Abstract: Cholic acid(HC) and deoxycholic acid(HDC) are main components of bile and have an important biological function in human body. In this paper the two polycrystalline molecules were studied via far-IR and THz absorption spectroscopy. Their characteristic bands in far-IR and THz region were observed. Some similar bands appear in the far-IR region for the two molecules because they have similar structures. There are two bands at 1.26 and 2.02(e.g. 42 and 67 cm^-1) in the THz spectrum of cholic acid molecule, and 1.13, 1.26, 1.69 and 2.17 THz(e.g. 38, 42, 56, 72 cm^-1) in the THz spectrum of deoxycholic acid. The results indicate that far-IR and THz results are consistent in some extent. The bands related to COOH were considered in the far-IR and THz region. Omnic 5.0 program was adopted to treat the data and the bands with a low signal-noise ratio were emphasized via this method. And many sub bands were observed, which indicate that complicated hydrogen bond networks exist. The difference in the structures of the two molecules is only one hydroxyl group, but their far-IR and THz absorption spectra have many differences, which may be caused by the differences of their molecule structures, for example, different hydrogen bonding networks and the various packing of CH chain, etc.. The results indicate that far-IR and THz absorption spectra are effective methods for investigating biomolecules and distinguishing their structures.

Key words: Cholic acid, Deoxycholic acid, Far-IR absorption spectrum, THz absorption spectrum, Hydrogen bonding