Low-Frequency Spectra of Amino Acids and Short-Chain Peptides Studied by Terahertz Time-Domain Spectroscopy
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Low-Frequency Spectra of Amino Acids and Short-Chain Peptides Studied by Terahertz Time-Domain Spectroscopy Carlito S. Ponseca Jr. & Ohki Kambara & Shintaro Kawaguchi & Kohji Yamamoto & Keisuke Tominaga
Received: 22 February 2010 / Accepted: 12 March 2010 / Published online: 28 March 2010 # Springer Science+Business Media, LLC 2010
Abstract The low-frequency spectra of the amino acids L-alanine and glycine and their peptides were studied using terahertz (THz) time-domain spectroscopy (TDS) at room temperature. In a previous work (Yamamoto et al., Biophys. J. 89, L22–L24 (2005)), the low-frequency spectra of amino acids (glycine and L-alanine) and their polypeptides (polyglycine and poly- L-alanine) were studied by THz-TDS, and it was found that there is a clear difference in low-frequency dynamics between the amino acids and the polypeptides. In the present study, amino acids and short peptides were chosen in order to investigate the effect of polymerization on low-frequency spectra. We focus on two physical quantities to represent the spectral features: (1) the intensity of the reduced absorption cross section (RACS), which we define from the absorption coefficient and refractive index, and (2) the exponent in the power law behavior of the RACS. We found that the two physical quantities show different dependences on peptide chain length, suggesting that the two physical quantities reflect different dynamics and interactions. The change in RACS intensity may be due to intermolecular or intrachain motion. The validity
C. S. Ponseca Jr. : O. Kambara : K. Tominaga (*) Molecular Photoscience Research Center, Kobe University, Nada-ku, Kobe, Hyogo 657-8501, Japan e-mail: [email protected] C. S. Ponseca Jr. e-mail: [email protected] O. Kambara e-mail: [email protected] S. Kawaguchi : K. Tominaga Graduate School of Science, Kobe University, Nada-ku, Kobe, Hyogo 657-8501, Japan K. Yamamoto Research Center for Development of Far-Infrared Region, Fukui University, 3-9-1 Bunkyo, Fukui 910-8507, Japan e-mail: [email protected]
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J Infrared Milli Terahz Waves (2010) 31:799–809
of the assumption of constant IR activity in the investigated frequency region is critical to understanding the origin of the variation in the exponent with chain length. Keywords Terahertz . Vibrational modes
1 Introduction The terahertz (THz) region, also referred to as the far-infrared (FIR) regime, spans frequencies from about 100 GHz to 10 THz, which are important for understanding the low-frequency dynamics of molecular systems related to intermolecular vibrations and large-amplitude intramolecular modes [1]. To date, extensive spectroscopic studies in this region have been carried out, and THz spectroscopy has been use in various fields of research, demonstrating that this method is complementary to other spectroscopic techniques. Specifically, THz spectroscopy has been shown to be a powerful technique for studying low-frequency spectra of biologically important molecules such as amino acids, polypeptides, and p
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