Theoretical and spectroscopic investigation of the oxidation and degradation of protocatechuic acid

In this work, we report a combined experimental and theoretical study on molecular structure and spectroscopic properties of the most stable conformers of PCA. 1H, 13C NMR and 2D COSY NMR, ESR, IR and electronic spectroscopies were coupled with DFT theoretical calculations performed at the B3LYP/6-31G∗∗ level. The calculated geometrical parameters for the neutral protocatechuic acid PCA-H3, its anions, its oxidized forms and the peroxo-derivative [PCA-H-O2]2− are in line with the experimental data. The neutral catecholate is the most stable form of PCA-H3 whilst the dianion [PCA-H]2− presents higher energy. This anion is (experimentally) stable only under argon, reacting with dioxygen, in the presence of air. The semiquinone [PCA-H-sq(3)]− is very close in energy from [PCA-H-sq(4)]− form and an equilibrium between these two oxidized radical forms might be expected. The energetically advantageous pathway for preparation of the symmetrically delocalized [PCA-sq]2− is to oxidize the [PCA]3−. The occurrence of this radical dianion form was justified experimentally by ESR, IR, UV-vis and NMR spectra. The structural calculations for [PCA-H-O2]2− indicate that C3 (and to a lesser extent C1) may undergo a nucleophilic attack from the “co-ordinated” peroxo-group. The conditions for the non-enzymatic degradation of PCA have been established and some new products are observed: ionization of PCA-H3, the presence of O2 and aprotic solvents provide the semiquinone-superoxo adduct which is then degraded to lactones, while in protic solvents, addition of H2O2 and the presence of air, are essential, providing aliphatic degradation products.