Browsing by Author "Garcia, Beatriz"
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- Electrochemical behavior of Hydroxyxanthones versus ROS scavenging activitiesPublication . Santos, Clementina M.M.; Garcia, Beatriz; Silva, Artur; Fernandes, EduardaXanthones are a class of naturally-occurring oxygenated heterocyclic compounds with a broad spectrum of biological activities of great interest for potential therapeutic applications. 1 IJ Thus, over the last decades a great number of publications have emerged focusing on the isolation and s~nthesis of these natural compounds or even in the search for novel bioactive derivatives.1- 1 Electrochemical methodologies can be applied to establish correlations between structure and oxidation potential of electroactive species and therefore to predict some of its biological activities. In fact, cyclic voltammetry has become an important and widely used electroanalytical technique in many relevant studies of redox ~rocesses for clarifying the electrochemical behavior of chemical and biochemical systems.1 1 Herein, we will report the electrochemical behavior of several phenolic and catecholic-substituted 2,3-diarylxanthones by cyclic voltammetry using a glassy carbon electrode, at pH 7.4, and the results obtained compared with their scavenging activities for reactive oxygen species (ROS) and reactive nitrogen species (RNS).141 Useful considerations about oxidation mechanism will be highlighted and the electrochemical profile of xanthones will corroborate their biological propetties.
- Electrochemical characterization of bioactive hydroxyxanthones by cyclic voltammetryPublication . Santos, Clementina M.M.; Garcia, Beatriz; Silva, Artur; Santus, René; Morlière, Patrice; Fernandes, EduardaThe present study reports the electrochemical behavior of several phenolic and catecholic-substituted 2,3-diarylxanthones on a glassy carbon electrode, challenged by cyclic voltammetry at different pH values (4.0, 7.4, and 11.0). Higher pH values required lower anodic and cathodic peak voltages. The oxidation of catecholic groups occurred at lower peak potentials in a reversible and pH dependent manner. Anodic peak potentials appeared at higher pH values and were attributed to the electrochemically irreversible oxidation of the phenolic groups. The number and position of hydroxyl substituents were the determinants for the electrochemical behavior and found to correlate with the scavenging activity for reactive oxygen (ROS) and nitrogen species (RNS). A xanthone with two catechol units presented the lowest anodic potential voltage (Epa = 0.15 V) and proved to be the most effective ROS and RNS scavenger.
- Electrochemical characterization of hydroxylated xanthones by cyclic voltammetryPublication . Santos, Clementina M.M.; Garcia, Beatriz; Silva, Artur; Cavaleiro, José; Fernandes, EduardaXanthones constitute a class of oxygenated heterocyclic compounds commonly identified in some higher plant families, fungi and lichens [1]. Their large variety of biological and pharmacological activities encouraged the scientific community for the search and isolation of natural derivatives for therapeutic applications [2]. Consequently, extensive studies have been devoted to the development of synthetic routes for the preparation of these naturally-occurring compounds or other synthetic bioactive derivatives [3,4]. For the present study, xanthones XH1-9 were obtained by synthesis and the diversity of substitution pattern includes the presence of phenolic and catecholic moieties. These structural features are important requisites for the scavenging activity against reactive oxygen and nitrogen species already demonstrated by these molecules [5]. During the last decades, electrochemical studies have been widely used for the evaluation of structure-antioxidant activity relationship of several phenolic compounds [6,7]. Thus, the electrochemical profile of xanthones XH1-9 can provide useful information about their electrochemical reactions, which may be used to predict or explain their biological properties. In this communication we will report the electrochemical characterization by cyclic voltammetry of various hydroxylated xanthones, in physiological conditions. The oxidation potential value versus structure relationship will be highlighted and some considerations about the oxidation mechanism will be presented.
- The dependence of alpha-tocopheroxyl radical reduction by hydroxy-2,3-diarylxanthones on structure and micro-environmentPublication . Morlière, Patrice; Patterson, Larry K.; Santos, Clementina M.M.; Silva, Artur; Marzière, Jean-Claude; Filipe, Paulo; Gomes, Ana Sara; Fernandes, Eduarda; Garcia, Beatriz; Santus, RenéThe flavonoid quercetin is known to reduce the α-tocopheroxyl radical (˙TocO) and reconstitute α-tocopherol (TocOH). Structurally related polyphenolic compounds, hydroxy-2,3-diarylxanthones (XH), exhibit antioxidant activity which exceeds that of quercetin in biological systems. In the present study repair of ˙TocO by a series of these XH has been evaluated using pulse radiolysis. It has been shown that, among the studied XH, only 2,3-bis(3,4-dihydroxyphenyl)-9H-xanthen-9-one (XH9) reduces ˙TocO, though repair depends strongly on the micro-environment. In cationic cetyltrimethylammonium bromide (CTAB) micelles, 30% of ˙TocO radicals are repaired at a rate constant of ∼7.4 × 106 M−1 s−1 by XH9 compared to 1.7 × 107 M−1 s−1 by ascorbate. Water-soluble Trolox (TrOH) radicals (˙TrO) are restored by XH9 in CTAB (rate constant ∼3 × 104 M−1 s−1) but not in neutral TX100 micelles where only 15% of ˙TocO are repaired (rate constant ∼4.5 × 105 M−1 s−1). In basic aqueous solutions ˙TrO is readily reduced by deprotonated XH9 species leading to ionized XH9 radical species (radical pKa ∼10). An equilibrium is observed (K = 130) yielding an estimate of 130 mV for the reduction potential of the [˙X9,H+/XH9] couple at pH 11, lower than the 250 mV for the [˙TrO,H+/TrOH] couple. A comparable value (100 mV) has been determined by cyclic voltammetry measurements.