A comparative study on the experimental and calculated results of mid-infrared and Raman vibrational modes of nucleic acid bases

Abstract The MIR (mid-infrared) and Raman spectra of the nucleic acid base powders were tested by Fourier transform infrared spectroscopy and laser Raman spectroscopy. The vibrational modes of nucleic acid base crystals were simulated by the generalized energy-based fragmentation approach under periodic boundary conditions (hereinafter referred as PBC-GEBF) together with Guassian09 software. Taking into account the intermolecular hydrogen bonds in the unit cell and the weak interactions between the unit cells, the PBC-GEBF method adopted by our group was for the first time used for calculating nucleic acid base crystals in the 400–2000 cm −1 band. The theoretical and experimental spectra turn out to be in good agreement with each other. Compared with the calculated results of the nucleic acid base monomer, the calculated results of crystals appear to be in better agreement with the experimental results. Some weak vibration peaks were reproduced by simulation, and the positions of the vibration peaks agree well. By analyzing the vibrational modes, it can be observed that the vibrational modes below 1000 cm −1 are mainly characterized by the collective vibrations involving all the molecules in the unit cell. In addition to intramolecular interactions, weak intermolecular interactions (including Van der Waals force and weak hydrogen bond) and the lattice stacking force also contribute to the overall vibrations. This study is of great importance to the analysis of the stability of RNA/DNA structures and their genetic properties.