Low-Frequency Spectra of Amino Acids and Short-Chain Peptides Studied by Terahertz Time-Domain Spectroscopy

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 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.