Theoretical exploration of the antioxidant activity of honokiol and magnolol

The radical scavenging mechanism of two bio-active compounds from Magnolia officinalis namely, honokiol and magnolol was explored on thermodynamic grounds employing density functional theory using M06-2X/6311+G** level. Three different possible mechanisms such as HAT, SET-PT and SPLET were scrutinized with the help of their respective molecular descriptors such as bond dissociation enthalpy (BDE), ionization potential (IP), proton dissociation enthalpy (PDE), proton affinity (PA) and electron transfer enthalpy (ETE). Whereas the HAT mechanism was found to be the most thermodynamically favored mechanism of the radical scavenging activity in gas phase, SPLET mechanism was found to be favored in polar solvent. Rate calculations additionally suggests a faster reaction in the case of conformation-2 of magnolol with intra-molecular H bonding. Our thermodynamic results and kinetic calculation predicts magnolol and honokiol as potent peroxyl radical scavengers. Moreover, the role of internal H bonding in tuning the high reactivity of one of the magnolol conformations is well established in our study.