Reaction dynamics of flavonoids and carotenoids as antioxidants - PubMed (original) (raw)

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Reaction dynamics of flavonoids and carotenoids as antioxidants

Rui-Min Han et al. Molecules. 2012.

Abstract

Flavonoids and carotenoids with rich structural diversity are ubiquitously present in the plant kingdom. Flavonoids, and especially their glycosides, are more hydrophilic than most carotenoids. The interaction of flavonoids with carotenoids occurs accordingly at water/lipid interfaces and has been found important for the functions of flavonoids as antioxidants in the water phase and especially for the function of carotenoids as antioxidants in the lipid phase. Based on real-time kinetic methods for the fast reactions between (iso)flavonoids and radicals of carotenoids, antioxidant synergism during protection of unsaturated lipids has been found to depend on: (i) the appropriate distribution of (iso)flavonoids at water/lipid interface, (ii) the difference between the oxidation potentials of (iso)flavonoid and carotenoid and, (iii) the presence of electron-withdrawing groups in the carotenoid for facile electron transfer. For some (unfavorable) combinations of (iso)flavonoids and carotenoids, antioxidant synergism is replaced by antagonism, despite large potential differences. For contact with the lipid phase, the lipid/water partition coefficient is of importance as a macroscopic property for the flavonoids, while intramolecular rotation towards coplanarity upon oxidation by the carotenoid radical cation has been identified by quantum mechanical calculations to be an important microscopic property. For carotenoids, anchoring in water/lipid interface by hydrophilic groups allow the carotenoids to serve as molecular wiring across membranes for electron transport.

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Figures

Scheme 1

Scheme 1

Backbone structures of flavone, isoflavone, flavanone, flavanol, and flavonol (upper), and molecular structures of representative (iso)flavonoids (lower).

Scheme 2

Scheme 2

Molecular structures of C40-carotenoids. The binding of the molecules at the water/lipid interface is considered important for the antioxidant efficiency

Scheme 3

Scheme 3

Hydrogen and electron transfer mechanisms of flavonoids. Acceleration of radical scavenging of galvoxyl radical (G•) by the presence of magnesium (II) as rate-determining is indicative of electron transfer [25].

Scheme 4

Scheme 4

Mechanism of radical generation and rearrangement via photoionization of puerarin monoanion. The initially formed radical is a stronger acid than the ground state anion [26].

Figure 1

Figure 1

Absorption spectra of β-carotene (β-Car) oxidized or reduced radicals (from left to right) including adduct radical ([β-Car···PyS]•, ∆) [37], neutral radical (β-Car•, ▲) [37], dication (β-Car2+, ■)[43], radical anion (β-Car•–, ○) [42], ion-pair or exciplex ([β-Car•+···CHCl3•−]or [Car···CHCl3]*, ●) [43], and radical cation (β-Car•+, □) [43].

Scheme 5

Scheme 5

Proposed reversible addition of oxygen to carotenoid-derived carbon-centered neutral radical [44].

Scheme 6

Scheme 6

Proposed reaction mechanism of carotenoid radical formation following direct photo-excitation or triplet sensitization of carotenoids (Car) in chlorinated solvent (chloroform). A solvent radical can initiate further carotenoid radical cation formation [43].

Scheme 7

Scheme 7

Proposed deposition and orientation of daidzein and different anisol daidzein derivatives in the phosphatidyl cholin bilayer of liposomes.

Scheme 8

Scheme 8

Respective radical scavenging mechanism and cooperation of astaxanthin and lycopene as antioxidants in liposomal membrane.

Scheme 9

Scheme 9

Proposed mechanism of synergistic antioxidation interactions between β-carotene (β-Car), vitamins C and E.

Scheme 10

Scheme 10

Proposed mechanism of synergistic antioxidant interaction between β-carotene and puerarin in liposome. Synergism is seen for 4′-propylpuerarin, where β-carotene (β-Car) is regenerated at the interface by 7-phenolate group. 7-Propylpuerarin is more lipophilic and less acidic and becomes an antioxidant in the lipid phase [60].

Scheme 11

Scheme 11

Schematic model of retinylisoflavonoid deposition in liposome [64].

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