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    • This progressive behaviour is summarized in

    2024-04-16

    This progressive behaviour is summarized in Fig. 4, which depicts the variation in convexity of the obtained cut-off patterns for the series of six antioxidants. The form and degree of convexity of the obtained cut-off curves should in principle correlate with the AOs hydrophobicities, and therefore with their logP values, listed in Table 1. The more Caspase-3, human recombinant proteinase and negative an AO logP value, the more convex its cut-off curve would be; conversely, the more hydrophobic and positive an AO logP value, the more concave its cut-off curve would be. A closer comparison of Fig. 4 with the logP values of Table 1 shows that this is not always the case. The ascorbate anion is a very hydrophilic antioxidant (logP = −1.54) but the rather shallow convex curve obtained in Fig. 3B points to a very poor selectivity by the probe series 1. α-Tocopherol is also a more hydrophobic (logP = 9.60) antioxidant than BHT (logP = 5.64), in spite of giving rise to a shallower concave cut-off curve than the latter. These discrepancies between trends in logP values and the results of the described protocol could be taken as an indication of the failure of the latter to identify the site of action of an antioxidant in a micro-heterogeneous medium. In what follows, we will show that the opposite is true. An adequate understanding of the origin of these discrepancies will in fact shed light on the limitations of using logP values as a reliable way of characterizing the site of action of an AO in such media. The mechanism of quenching of radical probes 1 by compounds 2–7 is the same – a hydrogen-abstraction from the hydroxylic AO by the nitroxide radical – for all antioxidants except the ascorbate anion. At pH 7, this anionic species reacts by an initial electron-transfer to the nitroxyl radical, followed by a proton abstraction by the formed N-oxide (Matsuoka et al., 2016). This is a much faster, and therefore less selective process, than a bimolecular H-abstraction, which is more strongly dependent on the proximity of the reacting groups. The shallow curve obtained for the ascorbate anion (Fig. 3B) thus reflects a process that takes place via a less selective mechanism, with a hydrophilic antioxidant. Practically no differences were observed in the rates of reaction of three antioxidants of different lipophilicities (methyl, propyl and octyl gallate) with DPPH in micellar solutions of Triton X-100 (Campos, Ponce, & Lissi, 2009). The much faster, less selective electron-transfer process by which DPPH reacts with phenolic substrates (Campos et al., 2009) should be held responsible for this observation, as argued for the reaction of probes 1 with the ascorbate anion (Fig. 3B), which takes place via the same mechanism. The more selective hydrogen-abstracting mechanism is operative for hydrophobic antioxidants α-tocopherol (logP = 9.60) and BHT (logP = 5.64), so that a larger degree of concavity, or a greater selectivity, should be expected for the more hydrophobic α-tocopherol (vitamin E). The opposite is observed by comparing the cut-off plots of Fig. 3E and F, which suggest that the site of reaction of α-tocopherol is less hydrophobic than that of BHT. This is in fact true as a result of the amphiphobic nature of α-tocopherol (vitamin E), its large, hydrophobic chain penetrates deeper into the micellar core, tilting the 6-hydroxychromane group towards a less hydrophobic environment, in the same way as depicted for the nitroxyl group of probe 1f in Fig. 1. This gives rise to the well-known paradoxical effect, which again operates here. LogP values simply measure the distribution of a given compound between two phases, but yield no information as to the average orientation of this compound in a micro-heterogeneous system. By contrast, cut-off curves like those of Fig. 4 yield precious information about the AO orientation and the proximity between its reactive H-donor group and the nitroxyl radical. In spite of being less hydrophobic than vitamin E, as suggested by their logP values, BHT’s phenolic group is less accessible to the nitroxyl radical than the 6-hydroxy substituent of the chromane ring in α -tocopherol. For all reasons, BHT reacts in a more hydrophobic microenvironment than vitamin E.