An experimental and theoretical study of the tautomeric composition of adenine (Ade) in water using Raman spectroscopy is reported. Experimental resonance Raman spectra of adenine at excitation wavelengths of 200, 218, and 266 nm were compared with quantum-mechanical calculations of N(9)H- and N(7)H-adenine tautomers and their cations. Both theoretical and experimental studies of nonresonance Raman spectra (457 nm excitation) of adenine were also performed for comparison. A satisfactory agreement of the calculated results with the experimental data was obtained. The Raman spectra are interpreted, and the basic regularities of the Raman intensity distribution are explained. On the basis of the analysis performed, the tautomeric composition of adenine in water is revealed. It is shown that the Ade-N(9),N(1)H(+) cation is the predominant form and that some neutral forms of Ade-N(9)H and Ade-N(7)H tautomers exist in water at pH 3.
543.42:547.751 Vibrational spectra of indole in the isolated state and in aqueous solution have been calculated using a B3LYP/6-311++G(d, p) method. The influence of water as a solvent on vibrational spectra of indole was examined with due regard for intermolecular interactions (SCRF method) and explicit consideration of the effect of hydrogen bonds (1:1 indole:water complex).
A complex experimental and theoretical study of the structural composition of cytosine in water was performed. Raman and resonance Raman spectra of cytosine in acidic, neutral, and alkaline water solutions (pH = 3, 7, and 10, respectively) were recorded at excitation wavelengths of 514, 266, 218, and 200 nm. The temperature dependence of the frequencies and intensities of the resonance Raman bands was obtained in the temperature interval of 4-80 °C. To interpret the experimental Raman and resonance Raman spectra and to determine the structural composition of the water solution of cytosine, the spectra of cytosine and its cation, anion, oxoimine, and hydroxoamine forms were calculated at the B3LYP/6-311++G(d,p) level using the SCRF method. The electronic spectra and geometric parameters of the cytosine and its molecular forms in the excited electronic states close to the excitation wavelengths of the resonance Raman spectra were calculated using the DFT method. The cation exists in the acidic solution together with cytosine and its oxoimine and hydroxoamine tautomeric forms. Cytosine with a small amount of anion dominates in the alkaline medium. The structural composition of the water solution of cytosine is confirmed by the results of quantum-mechanical calculations of the intensity distribution in the resonance Raman spectra.
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