius of gyration compare very well with those from full atomistic simulation and with available small angle x-ray experiment and small angle neutron scattering data. The radial monomer density shows very similar behavior with those obtained from the fully atomistic simulation. Our approach to deriving the coarse-grain model is general and straightforward to apply to other classes of dendrimers.

Universal features of water dynamics in solutions of hydrophilic polymers, biopolymers, and small glass-forming materials

A systematic investigation by dielectric spectroscopy of 18 different water-rich mixtures with very different hydrophilic substances shows universal features for the water dynamics. The temperature dependence of the relaxation times exhibits a crossover from non-Arrhenius to Arrhenius behavior at the T(g) range of the mixtures. Furthermore, the temperature dependence of the relaxation times presents a universal behavior both above and below the crossover temperature. We also show that these features suggest that the observed crossover is associated with the emergence of confinement effects.

Conformation, dynamics, solvation and relative stabilities of selected β-hexopyranoses in water: a molecular dynamics study with the gromos 45A4 force field

The present article reports long timescale (200 ns) simulations of four beta-D-hexopyranoses (beta-D-glucose, beta-D-mannose, beta-D-galactose and beta-D-talose) using explicit-solvent (water) molecular dynamics and vacuum stochastic dynamics simulations together with the GROMOS 45A4 force field. Free-energy and solvation free-energy differences between the four compounds are also calculated using thermodynamic integration. Along with previous experimental findings, the present results suggest that the formation of intramolecular hydrogen-bonds in water is an 'opportunistic' consequence of the close proximity of hydrogen-bonding groups, rather than a major conformational driving force promoting this proximity. In particular, the conformational preferences of the hydroxymethyl group in aqueous environment appear to be dominated by 1,3-syn-diaxial repulsion, with gauche and solvation effects being secondary, and intramolecular hydrogen-bonding essentially negligible. The rotational dynamics of the exocyclic hydroxyl groups, which cannot be probed experimentally, is found to be rapid (10-100 ps timescale) and correlated (flip-flop hydrogen-bonds interconverting preferentially through an asynchronous disrotatory pathway). Structured solvent environments are observed between the ring and lactol oxygen atoms, as well as between the 4-OH and hydroxymethyl groups. The calculated stability differences between the four compounds are dominated by intramolecular effects, while the corresponding differences in solvation free energies are small. An inversion of the stereochemistry at either C(2) or C(4) from equatorial to axial is associated with a raise in free energy. Finally, the particularly low hydrophilicity of beta-D-talose appears to be caused by the formation of a high-occurrence hydrogen-bonded bridge between the 1,3-syn-diaxial 2-OH and 4-OH groups. Overall, good agreement is found with available experimental and theoretical data on the structural, dynamical, solvation and energetic properties of these compounds. However, this detailed comparison also reveals some discrepancies, suggesting the need (and providing a solid basis) for further refinement.

Carbohydrate Clustering in Aqueous Solutions and the Dynamics of Confined Water

We use molecular dynamics simulations to investigate structure and dynamics of fructose aqueous solutions in the 1-5 M concentration range at ambient conditions. We analyze hydration structures, H-bond statistics, and size distribution of H-bonded carbohydrate clusters as functions of concentration. We find that the local tetrahedral order of water is reasonably well-preserved and that the solute tends to appear as scattered "isolated" molecules at low concentrations and as H-bonded clusters for less diluted solutions. The sugar cluster size distribution exhibits a sharp transition to a percolated cluster between 3.5 and 3.8 M. The percolated cluster forms an intertwined network of H-bonded saccharides that imprisons water. For the dynamics, we find good agreement between simulation and available experimental results for the self-diffusion coefficients. Water librational dynamics is little affected by sugar concentration, whereas reorientational relaxation is described by a concentration-independent bulk-like component attributed to noninterfacial water molecules and a slower component (strongly concentration dependent) that arises from interfacial solvent molecules and, hence, depends on the dynamics of the cluster structure itself. Analysis of H-bonding survival probability functions indicates that the formation of carbohydrate clusters upon increasing concentration enhances the H-bond relaxation time and slows down the entire system dynamics. We find that multiexponential or stretched-exponential fits alone cannot describe the H-bond survival probabilities for the entire postlibrational time span of our data (0.1-100 ps), as opposed to a combined stretched-plus-biexponential function, which provides excellent fits. Our results suggest that water dynamics in concentrated fructose solutions resembles in many ways that of protein hydration water.

Multiscale Coarse-Graining of Monosaccharides

A systematic multiscale coarse-graining (MS-CG) algorithm is applied to build coarse-grained models for monosaccharides in aqueous solution. The methodology is demonstrated for the example of alpha-D-glucopyranose. The nonbonded interactions are directly derived from the force-matching approach, whereas the bonded interactions are obtained through Boltzmann statistical analyses of the underlying atomistic trajectory. The MS-CG model is shown to reproduce many structural and thermodynamic properties in the constant NPT ensemble. Although the model is derived at a single temperature, pressure, and concentration, it is shown to be reasonably transferable to other thermodynamic states. In this model, long-range interactions are effectively mapped into short-range forces with a moderate cutoff and are evaluated by table look-up. As a result, molecular dynamics employing the MS-CG model is approximately 3 orders of magnitude more efficient than its atomistic counterpart. Consequently, the model is particularly suitable for simulating carbohydrate systems at large length and long time scales. Results for an alpha-(1-->4)-d-glucan with 14 glucose units are also presented, demonstrating that the MS-CG algorithm is also applicable to the coarse-graining of other saccharide systems.

Microscopic mechanism of water diffusion in glucose glasses

The preservation of biomaterials depends critically on the mobility of water in the glassy state, manifested as a secondary beta relaxation and diffusion. We use coarse grain simulations to elucidate the molecular mechanism underlying the relaxations for water-glucose glass, finding two pathways for water diffusion: (i) water jumps into neighbor water positions (linking to water structure), and (ii) water jumps into glucose positions (coupling to glucose rotation). This work suggests strategies for enhancing preservation by stiffening the segmental motions of the carbohydrates.