A Deep Insight in the Antioxidant Property of Carnosic Acid: From Computational Study to Experimental Analysis

Since the deep cause for the anti-oxidation of carnosic acid (CA) against oleic acid (OA) remains unclear, we focused on exploring the CA inhibition mechanism via a combined experimental and computational study. Atomic charge, total molecular energy, phenolic hydroxyl bond dissociation enthalpy (BDE), the highest occupied molecular orbital (HOMO), and the lowest unoccupied orbital (LUMO) energy were first discussed by the B3LYP/6-31G (d,p) level, a density functional method. A one-step hydrogen atom transfer (HAT) was proposed for the anti-oxidation of CA towards OA, and the Rancimat method was carried out for analyzing the thermal oxidation stability. The results indicate that the two phenolic hydroxyl groups located at C-7(O-15) and C-8(O-18) of CA exert the highest activity, and the chemical reaction heat is minimal when HAT occurs. Consequently, the activity of C-7(O-15) (303.27 kJ/mol) is slightly lower than that of C-8(O-18) (295.63 kJ/mol), while the dissociation enthalpy of phenol hydroxyl groups is much lower than those of alpha-CH2 bond of OA (C-8, 353.92 kJ/mol; C-11, 353.72 kJ/mol). Rancimat method and non-isothermal differential scanning calorimetry (DSC) demonstrate that CA outcompetes tertiary butylhydroquinone (TBHQ), a synthetic food grade antioxidant, both in prolonging the oxidation induction period and reducing the reaction rate of OA. The E-a (apparent activation energies of reaction) of OA, TBHQ + OA, and CA + OA were 50.59, 57.32 and 66.29 kJ/mol, revealing that CA could improve the E-a and thermal oxidation stability of OA.