Assignment of adenine ring in‐plane vibrations in adenosine on the basis of 15N and 13C isotopic frequency shifts and TUV resonance Raman enhancement

Infrared, Raman and UV resonance Raman spectra of adenosine and its 1,3-15N2, 2–13C, and 8–13C isotopic analogues were measured in neutral aqueous solution (Raman and UV Raman) and in the crystalline state (infrared and Raman). The observed isotopic wavenumber shifts are useful in distinguishing adenine ring vibrations from ribose vibrations. In-plane modes of the adenine ring are selectively enhanced in UV resonance Raman spectra, which facilitates the assignment of the in-plane vibrations. In addition to the in-plane modes, a ribose vibration coupled with adenine in-plane vibrations was identified in the UV resonance Raman spectra. The fundamental wavenumbers for 22 in-plane normal modes of the 9-substituted adenine ring of adenosine in the 1700–250 cm−1 region are proposed. Although the fundamental wavenumbers of adenosine correspond well with those of adenine above 1350 cm−l and below 800 cm−1 the vibrations in the 1350–800 cm−1, region are appreciably affected by the presence of the N-9—C-1′ glycosidic bond and the couplings between ribose and adenine ring vibrational motions. The adenosine fundamental wavenumbers and their isotopic shifts reported here may be useful in analysing vibrational spectra of adenine nucleosides and nucleotides and in improving the force field of the 9-substituted adenine ring.