Aza analogs of nucleic acid bases: infrared and Raman spectra of 5-azauracil and crystal structure of 5-azauracil monohydrate

Abstract

The X-ray crystal structure of 5-azauracil monohydrate (C₃H₃O₂N₃·H₂O, M_r=131.10 Da) has been determined from X-ray diffraction data. In the crystal structure all atoms of the 5-azauracil molecule and the solvated water, including hydrogens, lie exactly in the mirror plane perpendicular to b at y=1/4 in the monoclinic space group P2₁/m and therefore exhibit symmetry m (C_s). The crystal structure comprises solvate-promoted hydrogen-bonded layers in the ac plane, separated by b/2=3.1054 Å. The X-ray structure of 5-azauracil monohydrate is compared with the previously reported data for uracil, 6-azauracil and a complex of 5-azauracil with its own hydrolysis product. The presence of bands due to CO stretching vibrations in the infrared and Raman spectra confirms that 5-azauracil exists in its dioxo tautomeric form in both the anhydrous and hydrated crystal forms. The position of a strong Raman band due to the ring breathing vibration is found to be a marker for the state of hydration of 5-azauracil; it appears at 804 cm^-1 in the solid state Raman spectrum of 5-azauracil and shifts upwards in wavenumber to 816 cm^-1 in the spectrum of 5-azauracil monohydrate.