Analysis of vibrational spectra of solid-state adenine and adenosine in the terahertz region

The low-frequency vibrations of crystalline adenine and adenosine were studied by 5 K terahertz (THz) spectroscopy and solid-state density functional theory (DFT) based on a harmonic model. The accuracy of the calculations was assessed from aspects of frequencies and isotope shifts. The anharmonicity of the potential surface was discussed by observing the thermal shift of the sequence bands. An analytical method was proposed for quantifying the contributions of the intermolecular and intramolecular motions to the normal modes. Adenine exhibited weak coupling between the intermolecular and intramolecular vibrations. The adenine and ribose moieties of adenosine, which are linked through a glycosidic bond, showed intense relative vibrations strongly coupled with the intermolecular vibrations. This finding reveals that adenosine has high flexibility, which arises from the glycosidic bond. An experimental method was further proposed to identify the intermolecular or intramolecular modes.