Comparative study on the free radical scavenging mechanism exerted by geraniol and geranylacetone using the combined experimental and theoretical approach

In the current study the free radical scavenging mechanism of geraniol and geranylacetone has been investigated experimentally and theoretically. Two fragrant acyclic terpenoids were subjected to the ABTS and DPPH assay. Furthermore, the detailed quantum chemical calculations have been undertaken in order to evaluate the possible pathways underlying the radical scavenging activity of selected compounds in the non-polar and polar media. The density functional theory (DFT) using the hybrid Becke three-parameter exchange-correlation functional (B3LYP) was applied. In general both investigated compounds possessed very weak anti-radical activity at the applied experimental conditions but geranylacetone was significantly more effective as a ABTS•+ and DPPH• scavenger than geraniol. The comparison of DFT-predicted reaction enthalpies associated with the hydrogen atom transfer mechanism (HAT), the single electron and proton transfer process (SET-PT) and the sequential proton loss electron-transfer (SPLET), revealed that the former mechanism should be favoured in geraniol solvated in both the non-polar and polar media. On the contrary, an anti-radical activity of geranylacetone in the non-polar solvent was ascribed to the hydrogen donation mechanism while in the polar solvent it was assigned to the SPLET. According to the computational results the presence of allylic H-atom (at the position 1C) close to the –OH group seemed to be essential for the observed anti-radical activity of geraniol. The scavenging ability of geranylacetone was associated with the presence of both allylic and alkylic hydrogens in the close vicinity of the carbonyl functionality. Copyright © 2015 John Wiley & Sons, Ltd.