title: Elucidation of hydroxyl groups-antioxidant relationship in mono- and dihydroxyflavones based on O-H bond dissociation enthalpies.
abstract: Radical scavenging potential is the key to anti-oxidation of hydroxyflavones which generally found in fruits and vegetables. The objective of this work was to investigate the influence of hydroxyl group on the O-H bond dissociation enthalpies (BDE) from a series of mono- and dihydroxyflavones. Calculation at the B3LYP/6-31G(d,p) level reveals the important roles of an additional one hydroxyl group to boost the BDE of hydroxyflavones that were a stabilization of the generated radicals through attractive H-bond interactions, an ortho- and para-dihydroxyl effect, and a presence of the 3-OH in dihydroxyflavones. On the other hand, the meta-dihydroxyl effect and range-hydroxyl effect especially associated with the either 5-OH or 8-OH promoted greater BDE. Results did not only confirm that dihydroxyflavones had lower BDE than monohydroxyflavones but also suggest the selective potent hydroxyflavone molecules that are the 6'-hydroxyflavone (for monohydroxyflavone) and the 5',6'-, 7,8- and 3',4'-dihydroxyflavone which the corresponding radical preferable generated at C6'-O•, C8-O• and C4'-O•, respectively. Electron distribution was limited only over the two connected rings of hydroxyflavones while the expansion distribution into C-ring could be enhanced if the radical was formed especially for the 2',3'- and 5',6'dihydroxyflavone radicals. The delocalized bonds were strengthened after radical was generated. However the 5-O• in 5,6-dihydroxyflavone and the 3-O• in 3,6'-dihydroxyflavone increased the bond order at C4-O11 which might interrupt the conjugated delocalized bonds at the keto group.
authors.affiliation: Institute of Food Research and Product Development, Kasetsart University, Bangkok, 10900, Thailand, ifrwctw@ku.ac.th.
authors.forename: Witcha
authors.initials: W
authors.lastname: Treesuwan
authors.forename: Songwut
authors.initials: S
authors.lastname: Suramitr
authors.forename: Supa
authors.initials: S
authors.lastname: Hannongbua
mesh_terms: D000975:Antioxidants / Q000737:chemistry*; D005419:Flavonoids / Q000737:chemistry*; D006860:Hydrogen Bonding; D017665:Hydroxyl Radical / Q000737:chemistry; D008958:Models, Molecular
chemical_list: D000975:Antioxidants; D005419:Flavonoids; D017665:Hydroxyl Radical