The dielectric relaxation of mixtures of water and primary alcohol

Azologization and repurposing of a hetero-stilbene-based kinase inhibitor: towards the design of photoswitchable sirtuin inhibitors

The use of light as an external trigger to change ligand shape and as a result its bioactivity, allows the probing of pharmacologically relevant systems with spatiotemporal resolution. A hetero-stilbene lead resulting from the screening of a compound that was originally designed as kinase inhibitor served as a starting point for the design of photoswitchable sirtuin inhibitors. Because the original stilbenoid structure exerted unfavourable photochemical characteristics it was remodelled to its heteroarylic diazeno analogue. By this intramolecular azologization, the shape of the molecule was left unaltered, whereas the photoswitching ability was improved. As anticipated, the highly analogous compound showed similar activity in its thermodynamically stable stretched-out (E)-form. Irradiation of this isomer triggers isomerisation to the long-lived (Z)-configuration with a bent geometry causing a considerably shorter end-to-end distance. The resulting affinity shifts are intended to enable real-time photomodulation of sirtuins in vitro.

Eco-scalable baicalin loaded vesicles developed by combining phospholipid with ethanol, glycerol, and propylene glycol to enhance skin permeation and protection

A new class of biocompatible and scalable phospholipid vesicles was developed, aiming at improving the efficacy of baicalin on the skin. Phosphatidylcholine and baicalin (a natural polyphenol) were hydrated in two steps with a mixture of ethanol, glycerol, and propylene glycol at different ratios, and a low amount of water (4%). Hence, water was almost completely replaced by the co-solvents, which were never used before as predominant dispersing medium of phospholipid vesicles. The vesicles appeared three-dimensionally structured, forming a network that conferred a high viscosity to the dispersions. The vesicles were unilamellar, small in size (∼100 nm), and stable during 12 months of storage. They disclosed optimal performances in the transdermal delivery of baicalin, and high biocompatibility with skin cells (i.e., keratinocytes and fibroblasts). Furthermore, the vesicles promoted the efficacy of baicalin in protecting skin cells against oxidative stress in vitro and injured skin in vivo.

Characterization of the lossy dielectric materials using contour mapping

The imaginary part of the complex permittivity of a lossy dielectric material is large and couples with its real part. The resonant frequency of a cavity with the sample depends not only on the real part of the complex permittivity of the sample but also the imaginary part, resulting in serious ambiguity in determining the sample's complex permittivity. This work proposes a contour mapping method to determine the complex permittivity. The full-wave simulation gives us the contours of the resonant frequency and the quality factor, which are functions of the relative dielectric constant and the loss tangent. By mapping the measured resonant frequency and the measured quality factor, one can uniquely determine the complex permittivity of the sample. Five liquids were examined, including three low-loss materials for benchmarking and two lossy materials. The measured complex permittivities of the three low-loss materials agree very well with the other methods. As for the lossy materials, the measured relative dielectric constant and the loss tangent of alcohol are 6.786 and 0.895, respectively. Besides, the measured dielectric constant of glycerin is 6.811, and its loss tangent is 0.562. The proposed contour mapping technique can be employed to measure the complex permittivity of liquids and solids from lossless to lossy materials.

Distinguishing dynamical features of water inside protein hydration layer: Distribution reveals what is hidden behind the average

Since the pioneering works of Pethig, Grant, and Wüthrich on a protein hydration layer, many studies have been devoted to find out if there are any "general and universal" characteristic features that can distinguish water molecules inside the protein hydration layer from bulk. Given that the surface itself varies from protein to protein, and that each surface facing the water is heterogeneous, search for universal features has been elusive. Here, we perform an atomistic molecular dynamics simulation in order to propose and demonstrate that such defining characteristics can emerge if we look not at average properties but the distribution of relaxation times. We present results of calculations of distributions of residence times and rotational relaxation times for four different protein-water systems and compare them with the same quantities in the bulk. The distributions in the hydration layer are unusually broad and log-normal in nature due to the simultaneous presence of peptide backbones that form weak hydrogen bonds, hydrophobic amino acid side chains that form no hydrogen bond, and charged polar groups that form a strong hydrogen bond with the surrounding water molecules. The broad distribution is responsible for the non-exponential dielectric response and also agrees with large specific heat of the hydration water. Our calculations reveal that while the average time constant is just about 2-3 times larger than that of bulk water, it provides a poor representation of the real behaviour. In particular, the average leads to the erroneous conclusion that water in the hydration layer is bulk-like. However, the observed and calculated lower value of static dielectric constant of hydration layer remained difficult to reconcile with the broad distribution observed in dynamical properties. We offer a plausible explanation of these unique properties.

Origin of diverse time scales in the protein hydration layer solvation dynamics: A simulation study

In order to inquire the microscopic origin of observed multiple time scales in solvation dynamics, we carry out several computer experiments. We perform atomistic molecular dynamics simulations on three protein-water systems, namely, lysozyme, myoglobin, and sweet protein monellin. In these experiments, we mutate the charges of the neighbouring amino acid side chains of certain natural probes (tryptophan) and also freeze the side chain motions. In order to distinguish between different contributions, we decompose the total solvation energy response in terms of various components present in the system. This allows us to capture the interplay among different self- and cross-energy correlation terms. Freezing the protein motions removes the slowest component that results from side chain fluctuations, but a part of slowness remains. This leads to the conclusion that the slow component approximately in the 20-80 ps range arises from slow water molecules present in the hydration layer. While the more than 100 ps component has multiple origins, namely, adjacent charges in amino acid side chains, hydrogen bonded water molecules and a dynamically coupled motion between side chain and water. In addition, the charges enforce a structural ordering of nearby water molecules and helps to form a local long-lived hydrogen bonded network. Further separation of the spatial and temporal responses in solvation dynamics reveals different roles of hydration and bulk water. We find that the hydration layer water molecules are largely responsible for the slow component, whereas the initial ultrafast decay arises predominantly (approximately 80%) due to the bulk. This agrees with earlier theoretical observations. We also attempt to rationalise our results with the help of a molecular hydrodynamic theory that was developed using classical time dependent density functional theory in a semi-quantitative manner.

Anomalous water dynamics at surfaces and interfaces: synergistic effects of confinement and surface interactions

In nature, water is often found in contact with surfaces that are extended on the scale of molecule size but small on a macroscopic scale. Examples include lipid bilayers and reverse micelles as well as biomolecules like proteins, DNA and zeolites, to name a few. While the presence of surfaces and interfaces interrupts the continuous hydrogen bond network of liquid water, confinement on a mesoscopic scale introduces new features. Even when extended on a molecular scale, natural and biological surfaces often have features (like charge, hydrophobicity) that vary on the scale of the molecular diameter of water. As a result, many new and exotic features, which are not seen in the bulk, appear in the dynamics of water close to the surface. These different behaviors bear the signature of both water-surface interactions and of confinement. In other words, the altered properties are the result of the synergistic effects of surface-water interactions and confinement. Ultrafast spectroscopy, theoretical modeling and computer simulations together form powerful synergistic approaches towards an understanding of the properties of confined water in such systems as nanocavities, reverse micelles (RMs), water inside and outside biomolecules like proteins and DNA, and also between two hydrophobic walls. We shall review the experimental results and place them in the context of theory and simulations. For water confined within RMs, we discuss the possible interference effects propagating from opposite surfaces. Similar interference is found to give rise to an effective attractive force between two hydrophobic surfaces immersed and kept fixed at a separation of d, with the force showing an exponential dependence on this distance. For protein and DNA hydration, we shall examine a multitude of timescales that arise from frustration effects due to the inherent heterogeneity of these surfaces. We pay particular attention to the role of orientational correlations and modification of the same due to interaction with the surfaces.

Enhanced vibrational solvatochromism and spectral diffusion by electron rich substituents on small molecule silanes

Fourier transform infrared and two-dimensional IR (2D-IR) spectroscopies were applied to two different silanes in three different solvents. The selected solutes exhibit different degrees of vibrational solvatochromism for the Si-H vibration. Density functional theory calculations confirm that this difference in sensitivity is the result of higher mode polarization with more electron withdrawing ligands. This mode sensitivity also affects the extent of spectral diffusion experienced by the silane vibration, offering a potential route to simultaneously optimize the sensitivity of vibrational probes in both steady-state and time-resolved measurements. Frequency-frequency correlation functions obtained by 2D-IR show that both solutes experience dynamics on similar time scales and are consistent with a picture in which weakly interacting solvents produce faster, more homogeneous fluctuations. Molecular dynamics simulations confirm that the frequency-frequency correlation function obtained by 2D-IR is sensitive to the presence of hydrogen bonding dynamics in the surrounding solvation shell.

Translational and Rotational Diffusion of Two Differently Charged Solutes in Ethylammonium Nitrate-Methanol Mixture: Does the Nanostructure of the Amphiphiles Influence the Motion of the Solute?

In this Article, we have investigated the translational and rotational diffusion of two structurally similar but differently charged solutes (rhodamine 6G perchlorate and fluorescein sodium salt) in ethylammonium nitrate (EAN)-methanol (CH3OH) mixture to understand the effect of added ionic liquid on the motion of the solutes. EAN and CH3OH both are amphiphilic molecules and characterized by an extended hydrogen bonding network. Recently, Russina et al. found that a wide distribution of clusters exist in the CH3OH rich region (0.10 ≤ χEAN ≤ 0.15) and EAN molecules preserve their bulk-sponge-like morphology (Russina, O.; Sferrazza, A.; Caminiti, R.; Triolo, A. J. Phys. Chem. Lett. 2014, 5, 1738-1742). The effect of this microheterogeneous mixture on the solute's motion shows some interesting results compared to other PIL (protic ionic liquid)-cosolvent mixtures. Analysis of the time-resolved anisotropy data with the aid of Stokes-Einstein-Debye (SED) hydrodynamic theory predicts that the reorientation time of both of the solutes appears close to the stick hydrodynamic line in the methanol rich region. The hydrogen bond accepting solutes experience specific interaction with CH3OH, and with increasing concentration of EAN, the specific interaction between the solute and solvent molecules is decreased while the decrease is more prominent in the low mole fraction of EAN due to the large size of cluster formation. The temperature dependent anisotropy measurements show that the hydrogen bonding interaction between EAN and CH3OH is increased with increasing temperature. Moreover, fluorescence correlation spectroscopy (FCS) shows the dynamic heterogeneity of the mixture which is due to the segregation of the alkyl chain of the PIL. Formation of a large cluster at a low mole fraction of IL (0.10 ≤ χEAN ≤ 0.15) can be proved by the insensitivity of the translational diffusion and rotational activation energy of the solutes to the concentration of EAN. Thus, the result of the work suggests that the addition of EAN to the CH3OH affects the specific interaction between solute and solvent and, as a consequence, the translational motion as well as the rotational motion of the solutes are modulated.

Characterization of Water-Soluble Cucurbit[7]uril in Alcohol-Water Mixtures by High-Pressure Studies on the Inclusion Complexation with New Methylene Blue

The inclusion complexation of new methylene blue (NMB) with cucurbit[7]uril (CB[7]) was studied spectrophotometrically. It was found that CB[7] forms a 1:1 inclusion complex with NMB in alcohol-water mixtures and the structure of the inclusion complex has been established by 1H-NMR measurements. We have determined the association constants of CB[7] with NMB under high pressures. The association constants of CB[7] increase as the external pressure and solvent polarity increase. Pressure dependence experiments enabled us to calculate the reaction volume for inclusion of CB[7] to range from -15.8 to -10.8cm3 mol-1. From analyses of the high-pressure results, the volume change caused by water and/or alcohol molecules repelled from the CB[7] cavity in the alcohol-water mixtures was evaluated, which is indicative of the presence of water and/or alcohol molecules in the CB[7] cavity in the alcohol-water mixtures. The volume of molecules repelled from the CB[7] cavity plays an important role in the volume change upon inclusion.

Zeta potential of microbubbles in aqueous solutions: electrical properties of the gas-water interface

Microbubbles are very fine bubbles and appropriate for the investigation of the gas-water interface electrical charge, because of their long stagnation, due to slow buoyancy, in the electrophoresis cell observation area. This study investigated the zeta potential of microbubbles in aqueous solutions and revealed that the bubbles were negatively charged under a wide range of pH conditions. The potential was positive under strong acidic conditions, and the inorganic electrolytes decrease the potential by increasing the amount of counterions within the slipping plane. OH(-) and H(+) are crucial factors for the charging mechanism of the gas-water interface, while other anions and cations have secondary effects on the zeta potential, because counterions are attracted by the interface charge. The addition of a small amount of propanol and butanol provided significant information for considering the mechanism of the gas-water interface charge. Even though these alcohols did not have any electrical charge, they had a strong effect on the gas-water interface charge and dispersed the zeta potential of the microbubbles in the aqueous solution. These alcohols tended to adsorb to the interface and affect the hydrogen-bonding network at the interface, so that it was concluded that the gas-water interface electrical charge must be related to the difference of the construction of the hydrogen-bonding network between the bulk water and the gas-water interface.

Dielectric Properties of Ethyleneglycol−1,4-Dioxane Mixtures Using TDR Method

Complex permittivity has been determined for mixtures of ethyleneglycol-1,4-dioxane (EG-DX) with various concentrations in the frequency range from 100 MHz to 30 GHz at 25 degrees C by time domain reflectometry (TDR). A primary process with an asymmetric shape and a Debye-type small-amplitude high-frequency process are observed for each mixture. The deviation of the relaxation time for the primary process from that of the ideal mixture shows a maximum value at a mole fraction of 1,4-dioxane, xDX approximately =0.8. The static permittivity for the mixtures can be explained using the Luzar model by assuming the formation of two types of hydrogen-bonded dimers, one between EG-EG (pair 1) and the other between EG-DX (pair 2). The number of these pairs is also estimated as a function of concentration. These results of the relaxation time and static permittivity are interpreted on the basis of a model of two kinds of cooperative domains coexisting in the mixtures.

Broadband Dielectric Study of Dynamics of Polymer and Solvent in Poly(vinyl pyrrolidone)/Normal Alcohol Mixtures

Broadband dielectric measurements of poly(vinyl pyrrolidone) (PVP)-monohydroxyl alcohol mixtures of various normal alcohols with the number of carbon atoms per molecule ranging from 1 to 9 were made in the frequency range of 20 Hz to 20 GHz at 25 degrees C. Two relaxation processes due to the reorientation of dipoles on the PVP and alcohol molecules were observed. The relaxation process at frequencies higher than 100 MHz is the primary process of alcohols, and that at frequencies lower than 10 MHz is attributed to the local chain motion of PVP. For mixtures of alcohol molecules that are smaller than propanol, the relaxation time of the alcohol increases with increasing PVP concentration, whereas for mixtures of alcohol molecules larger than butanol, the relaxation time of the alcohol decreases with increasing PVP concentration. The increase in the density of hydrogen-bonding sites upon the addition of PVP reduces the relaxation time of alcohol in the mixture, and vice versa. The relaxation time of the local chain motion of PVP increases with PVP concentration and solvent viscosity. Different time scales of the molecular motions of polymer and solvent coexist in homogeneous mixtures with hydrogen-bonded polar solvent and polymer.

Broadband Dielectric Study of Dynamics of Poly(vinyl pyrrolidone)−Ethylene Glycol Oligomer Blends

Broadband dielectric measurements for blends of poly(vinyl pyrrolidone) (PVP) and ethylene glycol oligomer (EGO) from 0 to 40 wt % PVP were carried out at 25 degrees C in the frequency range from 20 Hz to 20 GHz. The EGOs used in this study were ethylene glycol (EG), diethylene glycol (2EG), and PEG400 (MW = 400). For the PVP-EG, -2EG, and -PEG400 blends, relaxation processes caused by the motion of EGO in the GHz range and the micro-Brownian motion of the PVP chain at 10 kHz-1 MHz were observed. Although the PVP-EGO blend is miscible, relaxation processes caused by the molecular motion of EGO and the local chain motion of PVP were observed individually. The relaxation time of the local chain motion of PVP showed a strong PVP concentration dependence and a solvent viscosity dependence, which are similar to those reported so far for the solutions in nonpolar solvents.

The Structure of Liquid Methanol

A combination of density functional calculations of molecular clusters with a quantum cluster equilibrium (QCE) model provides evidence that liquid methanol is dominated by cyclic and/or lasso structures. Only cluster populations including these structures fit the measured thermodynamic and spectroscopic properties, such as heat of vaporization, heat capacity, NMR chemical shifts, and quadrupole coupling constants. On the other hand, cluster populations comprising open-chain structures fail to reach the experimental values: the heat of vaporization is about 10 kJ mol(-1) too low, and the proton chemical shift is insufficiently downfield-shifted by about 1 ppm.

Ability of Polyvinylpyrrolidone and Polyacrylic Acid to Inhibit the Crystallization of Amorphous Acetaminophen

The inhibition of crystallization of amorphous acetaminophen (ACTA) by polyvinylpyrrolidone (PVP) and polyacrylic acid (PAA) was studied using amorphous solid dispersions prepared by melt quenching. Co-melting with PVP and PAA decreased the average molecular mobility, as indicated by increases in glass transition temperature and enthalpy relaxation time. The ACTA/PAA dispersion exhibited much slower crystallization than the ACTA/PVP dispersion with a similar glass transition temperature value, indicating that interaction between ACTA and polymers also contributed to the stabilizing effect of these polymers. The carboxyl group of PAA may interact with the hydroxyl group of ACTA more intensely than the carbonyl group of PVP does, resulting in the stronger stabilizing effect of PAA. Dielectric relaxation spectroscopy showed that the number of water molecules tightly binding to PVP per monomer unit was larger than that to PAA. Furthermore, a small amount of absorbed water decreased the stabilizing effect of PVP, but not that of PAA. These findings suggest that the stronger stabilizing effect of PAA is due to the stronger interaction with ACTA. The ability of PAA to decrease the molecular mobility of solid dispersion was also larger than that of PVP, as indicated by the longer enthalpy relaxation time.

Calorimetric studies of diclofenac sodium in aqueous solution of cyclodextrin and water-ethanol mixtures

The technique of solution calorimetry has been employed to study the interaction between diclofenac sodium and beta-cyclodextrin by determining the enthalpies of solution of the drug in water and in aqueous beta-cyclodextrin solution. Thermodynamic parameters characterizing the binding process such as enthalpy deltaH0, equilibrium constant K, free energy deltaG0 and entropy deltaS0 have been calculated to be 12.00 kJ mol(-1), 1670 dm3 mol(-1), -19.03kJ mol(-1) and 22.98 J K(-1) mol(-1), respectively. Enthalpies of solution of diclofenac sodium have also been determined in water-ethanol mixtures.