Antioxidant Mechanism of Flavonoids. Solvent Effect on Rate Constant for Chain-Breaking Reaction of Quercetin and Epicatechin in Autoxidation of Methyl Linoleate

Abstract

The rate of oxygen depletion, as measured by electron spin resonance spectroscopy (oximetry using a spin probe), in a homogeneous solution of peroxidating methyl linoleate (initiated by an azo initiator) in the presence or absence of antioxidants was converted to second-order rate constants for the inhibiting reaction of quercetin and epicatechin. In the non-hydrogen-bonding solvent chlorobenzene at 50 °C, k inh had values of 4.3 × 10 5 M -1 s -1 for quercetin and 4.2 × 10 5 M -1 s -1 for epicatechin, respectively. In the hydrogen-accepting "water-like" solvent tert-butyl alcohol, the values were 2.1 × 10 4 and 1.7 × 10 4 M -1 s -1 , respectively. The solvent effect (factor of 20) is more significant than for R-tocopherol (factor of 4), and the two flavonoids have efficiencies comparable to that of R-tocopherol in scavenging peroxyl radicals in the nonpolar solvent but not in the hydrogen-bonding solvent.

Antioxidant capacity of hesperidin fromCitruspeel using electron spin resonance and cytotoxic activity against human carcinoma cell lines

“…In polyphenol redox systems the solvents play a fundamental role in the chemical behavior of these compounds, and consequently their antioxidant property. This means that the compounds present different chemical forms related to the environment in which they are solved during the measurement of the antioxidant assay (Pedrielli et al, 2001;Finotti & Majo, 2003). Also, kinetic factors may be of great importance (Foti & Ruberto, 2001).…”

Reactions of Plant Phenolics with Food Proteins and Enzymes under Special Consideration of Covalent Bonds

“…In order to function as an antioxidant, the phenolic compound must be capable of forming a resonance stabilized phenoxyl radical, which in turn can react with other available radicals (e.g. peroxyl radical), consequently deactivating them (Robards et al, 1999;Pedrielli et al, 2001). However, the reactivity of the phenoxyl radicals should remain so low, such that the radical formation e.g.…”

Study of thermodynamics of complex formation of flavonoids of steviа (Stevіa rebaudіana Bertonі) leaves

“…In these classes, separate flavonoids differ by the presence of hydroxy-or oxysubstitutes in the A and B rings, which determine the antioxidant property [6]. The B ring hydroxy groups, the amount of which affect the complex formation degree and antioxidant effect, are involved in the formation of the complex [7]. There are two basic mechanisms of antioxidant action of flavonoids: acceptance of free radicals and chelation of metal ions with the formation of complexes [8].…”

“…The mechanisms of forming such complexes between flavonoids (quercetin, morin, taxifolin, silybin А) and ions of iron and copper [1] were studied. Today, much emphasis is placed on studying the molecular formation of complexes of vegetable triterpene glycosides with different bioactive compounds, which promote reducing therapeutic doses, improving solubility, expanding biological activity of medicinal substances [7,10]. In particular, it is acknowledged that stevia is a plant, rich in bioactive compounds and shows functional ability due to flavonoids [11,12].…”