Component dynamics in polyvinylpyrrolidone concentrated aqueous solutions

Relaxation and diffusion of an ionic plasticizer in amorphous poly(vinylpyrrolidone)

The present work focuses on the dynamics of the ionic constituents of 1-propyl-3-methyl-imidazolium-bis-(trifluormethylsulfonyl)-imide (PT), a paradigmatic ionic liquid, as an additive in poly(vinylpyrrolidone) (PVP). Hence, the resulting product can be regarded as a polymer electrolyte as well as an amorphous dispersion. Leveraging dielectric spectroscopy and oscillatory shear rheology, complemented by differential scanning calorimetry, the spectral shapes and the relaxation maps of the supercooled PVP-PT mixtures are accessed in their full compositional range. The study also presents dielectric and shear responses of neat PVP with a molecular weight of 2500 g mol-1. We discuss the plasticizing role of the PT additive and the decoupling between ionic dynamics and segmental relaxation in these mixtures. The extracted relaxation times, steady-state viscosities, and conductivities are employed to estimate the translational diffusivities of the ionic penetrants by means of the Stokes-Einstein, Nernst-Einstein, and Almond-West relations. While some of the estimated diffusivities agree with each other, some do not, pointing to the importance of the chosen hydrodynamic approximations and the type of response considered for the analysis. The present extensive dielectric, rheological, and calorimetric study enables a deeper understanding of relaxation and transport of ionic ingredients in polymers, particularly in the slow-dynamics regime which is difficult to access experimentally by direct-diffusivity probes.

Low-Temperature Behaviors, Cold Crystallization, and Glass Transition in Poly(vinylpyrrolidone) Aqueous Solution

Intermediate water is a kind of water around the biocompatible polymer, such as poly(vinylpyrrolidone) (PVP), that exhibits the phenomenon of cold crystallization. We investigate the low-temperature behavior of PVP aqueous solution using small- and wide-angle X-ray scattering and total neutron scattering measurements. The ice formation speed of the intermediate water is extremely reduced by confinement in the PVP moiety during the cooling process. However, around the glass transition temperature, the water-rich phase expands and orders the hydrogen-bond network, behaving as ice nuclei. During the heating process, cubic ice is formed first and then fills the water-rich region. After saturation of the cubic ice formation, the ice transforms from the cubic to the hexagonal ice form.

Solvent modulation in peptide sub-microfibers obtained by solution blow spinning

Peptides possess high chemical diversity at the amino acid sequence level, which further translates into versatile functions. Peptides with self-assembling properties can be processed into diverse formats giving rise to bio-based materials. Peptide-based spun fibers are an interesting format due to high surface-area and versatility, though the field is still in its infancy due to the challenges in applying the synthetic polymer spinning processes to protein fibers to peptides. In this work we show the use of solution blow-spinning to produce peptide fibers. Peptide fiber formation was assisted by the polymer poly (vinyl pyrrolidone) (PVP) in two solvent conditions. Peptide miscibility and further self-assembling propensity in the solvents played a major role in fiber formation. When employing acetic acid as solvent, peptide fibers (0.5 μm) are formed around PVP fibers (0.75 μm), whereas in isopropanol only one type of fibers are formed, consisting of mixed peptide and PVP (1 μm). This report highlights solvent modulation as a mean to obtain different peptide sub-microfibers via a single injection nozzle in solution blow spinning. We anticipate this strategy to be applied to other small peptides with self-assembly propensity to obtain multi-functional proteinaceous fibers.

Deuteron magnetic resonance study of glyceline deep eutectic solvents: Selective detection of choline and glycerol dynamics

Glyceline, a green solvent considered for various electrochemical applications, represents a multi-component glass former. Viewed from this perspective, the choline cation and the hydrogen bond donor glycerol, the two major constituents forming this deep eutectic solvent, were studied using nuclear magnetic resonance in a selective manner by means of suitably deuteron-labeled isotopologues. Carried out from far above to far below the glass transition temperature, measurements and analyses of the spin-lattice and spin-spin relaxation times reveal that the reorientational dynamics of the components, i.e., of glycerol as well as of chain deuterated choline chloride are slightly different. Possible implications of this finding regarding the hydrogen-bonding pattern in glyceline are discussed. Furthermore, the deuterated methyl groups in choline chloride are exploited as sensitive probes of glyceline's supercooled and glassy states. Apart from spin relaxometry, a detailed line shape analysis of the CD₃ spectra yields valuable insights into the broad intermolecular and intramolecular energy barrier distributions present in this binary mixture.

Characterization of the Features of Water Inside the SecY Translocon

Despite extended experimental and computational studies, the mechanism regulating membrane protein folding and stability in cell membranes is not fully understood. In this review, I will provide a personal and partial account of the scientific efforts undertaken by Dr. Stephen White to shed light on this topic. After briefly describing the role of water and the hydrophobic effect on cellular processes, I will discuss the physical chemistry of water confined inside the SecY translocon pore. I conclude with a review of recent literature that attempts to answer fundamental questions on the pathway and energetics of translocon-guided membrane protein insertion.

Dynamics of Uncrystallized Water, Ice, and Hydrated Polymer in Partially Crystallized Poly(vinylpyrrolidone)-Water Mixtures

In this study, we investigated the cooperative molecular dynamics of poly(vinylpyrrolidone) (PVP), ice, and uncrystallized water (UCW) in partially crystallized PVP-water mixtures by means of broadband dielectric spectroscopy. Three relaxation processes, denoted I, II, and III, were observed at temperatures ranging from immediately below the crystallization temperature (T_c) to approximately 200 K. At temperatures of 173-193 K, processes I and II cannot be distinguished. Below 168 K, process II separates into two processes: process IV at higher frequencies and process V at lower frequencies. Process I contributes to process V. In partially crystallized mixtures, process I originates from UCW in an uncrystallized phase with PVP. Process II is attributed to ice in the mixture, with a relaxation time that is 2 orders of magnitude smaller than that of pure ice. The concentration dependence of the strength of process II and the relaxation time relative to that of ice in bovine serum albumin (BSA)-water and gelatin-water mixtures strongly support this conclusion. Observation of processes IV and V indicates the presence of multiple ice relaxation processes. Process III is attributed to the α process of PVP in the uncrystallized phase in 40 and 50 wt % PVP mixtures. For mixtures with 30 wt % PVP or less, process III is attributed not only to the α process of PVP but also to interfacial polarization.

Structure and dynamics of titania - poly(N-vinyl caprolactam) composite hydrogels

The morphologies and dynamics of poly(N-vinyl caprolactam) (PVCL) based hydrogels with titania nanoparticles in different states (native, air-dried to a constant weight and swollen in H₂O or D₂O) are studied by a combination of complementary techniques: wide angle X-ray scattering, small angle neutron scattering, neutron spin echo spectroscopy, differential scanning calorimetry. The results suggest the presence of different structural types of water leading to different properties of the hydrogels. We propose a hierarchical structure of hydrogels spanning from the molecular to the microscopic scale consistent with both the static structure (polymer mesh size, association of the nodes of crosslinks and microchains of PVCL) and the dynamics (rate of relaxation of polymer chains, hydrodynamic polymer-polymer correlation length). The presence of nanoscale titania does not change the molecular structure and nanostructure due to its aggregation into meso-domains, but does affect the microstructure, changing the response rate to a temperature jump from 20 to 50 °C. Titania nanoparticles do not change the equilibrium swelling degree of hydrogels.

Synthesis and physico-chemical properties of poly(N-vinyl pyrrolidone)-based hydrogels with titania nanoparticles

Poly(N-vinyl pyrrolidone) (PVP)-based hydrogels with titania nanoparticles (TN) were synthesized by the sol-gel method for the first time and were characterized in different states (native, freeze-dried, air-dried to constant weight and ground to powder, or swollen to constant weight in H2O or D2O) by various methods such as wide-angle and small-angle X-ray and neutron scattering, neutron spin-echo (NSE) spectroscopy, and scanning electron microscopy. The static (static polymer-polymer correlation length (mesh size), associates of cross-links and PVP microchains) and dynamic (polymer chain relaxation rate, hydrodynamic polymer-polymer correlation length) structural elements were determined. The incorporation of titania nanoparticles into PVP hydrogel slightly increases the size of structural inhomogeneities (an increase in the static and dynamic polymer-polymer correlation length, the formation of associates of cross-links and PVP chains). Titania nanoparticles have an impact on the microstructure of the composite hydrogel and form associates with sizes from 0.5 to 2 µm attached to PVP hydrogel pore walls. The PVP and TN/PVP hydrogels show a high degree of water swelling. Moreover, the presence of titania nanoparticles in TN/PVP increases the number of water adsorption cycles compared to PVP hydrogel. The high swelling degree, bacteria-resistant and antimicrobial properties against Staphylococcus aureus allow considering NT/PVP hydrogels for medical applications as wound coatings.

Structural Relaxation Processes and Collective Dynamics of Water in Biomolecular Environments

In this simulation study, we investigate the influence of biomolecular confinement on dynamical processes in water. We compare water confined in a membrane protein nanopore at room temperature to pure liquid water at low temperatures with respect to structural relaxations, intermolecular vibrations, and the propagation of collective modes. We observe distinct potential energy landscapes experienced by water molecules in the two environments, which nevertheless result in comparable hydrogen bond lifetimes and sound propagation velocities. Hence, we show that a viscoelastic argument that links slow rearrangements of the water-hydrogen bond network to ice-like collective properties applies to both, the pure liquid and biologically confined water, irrespective of differences in the microscopic structure.

Synthesized tissue-equivalent dielectric phantoms using salt and polyvinylpyrrolidone solutions

PURPOSE

To explore the use of polyvinylpyrrolidone (PVP) for simulated materials with tissue-equivalent dielectric properties.

METHODS

PVP and salt were used to control, respectively, relative permittivity and electrical conductivity in a collection of 63 samples with a range of solute concentrations. Their dielectric properties were measured with a commercial probe and fitted to a 3D polynomial in order to establish an empirical recipe. The material's thermal properties and MR spectra were measured.

RESULTS

The empirical polynomial recipe (available at https://www.amri.ninds.nih.gov/cgi-bin/phantomrecipe) provides the PVP and salt concentrations required for dielectric materials with permittivity and electrical conductivity values between approximately 45 and 78, and 0.1 to 2 siemens per meter, respectively, from 50 MHz to 4.5 GHz. The second- (solute concentrations) and seventh- (frequency) order polynomial recipe provided less than 2.5% relative error between the measured and target properties. PVP side peaks in the spectra were minor and unaffected by temperature changes.

CONCLUSION

PVP-based phantoms are easy to prepare and nontoxic, and their semitransparency makes air bubbles easy to identify. The polymer can be used to create simulated material with a range of dielectric properties, negligible spectral side peaks, and long T₂ relaxation time, which are favorable in many MR applications. Magn Reson Med 80:413-419, 2018. © 2017 International Society for Magnetic Resonance in Medicine.

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.

Dynamics of hydrated proteins and bio-protectants: Caged dynamics, β-relaxation, and α-relaxation

BACKGROUND

The properties of the three dynamic processes, α-relaxation, ν-relaxation, and caged dynamics in aqueous mixtures and hydrated proteins are analogous to corresponding processes found in van der Waals and polymeric glass-formers apart from minor differences.

METHODS

Collection of various experimental data enables us to characterize the structural α-relaxation of the protein coupled to hydration water (HW), the secondary or ν-relaxation of HW, and the caged HW process.

RESULTS

From the T-dependence of the ν-relaxation time of hydrated myoglobin, lysozyme, and bovine serum albumin, we obtain Ton at which it enters the experimental time windows of Mössbauer and neutron scattering spectroscopies, coinciding with protein dynamical transition (PDT) temperature Td. However, for all systems considered, the α-relaxation time at Ton or Td is many orders of magnitude longer. The other step change of the mean-square-displacement (MSD) at Tg_alpha originates from the coupling of the nearly constant loss (NCL) of caged HW to density. The coupling of the NCL to density is further demonstrated by another step change at the secondary glass temperature Tg_beta in two bio-protectants, trehalose and sucrose.

CONCLUSIONS

The structural α-relaxation plays no role in PDT. Since PDT is simply due to the ν-relaxation of HW, the term PDT is a misnomer. NCL of caged dynamics is coupled to density and show transitions at lower temperature, Tg_beta and Tg_alpha.

GENERAL SIGNIFICANCE

The so-called protein dynamical transition (PDT) of hydrated proteins is not caused by the structural α-relaxation of the protein but by the secondary ν-relaxation of hydration water. "This article is part of a Special Issue entitled "Science for Life" Guest Editor: Dr. Austen Angell, Dr. Salvatore Magazù and Dr. Federica Migliardo".

Confined Water as Model of Supercooled Water

Water in confined geometries has obvious relevance in biology, geology, and other areas where the material properties are strongly dependent on the amount and behavior of water in these types of materials. Another reason to restrict the size of water domains by different types of geometrical confinements has been the possibility to study the structural and dynamical behavior of water in the deeply supercooled regime (e.g., 150-230 K at ambient pressure), where bulk water immediately crystallizes to ice. In this paper we give a short review of studies with this particular goal. However, from these studies it is also clear that the interpretations of the experimental data are far from evident. Therefore, we present three main interpretations to explain the experimental data, and we discuss their advantages and disadvantages. Unfortunately, none of the proposed scenarios is able to predict all the observations for supercooled and glassy bulk water, indicating that either the structural and dynamical alterations of confined water are too severe to make predictions for bulk water or the differences in how the studied water has been prepared (applied cooling rate, resulting density of the water, etc.) are too large for direct and quantitative comparisons.

Dynamics of tetrahydrofuran as minority component in a mixture with poly(2-(dimethylamino)ethyl methacrylate): A neutron scattering and dielectric spectroscopy investigation

We have investigated a mixture of poly(2-(dimethylamino)ethyl methacrylate) (PDMAEMA) and tetrahydrofuran (THF) (70 wt. % PDMAEMA/30 wt. % THF) by combining dielectric spectroscopy and quasielastic neutron scattering (QENS) on a labelled sample, focusing on the dynamics of the THF molecules. Two independent processes have been identified. The "fast" one has been qualified as due to an internal motion of the THF ring leading to hydrogen displacements of about 3 Å with rather broadly distributed activation energies. The "slow" process is characterized by an Arrhenius-like temperature dependence of the characteristic time which persists over more than 9 orders of magnitude in time. The QENS results evidence the confined nature of this process, determining a size of about 8 Å for the volume within which THF hydrogens' motions are restricted. In a complementary way, we have also investigated the structural features of the sample. This study suggests that THF molecules are well dispersed among side-groups nano-domains in the polymer matrix, ruling out a significant presence of clusters of solvent. Such a good dispersion, together with a rich mobility of the local environment, would prevent cooperativity effects to develop for the structural relaxation of solvent molecules, frustrating thereby the emergence of Vogel-Fulcher-like behavior, at least in the whole temperature interval investigated.

Cause of the fragile-to-strong transition observed in water confined in C-S-H gel

In this study, the rotational dynamics of hydration water confined in calcium-silicate-hydrate (C-S-H) gel with a water content of 22 wt.% was studied by broadband dielectric spectroscopy in broad temperature (110-300 K) and frequency (10(-1)-10(8) Hz) ranges. The C-S-H gel was used as a 3D confining system for investigating the possible existence of a fragile-to-strong transition for water around 220 K. Such transition was observed at 220 K in a previous study [Y. Zhang, M. Lagi, F. Ridi, E. Fratini, P. Baglioni, E. Mamontov and S. H. Chen, J. Phys.: Condens. Matter 20, 502101 (2008)] on a similar system, and it was there associated with a hidden critical point of bulk water. However, based on the experimental results presented here, there is no sign of a fragile-to-strong transition for water confined in C-S-H gel. Instead, the fragile-to-strong transition can be explained by a merging of two different relaxation processes at about 220 K.