pH-Dependent Antioxidant Mechanisms of Harmalol Toward HOO• Radicals in Aqueous Solution: A Quantum Chemical Study

Harmalol is a β-carboline alkaloid exhibiting promising antioxidant properties; however, a comprehensive understanding of its radical scavenging mechanisms in aqueous media across a wide pH range remains limited. In this study, the antioxidant activity of harmalol toward hydroperoxyl radicals was investigated theoretically at the M06-2X/6-311+G(d,p)/PCM(water) level by combining thermodynamic and kinetic analyses over the pH range 2–13. The calculations revealed that the antioxidant behavior of harmalol strongly depends on its protonation state, tautomeric form, and the surrounding pH. Under physiological conditions, the monocationic form predominates, with a smaller contribution from the neutral/zwitterionic I and II species, and radical scavenging proceeds predominantly via proton-coupled electron transfer (PCET)-type hydrogen-transfer reactions involving the monocationic, neutral and zwitterionic I forms as well as radical adduct formation (RAF) mechanism involving zwitterion I. Analysis of SOMO distributions, spin densities, and atomic charges confirmed that the hydrogen transfer reactions for monocationic, neutral and zwitterionic I forms do not follow a classical hydrogen atom transfer (HAT) mechanism. The zwitterion I and neutral forms of harmalol exhibited significantly higher apparent rate constants for the PCET reaction than the monocationic species. Under alkaline conditions, the monoanionic forms exhibit the most favorable thermodynamic parameters toward radical scavenging via formal hydrogen transfer mechanism. Relaxed potential energy surface scans suggest that hydrogen transfer from both monoanionic forms may proceed through a barrierless pathway, while radical adduct formation can also contribute to the antioxidant activity under strongly basic conditions. In addition, monoanion II efficiently participates in single-electron transfer (SET) reactions characterized by very high apparent rate constants. Overall, the results demonstrate that the antioxidant efficiency of harmalol increases with increasing pH and provide detailed insight into the pH-dependent radical scavenging mechanisms of β-carboline derivatives in aqueous environments.