Oxidation of caffeic acid by Fe(III) trapped in a Ca-polygalacturonate network

With the aim to verify if Fe(III) ions accumulated in a network of Ca-polygalacturonate (PGA) may promote the oxidation of caffeic acid (CAF) the interaction at pH 5.0 between CAF and Fe(III) ions trapped in a PGA was studied. The sorption kinetics evidenced a great affinity of CAF towards the Fe-PGA matrix. Chromatographic tests showed that the interaction leads to the formation of products which can be considered as CAF oligomers characterized by FT-IR spectra similar to those of natural humic acids. Tests carried out under nitrogen suggest that at pH 5.0 oxygen does not affect the nature of these oxidation products. Oxygen was hypothesized to exert a direct action on the redox process by oxidizing the Fe(II) ions, produced by oxidation of CAF, to Fe(III) thus regenerating oxidizing sites. A possible mechanism of formation of the polymers was proposed that implies that the CAF oxidation leads to highly reactive species such as semiquinones which give rise, by an oxidative coupling reaction, to the formation of oligomers that can aggregate through secondary bonds to produce more complex structures as those that characterize humic acids.