Investigação vibracional do comportamento anfótero de formamida: um composto modelo para sistemas biológicos

Abstract

Formamide (FA) is the simplest model capable of mimicking the interactions that take place between peptides, proteins and DNA structures, so that the understanding of its acidbase chemistry is paramount. Its behavior as an acid has been investigated towards two biologically relevant bases: pyridine (Py) and pyridazine (PRD), by means of Raman spectra of ternary mixtures in several compositions. Molecular association extent values (1-a) reveal that the concentration of the 1:1 Py:FA adduct is always greater in the equilibrium as compared to the 1:2 PRD:FA complex, thus confirming that Py is the stronger base, following the trend exhibited by the Proton Affinity (P.A.) values reported for such azabenzenes. The thermodynamic parameters of the PRD/FA system have been determined through temperature dependent Raman experiments, and the comparison between the values determined in this study and those available for the Py/FA system confirmed the conclusions drawn from the concentration dependent investigation. Furthermore, the calculated DfG° values show that the whole process is not spontaneous, due to an extremely negative DfS° value, which is explained in terms of the two hydrogen bonds formed in the complex containing PRD. The basic behavior of FA has been evaluated through coordination reactions with main group metal cations Mg(II), Ca(II) and Al(III), aiming to predict the catalytic potential of the solvates towards neutral amide hydrolysis. This has been accomplished by monitoring the changes on the vibrational modes nCO e nCN of FA provoked by coordination. From the downshift of the former vibrational mode and upshift of the second vibration, it was possible to observe the formation of [Mg(FA)6](ClO4)2, whose ionic FA form (I) is present and has been regarded as the active intermediate in the most likely hydrolysis mechanism. The trend observed in the presence of Ca(II) is markedly different, since both vibrational modes are upshifted on the formation of [Ca(FA)4](ClO4)2, implying that the molecular FA structure (II) prevails. Coordination of FA to Al(III) leads to changes in many regions and allows the observation of metal-ligand (nAlO e nAlN) vibrations for the first time. Both nCO e nCN modes are upshifted, indicating that FA form II is stabilized upon formation of [Al(FA)5]Cl3, which is stable up to around 110°C. Stabilization of different FA structures seems to be related to the size of the studied ions as compared to the ion radius values of the transition metals.