Uracil tautomers

TABLE 7.16 Relative Energies (in Real mol ) of Uracil Tautomers (16a-m)... 

Kryachko, E. S. Nguyen, M. T. Zeegers-Huyskens, T. Theoretical study of uracil tautomers. 2. Interaction with water, 7. Phys. Chem. A 2001,105, 1934-1943. 

The CNDO/2 calculations correctly predict tautomers 32 of uracil and thymine to be more stable than tautomers 28. It is interesting to observe that 5-fluorouracil is predicted to be more easily converted into lactim form 28 than uracil and thymine, considering that 5-fluorouracil is mutagenic. As to the relative stability of uracil tautomers, the different approaches give different results. For instance, the tt-SCF MO calculations including a-polarization effectpredict the relative stability of uracil tautomers to be 32 > 31 > 29 > 28 > 27, while the CNDO/2 approach gives the order 32 > 27 > 31 > 29 > 30 > 28. 

Relative Electronic Energies of the Uracil Tautomers (All Quantities in... 

It follows from Table 3 that the most stable uracil tautomer is the 2-hydroxy-4-oxo Ti with the tautomerization energy of 10.9 kcal/mol. Despite this fact, it is unable to function as a normal nucleobase due to the absence of the N1-H7 bond connected to the sugar-phosphate backbone although it appears in the modified nucleobases including pseudouridine. Among the normal ones, the 2-oxo-4-hydroxy T2 tautomer of uracil is the most stable one lying above Ti by only 0.9 kcal/mol. Its dipole moment exceeds that of Ti by more than 50% and, therefore, polar solvents may favor T2 rather than Ti (see Section 6). For these two reasons and also because it has the N1-H7 link to the sugar-phosphate backbone, it becomes of a major biophysical interest (see also Sections 4 and 6). 

Table 14. Ground and excited state optimized bond lengths (A), bond angles (°) and dihedral angles (°) of uracil tautomers and their hydrated forms. Excited state numbering corresponds to isolated species ...

Figure 8. Energy level diagrams of uracil tautomers (A) isolated form and (B) hydrated form. In the case of isolated tautomers (a),...

Electronic Excitation and Singlet—Triplet Coupling in Uracil Tautomers and Uracil-Water Complexes, a Quantum Chemical Investigation. 

The relative energies of the cytosine and uracil " tautomers have been evaluated up to the MP4(SDTQ)/6-31G(d,p) level, and comparisons with lower-level estimations are presented in Tables 4 and 5. A large energy difference of 11.5 kcal mol between the normal and the lowest-energy rare form (2-hydroxo-4-oxo) of uracil prevents experimental identification of the latter tautomer in isolated molecule experiments. 

Table 5 Relative Energies of Uracil Tautomers (kcal mol" ) ...

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