Article

Keto-enol tautomer of uracil and thymine

The Journal of Physical Chemistry 01/1988; 92(7):1760-1765. DOI: 10.1021/j100318a013
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    03/2012; , ISBN: 978-953-51-0372-1
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    ABSTRACT: A comparative analysis on the effect of the water on the molecular structure and spectroscopy of 5-halogenated uracils was carried out. Solvent effects were considered by using a variable number (1–10) of explicit water molecules surrounding the 5-halouracil derivatives in order to simulate the first hydration shell. More than 300 cluster structures with water were analyzed. B3LYP and MP2 quantum chemical methods were used. For cases where literature data are available, the computed values were in good agreement with previous experimental and theoretical studies. Several general conclusions were underlined.
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    ABSTRACT: In this work we investigated the lowest-lying electronic excitations for a series of methyl-substituted uracil derivatives, i.e., uracil, 1-methyluracil, 3-methyluracil, thymine, 1-methylthymine, 1,3-dimethyluracil, 3-methylthymine, 1,3-dimethylthymine, and their microhydrated complexes by means of coupled cluster singles and approximate doubles (CC2) and density functional theory (DFT) methods. The bulk water environment was mimicked by a combination of microhydration and the conductor-like screening model (COSMO). We find that the shift of the electronic excitation energies due to methylation and hydration depend on the character of the wave function and on the position of the methyl substituent. The lowest-lying singlet and triplet n-->pi* states are insensitive to methylation but are strongly blue-shifted by microhydration and bulk water solvation. The largest red-shift of the first (1)(pi-->pi*) excitation occurs upon methylation at N(1) followed by substitution at C(5) whereas no effect is obtained for a methylation at N(3). For this state, the effects of methylation and hydrogen bonding partially cancel. Upon microhydration with six water molecules, the order of the (1)(n-->pi*) and (1)(pi-->pi*) states is reversed in the vertical spectrum. Electrostatic solute-solvent interaction in bulk water leads to a further increase of their energy separation. The n-->pi* states are important intermediates for the triplet formation. Shifting them energetically above the primarily excited (1)(pi-->pi*) state will considerably decrease the triplet quantum yield and thus increase the photostability of the compounds, in agreement with experimental observations.
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