Barbituric and thiobarbituric acids: a conformational and spectroscopic study

Abstract

A conformational study on Barbituric (BA) and Thiobarbituric (TBA) acids was performed at ab initio MP2/6-31G** level on the neutral, protonated, mono- and di-anionic forms. Acid–base equilibria were studied by comparing the electronic transitions evaluated for the most stable conformations and the experimental spectra at different pH values. The electronic transitions were obtained through the ZINDO approach.

Introduction

Owing to their pharmacological activity, Barbituric (BA) and Thiobarbituric (TBA) acids, as well as their various substituted derivatives, are very important compounds in biological chemistry and medicine. Their biological activity is mainly related to tautomerism and acid–base equilibria and, in turn, to the nature of substituents. Both compounds show high reactivity, especially with unsaturated aldehydes [1], [2], [3], [4].

TBA differs from BA only for having a CS instead of a CO group (Fig. 1), but it shows some appreciable differences in the enolisation tendency, as well as in the spectral behaviour.

Several papers reporting the UV spectra, or the absorption maxima, of both compounds in aqueous solutions (at different pH values) and in non aqueous solvents [2], [5], [6], [7], [8], [9], [10], [11], [12], [13], [13](a), [13](b), [14], [14](a), [14](b), [15], [15](a), [15](b), [16], [17], [17](a), [17](b), [18], [19] are present in the literature. A systematic acid–base equilibrium study of variously substituted barbiturates and thiobarbiturates at different pH values has been made by Smyth et al. through polarographic and spectrophotometric measurements [9], [10], [11]; interesting differences concerning the environment and substituent effects on the dissociation equilibria of BA and TBA were pointed out.

In the solid state both BA and its dihydrated form prefer the 2,4,6-triketo structure [20], [21], while the 1,3-derivative of TBA attains geometrical parameters suggesting strong intermolecular hydrogen bondings supported by 5,6-enolisation [22].

From a kinetic study [23] it has been deduced that, in most solvents, BA is present essentially in its keto form (in water solution the enol, EH, and keto, KH, isomers are in a concentration ratio of 0.013). It also has been pointed out that the molecule undergoes two dissociation equilibria, characterised by pK_a values of about 4.0 and 12.6, respectively [24], [24](a), [24](b).

Debate exists on the acidic strengths of CH and NH groups. Indeed, a greater acidic nature of the CH group of the triketo isomer with respect to the NH groups has been evidenced in gas phase of BA through mass spectrometry [25], and by thermodynamic stydies in solution [26], [27].

Moreover, it was argued that the 5-substitution affects the CH dissociation in such a way that simultaneous dissociation of CH and NH groups (characterised by pK values of about 8) can occur at intermediate pH, while in strongly alkaline environment the dissociation of both the NH groups is prevailing [16].

For the alkylderivatives, pK values ranging from 7.97 to 8.52 were found; they were attributed to the NH dissociation. It was also pointed out that the lower (4 units) pK values of the unsubstituted and 5-substituted BA acids are justified by the acidity of the CH group [25].

NMR and UV studies [17], [17](a), [17](b), evidenced that in very acid solution BA undergoes a further equilibrium, in which a non planar protonated structure is involved; the protonation sites were attributed mostly to the 4 and 6 positions, depending on the nature of the 5-substituent.

TBA acid shows a pK value in the range 2.10–2.25 and a second one in the range 10.55–10.72 [10], [28], [29]. Which forms are involved in the two cases has been a debated problem. In contrast with a previous interpretation [29] (according to which the lower pK originates from enol dissociation) it was suggested that the former value is a consequence of the equilibrium between the N-protonated enol forms and the corresponding neutral one, while the higher pK was attributed to the equilibrium between neutral and anionic forms [9], [10]. In the same context, pK values in the range of 8, observed in the 5,5′ derivatives of TBA, were assigned to the N atoms protonation.

A NMR study concerning solvent and substituent effects on the tautomeric equilibria [12] indicated that keto and enol isomers of TBA and some of its 1,3-disubstituted derivatives are present in comparable amounts but that the enol percentage increases with increasing dielectric constant of the solvent, at variance with BA, which was found prevailingly in the keto form. In addition, the formation of a SH isomer is considered more probable than the corresponding OH one in BA.

Despite their capital role, few theoretical studies (carried out at a semiempirical level or with poor ab initio basis set) concerning molecular structures and UV spectra of BA and TBA were found in the literature [30], [31]; moreover, the assignment of the suggested conformations comes only from comparison between experimental spectra.

Aim of the present work is the rationalization of the equilibria of the title compounds through theoretical calculations, based on the relative energies of the most probable (neutral as well as ionic) forms, and by comparing experimental absorption bands recorded at various pH values with the calculated electronic transitions. For this purpose, an extensive conformational study at ab initio level, with inclusion of correlation energy, in gas phase as well as in aqueous solution, has been carried out. The studied forms (neutral, protonated, mono- and di-anionic isomers) are shown in Fig. 2, Fig. 3, Fig. 4, Fig. 5.