Computational assessment of the antioxidant activity of maculosin: a theoretical approach

Understanding the antioxidant mechanism of small bioactive molecules at the molecular level is essential for evaluating their potential application in biological and aqueous systems. In this work, the antioxidant activity of maculosin (MA) was systematically investigated in aqueous and lipid-like environments using quantum chemical approaches. Frontier molecular orbital and molecular electrostatic potential analyses reveal that MA possesses favorable electronic features for electron and hydrogen atom donation. Thermodynamic descriptors indicate that the O19-H, C3-H, and C7-H sites are the most reactive positions, while the N8-H site is inactive. Mechanistic evaluation shows that in water, MA predominantly scavenges radicals via a single-electron transfer pathway, leading to an overall rate constant markedly higher than that of the reference antioxidants Trolox and BHT. In contrast, in a lipid-like medium, the antioxidant activity of MA is governed by formal hydrogen atom transfer at carbon-centered sites, resulting in lower overall reactivity. These findings demonstrate a strong solvent-dependent antioxidant behavior and highlight MA as a highly efficient radical scavenger, particularly in polar media.

All calculations were performed using density functional theory. Geometry optimizations and frequency calculations were carried out at the DFT/M06-2X/6-311 +  + G(d,p) level to obtain stable structures and thermodynamic parameters in both aqueous and lipid-like phases using an implicit solvation model. Frontier molecular orbital energies, molecular electrostatic potential maps, and spin density distributions were analyzed to identify reactive sites. Thermodynamic descriptors including bond dissociation enthalpy, ionization potential, and proton affinity were calculated to assess the feasibility of fHAT, SETPT, and SPLET mechanisms. Reaction kinetics with the HOO• radical were evaluated following the QM-ORSA protocol to determine activation free energies, rate constants, and branching ratios. All quantum chemical calculations were performed using Gaussian 09 software packages.