The biomolecule of 5-bromocytosine: FT-IR and FT-Raman spectra and DFT calculations. Identification of the tautomers in the isolated state and simulation the spectra in the solid state

• The spectra of 5-BrC in the solid and in the isolated state were simulated.
• In the IR Ar matrix spectrum were identified the bands related to the two tautomers.
• The scaled wavenumbers of our tetramer model are in accordance to the experimental.
• The effect of the Br atom is more notorious in 5-BrC than in 5-bromouracil.
• All the cytosine tautomers have relative energies lower than those in uracil.

An accurate assignment of the IR spectrum in Ar matrix of 5-bromocytosine and of the IR and Raman spectra in the solid state was carried out. For this purpose Density functional calculations (DFTs) were performed to clarify wavenumber assignments of the experimental observed bands. The calculated values were scaled using scaling equations and they were compared with IR and Raman experimental data. Good reproduction of the experimental wavenumbers is obtained and the% error is very small in the majority of cases. In the isolated state all the tautomer forms of 5-bromocytosine were determined and optimized. The wavenumbers corresponding to C1 and C2b tautomers were identified and assigned in the IR experimental spectrum reported in Ar matrix. Our study confirms the existence of at least two tautomers, the amino-oxo and the amino-hydroxy in the isolated state.In the solid state the FT-IR and FT-Raman spectra of 5-bromocytosine in the powder form were recorded in the region 400–4000 cm−1 and 50–3500 cm−1, respectively. The unit cell found in the crystal was simulated as a tetramer form in three tautomers. Thus, it has been possible to assign all the 33 normal modes of vibration. The study indicates that the features, that are the characteristic of the vibrational spectra of cytosine, are retained by the spectra of 5-bromocytosine and it exists in the solid phase in the amino-oxo form.

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