Relaxation dynamics of liposomes in an aqueous solution.

The gel-liquid crystal phase transition has been studied by the temperature and frequency dependent dielectric relaxation behavior of liposomes in an aqueous solution (40 g L(-1) DPPC-water mixture). Four relaxation processes were observed in the frequency range from 40 Hz to 30 GHz which were ascribed to different molecular mechanisms, related to the structural units of the system. The gel-liquid crystal phase transition was also described very accurately from the temperature-dependent dielectric relaxation strength, relaxation time and symmetric shape parameter of the relaxation functions obtained from the fitting procedure. Relaxation process 3, obtained from the dielectric fitting procedure, was confirmed by dielectric modulus analysis. A comparison of the lipid membrane with non-biological systems like liquid crystals was performed. It was determined that the lipid membrane has a ferroelectric liquid crystal like behavior. Process 3 is comparable to the soft mode relaxation process observed in ferroelectric liquid crystals which was detected close to the smectic-C*-smectic-A phase transition. Differential scanning calorimetry was also used to confirm the gel-liquid crystal phase transition of this mixture.

Glass transition and dynamics in BSA-water mixtures over wide ranges of composition studied by thermal and dielectric techniques.

Protein-water dynamics in mixtures of water and a globular protein, bovine serum albumin (BSA), was studied over wide ranges of composition, in the form of solutions or hydrated solid pellets, by differential scanning calorimetry (DSC), thermally stimulated depolarization current technique (TSDC) and dielectric relaxation spectroscopy (DRS). Additionally, water equilibrium sorption isotherm (ESI) measurements were performed at room temperature. The crystallization and melting events were studied by DSC and the amount of uncrystallized water was calculated by the enthalpy of melting during heating. The glass transition of the system was detected by DSC for water contents higher than the critical water content corresponding to the formation of the first sorption layer of water molecules directly bound to primary hydration sites, namely 0.073 (grams of water per grams of dry protein), estimated by ESI. A strong plasticization of the T(g) was observed by DSC for hydration levels lower than those necessary for crystallization of water during cooling, i.e. lower than about 0.3 (grams of water per grams of hydrated protein) followed by a stabilization of T(g) at about -80°C for higher water contents. The α relaxation associated with the glass transition was also observed in dielectric measurements. In TSDC a microphase separation could be detected resulting in double T(g) for some hydration levels. A dielectric relaxation of small polar groups of the protein plasticized by water, overlapped by relaxations of uncrystallized water molecules, and a separate relaxation of water in the crystallized water phase (bulk ice crystals) were also recorded.

The protein "glass" transition and the role of the solvent.

Hydrated proteins undergo a change in their dynamical properties in the neighborhood of a temperature. The change of dynamics has been likened to glass transition of glass-forming substances because similar properties were found. However, a complete understanding of the conformation fluctuations of hydrated proteins and their relation to the dynamics of the solvent is still not available, possibly due to the protein molecules being more complex than ordinary glass-formers. For this reason, we turn our attention to the experimental findings of the dynamics of mixtures of water with simpler glass-formers (small molecules and polymers). Two major relaxation processes have been observed in these aqueous mixtures. One is the structural alpha-relaxation of the hydrophilic glass-former hydrogen bonded to the water, which is responsible for glass transition. The other one is the local secondary beta-relaxation of water in the mixture. Remarkably, these two relaxation processes in aqueous mixtures have analogues in hydrated proteins with the same properties. The conformation fluctuations of the protein and the relaxation of the solvent in hydrated proteins behave like the alpha-relaxation of the hydrophilic glass-former hydrogen bonded to the water and the beta-relaxation of water in other aqueous mixtures, respectively. At low temperatures, the Arrhenius activation energy of the relaxation time of the solvent in a hydrated protein is almost the same as that of the beta-relaxation of water in the glassy states of aqueous mixtures. The Arrhenius T-dependence of the solvent relaxation times no longer holds at temperatures that exceed the "glass" transition temperature of the hydrated protein, defined as the temperature at which the conformation relaxation time is very long. This behavior of the solvent in hydrated proteins is similar to that found in the beta-relaxation of water in aqueous mixtures when crossing the glass transition temperature of the mixture (Capaccioli, S.; Ngai, K. L.; Shinyashiki, N. J. Phys. Chem. B 2007, 111, 8197). Furthermore, the same dynamics were found in mixtures of two van der Waals glass-formers, which are even simpler systems than aqueous mixtures because of the absence of hydrogen bonding. The experimental data of these ideal mixtures of van der Waals glass-formers have been given a satisfactory theoretical explanation. Since the properties of hydrated proteins, aqueous mixtures, and the mixtures of van der Waals liquids are similar, we transfer the theoretical understanding gained in the study of the last system sequentially to the two other increasingly more complex systems.

The glass transition and dielectric secondary relaxation of fructose-water mixtures.

Broad-band dielectric measurements for fructose-water mixtures with fructose concentrations between 70.0 and 94.6 wt% were carried out in the frequency range of 2 mHz to 20 GHz in the temperature range of -70 to 45 degrees C. Two relaxation processes, the alpha process at lower frequency and the secondary beta process at higher frequency, were observed. The dielectric relaxation time of the alpha process was 100 s at the glass transition temperature, T(g), determined by differential scanning calorimetry (DSC). The relaxation time and strength of the beta process changed from weaker temperature dependences of below T(g) to a stronger one above T(g). These changes in behaviors of the beta process in fructose-water mixtures upon crossing the T(g) of the mixtures is the same as that found for the secondary process of water in various other aqueous mixtures with hydrogen-bonding molecular liquids, polymers, and nanoporous systems. These results lead to the conclusion that the primary alpha process of fructose-water mixtures results from the cooperative motion of water and fructose molecules, and the secondary beta process is the Johari-Goldstein process of water in the mixture. At temperatures near and above T(g) where both the alpha and the beta processes were observed and their relaxation times, tau(alpha) and tau(beta), were determined in some mixtures, the ratio tau(alpha)/tau(beta) is in accord with that predicted by the coupling model. Fixing tau(alpha) at 100 s, the ratio tau(alpha)/tau(beta) decreases with decreasing concentration of fructose in the mixtures. This trend is also consistent with that expected by the coupling model from the decrease of the intermolecular coupling parameter upon decreasing fructose concentration.

The Johari-Goldstein beta-relaxation of water.

There is a plethora of experimental data on the dynamics of water in mixtures with glycerol, ethylene glycol, ethylene glycol oligomers, poly(ethylene glycol) 400 and 600, propanol, poly(vinyl pyrrolidone), poly(vinyl methylether), and other substances. In spite of the differences in the water contents, the chemical compositions, and the glass transition temperatures Tg of these aqueous mixtures, a faster relaxation originating from the water (called the nu-process) is omnipresent, sharing the following common properties. The relaxation time tau(nu) has Arrhenius temperature dependence at temperatures below Tg of the mixture. The activation energies of tau(nu) all fall within a neighborhood of 50 kJ/mol. At the same temperature where mixtures are all in their glassy states, the values of tau(nu) of several mixtures are comparable. The Arrhenius temperature dependence of tau(nu) does not continue to higher temperatures and instead it crosses over to a stronger temperature dependence at temperatures above Tg. The dielectric relaxation strength of the nu-process, Deltaepsilon(nu)(T), has a stronger temperature dependence above Tg than below, mimicking the change of enthalpy, entropy, and volume when crossing Tg. These general property of the nu-process (except for the magnitude of the activation energy) had been found before in the secondary relaxation of the faster component in several binary nonaqueous mixtures. Other properties of the secondary relaxation in these nonaqueous mixtures have helped to identify it as the Johari-Goldstein (JG) secondary relaxation of the faster component. The similarities in properties lead us to conclude that the nu-processes in water mixtures are the JG secondary relaxations of water. The conclusion is reinforced by the processes behaving similarly to the nu-process found in 6 A thick water layer (two molecular layers) in fully hydrated Na-vermiculite clay, and in water confined in molecular sieves, silica hydrogels, and poly(2-hydroxyethyl methacrylate) hydrogels.

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.

Protein and water dynamics in bovine serum albumin-water mixtures over wide ranges of composition.

Dielectric dynamic behavior of bovine serum albumin (BSA)-water mixtures over wide ranges of water fractions, from dry protein until 40 wt % in water, was studied through dielectric relaxation spectroscopy (DRS). The α relaxation associated with the glass transition of the hydrated system was identified. The evolution of the low temperature dielectric relaxation of small polar groups of the protein surface with hydration level results in the enhancement of dielectric response and the decrease of relaxation times, until a critical water fraction, which corresponds to the percolation threshold for protonic conductivity. For water fractions higher than the critical one, the position of the secondary ν relaxation of water saturates in the Arrhenius diagram, while contributions originating from water molecules in excess (uncrystallized water or ice) follow separate relaxation modes slower than the ν relaxation.

Broadband dielectric study of the glass transition in poly(ethyleneglycol)-water mixture.

We performed broadband dielectric measurements of a polyethyleneglycol-water mixture in the frequency range between 10 GHz and 1 microHz and the temperature range between 300 and 133 K. One relaxation process is observed throughout the whole temperature range. The temperature dependence of the relaxation time clearly obeys the Vogel-Fulcher law above 183 K, and the Arrhenius law below 183 K. This observed relaxation process is the secondary process, and the primary process related to the glass transition is masked by the low-frequency ionic contribution below 183 K. The glass transition concerned with the masked primary process leads to the Vogel-Fulcher to Arrhenius transition of the secondary process.

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.

Microwave dielectric study on hydration of moist collagen.

Two dielectric relaxation peaks were found in moist collagen by the time domain reflectometry. The low-frequency peak around 100 MHz moves little as the water content is varied. Its relaxation strength depends on the content and vanishes for completely dried collagen. This process is concluded to be due to water molecules strongly bound to the tropocollagen. Amount of the bound water is estimated as 0.12 g water/g collagen. Twenty-one water molecules are bound to one repeat of the triple helix. The existence of stringlike water chains is suggested. If the water content is less than 0.5 g water/g collagen, the high frequency peak locates between those of bound and bulk water. Water among the tropo-collagen is weakly bound to the collagen. In the higher region it does not change much with the content, being close to that of bulk water. The bulk water appears in this region.

Globule-coil transition of denatured globular protein investigated by a microwave dielectric technique.

A mechanism for the gel-glass transition of denatured globular protein has been explained from the viewpoint of the globule-coil transition with microwave dielectric measurements using a time domain reflectometry (TDR) method. Boiled egg white, which is an aqueous gel of egg white prepared by heat treatment at 100 degrees C, becomes a glass on drying. In the gel state, the relaxation processes corresponding to the orientation of bulk water and the micro-Brownian motion of peptide chains of denatured protein were observed around 10 GHz and 10 MHz, respectively. When the gel-glass transition occurred, the relaxation strength for bulk water decreased rapidly as evaporation and breaking of water structure occurred. Simultaneously, the relaxation strength for micro-Brownian motion increased abruptly, as the structure of globular protein varied from globule state to coiled state. It is considered that the protein molecule spreads out and takes up a coiled state by reductions of hydrophobic and hydrophilic interactions of the globular protein. These reductions occur through a decrease in the amount of water.

Molecular dynamics of hinge-bending motion of IgG vanishing with hydrolysis by papain.

We have performed dielectric relaxation measurements via a time domain reflectometry (TDR) method to study dynamic behaviors of the segmental flexibility of immunoglobulin G (IgG) in aqueous solution without antigen binding. In general, an intermediate relaxation process due to bound water is observed around 100 MHz at 25 degrees C for common proteins between two relaxation processes due to overall rotation and reorientation of free water. However, the intermediate process observed around 6 MHz for IgG was due to both bound water and hinge-bending motion. The apparent activation energy of 33 kJ/mol was larger than 27 kJ/mol for only bound water, and the relaxation strength was about five times as large as expected for bound water. The shape of the relaxation curve was very broad and asymmetric. These characteristic differences arising from the hinge-bending motion of IgG disappeared for fragments decomposed from IgG hydrolyzed by papain, since the hinge-bending motion did not exist in this case. We have separated the relaxation processes due to hinge-bending motion and bound water for IgG and obtained the Fab-Fab angle of IgG as about 130 degrees by Kirkwood's correlation parameter and the activation energy of 34 kJ/mol for hinge-bending motion.