Abstract-Coexistence of Kosmotropic and Chaotropic Impacts of Urea on Water As Revealed by Terahertz Spectroscopy

Keiichiro Shiraga , Yuichi Ogawa, Koichiro Tanaka, Takashi Arikawa, Naotaka Yoshikawa, Masahito Nakamura, Katsuhiro Ajito, Takuro Tajima,

http://pubs.acs.org/doi/10.1021/acs.jpcb.7b11839

Whether urea can serve as a kosmotrope or chaotrope has long been a topic of debate. In this study, broad-band THz spectroscopy (0.2–12 THz) of aqueous solutions of urea was used to characterize the hydration state and the hydrogen bond structure of water around urea. Three low-frequency vibration modes of urea were found around 2, 4, and above 12 THz. After eliminating the contribution of these modes, the “urea-vibration-free” complex dielectric constant was decomposed into the relaxation modes of bulk water and the oscillation modes of water. When hydration water is defined to be reorientationally retarded relative to bulk, our analysis revealed that the hydration number is 1.9 independent of urea concentrations up to 5 M, and this number is in close agreement with that of water constrained by strong acceptor hydrogen bonds of urea oxygen. Regarding the hydrogen bond structure, it was found that the tetrahedral-like water structure is mostly preserved (though the hydrogen bond lifetime is significantly shortened) but the population of non-hydrogen-bonded water molecules fragmented from the network is markedly increased, presumably due to urea’s NH2 inversion. These experimental results point to the coexistence of apparently two contradictory aspects of urea: dynamical retardation (the kosmotropic aspect) by the −CO group and slight structural disturbance (the chaotropic aspect) by the −NH2 group.

Abstract-Coexistence of Kosmotropic and Chaotropic Impacts of Urea on Water as Revealed by Terahertz Spectroscopy

Keiichiro Shiraga, Koichiro Tanaka, Yuichi Ogawa, Takuro Tajima

https://www.researchgate.net/publication/322148885_Coexistence_of_Kosmotropic_and_Chaotropic_Impacts_of_Urea_on_Water_as_Revealed_by_Terahertz_Spectroscopy

Whether urea can serve as a kosmotrope or chaotrope has long been a topic of debate. In this study, broadband THz spectroscopy (from 0.2 to 12 THz) of aqueous solutions of urea was used to characterize the hydration state and the hydrogen bond structure of water around urea. Three low-frequency vibration modes of urea were found around 2 THz, 4 THz, and above 12 THz. After eliminating the contribution of these modes, the “urea-vibration-free” complex dielectric constant was decomposed into the relaxation modes of bulk water and the oscillation modes of water. When hydration water is defined to be reorientationally retarded relative to bulk, our analysis revealed that the hydration number is 1.9 independent of the urea concentrations up to 5 M and this number is close agreement with that of water constrained by the strong acceptor hydrogen bonds of urea oxygen. Regarding the hydrogen bond structure, it was found that the tetrahedral-like water structure is mostly preserved (though the hydrogen-bond lifetime is significantly shortened) but the population of non-hydrogen-bonded water molecules fragmented from the network is markedly increased, presumably due to the urea’s NH2 inversion. These experimental results point to coexistence of apparently two contradictory aspects of urea: dynamical retardation (the kosmotropic aspect) by the –CO group and slight structural disturbance (the chaotropic aspect) by the –NH2 group.

Abstract-Hydration and Hydrogen Bond Network of Water during the Coil-to-Globule Transition in Poly(N-isopropylacrylamide) Aqueous Solution at Cloud Point Temperature

Keiichiro Shiraga , Hirotaka Naito , Tetsuhito Suzuki , Naoshi Kondo , and Yuichi Ogawa

J. Phys. Chem. B, Just Accepted Manuscript

DOI: 10.1021/acs.jpcb.5b01021

Publication Date (Web): April 12, 2015

Copyright © 2015 American Chemical Society

http://pubs.acs.org/doi/abs/10.1021/acs.jpcb.5b01021?journalCode=jpcbfk

Aqueous solution of poly(N-isopropylacrylamide), P-NIPAAm, exhibits a noticeable temperature responsive change in molecular conformation at a cloud point temperature (Tcp). As the temperature rises above Tcp, the extended coil-like P-NIPAAm structure changes into a swollen globule-like conformation as hydration levels decrease and hydrophobic interactions. Though water plays an important role in this coil-to-globule transition of P-NIPAAm, the behavior of water molecules and the associated hydrogen-bond (HB) network of the surrounding bulk water are still veiled in uncertainty. In this study, we elucidated changes in the hydration state and the dynamical structure of the water HB network of P-NIPAAm aqueous solutions, during the coil-to-globule transition, by analyzing the complex dielectric constant in the terahertz (THz) region (0.25~12 THz), where bulk water reorientations and intermolecular vibrations of water can be selectively probed. The structural properties of the water HB network were examined in terms of the population of the non-HB water molecules (not directly engaged in the HB network or hydrated to P-NIPAAm), and the tetrahedral coordination of the water molecules engaged in the HB network. We found the hydration number (≈10) below Tcp was decreased to approximately 6.5 as temperature increased, in line with previous studies. The HB network of bulk water becomes more structured as the coil-to-globule phase transition takes place, via decreases in non-HB water and reduction in the orderliness of the tetrahedral HB architecture. Together these results indicate that the coil-to-globule transition is associated with a shift to hydrophobic dominated interactions that drive thermoresponsive structural changes in the surrounding water molecules.

Abstract-Characterization of Dielectric Responses of Human Cancer Cells in the Terahertz Region

Keiichiro Shiraga, 

Yuichi Ogawa, 

Tetsuhito Suzuki, 

Naoshi Kondo, 

Akiyoshi Irisawa, 

Motoki Imamura

http://link.springer.com/article/10.1007%2Fs10762-014-0067-y
Terahertz time-domain attenuated total reflection spectroscopy, in combination with a two-interface model, is used to determine the complex dielectric constants of cultured human cancer cells (DLD-1, HEK293 and HeLa). Picosecond and sub-picosecond water dynamics are dominant in the measured complex dielectric constants of these cells. We demonstrate that dielectric responses below 1.0 THz best characterize the particular water dynamics of cancer cells when compared with extracellular water. Debye-Lorentz fitting revealed that this is due to a significantly attenuated slow relaxation mode and enhanced fast relaxation mode of the water in these cancer cells. These findings could lead to a new procedure to digitally evaluate cellular activities or functions, in terms of intracellular water dynamics, and remove the veil from the mysterious intracellular milieu.

Abstract-Applicability of effective medium theory to wood density measurements using terahertz time-domain spectroscopy

• Soichi Tanaka, 
• Keiichiro Shiraga, 
• Yuichi Ogawa, 
• Yoshihisa Fujii, 
• Shogo 

• Okumura

http://link.springer.com/article/10.1007%2Fs10086-013-1386-7

The use of an effective medium theory is important when accurately measuring wood density using millimeter and terahertz wave techniques. To confirm the applicability of this theory to the evaluation of wood density, the relative permittivity and dielectric loss of oven-dry flat-sawn specimens of 11 different wood species were measured in a frequency range of 0.15–1.2 THz using a transmission measurement system for terahertz time-domain spectroscopy. A mixture model based on the effective medium theory well explained the density dependence of relative permittivity over the entire frequency range, while it did not fully explain that of dielectric loss, especially for higher frequencies. This indicates that wood scatters the terahertz wave with a wavelength close to the transverse sectional dimensions of the pores in wood in the same way as Mie scattering. It was found from the dielectric loss spectrum of wood substance that the frequency around 0.23 THz was preferable for the nondestructive evaluation of wood.