Investigation of the Antioxidant Activity of Hydroxycinnamic Acids, Hydroxybenzoic Acids, and Their Synthetic Diazomethane Derivatives

Phenolic-rich extracts from Satureja montana were evaluated before and after diazomethane treatment to determine how chemical derivatization influences their antioxidant capacity. Native and modified extracts were compared experimentally by measuring total phenolic content, ferric reducing antioxidant power (FRAP), and Fe2+-chelating ability. EN1 exhibited the highest concentration of phenolic compounds, reaching 1278.54 mmol/g, whereas EM2 retained only 1.99 mmol/g. In the FRAP assay, reducing power followed the order EN1 (9.36) > EN2 (3.72) > EM2 (2.08), with EM2 still exceeding caffeic, chlorogenic, and ferulic acids. In contrast, the modified extracts showed superior metal chelating capacity, with EM1 and EM2 displaying IC50 values of 0.70 and 0.82 mg/mL, respectively, both markedly lower than those of the native extracts and the pure standards. To rationalize these differences, a DFT study was performed at the B3LYP/6-311++G(d,p) level, examining 18 proposed phenolic acids and their methylated derivatives associated with the extracts. All methylation reactions were thermodynamically favorable, particularly for compounds 18 (−57.10 kcal/mol), 16 (−53.96), 6 (−53.34), and 3, 9, and 11 (−52.71). Solvent effects were found to be structure-dependent: caffeic acid showed BDE values of 72.29, 73.59, and 74.43 kcal/mol in the gas phase, water, and benzene, respectively, whereas syringic acid displayed values of 80.44, 77.09, and 80.65 kcal/mol under the same conditions. Likewise, the ionization potential of caffeic acid decreased from 180.09 kcal/mol in the gas phase to 133.26 kcal/mol in water and 154.22 kcal/mol in benzene. Among all analyzed species, methyl 3,4-dihydroxycinnamate exhibited the lowest BDE (71.60 kcal/mol) as well as the most favorable ΔG°r toward HOO• (−11.06 kcal/mol).