Evaluation of antioxidant properties of lycopene isomers using density functional theory

Lycopene, a naturally occurring carotenoid found in all-trans configuration in various fruits and vegetables, is recognized for its potential antioxidant properties. This study employed the Density Functional Theory (DFT) to investigate and compare lycopene isomers' antioxidant properties through the radical scavenging mechanism. The antioxidant capacity of the isomers is quantified using global descriptive parameters, ranking their potential from highest to lowest as 5-cis > all-trans > 9-cis > 13-cis. The study identifies the Hydrogen Atom Transfer (HAT) mechanism as the predominant mode of antioxidant action, evidenced by the lowest bond dissociation energies when compared to other mechanisms such as Sequential Electron Transfer Proton Transfer (SETPT) and Sequential Proton Loss Electron Transfer (SPLET). The 5-cis isomer exhibits the lowest bond dissociation energy, indicating a superior thermodynamic potential for antioxidant activity relative to the other isomers. Additionally, activation energy assessments reveal that the 5-cis and 13-cis isomers are the most kinetically favourable under the HAT radical scavenging mechanism, surpassing the 9-cis and all-trans configurations. This investigation highlights the 5-cis isomer as both thermodynamically and kinetically the most favourable antioxidant among the lycopene isomers studied.

The antioxidant potential and radical scavenging mechanism were computed using the ωB97X-D/6-31 + G (d,p) level of theory and the Gaussian 16 software package. The frontier molecular analysis and the global descriptive parameters were performed to compare the antioxidant properties of all-trans, 5-cis, 9-cis, and 13-cis isomers. Hydrogen atom abstraction (HAA), sequential electron transfer proton transfer (SETPT), and sequential proton loss electron transfer (SPLET) mechanisms were studied. The radical scavenging mechanism of isomers was performed using the QST3 (synchronization transition Quasi-Newton Searching) calculation, followed by the IRC (Intrinsic Reaction Coordinate) calculation.