Spontaneous self-association of adenine and uracil in polycrystals from low temperature FTIR spectra in the range below 1000 cm−1

FTIR spectra of solid samples of co-crystallized adenine and uracil were measured at 10 K in the range below 1000 cm−1. New bands ascribable to the N3H (uracil) and NH2 (adenine) out of plane vibrations, which disappear upon D-exchange, were revealed in comparison with the spectra of pure polycrystalline adenine and uracil obtained in the same conditions. The observed changes relate to the same groups that establish the H-bonds in base pairs of naturally occurring nucleic acids, despite the presence of an extra proton donor NH-group in both molecules. The well-established empirical correlation between the out of plane NH vibrational frequencies and H-bond energies was successfully applied for estimation of the latter in the mixed crystal.