β-carotene radical cation addition to green tea polyphenols. Mechanism of antioxidant antagonism in peroxidizing liposomes.
Green tea polyphenols, (-)-epicatechin (EC), (-)-epigallocatechin (EGC), (-)-epicatechin gallate (ECG), and (-)-epigallocatechin gallate (EGCG), all showed antioxidative effect in liposomes for lipid oxidation initiated in the lipid phase (antioxidant efficiency EC > EGCG > ECG > EGC) or in the aqueous phase (EC ≫ EGC > EGCG > ECG) as monitored by the formation of conjugated dienes. For initiation in the lipid phase, β-carotene, itself active as an antioxidant, showed antagonism with the polyphenols (EC > ECG > EGCG > EGC). The Trolox equivalent antioxidant capacity (TEAC EGC > EGCG > ECG > EC) correlates with the lowest phenol O-H bond dissociation enthalpy (BDE) as calculated by density functional theory (DFT). Surface-enhanced Raman spectroscopy (SERS) was used to assess the reducing power of the phenolic hydroxyls in corroboration with DFT calculations. For homogeneous (1:9 v/v methanol/chloroform) solution, the β-carotene radical cation reacted readily with each of the polyphenol monoanions (but not with the neutral polyphenols) with a rate approaching the diffusion limit for EC as studied by laser flash photolysis at 25 °C monitoring the radical cation at 950 nm. The rate constant did not correlate with polyphenol HOMO/LUMO energy gap (DFT calculations), and β-carotene was not regenerated by an electron transfer reaction (monitored at 500 nm). It is suggested that the β-carotene radical cation is rather reacting with the tea polyphenols through addition, as further evidenced by steady-state absorption spectroscopy and liquid chromatography-mass spectroscopy (LC-MS), in effect preventing regeneration of β-carotene as an active lipid phase antioxidant and leading to the observed antagonism.
Department of Chemistry, Renmin University of China, Beijing 100872, China.
Lin-Lin
LL
Song
Ran
R
Liang
Dan-Dan
DD
Li
Ya-Dong
YD
Xing
Rui-Min
RM
Han
Jian-Ping
JP
Zhang
Leif H
LH
Skibsted
D000975:Antioxidants; D002412:Cations; D005609:Free Radicals / Q000737:chemistry / Q000494:pharmacology; D015227:Lipid Peroxidation / Q000187:drug effects*; D008081:Liposomes / Q000737:chemistry*; D059808:Polyphenols / Q000037:antagonists & inhibitors* / Q000737:chemistry; D013662:Tea / Q000737:chemistry*; D019207:beta Carotene / Q000737:chemistry* / Q000494:pharmacology
D000975:Antioxidants; D002412:Cations; D005609:Free Radicals; D008081:Liposomes; D059808:Polyphenols; D013662:Tea; D019207:beta Carotene