Infrared absorption spectra of DNA and RNA as dry, lyophilized powders in KBr pellets, as hydrated KBr pellets, and in solution phase are reported. We find the symmetric and antisymmetric stretching vibrations of the phosphate linkage, νs(PO2-) and νas(PO2-), to be very sensitive to hydration, as intensity changes as well as frequency shifts are observed. An increase in water content causes DNA to undergo a structural transition form A-form to B-form. This observation leads us to believe that the frequency shifts are associated mostly with the conformational change, whereas the increase in intensities may be due to an increase in the local dielectric in the vicinity of the polar and solvent exposed phosphate groups. A similar effect was reported recently for the amide groups of proteins. The structure of RNA is less sensitive to hydration, and only the phophodiester stretching vibrations are affected by hydration. The solution transmission and ATR spectra of DNA and RNA solution are also compared and found to be identical to one another in the 950-1300 cm-1 frequency region. Furthermore, solution spectra resemble the fully hydrated solid-phase spectra.
Alex Pevsner and Max Diem, "Infrared Spectroscopic Studies of Major Cellular Components. Part II: The Effect of Hydration on the Spectra of Nucleic Acids," Appl. Spectrosc. 55, 1502-1505 (2001)