Masahiro Nakanishi

Dynamics of hydration water in sugars and peptides solutions.

We analyzed solute and solvent dynamics of sugars and peptides aqueous solutions using extended depolarized light scattering (EDLS) and broadband dielectric spectroscopies (BDS). Spectra measured with both techniques reveal the same mechanism of rotational diffusion of peptides molecules. In the case of sugars, this solute reorientational relaxation can be isolated by EDLS measurements, whereas its contribution to the dielectric spectra is almost negligible. In the presented analysis, we characterize the hydration water in terms of hydration number and retardation ratio ξ between relaxation times of hydration and bulk water. Both techniques provide similar estimates of ξ. The retardation imposed on the hydration water by sugars is ~3.3 ± 1.3 and involves only water molecules hydrogen-bonded (HB) to solutes (~3 water molecules per sugar OH-group). In contrast, polar peptides cause longer range perturbations beyond the first hydration shell, and ξ between 2.8 and 8, increasing with the number of chemical groups engaged in HB formation. We demonstrate that chemical heterogeneity and specific HB interactions play a crucial role in hydration dynamics around polar solutes. The obtained results help to disentangle the role of excluded volume and enthalpic contributions in dynamics of hydration water at the interface with biological molecules.

New sample cell configuration for wide-frequency dielectric spectroscopy: DC to radio frequencies

A new configuration for the sample cell to be used in broadband dielectric spectroscopy is presented. A coaxial structure with a parallel plate capacitor (outward parallel plate cell: OPPC) has made it possible to extend the frequency range significantly in comparison with the frequency range of the conventional configuration. In the proposed configuration, stray inductance is significantly decreased; consequently, the upper bound of the frequency range is improved by two orders of magnitude from the upper limit of conventional parallel plate capacitor (1 MHz). Furthermore, the value of capacitance is kept high by using a parallel plate configuration. Therefore, the precision of the capacitance measurement in the lower frequency range remains sufficiently high. Finally, OPPC can cover a wide frequency range (100 Hz-1 GHz) with an appropriate admittance measuring apparatus such as an impedance or network analyzer. The OPPC and the conventional dielectric cell are compared by examining the frequency dependence of the complex permittivity for several polar liquids and polymeric films.

Effect of Temperature and Hydration Level on Purple Membrane Dynamics Studied Using Broadband Dielectric Spectroscopy from Sub-GHz to THz Regions

To investigate the effects of temperature and hydration on the dynamics of purple membrane (PM), we measured the broadband complex dielectric spectra from 0.5 GHz to 2.3 THz using a vector network analyzer and terahertz time-domain spectroscopy from 233 to 293 K. In the lower temperature region down to 83 K, the complex dielectric spectra in the THz region were also obtained. The complex dielectric spectra were analyzed through curve fitting using several model functions. We found that the hydrated states of one relaxational mode, which was assigned as the coupled motion of water molecules with the PM surface, began to overlap with the THz region at approximately 230 K. On the other hand, the relaxational mode was not observed for the dehydrated state. On the basis of this result, we conclude that the protein-dynamical-transition-like behavior in the THz region is due to the onset of the overlap of the relaxational mode with the THz region. Temperature hysteresis was observed in the dielectric spectrum at 263 K when the hydration level was high. It is suggested that the hydration water behaves similarly to supercooled liquid at that temperature. The third hydration layer may be partly formed to observe such a phenomenon. We also found that the relaxation time is slower than that of a globular protein, lysozyme, and the microscopic environment in the vicinity of the PM surface is suggested to be more heterogeneous than lysozyme. It is proposed that the spectral overlap of the relaxational mode and the low-frequency vibrational mode is necessary for the large conformational change of protein.

Graphical Conversion between Compliance and Modulus, Permittivity and Electric Modulus, and Impedance and Admittance

Spectrometries probing relaxation and retardation phenomena, such as dielectric, mechanical, and impedance spectroscopies, often require the analyses with both susceptibilities spectra and its reciprocals (e.g., complex permittivity and electric modulus, mechanical compliance and mechanical modulus, and impedance and admittance). In the present paper, the geometric relation between and is derived and the procedure to convert into on a Cole-Cole diagram is proposed. This method helps us to relate them intuitively and yields clearer understanding on their interrelations. Moreover, it opens the new route for the geometric approach to derive many mathematical properties of spectra. The relation between peak position of spectrum and that of spectrum and the shape of spectra are discussed on the basis of this method.