Structure-antioxidant activity relationship of ferulic acid derivatives: Effect of ester groups at the end of the carbon side chain

Differences in the antioxidant activity of ferulic acid and its nine naturally occurring derivatives with the presence of different ester groups at the end of their carbon side chains were investigated using the density functional theory (DFT) method. The thermodynamics parameters related to the double H⁺/e⁻ free radical scavenging mechanisms were calculated in the gas, benzene and water phases. In the investigated phases, ferulic acid with the COOH group or COO⁻ group and its different ester derivatives show similar antioxidant capacities. They preferentially undergo the first H⁺/e⁻ reaction from the 4−OH group and then proceed to the second H⁺/e⁻ reaction from the 3−H group, forming a benzodioxole. The energy cost of the second H⁺/e⁻ reaction is considerably reduced. In the gas and benzene phases, ferulic acid and its derivatives have the highest probability of using the HAT mechanism in the first H⁺/e^‒ reaction. In water, the most possible mechanism is SPLET. In the studied phases, HAT is the most likely second H⁺/e^‒ reaction mechanism for ferulic acid and its ester derivatives trap free radicals.