Contents lists available at ScienceDirect Food Chemistry journal homepage: www.elsevier.com/locate/foodchem Research Article The location of amphiphobic antioxidants in micellar systems: The diving- swan analogy Amaia Lopez de Arbina a,b , Sonia Losada-Barreiro c , Marcos Caroli Rezende a , Matías Vidal a , Carolina Aliaga a,b, ⁎ a Facultad de Química y Biología, Universidad de Santiago de Chile, Av. B.O’Higgins 3363, Santiago, Chile b Centro para el Desarrollo de la Nanociencia y la Nanotecnología, CEDENNA, Chile c Departanento de Química Física, Universidad de Vigo, Spain ARTICLE INFO Keywords: Paradoxical or “cut-of” efect Hydrophobicity Antioxidant Micro-heterogeneous media Radical probe “Diving-swan” analogy ABSTRACT A protocol for determining the location of antioxidants (AOs) in a micro-heterogeneous medium was applied to three series of AOs with increasing hydrophobicities: chromancarboxylic acid (“Trolox”) esters, cafeic acid and its esters, and gallic acid and its esters. The observed paradoxical behaviour of these and other commonly encountered antioxidants was rationalized with the aid of a pictorial simile, the “diving-swan” analogy, that explains the orientation and location of an amphiphobic AO when it reacts with a radical probe in the micellar interface. 1. Introduction Understanding how the medium infuences the efectiveness of an- tioxidants (AOs) in food extracts, cosmetics or emulsions such as mayonnaise has been a major challenge and subject of interest in the recent research in this feld (Carvajal, Mozuraityte, Standal, Storrø, & Aursand, 2014; Lauberts et al., 2017; Qiu, Jacobsen, Villeneuve, Durand, & Sørensen, 2017). Evaluation of their efectiveness without taking into account their location and the structural features that de- termine their response and reactivity in these systems can lead to puzzling results. For example, the antioxidant efectiveness of cafeic acid esters of variable lipophilicity has been found to be diferent in fsh-oil enriched mayonnaise and milk (Alemán et al., 2015). A study with pomace extracts of kiwi and grape juice showed that their anti- oxidant activities do not only rely only on their total phenolic content but are also associated with the phenolic profles of the antioxidants (Zhu et al., 2019). A note on lipid oxidation in hetero-phasic systems has recently addressed the need of further studies and new models for a proper understanding of antioxidant efectiveness in such hetero- geneous media (Villeneuve, Durand, & Decker, 2018) The efectiveness of an antioxidant (AO) as a radical scavenger in a food depends not only on its intrinsic reactivity but also on its location in the heterogeneous environment. As shown for diferent phenolic antioxidants of variable hydrophobicity in a micellar system, the loca- tion of the antioxidant is a major factor to be considered when measuring its efectiveness (Carolina Aliaga, Rezende, & Arenas, 2009). In the same way, when measuring the antioxidant activity of complex mixtures, the location of the employed probe in the heterogeneous micro-environment cannot be ignored (Aliaga et al., 2016; Berton- Carabin, Coupland, & Elias, 2013; Leong et al., 2015). For a series of AOs of analogous structures and increasing hydro- phobicities, their efectiveness in a micellar system often varies in a paradoxical way: it increases (or decreases) with its increasing hydro- phobicity until a maximum (or minimum) is reached, after which it starts to decrease (or increase). This paradoxical or “cut-of” efect of AOs and/or probes in micellar solutions or emulsions has generally been ascribed to their diferent partitioning, location or orientation in these micro-heterogeneous systems (Aliaga et al., 2016; Lopez de Arbina, Rezende, & Aliaga, 2017). In previous works, we have shown that the location and the re- activity of the series of radical probes 4-alkanoyloxy-2,2,5,5-tetra- methylpiperidine-1-oxyl (4-alkanoyloxyTEMPO) 1 (Scheme 1) in a micellar system varies in a paradoxical way with the size of the lipo- philic alkanoyloxy side-chain RCO 2 -(Aliaga et al., 2016). The series of probes 1 (Scheme 1) share with many antioxidants like vitamin E or its derivatives (2, Scheme 1), the 6-hydroxy-2,5,7,8-tetramethylchroman- 2-carboxylic acid (“Trolox”) derivatives 3 (Scheme 1), the 3-(3,4-di- hydroxyphenyl)propenoic acid (“cafeic acid”) derivatives 4 (Scheme 1) or the 3,4,5-trihydroxybenzoic acid (“gallic acid”) derivatives 5 (Scheme 1), a common structural feature. These amphiphobic https://doi.org/10.1016/j.foodchem.2018.12.020 Received 13 September 2018; Received in revised form 3 December 2018; Accepted 5 December 2018 ⁎ Corresponding author at: Facultad de Química y Biología, Universidad de Santiago de Chile, Av. B.O’Higgins 3363, Santiago, Chile. E-mail address: carolina.aliaga@usach.cl (C. Aliaga). Food Chemistry 279 (2019) 288–293 Available online 11 December 2018 0308-8146/ © 2018 Elsevier Ltd. All rights reserved. T