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Rosmarinic acid ( RA ) is reported in separate studies to be either an inducer or reliever of oxidative stress, and this contradiction has not been resolved. In this study, we present a comprehensive examination of the radical scavenging activity of RA using density functional theory calculations in comparison with experimental data. In model physiological media, RA exhibited strong HO • radical scavenging activity with overall rate constant values of 2.89 × 10 10 and 3.86 × 10 9 M -1 s -1 . RA is anticipated to exhibit excellent scavenging properties for HOO • in an aqueous environment ( k overall = 3.18 × 10 8 M -1 s -1 , ≈2446 times of Trolox) following the hydrogen transfer and single electron transfer pathways of the dianion state. The neutral form of the activity is equally noteworthy in a lipid environment ( k overall = 3.16 × 10 4 M -1 s -1 ) by the formal hydrogen transfer mechanism of the O6(7,15,16)-H bonds. Chelation with RA may prevent Cu(II) from reduction by the ascorbic acid anion (AA - ), hence blocking the OIL-1 pathway, suggesting that RA in an aqueous environment also serves as an OIL-1 antioxidant. The computational findings exhibit strong concurrence with the experimental observations, indicating that RA possesses a significant efficacy as a radical scavenger in physiological environments.

Syringic acid (SA) is a natural phenolic acid found in vegetables, fruits, and other plant-based foods. A range of biological activities were proposed for this compound including anticancer, antimicrobial, anti-inflammation, and anti-diabetic activities, as well as antioxidant and antinitrosant properties. In this study, the focus is on the latter two. The HO • , HOO • , NO, and NO 2 scavenging activities of SA were evaluated in physiological environments by kinetic and thermodynamic calculations. The computed rate constants of the HO • radical scavenging of SA were 4.63 × 10 9 and 9.77 × 10 7 M -1 s -1 in polar and nonpolar solvents, respectively. A comparison with the experimentally determined rate constant in aqueous solution yields a k calculated / k experimental ratio of 0.3, thus the computed kinetic data are reasonably accurate. SA exhibited excellent HOO • and NO 2 scavenging activity in water ( k overall (HOO • ) = 1.53 × 10 8 M -1 s -1 and k overall (NO 2 ) = 1.98 × 10 8 M -1 s -1 ), whereas it did not show NO scavenging activity in any of the studied environments. In lipid medium, SA exhibited weak activity. Thus, in polar environments, the HOO • radical scavenging of SA is 1.53 times higher than that of ascorbic acid. Consistently, SA is a promising antioxidant and antinitrosant agent in polar environments.