H-atom transfer is a preferred antioxidant mechanism of curcumin

被引:370
作者
Jovanovic, SV
Steenken, S
Boone, CW
Simic, MG
机构
[1] Helix Int, Nepean, ON K2E 7R9, Canada
[2] Max Planck Inst Strahlenchem, D-4330 Mulheim, Germany
[3] Techlog Inc, Gaithersburg, MD USA
[4] NCI, Bethesda, MD 20892 USA
关键词
D O I
10.1021/ja991446m
中图分类号
O6 [化学];
学科分类号
0703 ;
摘要
Antioxidant mechanisms of curcumin, bis(4-hydroxy-3-methoxyphenyl)-1,6-heptadiene-3,5-dione, have been studied by laser flash photolysis and pulse radiolysis. The keto-enol-enolate equilibrium of the heptadienone moiety of curcumin determines its physicochemical and antioxidant properties. In neutral and acidic aqueous solutions (from pH 3 to 7), the keto form dominates, and curcumin acts as an extraordinarily potent H-atom donor. The reaction rate constant with the methyl radical (3.5 +/- 0.3) x 10(9) M-1 s(-1) is close to diffusion control in 40% aqueous DMSO at pH 5. The tert-butoxyl radical reacts with curcumin in acetonitrile solutions at a diffusion controlled rate, k = (7.5 +/- 0.8) x 10(9) M-1 s(-1). The apparent site of reaction is the central CH2 group in the heptadienone link, which has two labile hydrogens. This is supported by comparing the reaction patterns of curcumin and dehydrozingerone (DHZ) ("half-curcumin", 4-(4-hydroxy-3-methoxyphenyl)-3-buten-2-one). DHZ does not react With the methyl radical, indicating that the presence of the labile hydrogens is crucial for the H-atom donating ability of curcumin. The tert-butoxyl radical reacts with DHZ at almost an order of magnitude lower rate (1.1 +/- 0.1) x 10(9) M-1 s(-1), clearly abstracting an H-atom from the phenolic OH group. The reaction mechanism of curcumin changes dramatically above pH 8, where the enolate form of the heptadienone link predominates. As a consequence, the reaction of the methyl radical diminishes completely in alkaline media, and the phenolic part of the molecule takes over as (electron donor) reaction site. The electron donating ability of curcumin is assessed from the measurements of one-electron-transfer equilibria of DHZ radicals. Reduction potential of the DHZ phenoxyl radical, E(pH = 6.5) = 0.83 +/- 0.06 V, and E(pH = 13.0) = 0.47 +/- 0.06 V vs NHE, which may be expected for an ortho-methoxy-substituted phenoxyl radical, indicate only moderate electron-donating ability. The importance of H-atom donation vs electron donation in free radical scavenging and antioxidant mechanisms of curcumin is discussed.
引用
收藏
页码:9677 / 9681
页数:5
相关论文
共 35 条
  • [1] FORMATION OF CYCLIC SULFURANYL RADICALS OF THE TYPE [RSSR2]. FROM 1,3-BIS(ALKYLTHIO)PROPANES BY LASER FLASH-PHOTOLYSIS OF 1,N-BIS(ALKYLTHIO)ALKANES
    ANKLAM, E
    STEENKEN, S
    [J]. JOURNAL OF PHOTOCHEMISTRY AND PHOTOBIOLOGY A-CHEMISTRY, 1988, 43 (02) : 233 - 235
  • [2] Anti-tumour and free radical scavenging activity of synthetic curcuminoids
    Anto, RJ
    Kuttan, G
    Babu, KVD
    Rajasekharan, KN
    Kuttan, R
    [J]. INTERNATIONAL JOURNAL OF PHARMACEUTICS, 1996, 131 (01) : 1 - 7
  • [3] CHENG AL, AM SOC CLIN ONC 1998
  • [4] SPECTRAL AND PHOTOCHEMICAL PROPERTIES OF CURCUMIN
    CHIGNELL, CF
    BILSKI, P
    RESZKA, KJ
    MOTTEN, AG
    SIK, RH
    DAHL, TA
    [J]. PHOTOCHEMISTRY AND PHOTOBIOLOGY, 1994, 59 (03) : 295 - 302
  • [5] PICOSECOND AND NANOSECOND STUDIES OF THE PHOTOREDUCTION OF BENZOPHENONE BY N,N-DIETHYLANILINE AND TRIETHYLAMINE
    DEVADOSS, C
    FESSENDEN, RW
    [J]. JOURNAL OF PHYSICAL CHEMISTRY, 1991, 95 (19) : 7253 - 7260
  • [6] CURCUMIN-DERIVED TRANSIENTS - A PULSED-LASER AND PULSE-RADIOLYSIS STUDY
    GORMAN, AA
    HAMBLETT, I
    SRINIVASAN, VS
    WOOD, PD
    [J]. PHOTOCHEMISTRY AND PHOTOBIOLOGY, 1994, 59 (04) : 389 - 398
  • [7] GORMAN AA, 1989, HDB ORGANIC PHOTOCHE, V2, P229
  • [8] A SURVEY OF HAMMETT SUBSTITUENT CONSTANTS AND RESONANCE AND FIELD PARAMETERS
    HANSCH, C
    LEO, A
    TAFT, RW
    [J]. CHEMICAL REVIEWS, 1991, 91 (02) : 165 - 195
  • [9] ONE-ELECTRON REDUCTION OF NITROBENZENES BY ALPHA-HYDROXYALKYL RADICALS VIA ADDITION ELIMINATION - AN EXAMPLE OF AN ORGANIC INNER-SPHERE ELECTRON-TRANSFER REACTION
    JAGANNADHAM, V
    STSTEENKEN
    [J]. JOURNAL OF THE AMERICAN CHEMICAL SOCIETY, 1984, 106 (22) : 6542 - 6551
  • [10] O-H BOND STRENGTHS AND ONE-ELECTRON REDUCTION POTENTIALS OF MULTISUBSTITUTED PHENOLS AND PHENOXYL RADICALS - PREDICTIONS USING FREE-ENERGY RELATIONSHIPS
    JONSSON, M
    LIND, J
    ERIKSEN, TE
    MERENYI, G
    [J]. JOURNAL OF THE CHEMICAL SOCIETY-PERKIN TRANSACTIONS 2, 1993, (09): : 1567 - 1568