Intermolecular weak interactions of crystalline purine and uric acid investigated by terahertz spectroscopy and theoretical calculation

The terahertz (THz) absorption spectra of purine and its derivative, uric acid, have been measured by using terahertz time-domain spectroscopy (THz-TDS) system in the frequency range from 0.3 to 1.6 THz. In parallel with the experimental study, the mechanism of the THz characteristic absorption peaks and the weak interaction type of purine and uric acid were investigated by density functional theory (DFT), potential energy distribution (PED), and energy decomposition analysis based on molecular forcefield (EDA-FF) methods. In particular, the specific vibration information provided by the PED analysis is applied to interpret the absorption peak mechanism of the theoretical spectra predicted by DFT. Moreover, the EDA-FF reveals that the weak interactions both for purine and uric acid are dominated by electrostatic interaction, compared with purine the existence of oxygen atoms in uric acid formed more hydrogen bonds among hydrogen and oxygen atoms. Results demonstrate that THz spectroscopy combined with the above three analysis methods provide a valuable reference for the analysis of molecular non-bonded interaction and biomolecules with similar structures.