Spectral features of peptides in the terahertz region

The complex refractive index and the absorption coefficient of polycrystalline L-alanyl-L-alanine have been measured in the frequency range from 5 to 50 cm^-1 by the terahertz time domain spectroscopy. One band at 42 cm^-1 is observed in the absorption spectrum at room temperature. This band is broader than those of polycrystalline L-alanine at room temperature. It suggests that the band width of L-alanyl-L-alanine undergoes larger inhomogeneous broadening than those of L-alanine. The absorption continuum of L-alanyl-L-alanine, which increases with frequency, is found larger than that of L-alanine. The absorption feature can be explained by considering larger unit cell for L-alanyl-L-alanine than that for L-alanine and crystalline disorder.