Naoki Shinyashiki

Glass Transitions in Aqueous Solutions of Protein (Bovine Serum Albumin)

Measurements by adiabatic calorimetry of heat capacities and enthalpy relaxation rates of a 20% (w/w) aqueous solution of bovine serum albumin (BSA) by Kawai, Suzuki, and Oguni [Biophys. J. 2006, 90, 3732] have found several enthalpy relaxations at long times indicating different processes undergoing glass transitions. In a quenched sample, one enthalpy relaxation at around 110 K and another over a wide temperature range (120-190 K) were observed. In a sample annealed at 200-240 K after quenching, three separated enthalpy relaxations at 110, 135, and above 180 K were observed. Dynamics of processes probed by adiabatic calorimetric data are limited to long times on the order of 10(3) s. A fuller understanding of the processes can be gained by probing the dynamics over a wider time/frequency range. Toward this goal, we performed broadband dielectric measurements of BSA-water mixtures at various BSA concentrations over a wide frequency range of thirteen decades from 2 mHz to 1.8 GHz at temperatures from 80 to 270 K. Three relevant relaxation processes were detected. For relaxation times equal to 100 s, the three processes are centered approximately at 110, 135, and 200 K, in good agreement with those observed by adiabatic calorimetry. We have made the following interpretation of the molecular origins of the three processes. The fastest relaxation process having relaxation time of 100 or 1000 s at ca. 110 K is due to the secondary relaxation of uncrystallized water (UCW) in the hydration shell. The intermediate relaxation process with 100 s relaxation time at ca. 135 K is due to ice. The slowest relaxation process having relaxation time of 100 s at ca. 200 K is interpreted to originate from local chain conformation fluctuations of protein slaved by water. Experimental evidence supporting these interpretations include the change of temperature dependence of the relaxation time of the UCW at approximately T(gBSA) approximately = 200 K, the glass transition temperature of protein in the hydration shell, similar to that found for the secondary relaxation of water in a mixture of myoglobin in glycerol and water [Swenson et al. J. Phys.: Condens. Matter 2007, 19, 205109; Ngai et al. J. Phys. Chem. B 2008, 112, 3826]. The data all indicate in hydrated BSA or other proteins that the secondary relaxation of water and the conformation fluctuations of the protein in the hydration shell are inseparable or symbiotic processes.

Evaluation of complex permittivity of aqueous solution by time domain reflectometry

Abstract A time domain reflectometry (TDR) method has been developed in order to measure dielectric relaxation process with a weak relaxation strength of the order of 0.1 in aqueous solution. Application of the TDR measurement have been made for poly(L-glutamic acid) in aqueous solution, which exhibits a helix-coil transition with changing the pH value. Dielectric relaxation process observed around 100MHz shows a definite transition in its strength in the vicinity of pH=6. The TDR method has been also applied to a DNA in aqueous solution with 0.1SSC buffer. A double helix structure of DNA melts at about 75°C to a coiled structure. Relaxation process around 100MHz shows a transition in the strength and also in the relaxation time around this temperature. In both cases, relaxation process caused by water molecules could be observed separately from the process observed around 100MHz. The relaxation strength and the relaxation time are nearly the same as those of the free water. A bilinear analysis developed by Cole has been used to measure methanol-water mixtures. A relaxation process could be observed continuously with the composition. It has been concluded that the bilinear analysis is quite powerful if the present TDR method is used together for the dielectric measurement covering a wide frequency region from 1MHz to 15GHz.

Shape of dielectric relaxation curves of ethylene glycol oligomer–water mixtures

Dielectric measurements of water mixtures of ethylene glycol oligomer (EGO) with 1–6 repeat units were carried out in the frequency range of 100 MHz–30 GHz at 25 °C. One relaxation process due to water and EGO was observed for each mixture. If the number of repeat units of EGO is larger than three, the water mixtures show a broad and symmetric relaxation curve. On the other hand, if the number of repeat units of EGO is two or less, the mixtures show a broad and asymmetric relaxation curve. The two types of relaxation curves observed in the EGO–water mixtures reflect the size of the EGO molecule. The asymmetric relaxation curve is due to the cooperative motion of water and EGO molecules in the EGO–water cluster for smaller EGO–water mixtures. In contrast, the symmetric dielectric relaxation curve is a result of the variation of local structure in larger EGO–water mixtures. The larger EGO molecules cannot move cooperatively and behave as a geometrical constraint to the motion of water clusters.

Broadband dielectric study of α–β separation for supercooled glycerol–water mixtures

Broadband dielectric measurements for 60, 65, 70, 80, and 100 wt% glycerol–water mixtures were performed in the frequency range of 10 μHz–30 GHz and in the temperature range of 148–298 K. In the lower temperature range, the separation of the β- and α-processes occurred for all the mixtures. The relaxation strength of the α-process below the separation temperature exhibits a maximum at a mole fraction of water, xw≅0.55, and the relaxation strength of the β-process increases with increasing xw. In our previous study, the concentration dependence of the dielectric behavior of the alcohol–water mixtures at 25 °C was interpreted using a cooperative domain (CD), in which the reorientation of molecules occurred cooperatively. The concentration dependence of the dielectric behavior for the glycerol–water mixtures is interpreted using a CD model.

The symmetric broadening of the water relaxation peak in polymer–water mixtures and its relationship to the hydrophilic and hydrophobic properties of polymers

The dielectric relaxation of water molecules in polymer–water mixtures is discussed. The memory function approach and scaling relationships are used as a basis for the model of symmetric dielectric spectrum broadening. The correspondence between the relaxation time, the geometrical properties, the self-diffusion coefficient, and the Cole–Cole exponent is established. The relationship between the hydrophilic and hydrophobic properties of the polymers and the dielectric relaxation parameters is discussed.

Dielectric study on dynamics of water in polymer matrix using a frequency range 106–1010 Hz

Dielectric measurements were made on polyethylene glycol (PEG) and polyvinyl pyrolidone (PVP)–water systems over a frequency range 106 –1010 Hz by a time domain reflectometry. Two relaxation peaks were observed in the PVP system. The high frequency process is caused by rotational diffusion of water clusters and concentration dependence of the relaxation time is well explained by the free volume theory. The low frequency process is attributed to water molecules bound to the polymer and its relaxation time is reasonably irrespective of the concentration. On the other hand, the PEG system shows a single relaxation process which is caused by the rotational diffusion of water clusters. A sign of the segmental motion was recognized barely for highly concentrated system of PEG.

Relaxation processes of water in the liquid to glassy states of water mixtures studied by broadband dielectric spectroscopy

The relaxation processes of water mixtures of glycerol, ethylene glycol, ethylene glycol oligomers with two to six repeat units, poly(ethylene glycol) 400 and 600, fructose, and propanol have been studied by broadband dielectric spectroscopy at different water contents in the frequency range 10 µHz–20 GHz and in the temperature range 300–80 K without water crystallization. The results show that, in the vicinity of the glass transition temperature of the mixtures, two kinds of water exist. Part of the water behaves as excess water retaining its inherent mobility and appearing as a separate relaxation process (named here the ν-process) at frequencies higher than the structural α-process at subzero temperatures. Another part of the water moves cooperatively with solute molecules and contributes to the α-process.

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.

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.

The dielectric relaxation of supercooled ethyleneglycol-water mixtures

Abstract Broadband dielectric measurements for 60, 70, and 80 wt% ethyleneglycol-water mixtures were performed in the frequency range of 0.1 mHz–30 GHz and in the temperature range of 140.5 K–298.0 K. In the lower temperature range for mixtures of 60 and 70 wt%, separation of the β-process from α-process occurred. The glass transition temperature depended on the water content for the α-process but fragility did not. The peak frequency of the β-process depended on the water content. The β-process exhibiting larger activation energy at the lower frequency was caused by the large cooperative rearranging region constructed by strongly interacting molecules through hydrogen-bond.