Non-freezing Water Molecules Research Articles

Elucidating the Feature of Intermediate Water in Hydrated Poly(ω-methoxyalkyl acrylate)s by Molecular Dynamics Simulation and Differential Scanning Calorimetry Measurement.

Intermediate water (IW) has been reported to play an important role in nonthrombogenicity of biomaterials. However, clear insights into the IW in the hydrated polymer are still debated. In this study, a series of molecular dynamics simulations was performed to identify the IW structure in hydrated poly(ω-methoxyalkyl acrylate)s (PMCxAs, where x indicates the number of methylene carbons) with x = 1-6. Through the quantitative comparison with experimental measurements, IW molecules were suggested to mainly come from the water interacting with an oxygen atom of the polymers, while most of the nonfreezing water (NFW) molecules corresponded to the water interacting with two polymer oxygen atoms. In addition, the IW molecules were found to effectively enhance the flexibility of the PMCxA side chains in comparison with the NFW molecules. The variations of the saturated IW content and the side-chain flexibility with the methylene carbon chain length of PMCxA were also found to be correlated with the experimental nonthrombogenicity of PMCxA, suggesting that the polymer with the more saturated IW content and higher chain flexibility possesses better nonthrombogenicity. Furthermore, through the analyses of the interplays between the IW and polymer and between IW and its adjacent water, we found that the presence of the unique interaction between IW and its adjacent water in the hydrated poly(2-methoxyethyl acrylate) (PMEA) is the main factor causing different cold crystallization behaviors of PMEA from the other PMCxAs rather than the interaction between water and the PMCxA matrix. The findings will be useful in the development of new nonthrombogenic materials.

Controlling water dynamics for kinetic inhibition of clathrate hydrate

Subsea natural gas drilling and gas transport is challenged by the catastrophic formation of gas clathrate hydrate in production lines. Polymer inhibitors are vital to mitigate this technical problem in subsea gas drilling operation. However, physico- chemical factors affecting their performance are not yet completely known. Herein, we report that the activities of synthetic kinetic inhibitors of clathrate hydrates are correlated to the surrounding water dynamics. Two types of linear polyamide inhibitors are found to be capable of producing immobilized nonfreezable water molecules with fast transverse relaxation in sub-millisecond to millisecond time-scale as revealed by NMR relaxometry. Such a unique state of water can be construed as a form of nonfreezable bound water measured by differential scanning calorimetry. The quantity of such water species can be tuned by the size of the hydrophobic groups introduced to the polyamides and result in longer inhibition time before the rapid growth of the hydrate crystals. Addition of a single hydroxyl side group in the polymer structural unit can also affect the dependence of the kinetic inhibition time on the amount of the supercooled water. Both the hydrophobic and hydrophilic groups in the polymer inhibitors exert coordinated influence on the dynamic properties of the bound water, which are responsible for the early recognition and inhibition of the clathrate hydrate clusters.

Interfacial Effect of Hydration Structures of Hydroxyapatite Nanoparticle Films on Protein Adsorption and Cell Adhesion States.

The synthesized elliptical hydroxyapatite (E-HAp) and needle-like HAp (N-HAp) nanoparticles (NPs) were electrophoretically deposited on a gold (Au) substrate. A comparative study of the hydration layers on E-HAp, N-HAp, and Au films was achieved to investigate the interfacial effect of the hydration layers on the conformation of the adsorbed fibrinogen (Fgn) and fibroblast adhesion properties. As a result, the ratios of three types of hydration layer states (free water, intermediate water, nonfreezing water) analyzed by a Fourier transform infrared (FT-IR) spectral deconvolution of the O-H stretching absorption band were investigated. The ratio of the bonding water state (i.e., intermediate and nonfreezing water molecules) is almost the same between two HAp films, and the E-HAp film with an elliptical shape and smaller particle size exhibited the smallest ratio of nonfreezing water, which can suppress the denaturation of the adsorbed protein. Subsequently, FT-IR spectral deconvolution results of the amide I band of the adsorbed Fgn on the E-HAp film indicated the higher proportion of α-helix and β-sheet structures as compared with those on the N-HAp and Au films, suggesting that the smaller proportion of nonfreezing waters would play a significant role in the stereoscopic Fgn conformation. In the culture of fibroblasts, FT-IR spectra of the adhered cells on the E-HAp, N-HAp, and Au films exhibited different absorbance intensities of the amide A, I, II, and III bands, suggesting a different amount of collagen-producing states by the cells, which were also supported by immunostaining results of the collagen type I. Therefore, the different hydration structures on the films clearly influenced the conformation of the adsorbed protein, and the preferential conformation was found at the interfaces between the fibroblasts and the underground E-HAp films.

Nonfreezable Water and Polymer Swelling Control the Marine Antifouling Performance of Polymers with Limited Hydrophilic Content.

Zwitterionic chemical groups have well-documented resistance to marine fouling species when presented as homogeneous polymer brushes. These model formulations are not, however, suitable for practical fouling-control applications. It is presently unknown if a uniform film of zwitterions is required to elicit nonfouling character via the binding of interfacial water or if the incorporation of zwitterionic functionality into a more practical bulk polymer system will suffice. Here, copolymers of n-butyl methacrylate were synthesized with low incorporation levels (up to 20 mol %) of hydrophilic functionality, including zwitterionic moieties. Their antifouling (AF) properties were evaluated using barnacle cyprids (Balanus improvisus), diatom cells (Navicula incerta), and a multispecies biofilm. The laboratory assays revealed higher resistance of ionic copolymers toward cyprid settlement, which was attributed to their swelling and the presence of nonfreezable water molecules bound tightly to the polymer chains. Additionally, cells of N. incerta and the multispecies biofilm were removed more effectively on polymers containing sulfobetaine methacrylate and sulfopropyl methacrylate moieties. The results indicate that the presence of tightly bound interfacial water is not limited to model systems of pure hydrophilic homopolymers, but that this mechanism can also reduce the settlement and adhesion of fouling species via bulk copolymer systems with limited hydrophilic content. The swelling of polymers with hydrophilic content may also contribute to their AF efficacy, and such materials may therefore represent a route to translation of the well-documented nonfouling character of zwitterions into practical, industrially relevant coating formulations.

Hydration level dependence of the microscopic dynamics of water adsorbed in ultramicroporous carbon

Even when not functionalized intentionally, most carbon materials are not hydrophobic and readily adsorb water molecules from atmospheric water vapor. We have equilibrated an ultramicroporous carbon at several levels of relative humidity, thereby attaining various hydration levels. The water molecules were adsorbed on the pore walls (but did not fill completely the pore volume) and thus could be better described as hydration, or surface, rather than confined, water. We used quasielastic neutron scattering to perform a detailed investigation of the dependence of microscopic dynamics of these adsorbed water species on the hydration level and temperature. The behavior of hydration water in ultramicroporous carbon clearly demonstrates the same universal traits that characterize surface (hydration) water in other materials that are surface-hydrated. Thus, unless special treatment is intentionally applied to ultramicroporous carbon, the species filling its pores in various applications, ranging from hydrogen molecules to electrolytes, likely find themselves in contact with non-freezing water molecules characterized by rich microscopic dynamics.

2H-NMR and 13C-NMR Study of the Hydration Behavior of Poly(2-methoxyethyl acrylate), Poly(2-hydroxyethyl methacrylate) and Poly(tetrahydrofurfuryl acrylate) in Relation to Their Blood Compatibility as Biomaterials

We recorded 2H-NMR spectra of (deuterated) water in the presence of poly(2-methoxyethyl acrylate) (PMEA), poly(2-hydroxyethyl methacrylate) (PHMEA) and poly(tetrahydrofurfuryl acrylate) (PTHFA). The observed 2H-NMR peak intensities varied substantially with water content and temperature, depending upon either strong binding to polymer surface or suppressed peaks due to freezing. Indeed, 2H-NMR signals in the presence of PHEMA were strongly dependent upon its water content, while those of hydrated PMEA and PTHFA remained unchanged even at −30°C and −20°C. The latter were considerably broadened at −50°C and −30°C, respectively, due to freezing water from the super-cooled state. As a result, the states of the water molecules in PMEA and PTHFA can be classified into three types; free, freezing bound and non-freezing water molecules. The states of the water in PHEMA depend on the water content, and the water can be classified into two types, free and non-freezing water, which exhibit rapid fluctuation and restricted mobility because of the presence of macromolecules, respectively. A kind of freezing bound water, however, should exist in PHEMA. This is also consistent with the substantially decreased 2H spin–lattice relaxation times of hydrated PHEMA as compared with those of PMEA or PTHFA. It is also interesting to note that the flexibility of bound water or polymer (PMEA > PTHFA > PHEMA) is related to a characteristic parameter for biocompatibility such as the production of TAT (thrombin–antithrombin III complex) as a marker of activation of the coagulation system. Therefore, it is naturally recognized that such differential polymer dynamics might be responsible for concomitant changes in structure and dynamics of surrounding water molecules in the vicinity of constituent polymer network.

Effect of Sodium Sulfate on the Gelling Behavior of Agarose and Water Structure Inside the Gel Networks

Agarose hydrogels which constitute a special class of soft matter are undoubtedly one of the most studied biopolymer gels. However, certain issues such as why the sulfate salts and sulfate content in the agarose molecules reduce the gel strength are still not very clear. The present work provides a detailed analysis of structural changes with respect to coil-helix transition or aggregation of helices in the aqueous agarose solutions and hydrogels that accompanied the systematic addition of sodium sulfate. A combined approach which includes the differential scanning calorimetry and temperature-dependent vacuum-ultraviolet circular dichroism measurements permitted the accurate estimates of the energy changes for coil-helix transition and helix-helix interactions. The hydration process of agarose molecule investigated from differential scanning calorimetry and concentration-dependent ultrasonic measurements indicated the loss of both the freeze bound and nonfreezable water molecules with the increase of sulfate content in the solution. Temperature-dependent fluorescence measurements using pyrene as a probe indicated polarity changes when the gel network is created in waters of different salt concentration. Changes in the hydrogen bonding of the water molecules confined in the gel network have been monitored from the intensity ratios of ice-like and liquid-like -OH stretching band of water Fourier transform infrared (FTIR) spectra. Analysis of the -OH stretching band showed the strengthening of hydrogen bonding of water molecules in the gel which tend to weaken with the subsequent addition of sodium sulfate.

State Analysis of Water in Anion-Exchange Resins by Differential Scanning Calorimetry

The state of water in anion-exchange resins of various anion forms was studied by differential scanning calorimetry (DSC). The amounts of non-freezing water molecules in both MR and Gel type resins were much the same in the order of F>OH>Cl>Br>I. The melting peak of the water in the Gel-type resins shifted to the high-temperature side in the order of I>Br>Cl>OH>F when the freezing water content of each ion form was equal. Moreover, the free water was detected in the I- form. These results indicated that the water molecules in the resins were strongly bound in the order of F>OH>Cl>Br>I.

States of water in different hydrophilic polymers — DSC and FTIR studies

The structure of water molecules sorbed in different hydrophilic polymers was studied by DSC and FTIR. The obtained data shows that, first, the sorbed water molecules are directly bound to the hydrophilic site to form non-freezable water. Then, beyond a certain water content threshold, the sorbed water molecules become freezable, but with a melting point lower than 0°C, due to their location in the second hydration layer. Bulk-like water which can be frozen at 0°C appears at higher water contents, and the two types of freezable water finally merge together at very high water contents. The average number of non-freezable water molecules per site depends on the chemical nature of the polar site: ca. 1 for a hydroxyl, and 4.2 for an amide group. For a polymer with carboxylate sites, it increases with the size of the alkaline counter-ion of the site, due to the increasing ability of the carboxylate counter-ion pair to undergo dissociation.

Interactions between hydroxypropylcelluloses and vapour/liquid water

Understanding of the uptake of water vapour or liquid water by cellulose-based polymers is important because of the influence of these processes on many of the biologically or technologically relevant properties of these polymers. In this work we studied these processes in the cases of twelve hydroxypropylcelluloses with low or medium-high degrees of substitution (L-HPCs and HPCs, respectively), characterization of which showed significant differences in structural and physical parameters (substitution pattern, crystallinity, particle size, specific surface area, and intraparticular porosity). Water vapour sorption–desorption isotherms determined to characterize the uptake of water vapour were fitted well by the Young–Nelson model, the optimized parameters of which indicated that at all relative humidities the capacity to bind water vapour as a surface monolayer is greater for HPCs than L-HPCs, but the capacity to absorb water vapour internally is greater for L-HPCs than HPCs. Guggenheim–Anderson–deBoer (GAB) models fitted the sorption–desorption isotherms less well. Differential scanning calorimetry (DSC) experiments showed all sorbed water vapour to be held as non-freezing water. Isoperibol microcalorimetry experiments carried out to investigate interactions with liquid water showed enthalpies of hydration/dissolution of between −62.86 and −71.35 J g −1 for L-HPCs and between −82.95 and −99.80 J g −1 for HPCs, and DSC showed average numbers of non-freezing water molecules per polymer repeat unit of 2.65–4.19 for L-HPCs and 18.10–22.42 for HPCs. DSC characterization of the kinetics of the water uptake by 10 mg compacts obtained by direct compression of hydroxypropylcelluloses showed faster uptake by L-HPC compacts than by HPC compacts, among which there were significant differences in capacity for diffusive uptake. The explanations of the above differences in terms of the different substituent contents, particle sizes and porosities of the HPCs is supported by multiple linear regression analyses.

Freezing of phosphocholine headgroup in fully hydrated sphingomyelin bilayers and its effect on the dynamics of nonfreezable water at subzero temperatures

Differential scanning calorimetry (DSC) and nuclear magnetic resonance (NMR) spectroscopy are applied to characterize the nonfreezable water molecules in fully hydrated D2O/sphingomyelin at temperatures below 0 degrees C. Upon cooling, DSC thermogram displays two thermal transitions peaked at -11 and -34 degrees C. The high-temperature exothermic transition corresponds to the freezing of the bulk D2O, and the low-temperature transition, which has not previously been reported, can be ascribed to the freezing of the phosphocholine headgroup in the lipid bilayer. The dynamics of nonfreezable water are also studied by 2H NMR T1 (spin-lattice relaxation time) and T2e (spin-spin relaxation time obtained by two pulse echo) measurements at 30.7 MHz and at temperatures down to -110 degrees C. The temperature dependence of the T1 relaxation time is characterized by a distinct minimum value of 2.1 +/- 0.1 ms at -30 degrees C. T2e is discontinuous at temperature around -70 degrees C, indicating another freezing-like event for the bound water at this temperature. Analysis of the relaxation data suggest that nonfreezable water undergoes both fast and slow motions at characteristic NMR time scales. The slow motions are affected when the lipid headgroup freezes.

Phase transitions of the water-xanthan system

The phase transitions of the water-xanthan system are studied using differential scanning calorimetry (d.s.c.). Upon heating, a glass transition, the cold crystallization of water, melting of water and transition from the mesophase to the isotropic liquid were observed for samples with water content ( W c) ranging from 0.5 to 1.4. These transition temperatures vary depending on the water content. The glass transition was observed for water content ranging from 0.5 to 2.0. Enthalpy relaxation was observed to occur after annealing at a temperature lower than the glass transition temperature. From the heat of fusion of water, the amounts of non-freezing ( W nf) and freezing water ( W f) were calculated. There were 50 non-freezing water molecules per repeat unit (four pyranose units). The heat of transition from the mesophase to the isotropic liquid decreased with increasing W c. The heat capacity difference ( ΔC p) between the glassy and liquid states was measured for the system, and it was found that this showed a good correlation with W c. The d.s.c. results indicate that both transitions, the liquid crystalline to the isotropic liquid state and the glass.transition, are caused by the molecular motion of xanthan sorbed on non-freezing water.

Hydration of [formula omitted] compared to monosaccharide hydration

Differential scanning calorimetry (DSC) studies of the ice-water transition of N- palmitoylgalactosylsphingosine (NPGS) (cerebroside)/water mixtures indicate 4 ± 1 non-freezable water molecules per molecule NPGS. This hydration level, representing strongly bound water, is identical to that observed previously for human glucocerebroside (Bach, D., Sela, B. and Miller, I.R. (1982) Chem. Phys. Lipids 31, 381). Comparison of gluco- and galacto-cerebroside hydration with hydration measurements on simple monosaccharides suggests a favored orientation of the glycosyl polar group at the cerebroside-water interface.