The role of intramolecular H-bonds predominant effects in myricetin higher antioxidant activity

- The sequential active rings in Myc structure are B-ring > pyrone C-ring > A-ring.
- Trend of radical reduction in Myc sites is 4′-OH > 3′-OH ≈ 5′-OH > 3-OH > 7-OH > 5-OH.
- Higher activity of B-ring corresponds to “OHs numbers” and “OHs ortho location”.
- The intramolecular H-bonds in Myc rings contributed to stabilize the Myc radicals.
- The electrons delocalization play second possible role for easy H atom donation.

Flavonoid antioxidants play essential roles in the prevention of damage caused by free radicals, which is concerned in many chemical and biological processes. The structural electronic properties of myricetin (Myc) and six respective radicals such as total electronic energies, bond dissociation enthalpy magnitudes (BDEs), intramolecular H-bonds strength and charges density distributions have been computed using density functional theory approach with B3P86 level. The important Myc molecular characteristics, the antioxidant activities, electronic structure-activity relationships, OH numbers and positions-activity relationships as well as radical scavenging mechanisms were discussed. The sequential active OH groups for H atom abstraction was predicted as 4′-OH > 3′-OH ≈ 5′-OH > 3-OH > 7-OH > 5-OH that concomitant with B-ring > pyrone C-ring > A-ring activity in Myc structure. The vital role of intramolecular H-bonds was comprehensively monitored for the stabilization of B-ring radicals (4′-O, 3′-O, 5′-O), that lead to the highest activity of 4′-OH, 3′-OH, 5′-OH in B-ring. Also, the electrons delocalization among A, B and C-ring (increasing co-planarity) play second possible role for easy H atom donation. The number of three ortho-phenolic OH groups and radical stabilization by the strength of intramolecular H-bonds make B-ring to be very active and excellent to react with free radicals among three-rings in Myc structures. Our findings may be useful to elucidate the radical scavenging mechanism of other flavonoid antioxidants for the aim to design novel antioxidants.

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