Abstract-A Quantitative Interpretation for the Difference of Terahertz Spectra of dl- and l-Alanine: Origins of Infrared Intensities in Terahertz Spectroscopy

 

Feng Zhang, Keisuke Tominaga, Michitoshi Hayashi,  Masahiko Tani, 

https://pubs.acs.org/doi/10.1021/acs.jpcc.1c02854?goto=supporting-info

The absorption spectra of l-, d-, and dl-alanine in the low-frequency region were measured by terahertz (THz) time-domain spectroscopy ( Appl. Phys. Lett. 2005, 86, 053903, DOI: 10.1063/1.1857080). It has been observed that several absorption bands have a significant difference between the enantiomers (l- and d-alanine) and the racemic compound (dl-alanine) in their peak frequencies. In this work, we calculate the THz spectra of dl- and l-alanine by solid-state density functional theory to quantitatively interpret the spectral difference. The two systems give rise to similar distributions of normal modes in the 5–80 cm–1 frequency range; however, the THz spectra of both crystals are different. During our work, we found two critical aspects regarding the intensities of the THz bands. One aspect, as is known for a molecular crystal, is that the cancellation of the transition dipole moments in the unit cell: Even if an individual molecule in a unit cell has a nonzero transition dipole moment, the total transition dipole moment of the unit cell is zero due to the cancellation of the contributions from the molecules. The second aspect is that these THz normal modes of dl- and l-alanine are dominated by the intermolecular translations, which are intrinsically infrared (IR)-inactive if the molecules are not polarized. Our analysis shows that the IR intensities of these “almost-pure” intermolecular translations are due to a subtle balance between the IR activities induced by the polarization effect of translations and the IR activities of the librations and intramolecular vibrations, which have marginal presences in these normal modes.

Abstract-High-Resolution THz Spectroscopy and Solid-State Density Functional Theory Calculations of Polycyclic Aromatic Hydrocarbons

Feng Zhang, Houng-Wei Wang, Keisuke Tominaga, Michitoshi Hayashi, Tetsuo Sasaki

https://link.springer.com/article/10.1007%2Fs10762-019-00621-0

High-resolution and broadband THz spectra of the crystals of nine polycyclic aromatic hydrocarbons (PAHs) are presented. Five PAHs are comprised of ortho-fused benzene rings and the other four of peri-fused benzene rings. THz mode assignment is performed by using the anthracene and pyrene crystals as examples. The performance of the PBE functional augmented by Grimme’s two dispersion correction terms, D* and D3, respectively, are rigorously evaluated against the experimental criteria of frequency and isotope shift (IS). The D* and D3 terms use empirical and semi-classical approach for correcting the London-type dispersion interactions, respectively. The nature of each THz mode simulated by PBE-D* and that by PBE-D3 is quantitatively compared in terms of the percentage contributions of the intermolecular and the intramolecular vibrations to the vibrational energy. We find that the two methods have equivalent performance in reproducing the frequencies, ISs, and nature of THz modes of both the anthracene and pyrene crystals.

Abstract-Temperature Dependent Conductivity of Graphene Oxide and Graphene Oxide-Polyaniline Nanocomposites Studied by Terahertz Time-Domain Spectroscopy

Partha Dutta, Jessica Afalla, Arnab Halder, Sudeshna Datta, and Keisuke Tominaga

http://pubs.acs.org/doi/abs/10.1021/acs.jpcc.6b10412

In this work, we have studied the temperature-dependent conductivity of graphene oxide (GO) and graphene oxide-polyaniline (GO-PANI) nanocomposites by terahertz time-domain spectroscopy (THz-TDS) from 78 K to 293 K. The refractive index and absorption coefficient are related to the conductivity, and it has been found that in the THz region, the real part of the complex conductivity of GO is less than that of GO-PANI over the entire range of experimental temperatures. Both the systems exhibit an increase in these physical parameters with increasing temperature. The complex conductivity spectra of these systems in the THz region are well fitted by an analytical model that has contributions from the scattering of free electrons (Drude-Smith term) and from bound-electron oscillation (Lorentz term). The fitting analysis suggests a more ordered structure and anharmonicity for both the systems with increasing temperature and reports that GO-PANI has a greater free electron density and damping frequency of oscillation and more ordered structures than GO at all temperatures.

Abstract-Elucidation of Chiral-symmetry Breaking in a Racemic Polymer System with Terahertz Vibrational Spectroscopy and Crystal Orbital Density Functional Theory

Feng Zhang, Houng-Wei Wang, Keisuke Tominaga, Michitoshi Hayashi, Sunglin Lee, and Takashi Nishino

J. Phys. Chem. Lett., Just Accepted Manuscript

DOI: 10.1021/acs.jpclett.6b02213

Publication Date (Web): November 1, 2016

Copyright © 2016 American Chemical Society

http://pubs.acs.org/doi/abs/10.1021/acs.jpclett.6b02213

The conservation of chiral symmetry has been used as a fundamental rule to determine polymer packing-conformations in racemic systems. We have illustrated, through the interplay of polarization THz spectroscopy and solid-state density functional theory, that the chiral symmetry is not conserved in a poly(lactic acid) stereocomplex (scPLA) system. poly(L-lactic acid) (PLLA) displays a weaker violation of the 31 screw symmetry than poly(D-lactic acid) (PDLA), and possesses a stronger intramolecular vibrational energy, on average, in the low-frequency gamma-point phonon modes than does PDLA. Polarization THz spectroscopy adds a new dimension to polymer crystallography through which new phenomena are expected to be revealed.

Abstract-Broadband Dielectric Spectroscopy on Lysozyme in the Sub-Gigahertz to Terahertz Frequency Regions: Effects of Hydration and Thermal Excitation

Naoki Yamamoto†, Kaoru Ohta‡, Atsuo Tamura†, and Keisuke Tominaga*†‡

†Graduate School of Science and ‡Molecular Photoscience Research Center, Kobe University, Rokkodai-cho 1-1, Nada, Kobe 657-8501, Japan

J. Phys. Chem. B, Article ASAP

DOI: 10.1021/acs.jpcb.6b01491

Publication Date (Web): May 09, 2016

Copyright © 2016 American Chemical Society

*E-mail: tominaga@kobe-u.ac.jp. Tel./ Fax: +81-78-803-5684.

http://pubs.acs.org/doi/abs/10.1021/acs.jpcb.6b01491

We have performed dielectric spectral measurements of lysozyme in a solid state to understand the effects of hydration and thermal excitation on the low-frequency dynamics of protein. Dielectric measurements were performed under changing hydration conditions at room temperature in the frequency region of 0.5 GHz to 1.8 THz. We also studied the temperature dependence (83 to 293 K) of the complex dielectric spectra in the THz frequency region (0.3 THz to 1.8 THz). Spectral analyses were performed using model functions for the complex dielectric constant. To reproduce the spectra, we found that two relaxational modes and two underdamped modes are necessary together with an ionic conductivity term in the model function. At room temperature, the two relaxational modes have relaxation times of ∼20 ps and ∼100 ps. The faster component has a major spectral intensity and is suggested to be due to coupled water–protein motion. The two underdamped modes are necessary to reproduce the temperature dependence of the spectra in the THz region satisfactorily. The protein dynamical transition is a well-known behavior in the neutron-scattering experiment for proteins, where the atomic mean-square displacement shows a sudden change in the temperature dependence at approximately 200 K, when the samples are hydrated. A similar behavior has also been observed in the temperature dependence of the absorption spectra of protein in the THz frequency region. From our broadband dielectric spectroscopic measurements, we conclude that the increase in the spectral intensities in the THz region at approximately 200 K is due to a spectral blue-shift of the fast relaxational mode.

Abstract-Broadband Dielectric Spectroscopy on Lysozyme in the Sub-Gigahertz to Terahertz Frequency Regions: Effects of Hydration and Thermal Excitation

Naoki Yamamoto, Kaoru Ohta, Atsuo Tamura, and Keisuke Tominaga

J. Phys. Chem. B, Just Accepted Manuscript

DOI: 10.1021/acs.jpcb.6b01491

Publication Date (Web): May 9, 2016

Copyright © 2016 American Chemical Society

http://pubs.acs.org/doi/abs/10.1021/acs.jpcb.6b01491?journalCode=jpcbfk

Abstract

We have performed dielectric spectral measurements of lysozyme in a solid state to understand the effects of hydration and thermal excitation on the low-frequency dynamics of protein. Dielectric measurements were performed under changing hydration conditions at room temperature in the frequency region of 0.5 GHz to 1.8 THz. We also studied the temperature dependence (83 K to 293 K) of the complex dielectric spectra in the THz frequency region (0.3 THz to 1.8 THz). Spectral analyses were performed using model functions for the complex dielectric constant. To reproduce the spectra, we found that two relaxational modes and two underdamped modes are necessary together with an ionic conductivity term in the model function. At room temperature, the two relaxational modes have relaxation times of ~20 ps and ~100 ps. The faster component has a major spectral intensity and is suggested to be due to coupled water-protein motion. The two underdamped modes are necessary to reproduce the temperature dependence of the spectra in the THz region satisfactorily. The protein dynamical transition is a well-known behavior in the neutron scattering experiment for proteins, where the atomic mean-square displacement shows a sudden change in the temperature dependence at approximately 200 K, when the samples are hydrated. A similar behavior has also been observed in the temperature dependence of the absorption spectra of protein in the THz frequency region. From our broadband dielectric spectroscopic measurements, we conclude that the increase in the spectral intensities in the THz region at approximately 200 K is due to a spectral blue-shift of the fast relaxational mode.

Abstract-Temperature and Hydration Dependence of Low-Frequency Spectra of Lipid Bilayers Studied by Terahertz Time-Domain Spectroscopy

Naoki Yamamoto , Tomoyo Andachi , Atsuo Tamura , and Keisuke Tominaga

J. Phys. Chem. B, Just Accepted Manuscript

DOI: 10.1021/jp5099766

Publication Date (Web): December 4, 2014

Copyright © 2014 American Chemical Society

http://pubs.acs.org/doi/abs/10.1021/jp5099766


We have studied temperature and hydration dependent low-frequency spectra of lipid bilayers of 1,2-dimyristoyl-sn-glycero-3-phosphoryl-3’-rac-glycerol (DMPG) and 1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC) by terahertz time-domain spectroscopy (THz-TDS). We measured X-ray diffraction patterns and mid-infrared spectra of these lipid bilayers and found that the lipid bilayer have two different types of phases, i.e. the gel phase and the crystalline phase, depending on preparation methods of the samples. In both the phases a few distinct bands were observed in the THz region. For DMPG the peak wavenumbers of the absorption bands did not change upon hydration, while the bandwidth in the crystalline phase was smaller than that in the gel phase. We performed spectral analyses for the complex dielectric spectra for DMPG and DMPC with a model function, mainly to determine the peak wavenumbers of the absorption bands. In contrast to the case of the DMPG bilayers the peak wavenumber of the absorption band of the DMPC bilayer shifts upon hydration. In the hydrated DMPC bilayer it was suggested fast reorienting water molecules exist with a relaxation time of sub-picoseconds. It is suggested that the THz absorption patterns reflect the lipid packing pattern in the bilayers. The temperature dependence of the absorption band was analyzed by an empirical equation, and the anharmonicity of the vibrational potential of the low-frequency mode was quantitatively evaluated.

Abstract-Low-frequency Spectra of a Phospholipid Bilayer Studied by Terahertz Time-domain Spectroscopy

• Tomoyo Andachi, 
• Naoki Yamamoto, 
• Atsuo Tamura, 
• Keisuke Tominaga

http://link.springer.com/article/10.1007%2Fs10762-013-0003-6

We have investigated the low-frequency spectra of a phospholipid bilayer composed of 1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC) by terahertz time-domain spectroscopy (THz-TDS). We focused on the temperature and hydration dependence of the low-frequency spectra of a gel-phase sample. The spectra of the dehydrated and hydrated samples showed shoulder bands at 45 and 30 cm^-1, respectively. In contrast to the dehydrated sample, in the hydrated sample spectra the slope of the temperature change of the absorption coefficient increased sharply around 240 K. This result suggests that water molecules affect the change in the low-frequency dynamics. We obtained the absorption coefficient difference spectra for different hydration levels to clarify the mechanism of the spectral change.