Abstract
Major phenolic compounds from olive oil (ArOH-EVOO), oleuropein (Ole), tyrosol (Tyr), and p-coumaric acid (p-Cou), are known for their antioxidant and neuroprotective properties. We previously demonstrated that their combination could potentiate their antioxidant activity in vitro and in cellulo. To further our knowledge of their electron-transfer properties, Ole, Tyr, and p-Cou underwent a spectroelectrochemical study, performed either individually or in equimolar mixtures. Two mixtures (Mix and Mix-seq) were prepared in order to determine whether distinct molecules could arise from their simultaneous or sequential oxidation. The comparison of Liquid Chromatography–Electrospray Ionization–Tandem Mass Spectrometry (LC-ESI-MS2) profiles highlighted the presence of specific oxidized products found in the mixes. We hypothesized that they derived from the dimerization between Tyr and Ole or p-Cou, which have reacted either in their native or oxidized forms. Moreover, Ole regenerates when the Mix undergoes oxidation. Our study also showed significant neuroprotection by oxidized Ole and oxidized Mix against H2O2 toxicity on SK-N-SH cells, after 24 h of treatment with very low concentrations (1 and 5 nM). This suggests the putative relevant role of oxidized Ole products to protect or delay neuronal death.
Highlights
Phenolic compounds from olive oil (ArOH-EVOO) are studied for their antioxidant and neuroprotective properties
Two mixtures (Mix and Mix-seq) of three major phenolic compounds from olive oil (Ole, Tyr, and p-coumaric acid (p-Cou)) were prepared in order to investigate whether specific interesting molecules could arise from their simultaneous or sequential oxidation via an electrochemical technique
As it is well-known, the ortho-diphenol function was oxidized in a two-electron-proton mechanism in 9,its ortho-quinonic form on the anodic scan and was reversibly reduced back to the Biomolecules 2019, 6 of catechol form on the reverse cathodic scan [30,31]
Summary
Phenolic compounds from olive oil (ArOH-EVOO) are studied for their antioxidant and neuroprotective properties. The ortho-diphenol oleuropein (Ole) and its derivative hydroxytyrosol (OH-Tyr) have shown some interesting reactivities in non-cellular in vitro assays, such as DPPH (2,2-diphenyl-1-picrylhydrazyl), superoxide, peroxyl radicals inhibition, and reducing capacity methods [1,2,3] (Figure 1). The chemistry behind these assays involves H-atom transfer (HAT), radical-chain breaking, and electron transfer (ET) mechanisms, which have been detailed by Apak [4]. An interesting highlight is that some molecule combinations enhance the antioxidant efficacy in both non-cellular and cellular in vitro assays by synergic effects [1,21,22,23]. Our previous work has shown for instance that a low concentration equimolar mix of Ole, Tyr, and p-Cou (0.1 and 1 μM) reduces intracellular ROS and protein carbonyl levels, and prevents the activation of
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