Abstract
The acid–base chemistry of three zwitterionic compounds, namely anthranilic (2-aminobenzoic acid), N-methylanthranilic and N-phenylanthranilic acid has been characterized in terms of the macroconstants K a1, K a2, the isoelectric point pH I, the tautomerization constant K z and microconstants k 11, k 12, k 21, k 22. The potentiometric titration method was used to determine the macrodissociation constants. Due to the very poor water solubility of N-phenylanthranilic acid the dissociation constants pK a1 and pK a2 were determined in MDM–water mixtures [MDM is a co-solvent mixture, consisting of equal volumes of methanol (MeOH), dioxane and acetonitrile (MeCN)]. The Yasuda–Shedlovsky extrapolation procedure has been used to obtain the values of pK a1 and pK a2 in aqueous solutions. The pK a1 and pK a2 values obtained by this method are 2.86 ± 0.01 and 4.69 ± 0.03, respectively. The tautomerization constant K z describing the equilibrium between unionized form ⇌ zwitterionic form was evaluated by the K z method based on UV–VIS spectrometry. The method uses spectral differences between the zwitterionic form (found at isoelectric pH in aqueous solution) and the unionized form (formed in an organic solvent of low dielectric constant). The highest value of the K z constant has been observed in the case of N-methylantranilic acid (log10 K z = 1.31 ± 0.04). The values of log10 K z for anthranilic and N-phenylanthranilic acids are similar and have values of 0.93 ± 0.03 and 0.90 ± 0.05, respectively. The results indicate that the tested compounds, in aqueous solution around the isoelectric point pHI, occur mainly in the zwitterionic form. Moreover, the influence of the type of substituent and pH of the aqueous phase on the equilibrium were analyzed with regard to the formation and the coexistence of different forms of the acids in the examined systems.
Highlights
Ampholytes are molecules containing both one acidic and one basic groups
The acid–base chemistry of three zwitterionic compounds, namely anthranilic (2-aminobenzoic acid), N-methylanthranilic and N-phenylanthranilic acid has been characterized in terms of the macroconstants Ka1, Ka2, the isoelectric point pHI, the tautomerization constant Kz and microconstants k11, k12, k21, k22
N-phenylanthranilic acid is poorly soluble in water and its solubility in the methanol–water system is limited; the dissociation constants pKa1 and pKa2 were determined in the MDM–water system
Summary
Ampholytes are molecules containing both one (or more) acidic and one (or more) basic groups. The microscopic constants and tautomeric ratios describing zwitterionic ampholytes play an important role in understanding the ionic composition of many biologically active molecules and their physicochemical properties. The determination of these microconstants needs the combination of at least two experimental approaches. The pKa value of a derivative can be regarded as the relevant microconstant of the parent molecule Both the microconstant and the tautomeric constant can be determined by using the direct method. It is suitable for the compounds in which protonation of both functional groups has an influence on spectral changes This method was developed by Metzler and Snell and was used to determine the microscopic constants of pyridoxine in the dioxan–water system [6]. We have analyzed the influence of the type of substituent and pH of the aqueous phase on the equilibrium with regard to the formation and coexistence of different forms of acids in the examined systems
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