The fluorescence excitation and dispersed fluorescence spectra of jet-cooled uracil, thymine, and their derivatives have been observed. Two band systems having well-resolved vibrational structures were found for uracil and thymine in the frequency region from 31 000 to 38 000 cm-1, which corresponds to the region of the long tail in the vapor absorption spectrum. The shorter wavelength band system (system I) was identified as the S1(n,π*) ← S0 transition of the diketo tautomer, while the longer wavelength system (system II) was assigned to the S1(n,π*) ← S0 transition of one of the keto-enol tautomers. Successful detection of a very small amount of the keto-enol tautomer is due to a high fluorescence yield of the keto-enol tautomer. The nature of the S1,(n,π*) states of the tautomers and the hydrogen-bonded complexes with water are also discussed.
[Show abstract][Hide abstract] ABSTRACT: The relative stability orders in the tautomers of uracil and its derivatives (5-fluorouracil, 5-chlorouracil, 5-aminouracil, 5-hydroxyuracil, 5-methyluracil, 6-methyluracil, 5-hydroxy-6-methyluracil and 5-amino-6-methyluracil) were established using composite (G3MP2B3) and DFT (TPSS) methods. The stability orders were determined both in the gas phase and water solutions, taking into account specific and non-specific hydration. The primary solvation shell of uracils was modeled as a complex of a tautomer with 5 water molecules. An analysis of the factors which determine the stability of the enol forms of uracils was performed. The most important factor was found to be changes in the intramolecular conjugation at tautomerization. As was shown by the NBO analysis, the stabilization energy due to the nN → π∗ (or σ∗) interaction in the diketo tautomer is lost in the enol forms, but is partially compensated by an increase in the conjugation length. The effect of the substituent in the fifth position of the pyrimidine ring on the energy of tautomers is less prominent. It was shown that the hydration energy considerably differs for tautomers, and leads to substantial redistribution in the stability series of uracil tautomers. Both specific and non-specific solvation are of vital importance for stabilization of tautomers.
"It was also found that the Ade imino tautomer is more stabilized under the influence of charged platinum (Burda et al., 2000) or mercury (Zamora et al., 1997) cations. It is generally believed that Thy exists in the canonical diketo form in the gas phase as well as in the aqueous solution (Kwiatkowski & Pullman, 1975), but there is experimental evidence of small amounts of its rare tautomeric forms in the gas phase (Fujii et al., 1986; Tsuchiya et al., 1988) and in the solution (Hauswirth & Daniels, 1971; Katritzky & Waring, 1962; Morsy et al., 1999; Samijlenko et al., 2010; Suwaiyan et al., 1995). Also laser ablation in combination with MB-FTMW spectroscopy spectroscopy has been used to establish unambiguously the presence of the diketo form of thymine in the gas phase and to obtain its structure (López et al., 2007). "
[Show abstract][Hide abstract] ABSTRACT: The microwave spectra of the nucleic acid base thymine and of two of its mono-deuteriated species have been analysed indicating that the diketo tautomer is the most abundant form in the gas phase.
Journal of the Chemical Society Chemical Communications 01/1989; 1(1). DOI:10.1039/c39890000037
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