Far-infrared spectra of amino acids

The far-infrared absorption of polycrystalline samples of 18 amino acids has been measured in the spectral range from 10 to 650 cm−1. The assignment of the observed bands is based on comparing the spectra of different amino acids and normal modes calculations present in literature. The absorption peaks can be grouped in five regions that are characteristic for all the specimens. In the region 480-650 cm−1 COO− rocking/wagging/bending modes are observed. NH3+ torsion is dominant in the range 380-480 cm−1. Between 270 and 380 cm−1 the CCαN deformation mode dominates, followed by a region between 220 and 270 cm−1 specific for COO− vibrations. Below 220 cm−1 a one-to-one correspondence of absorption frequencies to certain modes (COO− torsion, NCαC deformation, NH3+ torsion, and omnipresent H bonds) is no more possible.