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416 Z. NOWAKOWSKA, E. WYRZYKIEWICZ

ABSTRACT

The essential biological importance of uracil and its thio-derivatives, as well as the rełatively smali size of the parent uracil molecule, have motivated a number of experimental and theoretical studies of this species. There are many examples of biological processes in which the knowledge of the relative stability of different tautomers of one molecule can be very helpful in under-standing the molecular mechanism involved.

Tautomerism of pyrimidine bases has been studied extensively in the past decades. Both theoretical [1-3] and experimental [3, 4] efiorts have been direc-ted toward the prediction of the molecular structure and properties of possible tautomers. 2-Thiouracil and 4-thiouracil can exist in six tautomeric forms with completed cyclic conjugation (aromatic tautomers). Non-ring conjugated tautomers also exist, but are likely to be of little importance [5, 6]. 2-Thiouracil and 4-thiouracil can form four mono O-, S- or N-methyl derivatives, each of which can exist in three tautomeric forms. It has been realized that the relative population of these forms strongly depends on the environment [7]. In par-ticular, the form dominating in the gas phase, or in the nonpolar solvents, may completely disappear in the crystalline State, or in the polar solvents and may be replaced by another tautomeric form. Theoretical calculations based on the ąuantum mechanical principles may certainly help to resolve the question of the relative stability of various tautomeric forms of thiouracils and to predict the relative gas-phase abundances of those forms. Because of the considerable size of thiouracils by ąuantum mechanical standard, only semiempirical ąuasi-ab initio methods have been applied so far.

Crystallographic studies of nucleosides and nucleobases and their derivat-ives are informative [8, 9] and can be used as a basis for the theoretical evaluation of their electronic structures by a ąuantum-mechanical approach [10, 11].