H-atom transfer is a preferred antioxidant mechanism of curcumin

Article abstract of DOI:10.1021/ja991446m

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) × 10⁹ M_-1 s^-1 is close to diffusion control in 40% aqueous DMSO at pH 5. The terf-butoxyl radical reacts with curcumin in acetonitrile solutions at a diffusion controlled rate, k = (7.5 ± 0.8) × 10⁹M^-1 s^-1. The apparent site of reaction is the central CU₂ 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 terf-butoxyl radical reacts with DHZ at almost an order of magnitude lower rate (1.1 ± 0.1) × 10⁹ 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.

Full text of DOI:10.1021/ja991446m

J. Am. Chem. Soc. 1999, 121, 9677-9681
9677
H-Atom Transfer Is A Preferred Antioxidant Mechanism of Curcumin
Slobodan V. Jovanovic,*,† Steen Steenken,‡ Charles W. Boone,§ and Michael G. Simic|
Contribution from Helix International, 381 Viewmount DriVe, Nepean, Ontario, Canada K2E 7R9,
Max-Planck-Institut fu¨r Strahlenchemie, Mu¨lheim, Germany, National Cancer Institute,
Bethesda, Maryland, Techlogic Inc., Gaithersburg, Maryland
ReceiVed May 3, 1999. ReVised Manuscript ReceiVed July 7, 1999
Abstract: 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) × 109 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) × 109 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-methoxy-
phenyl)-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) × 109 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.
Introduction
Curcumin (1) is a yellow pigment of turmeric, a spice
manufactured from the root of Curcuma longa. Turmeric is one
of the major spices in certain Asian cuisines, notably Indian. It
has been argued that increased dietary curcumin leads to
improved chemoprevention of cancer,1-6 possibly through
inhibition of the growth of blood capillaries in the cancerous
tissue.7,8 Curcumin also has bactericidal action9 and may
minimize oxidative damage through free radical scavenging.6,9-14
In view of the importance of dietary antioxidants in chemo-
prevention of degenerative diseases, such as cancer, Alzheimer’s,
Parkinson’s, and cardiovascular diseases, antioxidant and chemo-
preventive mechanisms of curcumin have been thoroughly
investigated. Bactericidal action has been linked to a photody-
namic effect.9,10 Low photochemical yield of singlet oxygen in
benzene, φ ) 0.12,9 is characteristic for an n,π* triplet
precursor.15 On the other hand, the triplet energy 191 kJ/mol in
benzene9 is fairly low and consistent with a π,π* triplet.
Quenching of singlet oxygen by curcumin is surprisingly slow,
k ) 2.5 × 105 M-1 s-1.9 Free radical scavenging ability2,6,9-14
† Helix International. Telephone/fax: (613) 225-1105. E -mail: bq295@
freenet.carleton.ca.
‡ Max-Planck-Institut fu¨r Strahlenchemie.
§ National Cancer Institute.
| Techlogic Inc.
(1) Cheng, A. L.; Lin, J. K.; Hsu, M. M.; Shen, T. S.; Ko, J. K.; Lin, J.
T.; Wu, M. S.; Yu, H. S.; Jee, S. H.; Chen, G. S.; Chen, T. M.; Chen, C.
A.; Lai, M. K.; Pu, Y. S.; Pan, M. H.; Wang, Y. J.; Tsai, C. C.; Hsieh, C.
Y. American Society of Clinical Oncology 1998 Annual Meeting. Ab-
stract: 2140. http://www.asco.org/prof/sh/f•sh.htm.
(2) Venkatesan, N. Br. J. Pharmacol. 1998, 124, 425.
(3) Singh, S. V.; Hu, X.; Srivastava, S. K.; Singh, M.; Xia, H.; Orchard,
J. L.; Zaren, H. A. Carcinogenesis 1998, 19, 1357.
(4) Rajakrishnan, V.; Viswanathan, P.; Rajasekaran, K. N.; Gunashekaran,
G.; Menon, V. P. Med. Sci. Res. 1998, 26, 715.
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Carcinogenesis 1998, 19, 419.
(6) Anto, R. J.; Kuttan, G.; Babu, K. V. D.; Rajasekharan, K. N. Int. J.
Pharm. 1996, 131, 1.
(7) Stavric, B. Clin. Biochem. 1994, 27, 319.
(8) Stavric, B. Food Chem. Toxicol. 1994, 32, 79.
(9) Gorman, A. A.; Hamblett, I.; Srinavasan, V. S.; Wood, P. D.
Photochem. Photobiol. 1994, 59, 389.
(10) Chignell, C. F.; Bilski, P.; Reszka, K. J.; Motlen, A. G.; Sik, R. H.;
Dahl, T. A. Photochem. Photobiol. 1994, 59, 295.
(11) Priyadarsini, K. I. Free Radical Biol. Med. 1997, 23, 1997.
(12) Sreejayan, N.; Rao, M. N. A. Arzneim.-Forsch. 1996, 46, 169.
(13) Sreejayan, N.; Rao, M. N. A. J. Pharm. Pharmacol. 1997, 49, 105.
(14) Unnikrishnan, M. K.; Rao, M. N. A. Mol. Cell. Biochem. 1995,
146, 35.
(15) Gorman, A. A.; Rodgers, M. A. J. In Handbook of Organic
Photochemistry; Scaiano, J. C., Ed.; CRC Press: Boca Raton, FL, 1989; p
229.
10.1021/ja991446m CCC: $18.00 © 1999 American Chemical Society
Published on Web 10/04/1999

9678 J. Am. Chem. Soc., Vol. 121, No. 41, 1999
JoVanoVic et al.
UV-vis spectra were measured on a Shimadzu UV-vis, and a HP
8450A diode array spectrophotometer. Supra-sil quartz cuvettes (1 and
10 mm) were used.
and inhibition of lipid peroxidation have been attributed to
electron donation from the phenolic part of curcumin.
Phenolic antioxidants usually scavenge damaging free radicals
by an electron-transfer mechanism. The electron-donating ability
is determined by the one-electron oxidation potential of the
parent antioxidants, expressed by definition as the reduction
potential of the corresponding phenoxyl radicals. Using empiri-
cal linear-energy relations,16-18 the oxidation potential of
curcumin can be estimated at pH 7 as E7 ) 0.77 V (similar to
that of hesperidin).19 This is considerably higher than E7(vitamin
E) ≈ 0.48 V or E7(vitamin C) ) 0.28 V,20 which means that
the phenoxyl radical of curcumin can oxidize vitamin E or
vitamin C. It is also higher than the potential of some other
lipid soluble antioxidants, such as methyl gallate (E7 ) 0.52
V) or quercetin (E7 ) 0.33 V).21 Surely, such high oxidation
potential cannot be the basis for the observed excellent
antioxidant properties of curcumin.
The 3 MeV van-de-Graaff pulse radiolysis equipment with optical
detection at the Max-Planck-Institut fu¨r Strahlenchemie22 was used for
the pulse radiolysis studies. A 2 cm Supra-sil quartz cell with
temperature variation through a thermostatically controlled liquid jacket
was used for sample irradiation. The spectra of the radicals were
measured at 5-10 Gy/pulse, whereas the rate constants were determined
at considerably lower 1-2 Gy/pulse to minimize interference from
radical-radical reactions. Thiocyanate dosimetry was used in dose
determinations, assuming G[(SCN)2•-]) 6.0 in N2O-saturated 10 mM
KSCN aqueous solutions.
Fully computerized laser photolysis (λ) 248 nm) at the Max-Planck-
Institut23 was used for photochemical investigations.
Results and Discussion
H-Atom Transfer Reactions of Curcumin. Curcumin, bis-
(4-hydroxy-3-methoxyphenyl)-1,6-heptadiene-3,5-dienone, un-
dergoes proton-transfer equilibria with pKa1 ) 8.55 ( 0.05 and
pKa2 ) 10.41 ( 0.05 (the pKa values were determined from
the pH dependent spectral changes). The ionized curcumin is
more water soluble and expected to be a better electron donor
than the nonionized form. On the other hand, the nonionized
curcumin may exist in the keto form, which may be an excellent
H-atom donor as discussed above. It is conceivable that the keto
form of curcumin predominates in acidic and neutral aqueous
solutions and in the cell membrane.
What makes curcumin a superb antioxidant? In the keto form
of curcumin, the heptadienone linkage between the two meth-
oxyphenol rings contains a highly activated carbon atom
(highlighted in 1). It is obvious that the C-H bonds on this
carbon should be very weak, due to delocalization of the
unpaired electron on the adjacent oxygens:
•
The methyl radical, CH3, is generated from (CH3)2SO by
pulse radiolysis of N2O-saturated aqueous solutions of 5 M
dimethyl sulfoxide at pH ) 5.0 in the following series of
reactions,24
H2O ' H, •OH, e-aq, H3O+, etc...
(1)
(2)
If this is the case, this group can serve as an H-atom donor.
We have investigated by laser flash photolysis and pulse
radiolysis in aqueous and acetonitrile solutions the ability of
curcumin to donate an H-atom. Our results clearly show that
the H-atom donation is a preferred reaction of curcumin at pH
e 7 and in nonprotic solvents. The antioxidant mechanisms of
curcumin in vivo, and its interactions with other physiological
and dietary antioxidants are discussed in the light of this novel
possibility of action.
e-aq + N2O + H3O+ f •OH + H2O + N2
•OH + (CH3)2SO f •CH3 + CH3SO2- + H3O+ (3)
The methyl radical was found to react rapidly with curcumin
in the concentration range from 0.09 to 0.28 mM. The spectrum
of the resulting curcumin radical is shown in Figure 1.
The rate of the methyl radical reaction is found to be
dependent on the concentration of curcumin, from which the
reaction rate constant is derived as k ) (3.5 ( 0.3) × 109 M-1
s-1. To the best of our knowledge, this is the highest rate of
the reaction of the methyl radical with any substrate. It is
considerably higher than the rates of H-abstraction25,26 from
Materials and Methods
Curcumin (99.5% purity) was a generous gift of Mr. R. Kaskey of
R-Kane, Inc, Pennsauken, NJ. All other chemicals were of the highest
purity commercially available. 4-(4-hydroxy-3-methoxybenzene)-buten-
3-one() DHZ, i.e., “half-curcumin”), benzophenone, and diethylamine
were obtained from Aldrich, promethazine hydrochloride from Sigma,
N,N,N′,N′-tetramethylene-p-phenylenediamine hydrochloride from Flu-
ka, dimethyl sulfoxide, acetonitrile, 2-propanol, acetone, and phosphate
buffer were the products of Merck. Water was purified through a
Millipore MilliQ system to a resistivity better than 18 MΩ/cm. Prior
to pulse radiolysis or flash photolysis experiments, the solutions were
either deaerated by passing high purity Ar for 20 min or saturated with
N2O to convert hydrated electrons to OH radicals.
either aliphatic k(•CH3 + GSH, at pH ) 7) ) 5 × 107 M-1 s-1
,
aromatic k(•CH3 + 4-methoxythiophenol, at pH ) 3) ) 1.3 ×
108 M-1 s-1 or heterocyclic thiols, k(•CH3 + 1-methyl-5-ethyl-
4-mercaptoimidazole, at pH ) 7.0)) 1.5 × 105 M-1 s-1
.
The reaction of the methyl radical with curcumin is pH-
dependent. The rate of the reaction decreases with the increase
in the pH from 5 to 9. This is interpreted as due to concomitant
decrease in the concentration of the keto form. At pH 9.5, the
build up of the curcumin radical is completely suppressed.
(16) Lind, J.; Shen, X.; Eriksen, T. E.; Merenyi, G. J. Am. Chem. Soc.
1990, 112, 479.
(17) Jonsson, M.; Lind, J.; Eriksen, T. E.; Merenyi, G. J. Chem. Soc.,
Perkin Trans. 2 1993, 1567.
(18) Jovanovic, S. V.; Tosic, M.; Simic, M. G. J. Phys. Chem. 1991,
95, 10824.
(19) Jovanovic, S. V.; Steenken, S.; Tosic, M.; Marjanovic, B.; Simic,
M. G. J. Am. Chem. Soc. 1994, 116, 4846.
(20) Steenken, S.; Neta, P. J. Phys. Chem. 1982, 86, 3661.
(21) Jovanovic, S. V.; Steenken, S.; Simic, M. G.; Hara, Y. In FlaVonoids
in health and disease; Rice-Evans, C. A., Packer, L., Eds.; Marcel Dekker:
New York, 1998; p 137.
(22) Jagannadham, V.; Steenken, S. J. Am. Chem. Soc. 1984, 106, 6542.
(23) Anklam, E.; Steenken, S. J. Photochem. Photobiol. 1988, 43A, 233.
(24) Veltwisch, D.; Janata, E.; Asmus, K.-D. J. Chem. Soc., Perkin Trans.
2 1980, 146.
(25) Jovanovic, S. V.; Simic, M. G. In Anticarcinogenesis and radiation
Protection 2; Nygaard, O. F., Upton, C. A., Eds.; Plenum Press: New York,
1991; p 229.
(26) Marjanovic, B.; Simic, M. G.; Jovanovic, S. V. Free Radical Biol.
Med. 1995, 18, 679.

Antioxidant Mechanism of Curcumin
J. Am. Chem. Soc., Vol. 121, No. 41, 1999 9679
Figure 1. Spectra of curcumin transients obtained by -O- pulse
radiolysis of 0.087 mM curcumin in N2O-saturated aqueous solution
of 40% DMSO at pH 4.80, D ) 2 Gy/pulse, and -9- 248 nm laser
photolysis of Ar-saturated acetonitrile solution of 0.074 mM curcumin
and 1.27 M tert-butylperoxide.
abstraction from the phenolic O-H in curcumin would account
for only ∼15% of the reaction.
H-atom transfer reactions of curcumin were also investigated
using the benzophenone ketyl and 2-propanol radicals.
The methyl radical did not react (k < 105 M-1 s-1 at pH 5.0)
with dehydrozingerone, DHZ, 4-(4-hydroxy-3-methoxyphenyl)-
3-buten-2-one:
The photochemically generated (by 248 nm laser photolysis
of 1 mM benzophenone, 0.1 M diethylamine, 0.1 M HCl in
acetonitrile) benzophenone ketyl radical, Ph2C•OH, reacted with
curcumin at k ) (1.5 ( 0.1) × 1010 M-1 s-1 in acetonitrile
solutions.
The radiolytically generated (pulse radiolysis of N2O-saturated
aqueous solution of 40% 2-propanol, 10% acetone at pH 5.0)
2-propanol radical, (CH3)2C•-OH, reacted with curcumin at k
) (7.1 ( 0.7) × 107 M-1 s-1
.
The spectra of the resulting curcumin radicals from the ketyl
and 2-propanol radical reactions are shown in Figure 2. As seen
from Figure 2, in addition to the 490 nm band characteristic of
the carbon centered curcumin radical, there is a broad band at
∼750 nm. Clearly, the spectra of curcumin radicals generated
by the ketyl and 2-propanol radicals indicate that the H-atom
transfer from the central CH2 group is a predominant mechanism
of reaction (see Figures 1 and 2). However, on the basis of the
similarity of the spectra in Figure 2 with the spectra of
benzophenone27,28 and naphthoketone29 ketyl radicals, and the
well-known reducing nature of 2-propanol radical,30 we hy-
pothesize that the 750 nm band is due to the curcumin ketyl
radical, generated in an apparently minor reaction by the
reduction of a carbonyl group followed by rapid protonation of
incipient curcumin radical anion.
At this pH DHZ (“half-curcumin”) is in the keto form, but
the terminal CH3 group is apparently a considerably poorer
H-atom donor than the central CH2 in curcumin.
We conclude that the methyl radical abstracts an H-atom from
the central CH2 group in curcumin, as indicated in reaction 4,
Electron-Transfer Reactions. Electron-transfer reactions of
curcumin have been the focus of interest of free radical
chemists.9-11 The methoxyphenol moiety was singled out as
the most probable antioxidant reaction site. It is a common
prejudice that a phenolic part of any molecule is always
responsible for the antioxidant activity. Actually, the two ortho-
methoxyphenols in curcumin can only be very moderate electron
donors on the basis of the reduction potentials of the corre-
sponding ortho-phenoxyl radicals. Taking the empirical for-
mula21
Photochemically generated (by 248 nm laser photolysis of
Ar-saturated acetonitrile solutions of 1.3 M tert-butylperoxide)
tert-butoxyl radical, (CH3)3CO•, was found to react with
curcumin by abstracting the H-atoms from the central CH2 group
and, to a considerably smaller extent if at all, from the phenolic
OH group as indicated below (spectrum of the resulting
curcumin radicals is shown in Figure 1).
E7 (in V vs NHE) ) 0.95 + 0.31Σσ+
The reaction rate constant, k ) (7.5 ( 0.8) × 109 M-1 s-1
,
is close to diffusion controlled. This should be compared with
almost an order of magnitude slower H-abstraction from the
phenolic O-H group in DHZ, k[(CH3)3C-O• + DHZ] ) (1.1
( 0.1) × 109 M-1 s-1. From the difference between the rates
of reaction with curcumin and DHZ (“half-curcumin”), H-
(27) Devadoss, C.; Fessenden, R. W. J. Phys. Chem. 1991, 95, 7253.
(28) Peters, K. S.; Lee, J. J. Phys. Chem. 1993, 97, 3761.
(29) Jovanovic, S. V.; Morris, D. G.; Pliva, C. N.; Scaiano, J. C. J.
Photochem. Photobiol. Chem. 1997, 107, 153.
(30) Wardman, P. J. Phys. Chem. Ref. Data 1989, 18, 1637.

9680 J. Am. Chem. Soc., Vol. 121, No. 41, 1999
JoVanoVic et al.
) 0.99 V, we calculate the reduction potential of the DHZ
phenoxyl radicals as
E6.5 ) 0.99 - 0.13 ) 0.86 V
which is in a fair agreement with the calculated E6.5 ) 0.80
V.33
Our value of the reduction potential of the DHZ phenoxyl
radical is considerably lower than the recently published E6 )
1.09 V.34 The latter value was determined against the azide
redox couple as a reference, with an equilibrium constant of
∼104. We believe that our value is more accurate because the
promethazine radical cation oxidizes half-curcumin, which
means that the reduction potential of corresponding phenoxyl
radicals must be lower than 0.99 V. In addition, the accuracy
of any equilibrium measurements in the azide-phenol system,
where both azide and phenoxyl radicals decay at k ≈ 109 M-1
s-1, is bound to large experimental error.
We have also studied the one-electron reduction of the DHZ
phenoxyl radicals at pH 13.0 by N,N,N′,N′-tetramethyl-p-
phenylenediamine‚ (TMPD), E13(TMPD•+/TMPD) ) 0.26 V.20
The DHZ phenoxyl radical was generated in an N2O-saturated
aqueous solution containing from 1 to 2.1 mM half-curcumin,
0.1 M KOH, and 0.1 M KBr. The DHZ phenoxyl radical
oxidized TMPD to the corresponding radical
Figure 2. Spectra of curcumin transients obtained by -O- pulse
radiolysis of N2O-saturated aqueous solution of 0.73 mM curcumin,
35% 2-propanol, and 6% acetone at pH 5.0, D ) 2 Gy/pulse and -9-
248 nm laser photolysis of Ar-saturated acetonitrile solution of 0.029
mM curcumin, 1 mM benzophenone, 1 M Diethylamine.HCl.
where σ+ is the Brown substituent constant,31 the calculated
reduction potential of the orthomethoxyphenoxyl curcumin
radical is
E7 ) 0.95 + 0.31(+0.39 - 0.96) ) 0.77 V
The actual reduction potential of the curcumin phenoxyl
radicals is very difficult to determine from the equilibrium
kinetics, given the very low solubility of curcumin in aqueous
solutions, intensely colored solutions at high pH and the
possibility of generating more than one curcumin radical. We
have, therefore, used DHZ (“half-curcumin”), from which the
phenoxyl radical can be generated exclusively and which is
somewhat more soluble in aqueous solutions.
MeO-Ph-O• + TMPD f TMPD•+ + MeO-Ph-O-
(7)
From the TMPD concentration dependent build-up of absor-
bance at 560 nm
kf ) (2.6 ( 0.3) × 109 M-1 s-1
The reduction potential of the DHZ phenoxyl radical, E6.5
)
The reduction potential difference is calculated from the
equilibrium constant derived from the absorbances of the TMPD
radical cation at equilibrium, Kabs) 375
0.86 V vs NHE, was determined from the electron-transfer
equilibrium with promethazine radical cation at pH 6.5. The
promethazine radical cation was generated by pulse radiolysis
in an N2O-saturated aqueous solution of 3.11-4.18 mM
promethazine‚HCl, 0.1 M KBr, and 3 mM phosphate buffer.
Upon addition of DHZ in concentrations from 0.016 to 0.17
mM, the promethazine radical cation, Pz•+, oxidized DHZ,
MeO-Ph-OH, to the corresponding phenoxyl radical in an
apparent electron-transfer reaction
∆E ) 0.059 log 375 ) 0.15 V
Taking E13(TMPD•+/TMPD) ) 0.26 V20 we calculate the
reduction potential of the DHZ radicals as E13 ) 0.41 V.35
pKa ) 8.0 ( 0.1 of DHZ, determined by UV-vis spectros-
copy, cannot account for a ∼0.4 V drop in the reduction
potential of the radical from pH 6.5 to pH 13. However, we
have not investigated further the pH dependence of the reduction
potential of the DHZ phenoxyl radical.
kf
Pz•+ + MeO-Ph-OH y
\
kz Pz + MeO-Ph-O• + H+ (6)
r
The reduction potential of the phenoxyl radical of DHZ is
relatively high in comparison with well-known antioxidants-
electron donors, such as vitamin E (E7 ) 0.48 V; E13 ) 0.19
V),20 gallocatechins (E7 ) 0.44 V)36 or vitamin C (E7 ) 0.28
V).20 In fact, it was reported9 that curcumin phenoxyl was
From the measured pseudo-first-order rate constants, using
the standard procedure,32 the following values are obtained:
kf ) (1.8 ( 0.2) × 108 M-1 s-1
,
kr ) (7.0 ( 1.4) × 105 M-1 s-1, and Kkin ) 260
(33) While our manuscript was being reviewed we found that 4-meth-
oxyphenoxyl radical, generated by pulse radiolysis in N2O-saturated aqueous
solution of 0.1 M KBr, oxidizes DHZ with kf ) 2.2 × 108 M-1 s-1 and kr
) 8 × 107 M-1 s-1 (Kkin ) 3; Kabs ) 3). Based on E6.5(4-methoxyphenol)
) 0.80 V/NHE, E6.5 ) 0.77 V/NHE is derived for DHZ radical. Taking
this value and adding E6.5 ) 0.86 V vs Pz, we derive E6.5 ) 0.83 ( 0.06
V.
We choose the more accurate equilibrium constant, which is
derived from the absorbances of the promethazine radical at
equilibrium, Kabs ) 146, to calculate the redox potential
difference
(34) Priyadarsini, K. I.; Devasagayam, T. P. A.; Rao, M. N. A.; Guha,
S. N. Radiat. Phys. Chem. 1999, 54, 551.
∆E ) 0.059 log 146 ) 0.13 V
(35) While our manuscript was being reviewed, we found that DHZ
radical oxidized 3,5-dihydroxyanisole at pH 13 in N2O-saturated aqueous
solution of 0.1 M KBr, with kf ) 8.6 × 108 M-1 s-1 and kr ) 1.5 × 108
M-1 s-1 (Kkin ) 5.7; Kabs ) 4.6). Based on E13(3,5-DHA) ) 0.46 V, we
calculate E13(DHZ) ) 0.5 V. Taking this as a more accurate and adding
0.41 V determined against TMPD, E13(DHZ) ) 0.47 ( 0.06 V.
(36) Jovanovic, S. V.; Hara, Y.; Steenken, S.; Simic, M. G. J. Am. Chem.
Soc. 1995, 117, 9881.
From the redox potential difference, and the literature value of
the reduction potential of the promethazine radical cation,32
E
(31) Hansch, C.; Leo, A.; Taft, R. W. Chem. ReV. 1991, 91, 165.
(32) Jovanovic, S. V.; Steenken, S.; Simic, M. G. J. Phys. Chem. 1990,
94, 3583.

Antioxidant Mechanism of Curcumin
J. Am. Chem. Soc., Vol. 121, No. 41, 1999 9681
capable of oxidizing vitamin E and vitamin C, in complete
agreement with the reduction potentials of corresponding
radicals.
On the basis of the rate constants of H-atom transfer reactions,
curcumin is a superb H-atom donor. It appears to be better than
such well-known, “classical” H-atom donors as thiols. k(•CH3
+ curcumin) ) 3.5 × 109 M-1 s-1 is more than an order of
magnitude higher than the fastest known k(•CH3+ 4-methoxy-
thiophenol, at pH 3) ) 1.3 × 108 M-1 s-1. It may, therefore,
be concluded that H-atom transfer plays a crucial role in the
antioxidant action of curcumin.
The one-electron reduction potential of the methoxyphenol
moiety in curcumin, as estimated from the measurements of
the reduction potential of the DHZ (“half-curcumin”) phenoxyl
radical, is relatively high, E6.5 ) 0.83 ( 0.06 V.33 Although
this enables scavenging of alkylperoxyl radicals by electron
transfer, it is much higher than E7 ) 0.48 V of vitamin E and
E7 ) 0.28 V of vitamin C. In agreement with this, curcumin
transients can oxidize vitamin E and C to the corresponding
free radicals.
The oxidation potential of DHZ (which is very similar to that
of curcumin) is lower than E7 ) 1.05 V of alkyl peroxyl
radicals,37 which means that curcumin can neutralize alkyl
peroxyl radicals, such as lipid peroxyl, by donating an electron.
Such moderate electron donating ability and relative insolu-
bility in water, which is an ideal medium for electron-transfer
processes involving peroxyl radicals,38 are likely to diminish
the importance of electron transfer in antioxidant mechanisms
of curcumin.
Conclusions
Curcumin is practically insoluble in water at neutral pH. In
slightly acidic media and possibly in the interior of cell
membranes, it is likely to exist in the keto form. This form
appears to favor H-atom transfer reactions.
JA991446M
(38) Neta, P.; Huie, R. E.; Maruthamuthu, P.; Steenken, S. J. Phys. Chem.
1989, 93, 7654.
(37) Jovanovic, S. V.; Jankovic, I.; Josimovic, L. 1992, 114, 9018.