Angle-Dependent Terahertz Time-Domain Spectroscopy of Amino Acid Single Crystals

Abstract

The measurement of absorption spectra using angle-dependent terahertz (THz) time-domain spectroscopy for amino acid single crystals of l-cysteine and l-histidine is reported for the first time. Linearly polarized THz radiation enables us to observe angle-dependent far-infrared absorption spectra of amino acid single crystals and determine the direction of the oscillating dipole of the molecules in the 20-100 cm(-1) range. By comparing the THz spectra of a single crystal and powder, we found that there was a clear hydrogen-bond peak in the crystal spectrum as a result of the larger hydrogen-bond network. The low-temperature THz spectra of amino acid microcrystals showed more intermolecular vibrational modes than those measured at room temperature. An ab initio frequency calculation of a single amino acid molecule was used to predict the intramolecular vibrational modes. The validity of the calculation models was confirmed by comparing the results with experimentally obtained data in the Raman spectral region.

Terahertz Time-domain Spectra of Aromatic Carboxylic Acids Incorporated in Nano-sized Pores of Mesoporous Silicate

“…We found that the observed THz absorption band resolves into narrower peaks as the temperature is decreased and some of these shift towards higher frequencies. 14,27 Since the behavior of each band is different because of the different nature of the THz absorption, inter-and intramolecular modes can be roughly distinguished by analyzing the temperature-induced shifts of the peaks. For example, the average hydrogen bond strength may decrease with increasing temperature and cause a redshift of the hydrogen bond stretching frequency in the THz region 27 whereas the shifts in the frequency of the intramolecular modes including torsion and a deformation skeleton are rather small with changing temperature.…”

“…The optical set-ups and experimental conditions are described elsewhere. 13 Since heat induced frequency shifts and resonance broadening are generally observed for THz peaks, 10,11,14 we measured the THz-TD spectra at room temperature (298 K), 230, 170 and 90 K to observe the spectral properties in detail. The aromatic carboxylic acids incorporated in SBA-16 were packed in the cavity of a plastic cell.…”

“…The THz-TD absorption spectra of DNA 10 and amino acids [11][12][13][14] have revealed that specific resonance patterns can be identified as the vibrational modes of intermolecular hydrogen bond networks. Recently, we found that incorporating target molecules in nanosized spaces is effective in reducing the influence of the intermolecular modes of the target molecule.…”

Analytical Terahertz Spectroscopy

“…Moreover, the calculations have not agreed sufficiently well with experimental results, even with small molecules, because the theoretical models do not take complete account of the appropriate crystalline conditions or hydrogen bonding effects. 32 THz-TDS employs several types of optical arrangement classified as transmission, reflection, ATR, and polarization arrangements including ellipsometry. The principles have been detailed elsewhere 23 and here we provide example applications of these methods.…”

The effect of the flexibility of hydrogen bonding network on low-frequency motions of amino acids. Evidence from Terahertz spectroscopy and DFT calculations

“…The photon energy of THz radiation can give rise to collective atomic motions of compound. These low-frequency vibrational modes are usually associated with motions of hydrogen-bonding networks which play essential roles on stabilization of biomolecules in solid state [1,2], folding of proteins [3] and solvation of proteins [4] and some other biomolecules [5,6,7,8] in aqueous phase.…”