Organic & Biomolecular Chemistry Dynamic Article Links Cite this: Org. Biomol. Chem., 2011, 9, 3965 www.rsc.org/obc PAPER Structure–activity relationships in hydroxy-2,3-diarylxanthone antioxidants. Fast kinetics spectroscopy as a tool to evaluate the potential for antioxidant activity in biological systems† Clementina M. M. Santos, a,b Artur M. S. Silva, a Paulo Filipe, c Ren´ e Santus, d ,e Larry K. Patterson, f ,e Jean-Claude Mazi` ere, e,g,h Jos´ e A. S. Cavaleiro a and Patrice Morli` ere* e,g,h Received 6th October 2010, Accepted 28th February 2011 DOI: 10.1039/c0ob00841a A structure–activity relationship has been established for eight hydroxy-2,3-diarylxanthones (XH) bearing hydroxy groups on the two aryl rings. One-electron oxidation by superoxide radical-anions ( ∑ O 2 - ) and ∑ Trp radicals as well as reaction with ∑ CCl 3 O 2 and ∑ CHCl 2 O 2 radicals demonstrates that two OH groups are required for efficient antioxidant reactivity in cetyltrimethylammonium bromide micelles. Hydroxy groups at the meta and para positions on either of the two phenyl rings confer enhanced reactivity, but XH bearing an OH at the para position of either phenyl ring is unreactive. While oxidation is favoured by OH in both meta and para positions of 2-aryl xanthone substituents, addition of a third and/or fourth OH enhances electron-donating capacity. In Cu 2+ -induced lipid peroxidation of human LDL, the lag period preceding the commencement of lipid peroxidation in the presence of XH bearing OH at meta and para positions on the 3-phenyl ring is extended to twice that observed with a comparable concentration of quercetin, a reference antioxidant. These antioxidants are also superior to quercetin in protecting human skin keratinocytes against tert-butylhydroperoxide- induced oxidative stress. While XH antioxidant activity in model biological systems is consistent with the structure–activity relationship, their response is also modulated by the localization of XH and by structural factors. Introduction Xanthones are natural heterocyclic compounds found in higher plants, such as lichens and fungi. 1 Antiallergic, anti-inflammatory and antitumor activities have been demonstrated for this class of compounds. 2 Among their most interesting properties is their strong antioxidant activity due to the presence of hydroxy groups and/or a catechol moiety at key positions. 3 Oxygenated and a University of Aveiro, Department of Chemistry & QOPNA, 3810-193 Aveiro, Portugal b Polytechnic Institute of Braganc ¸a, School of Agriculture, Department of Vegetal Production and Technology, 5301-855 Braganc ¸a, Portugal c Universidade de Lisboa, Hospital de Santa Maria, Clinica Universit´ aria de Dermatologia, 1600 Lisbon, Portugal d Mus´ eum National d’Histoire Naturelle, D´ epartement RDDM, F-75231 Paris, France e INSERM ERI12, Laboratoire de Biochimie, CHU Amiens – Hˆ opital Nord, place Victor Pauchet, 80054 Amiens Cedex 1, France. E-mail: morliere. patrice@chu-amiens.fr; Fax: +33 3 22 66 85 93; Tel: +33 3 22 66 86 69 f University of Notre Dame, Radiation Laboratory, Notre Dame, Indiana 46556, USA g INSERM, ERI12, F-80054 Amiens, France h Universit´ e de Picardie Jules Verne, Facult´ e de M´ edecine et de Pharmacie, EA 4292, F-80036 Amiens, France † Electronic supplementary information (ESI) available. See DOI: 10.1039/c0ob00841a prenylated xanthones, extracted from mango, have been shown to be powerful antioxidants in animals and humans. 4 These findings have stimulated the synthesis of polyphenolic xanthone derivatives with well-defined chemical structures. 5 A preliminary study on the ability of polyhydroxylated 2,3-diarylxanthones to scavenge reactive oxygen and nitrogen species ( ∑ O 2 - , HOCl, ∑ NO, ONOO - and singlet oxygen) has been performed using chemical methods. 6 In view of encouraging results, a physical chemical study has been performed to specify the fine structural features determining the antioxidant properties of nine 2,3- diarylxanthones (XH) bearing up to 4 hydroxy groups at various sites on the two aryl rings. The first objective has been to determine by means of a fast spectroscopic technique a ranking of their comparative interactions with reactive oxygen species (ROS) in order to establish a structure–activity relationship within this class of molecules as well as to highlight the most promising molecules for further investigation in biological systems. The antioxidant and redox properties of these rather hydrophobic molecules have been assayed in a model multiphase aqueous medium which mimics the complexity of microenvironmental conditions inherent to biolog- ical structures. The present investigation has been carried out in cationic cetyltrimethylammonium bromide (CTAB) buffered (pH 7) micellar solutions, using pulse radiolysis to determine reaction kinetics. With this technique one may convert all primary species This journal is © The Royal Society of Chemistry 2011 Org. Biomol. Chem., 2011, 9, 3965–3974 | 3965