Fast Regeneration of Carotenoids from Radical Cations
J. Phys. Chem. A, Vol. 114, No. 1, 2010 131
-5
TABLE 2: Second Order Rate Constants for Electron Transfer from Puerarin Dianion (1.2 × 10 M) and Daidzein Dianion
-5
#
-1
(
1.2 × 10 M) to Carotenoid Radical Cation in Methanol/Chloroform (1/9) and Activation Free Energy ∆G (kJ·mol ) at 25°
puerarin2
-
daidzein2-
k (l·mol- ·s-1
1
)
∆G /kJ·mol
#
-1
k (l·mol-1 ·s-1
)
∆G /kJ·mol
#
-1
carotenoid
9
9
ꢀ
-carotenea
zeaxanthin
(5.5 ( 0.1) × 10
(8.5 ( 0.1) × 10
(6.5 ( 0.1) × 10
(11.1 ( 0.1) × 10
17.4
16.3
11.3
10.0
(5.8 ( 0.3) × 10
(8.3 ( 0.2) × 10
(5.7 ( 0.1) × 10
(9.2 ( 0.1) × 10
17.3
16.4
11.6
10.4
a
9
9
b
10
10
10
10
canthaxanthin
astaxanthin
a
a
Reaction followed at 950 nm. b Reaction followed at 900 nm.
the more important group for electron transfer than the hydroxyl.
The presence of both hydroxyl and keto groups as in astaxanthin
therefore yields the highest rate of reaction. As for the
isoflavonoids, the rate of puerarin dianion reacting with caro-
tenoid radical cations is quite similar to that of daidzein dianion,
and the presence of glucoside in puerarin is not resulting in
any steric hindrance. The 4′-phenolate group, which notably is
the more reducing in both isoflavonoids, may be identified as
electron donor. The π-bond of the keto group of the carotenoid
δ-
δ+
is polarized according to O ) C accelerating the interaction
with the phenolate groups of the isoflavonoids.
#
Figure 6. Activation free energy ∆G as a function of ∆∆G°, reaction
free energy relative to reaction with ꢀ-carotene radical cation, for
electron transfer to carotenoid radical cations from puerarin or daidzein
Astaxanthin is found to be an efficient antioxidant despite
its low rank in the regeneration hierarchy among the carotenoids
as shown both experimentally and based on theoretical
#
dianion. From linear regression, the relation ∆G ) 0.55 · ∆∆G° +
1
7.0 is obtained.
calculations.2
1,37,38
On the basis of the high electron accepting
index, astaxanthin was recently classified as the best antiradical
substance among the carotenoids, while canthaxanthin, zeax-
anthin and especially ꢀ-carotene were concluded to be less
effective. This classification relates, however, to carotenoids
as “antireductants” rather than as an antioxidants, and under
conditions of oxidative stress, the reverse ordering applies with
Astaxanthin contains two hydroxyl and two keto groups which
make this carotenoid more hydrophilic than the parent ꢀ-car-
otene, a feature which has been associated with the antioxidative
properties of astaxanthin. In order to explore the roles of these
two functional groups, we have accordingly selected the four
carotenoids seen in Scheme 1, in which hydroxyl and keto
groups vary systematically facilitating structural comparison.
The reduction of the radical cations of these carotenoids by
the isoflavonoid dianions followed second-order kinetics and
the second-order rate constants are collected in Table 2. The
second-order rate constants are approaching the diffusion limit,
which may indicate that some preassociation is involved prior
to electron transfer as shown in eq 12. The reaction is an electron
transfer and the driving force for the reaction may be calculated
from the standard reduction potential for the carotenoid radical
cations and for the isoflavonoid radicals shown in Table 1. Since
the potentials for the carotenoid in dichloromethane need a
correction not available for specific solvation effects to be
comparable with the potential determined for puerarin and
daidzein dianions in aqueous solution, the ∆G° calculated for
the electron transfer reactions are in Table 1, expressed relative
38
ꢀ
-carotene as the best antiradical substance due to its highest
38
relative electron donating index among these carotenoids. Plant
phenols, like isoflavonoids, are good antioxidants and, as such,
also good antiradical substance. The major finding of the
39
present study is that when carotenoids under oxidative stress
act as electron donors and antioxidants rather than electron
acceptors and “antireductants”, the carotenoid radical cation
formed is efficiently reduced by the isoflavonoids, acting as good
electron donors. Moreover, the rate for this electron transfer
depends on the driving force as calculated from the standard
reduction potential, and the rate is approaching the diffusion
limit for the radical cation of astaxanthin, the carotenoid with
the highest electron accepting index.
Acknowledgment. This work has been supported by grants-
in-aid from Natural Science Foundation of China (Nos. 20673144
and 20803091) and from the Ministry of Science and Technol-
ogy of China (Nos. 2009CB220008 and 2006BAI08B04-06).
Continuing support from LMC (U-10 grant), Centre for
Advanced Food Studies to the Food Chemistry group at
University of Copenhagen, is gratefully acknowledged.
to the value for ꢀ-carotene as ∆∆G°. The activation free energy,
#
∆
G , is calculated using transition state theory from the second-
3
5
order rate constants and shown in Table 2. These thermody-
namic data together establish a free energy linear relationship
#
between ∆G and ∆∆G° with the slope 0.55 ( 0.04 V as seen
in Figure 6. The linearity indicates that a common mechanism
is operative for the different carotenoids, and the intermediate
value of the slope indicate that both the reactants and the
products have important contributions to the structure of the
transition state for electron transfer. A similar analysis for 13
vitamin E analogues and the stable 2,6-ditert-butyl-4-(4-meth-
References and Notes
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(
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36
oxylhexyl)phenoxyl radical led to a slope of 0.32 ( 0.04.
(
From a comparison of the four carotenoids, it is seen that
the astaxanthin radical cation as the most oxidizing radical is
reacting with the highest rate and faster than the radicals of
both canthaxanthin and zeaxanthin. Furthermore, canthaxanthin
reacts faster than zeaxanthin, suggesting the keto-group to be
(4) McNulty, H. P.; Byun, J.; Lockwood, S. F.; Jacob, R. F.; Mason,
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(
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(