Terahertz Spectroscopy of Biomolecules in Nanospace
Terahertz (THz) wave region around 0.3 - 10 terahertz has attracted much attention in spectroscopy since it can reveal weak chemical bonds in molecules. We have recently shown that THz time-domain spectroscopy (THz-TDS) using THz wave pulses exhibits sufficient sensitivity to molecules to be used as a tool in analytical chemistry [1, 2].
Katsuhiro Ajito1, Yuko Ueno1,2, Rakchanok Rungsawang1, and Isao Tomita1,3
1Materials Science Laboratory,
2NTT Microsystem Integration Laboratories, 3NTT Photonics Laboratories
Figure 1 shows angle-dependent THz-TD absorption spectra of cystein single crystal measured by using linearly polarized THz wave. An ab initio frequency calculation of a single amino acid molecule was used to predict the intramolecular vibrational modes. These provide high absorption when molecules are probed with a THz electric field whose polarization is parallel to the oscillating dipole directions .
Figure 2 shows THz-TD absorption spectra of two steric isomers, namely fumaric acid and maleic acid molecules in nanosized pores of a mesoporus silicate and their crystals in polyethylene pellets. The intermolecular modes observed with fumaric acid crystals are inactivated by incorporating the molecules in the nano-sized pores due to the separation of the molecules, whereas the intramolecular modes of maleic acid for the crystal and the incorporated samples are similar . Incorporating target molecules in nano-sized spaces is effective in reducing the concentration of the intermolecular hydrogen bonds of the sample, and it is possible to distinguish intramolecular hydrogen-bonding vibrational modes from intermolecular hydrogen-bonding vibrational modes.
We believe that THz spectroscopy will open a new filed in biological analysis in nanospace.
 K. Ajito, R. Rungsawang, I. Tomita, and Y. Ueno, Electrochemistry 74 (2006) 506.
 Y. Ueno, R. Rungsawang, I. Tomita, and K. Ajito, Anal. Chem. 78 (2006) 5424.
 R. Rungsawang, Y. Ueno, I. Tomita, and K. Ajito, J. Phys. Chem. B, 110 (2006) 21259 .
 Y. Ueno, R. Rungsawang, I. Tomita, and K. Ajito, Chem. Lett. 35 (2006) 1128.
Fig. 1. Angle-dependent THz-TD absorption spectra of cystein single crystal at room temperature. 0゜is defined when the c-axis of the crystal is parallel to the polarization of the THz field.
Fig. 2. THz-TD absorption spectra of fumaric acid and mareic acid molecules (upper and lower) in nanosized pores of a mesoporus silicate (solid lines) and their crystals in polyethylene pellets (dotted lines) at 77K.
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