Determining microscopic dissociation constants of polyprotic polyphenols using NMR spectroscopy

Protolytic equilibrium represents one of the crucial chemical features of compounds, which strongly affects their properties. The influence ranges from compound reactivity and stability to its bioavailability and pharmacokinetics. In the case of ellagitannins, the deprotonation properties explain their ability to chelate metal ions, oxidative properties and antibacterial activity. However, methods for accurately determining microscopic pK_as of polyprotic acids are still limited because the majority of most used methods, such as potentiometric titrations and UV-Vis spectroscopy, do not provide information on deprotonation position.

The present work exploits NMR spectroscopy to elucidate structural details of deprotonations occurring in model compound methyl gallate and four ellagitannins (vescalin, castalin, vescalagin, and castalagin). The pH-dependence of ¹³C chemical shifts was examined, and microscopic pK_a values of most acidic hydroxyl groups for individual aromatic rings of methyl gallate and four polyprotic ellagitannins were determined. The deprotonation positions of phenolic protons were established by comparing experimental and density functional theory (DFT)-calculated NMR spectra. The chosen compounds enabled us to determine how different structural motifs and stereconfiguration of ellagitannins affect pK_a values. These results can further explain the oxidation of the studied ellagitannins and their chelation of metal ions.

The developed method enables us to determine microscopic pK_a values and deprotonation positions of ellagitannins, which is pivotal for the understanding of their biological activity. Finally, this method can be applied to determining the positions of deprotonation and the corresponding pK_a values of a wide range of polyprotic compounds.