Structure-property studies on the antioxidant activity of flavonoids present in diet.
The screening of natural flavonoids for their bioactivity as antioxidants is usually carried out by determinination of their profile as chain-breaking antioxidants, by the evaluation of their direct free radical-scavenging activity as hydrogen- or electron-donating compounds. Since this may not be the only mechanism underlying the antioxidant activity it is important to check the ability of these compounds to act as chelators of transition metal ions. Accordingly, in the present study the acidity constants of catechin and taxifolin, as well as the formation constants of the corresponding copper (II) complexes, were investigated by potentiometry and/or spectrophotometry. Moreover, a detailed quantitative examination of the coordination species formed is presented. In addition, the partition coefficients of both catechin and taxifolin in a biomimetic system (micelles) were determined, since these properties may also contribute to the antioxidant behavior of this type of compound. The log P values determined depend on the electrostatic interactions of the compounds with the differently charged micelles (the highest values were obtained for zwitterionic and cationic micelles). The prooxidant behavior of the compounds was assessed through the oxidation of 2'-deoxyguanosine, induced by a Fenton reaction, catalyzed by copper. The data obtained reveal that the flavonoids under study did not present prooxidant activity, in this particular system. The results obtained are evidence of a clear difference among the pKa, the complexation properties, and the lipophilicity of the flavonoids studied, which can partially explain their distinct antioxidant activity. The most stable geometries of the free compounds were determined by theoretical (ab initio) methods, in order to properly account for the electron correlation effects which occur in these systems, thus allowing a better interpretation of the experimental data.
Teixeira
Susana
S
REQUIMTE, Departamento de Química-Física, Faculdade de Farmácia, Universidade do Porto, Rua Anibal Cunha 164, 4099-030 Porto, Portugal.
Siquet
Christophe
C
Alves
Carla
C
Boal
Isabel
I
Marques
M Paula
MP
Borges
Fernanda
F
Lima
José L F C
JL
Reis
Salette
S
D000975:Antioxidants / Q000737:chemistry*; D002392:Catechin / Q000737:chemistry*; D003849:Deoxyguanosine / Q000737:chemistry; D004032:Diet; D005419:Flavonoids / Q000737:chemistry*; D044948:Flavonols / Q000737:chemistry; D010084:Oxidation-Reduction; D011794:Quercetin / Q000031:analogs & derivatives* / Q000737:chemistry; D013056:Spectrophotometry, Ultraviolet; D013329:Structure-Activity Relationship
D000975:Antioxidants; D005419:Flavonoids; D044948:Flavonols; D002392:Catechin; D011794:Quercetin; C003377:taxifolin; D003849:Deoxyguanosine