Autoxidation of gallic acid as a viable alternative to enzymatic polymerization: spectroscopic characterization of autoxidation products

Abstract

Autoxidation of gallic acid was carried out over 96 hours in aqueous solutions at pH 9, both in the absence of metal ions and in the presence of Mg(II) or Ca(II) ions. UV-vis spectrophotometry was used to monitor the reaction and provided the first indication that oligomerization/polymerization of gallic acid occurred during its autoxidation. This was confirmed by comparing FTIR spectra of the autoxidation products with that of pure gallic acid. LDI-TOF mass spectra revealed that the stable autoxidation products mainly consisted of 3 to 4 monomeric units with up to 6 monomeric units in the absence of metal ions and in the presence of Mg(II) ions and possibly up to one more monomeric unit in the presence of Ca(II) ions. These mass spectral data in combination with the literature data about similar materials indicate that the autoxidation products obtained in this study may be supramolecular structures composed of oligomerized gallic acid forms held together by the non-covalent interactions. ESR spectroscopy indicated that main radical species in autoxidation products are slightly anisotropic carbon-centered radicals with a high degree of unpaired spin delocalization and, to a lesser extent, highly anisotropic oxygen-centered radicals embedded in a polymeric matrix. In general, the spectroscopic characteristics of gallic acid autoxidation products showed marked similarity to those of products obtained via enzymatic oxidative polymerization of gallic acid and other simple phenolic compounds. These results suggest that autoxidation could serve as a viable, more cost-effective alternative to the enzymatic oxidative polymerization of simple phenolic compounds for various applications.