Free radical scavenging properties of thienyl and furyl linalool analogues: an experimental and DFT/B3LYP study

In the present study, thiophene (2a-b) and furane (3a-b) linalool analogues were screened for antioxidant/pro-oxidant activity using experimental and theoretical methods. The new compounds were subjected to two different types of assays: a hydrogen transfer-based test (i.e.ORAC(FL) measurements) and an electron transfer-based test (i.e.ABTS assay), respectively. Furthermore, the PM3 semi-empirical method and the density functional theory (DFT) method with the hybrid B3LYP exchange-correlation functional have been applied to investigate the free radical scavenging properties of synthetic compounds. The experimental results showed that linalool and its analogues possessed an almost identical ability to neutralize peroxyl radicals. In contrast, new compounds were better ABTS(center dot+) scavenges in comparison with the starting compound but they, in general, exerted very weak antiradical activity in comparison with the synthetic antioxidant, i.e.Trolox. Taking into account the DFT-predicted reaction enthalpies associated with two possible mechanisms which might be responsible for the antioxidant activity of investigated compounds, i.e.the single electron and proton transfer process (SET-PT) and the direct H-atom transfer (HAT) mechanism, it could be concluded that the latter mechanism should be favoured in the polar medium. The theoretical results also revealed that the new odourants may exhibit pro-oxidant activity by auto-oxidation. The antioxidant and pro-oxidant properties of linalool and its analogues were determined by the low bond dissociation enthalpy of a C-H bond located at allylic carbon atom 5C and the relatively low ionization potential, respectively. Copyright (c) 2014 John Wiley & Sons, Ltd.