Kinetics and mechanism of caffeic acid autoxidation in weakly alkaline aqueous solutions in the presence of Mg(II) ions

A large number of studies indicated that caffeic acid represents an important natural phenolic compound with many potential uses in medications and cosmetics, but also with therapeutic implications in cardiovascular diseases, various types of cancer, and neurological diseases. In this work we studied kinetics and reaction mechanism of caffeic acid autoxidation in weakly alkaline aqueous solutions (pH 7.4 and 8.4) in the presence of Mg(II) ions by applying high performance liquid chromatography with diode-array detection (HPLC-DAD) and electron spin resonance (ESR) spectroscopy. The results of HPLC-DAD analysis revealed that the presence of Mg(II) ions enhanced the autoxidation rate to a much greater extent than the increase in pH from 7.4 to 8.4. Analysis of DAD UV-Vis and ESR spectra, as well as comparison of the calculated logP values with retention times of chromatographically separated compounds, enabled the identification of the main initial products of caffeic acid autoxidation in the presence of Mg(II) ions as caffeic acid quinone and its dimer and indicated the nature of some minor products (hydroxylated derivatives of caffeic acid and their quinones). ESR spectroscopy disclosed that prolonged autoxidation of caffeic acid leads to the formation of polymeric, humic acid like, products. These findings may be important when considering treatments and/or storage of medications, cosmetics, and foods containing caffeic acid.