Cytosine and cytosine monohydrate are representative biomolecules for investigating the effect of hydrogen bonds in deoxyribonucleic acid. to better understand intermolecular interactions, such as hydrogen bonds, between nucleobases it is necessary to identify the low-frequency vibrational modes associated with intermolecular interactions and crystalline structures. in this study, we investigated the characteristic low-frequency vibrational modes of cytosine and cytosine monohydrate using terahertz time-domain spectroscopy (tHz-tDS). the crystal geometry was obtained by the powder X-ray diffraction technique. The optimized atomic positions and the normal modes in the terahertz region were calculated using density functional theory (Dft), which agreed well with the experimental results. We found that overall terahertz absorption peaks of cytosine and cytosine monohydrate consist of collective vibrations mixed with intermolecular and intramolecular vibrations in mode character analysis, and that the most intense peaks of both samples involve remarkable intermolecular translational vibration. these results indicate that tHz-tDS combined with Dft calculations including mode character analysis can be an effective method for understanding how water molecules contribute to the characteristics of the low-frequency vibrational modes by intermolecular vibrations with hydrogen bonding in biological and biomedical applications.

The resonant peaks of biomolecules provide information on the molecules’ physical and chemical properties. Although many biomolecules have resonant peaks in the terahertz region, it is difficult to observe their specific signals in aqueous environments. Hence, this paper proposes a method for determining these peaks. We found the specific resonant peaks of a modified nucleoside, 5-methlycytidine and modified HEK293T DNA in an aqueous solution through baseline correction. We evaluated the consistency of various fitting functions used for determining the peaks with various parameters. We separated two resonance peaks of 5-methlycytidine at 1.59 and 1.97 THz and for artificially methylated HEK293T DNA at 1.64 and 2.0 THz.