Terahertz time-domain spectroscopy (THz-TDS) was used to measure the far-infrared absorption spectra of some important biological materials such as Bacillus subtilis spores and dipicolinic acid in the frequency region of 0.2 to 2.0 THz (6.7-67 cm-1). A distinct absorption peak at 1.538 THz was observed in the B. subtilis spores. A dominate absorption mode at 1.54 THz was also observed in DPA powder.
The low-frequency torsional modes of tryptophan were measured by terahertz time-domain spectroscopy (THz-TDS). It was found that there are two dominated torsional vibrational modes at around 1.435 and 1.842 THz. The origin of the observed torsional vibrations are assigned to the chain and ring of the tryptophan molecule.