title: Electron transfer from plant phenolates to carotenoid radical cations. Antioxidant interaction entering the Marcus theory inverted region.
abstract: β-Carotene, lycopene, and zeaxanthin are maximally regenerated by plant phenolates from their radical cations formed during laser flash photolysis in 9:1 (v/v) chloroform/methanol for a driving force corresponding to the reorganization energy according to the Marcus theory. For β-carotene, the reorganization energy has values of 0.41 ± 0.04 and 0.40 ± 0.04 eV for the plant phenols in the presence of 1 and 2 equiv of base, respectively, at 23 °C. For a driving force lower than the reorganization energy, regeneration of the carotenoids is less efficient as is seen for m-hydroxybenzoic acid, vanillic acid, and p-coumaric acid. For a driving force above the maximum rate as determined to have kET = 6.3 × 10(9) L·mol(-1)·s(-1) for syringic acid and β-carotene, the reaction becomes gradually slower and regeneration less efficient as is seen for the more reducing caffeic acid, rutin, and quercetin corresponding to an inverted region for the rate of electron transfer. Lycopene and zeaxanthin show a similar behavior for the same series of plant phenols with slightly lower reorganization energy, in agreement with the lower reduction potential of their radical cations, while, for the ketocarotenoids astaxanthin and canthaxanthin, fast reactions with a solvent of radical cations inhibit regeneration from being detected. Intermediate reducing plant phenols accordingly yield maximal protection of carotenoids against photobleaching in foods and beverages.
authors.affiliation: Department of Chemistry, Renmin University of China , Beijing 100872, China.
authors.forename: Hong
authors.initials: H
authors.lastname: Cheng
authors.forename: Rui-Min
authors.initials: RM
authors.lastname: Han
authors.forename: Jian-Ping
authors.initials: JP
authors.lastname: Zhang
authors.forename: Leif H
authors.initials: LH
authors.lastname: Skibsted
mesh_terms: D000975:Antioxidants / Q000737:chemistry*; D002338:Carotenoids / Q000737:chemistry*; D002412:Cations / Q000737:chemistry; D055598:Chemical Phenomena; D004579:Electron Transport; D005609:Free Radicals / Q000737:chemistry; D062385:Hydroxybenzoates / Q000737:chemistry*; D000077276:Lycopene; D010782:Photolysis; D010944:Plants / Q000737:chemistry*; D024341:Xanthophylls / Q000737:chemistry; D065146:Zeaxanthins; D019207:beta Carotene / Q000737:chemistry
chemical_list: D000975:Antioxidants; D002412:Cations; D005609:Free Radicals; D062385:Hydroxybenzoates; D024341:Xanthophylls; D065146:Zeaxanthins; D019207:beta Carotene; D002338:Carotenoids; C017616:phenolic acid; D000077276:Lycopene