Low-temperature FTIR spectra and hydrogen bonds in polycrystalline adenosine and uridine

FTIR spectra of polycrystalline samples of adenosine and uridine, pure and containing small (<10%) quantity of N(O)H or N(O)D groups, were measured in KBr pellets from 4000 to 400 cm−1 at temperatures from 300 to 20 K. For the first time, the bands of narrow isotopically decoupled proton stretching vibration ν1 mode of NH and OH groups were found and assigned to ordered hydrogen bonds according to crystal structural data for both nucleosides. The FTIR adenosine spectra in the out-of-plane bending proton ν4 mode range (lower than 1000 cm−1) of N(O)H groups revealed at low temperature at least twice more bands, than in the ν1 range, which are influenced by isotopic exchange and (or) cooling. Almost all of them have their counterparts in the N(O)D substance spectrum with an isotopic frequency ratio of 1.30-1.40. These bands were assigned to the differently H-bound disordered NH and OH protons, which could not be seen with crystal structural methods. The energy and length of different H-bonds were estimated from peak positions of both mode bands (as the red shift of ν1 or blue shift of ν4 relatively free molecules) with well-established empirical correlations between spectral, thermodynamic and structural parameters of hydrogen bonds. The results were compared with independent experimental data.