title: Toward the prediction of the activity of antioxidants: experimental and theoretical study of the gas-phase acidities of flavonoids.
abstract: The relative gas-phase acidities were determined for eight flavonoids, applying the kinetic method, by means of electrospray-ion trap mass spectrometry. The experimental acidity order, myricetin > luteolin > quercetin > (+/-)-taxifolin > kaempferol > apigenin > (+)-catechin > (+/-)-naringenin shows good agreement with the order obtained by theoretical calculations at the B3LYP/6-311 + G(2d,2p)//HF/6-31G(d) level. Moreover, these calculations provide the gas-phase acidities of the different OH groups for each flavonoid. The calculated acidity values (Delta(ac)H), corresponding to the most favorable deprotonation, cover a narrow range, 314.8-330.1 kcal/mol, but the experimental method is sensitive enough to differentiate the acidity of the various flavonoids. For all the flavones and the flavanol, catechin, the 4'-hydroxyl group is the most favored deprotonation site whereas for the flavanones studied, taxifolin and naringenin, the most acidic site is the 7-hydroxyl group. On the other hand, the 5-hydroxyl, in flavones and naringenin, and the 3-hydroxyl, in taxifolin and catechin, are always the less acidic positions. The acidity pattern observed for this family of compounds mainly depends on the following structural features: The ortho-catechol group, the 2,3 double bond and the 4-keto group.
authors.affiliation: Departamento de Química e Bioquímica, Faculdade de Ciências, Universidade de Lisboa, Lisbon, Portugal.
authors.forename: Hugo F P
authors.initials: HF
authors.lastname: Martins
authors.forename: J Paulo
authors.initials: JP
authors.lastname: Leal
authors.forename: M Tereza
authors.initials: MT
authors.lastname: Fernandez
authors.forename: Victor H C
authors.initials: VH
authors.lastname: Lopes
authors.forename: M Natália D S
authors.initials: MN
authors.lastname: Cordeiro
mesh_terms: D000975:Antioxidants / Q000737:chemistry*; D005419:Flavonoids / Q000032:analysis* / Q000737:chemistry*; D005740:Gases / Q000737:chemistry*; D006863:Hydrogen-Ion Concentration; D008958:Models, Molecular; D008968:Molecular Conformation; D021241:Spectrometry, Mass, Electrospray Ionization; D013816:Thermodynamics
chemical_list: D000975:Antioxidants; D005419:Flavonoids; D005740:Gases