First direct observation of reversible oxygen addition to a carotenoid-derived carbon-centered neutral radical

Article abstract of DOI:10.1021/ol051436s

(Chemical Equation Presented) Direct observation of reversible oxygen addition to a carotenoid-derived carbon-centered neutral radical is reported for the first time. The influence of temperature on the observed reaction kinetics has been used to obtain k

Full text of DOI:10.1021/ol051436s

ORGANIC
LETTERS
2005
Vol. 7, No. 18
3957-3960
First Direct Observation of Reversible
Oxygen Addition to a
Carotenoid-Derived Carbon-Centered
Neutral Radical
Ali El-Agamey† and David J. McGarvey*
School of Physical and Geographical Sciences, Keele UniVersity,
Keele, Staffordshire ST5 5BG, U.K.
d.j.mcgarVey@chem.keele.ac.uk
Received June 21, 2005
ABSTRACT
Direct observation of reversible oxygen addition to a carotenoid-derived carbon-centered neutral radical is reported for the first time. The
influence of temperature on the observed reaction kinetics has been used to obtain kinetic and thermodynamic parameters relating to the
•
reversible addition of oxygen to the carotenoid radical obtained from reaction of 7,7
′
-dihydro-
â
-carotene (77DH) with phenylthiyl radical (PhS )
in benzene. In addition, the rate constant for oxygen addition to the equivalent -carotene (â-CAR) derived radical is also reported.
â
The reactions of carbon-centered free radicals with oxygen
have been the subject of numerous studies,1 and an under-
standing of such reactions is important for elucidation of the
mechanisms of free-radical-mediated oxidations in biological
and polymer systems, as well as for understanding the
effectiveness of the protection against such oxidations
afforded by radical-scavenging chain-breaking antioxidants.
of a number of radicals derived from synthetic organic
antioxidants. In addition, Porter et al.4 have reported the
results of detailed experimental and theoretical studies of
the kinetics of lipid peroxidation, where it is clear that the
dynamics of the reversible addition of oxygen to conjugated
lipid-derived carbon-centered radicals is important in ac-
counting for peroxidation rates and product distributions.
Recent work by Scaiano,2 and others3 has highlighted
various structural influences on the reactivity toward oxygen
(2) (a) Frenette, M.; Aliaga, C.; Font-Sanchis, E.; Scaiano, J. C. Org.
Lett. 2004, 6, 2579. (b) Font-Sanchis, E.; Aliaga, C.; Cornejo, R.; Scaiano,
J. C. Org. Lett. 2003, 5, 1515. (c) Font-Sanchis, E.; Aliaga, C.; Focsaneanu,
K.-S.; Scaiano, J. C. Chem. Commun. 2002, 1576. (d) Bejan, E. V.; Font-
Sanchis, E.; Scaiano, J. C. Org. Lett. 2001, 3, 4059. (e) Scaiano, J. C.;
Martin, A.; Yap, G. P. A.; Ingold, K. U. Org. Lett. 2000, 2, 899.
(3) (a) Perez-Prieto, J.; Galian, R. E.; Miranda, M. A.; Catalina, F.;
Martin-Vargas, N.; Lopez-Ortiz, F. Org. Lett. 2004, 6, 561. (b) Magnus,
P.; Venable, J. D.; Shen, L.; Lynch, V. Tetrahedron Lett. 2005, 46, 707.
(c) Pratt, D. A.; Porter, N. A. Org. Lett. 2003, 5, 387.
† Permanent address: Chemistry Department, Faculty of Science, New-
Damietta, Damietta, Egypt.
(1) (a) Neta, P.; Huie, R. E.; Mosseri, S.; Shastri, L. V.; Mittal, J. P.;
Maruthamuthu, P.; Steenken, S. J. Phys. Chem. 1989, 93, 4099. (b)
Hasegawa, K.; Patterson, L. K. Photochem. Photobiol. 1978, 28, 817. (c)
Maillard, B.; Ingold, K. U.; Scaiano, J. C. J. Am. Chem. Soc. 1983, 105,
5095. (d) Neta, P.; Huie, R. E.; Ross, A. B. J. Phys. Chem. Ref. Data 1990,
19, 413.
10.1021/ol051436s CCC: $30.25
© 2005 American Chemical Society
Published on Web 08/11/2005

In 1984, Burton and Ingold5 suggested that â-CAR
scavenges peroxyl radicals by addition to the conjugated
system of double bonds, yielding a resonance-stabilized
carbon-centered radical (ROOCAR•). They presented evi-
dence that â-CAR functions as a very effective chain-
breaking antioxidant, but as oxygen pressure is increased the
effectiveness of â-CAR as an antioxidant is reduced. The
reversible reaction of resonance-stabilized radicals (e.g.,
ROOCAR•) with oxygen to generate potential chain-carrying
peroxyl radicals (ROOCAROO•) was postulated (Scheme S1,
Supporting Information).
much longer lived, lasting tens to hundreds of milliseconds
rather than hundreds of microseconds as in the case of
carotenoid radicals derived from acylperoxyl radical addition
to carotenoids. The carotenoids studied are shown in Figure
1. Studies of the addition reactions of thiyl radicals with
Effective chain-breaking antioxidants (e.g., R-tocopherol)
are characterized by potent radical-scavenging properties
coupled with poor chain-carrying properties of the antioxi-
dant-derived radical that reflect its lack of reactivity toward
oxygen.6 The radical scavenging properties of carotenoids
in the context of their antioxidant properties have been the
subject of numerous studies.7 However, the reaction of
carotenoid-derived carbon-centered neutral radicals (e.g.,
ROOCAR•) with oxygen has not previously been directly
observed. However, reported rate constants for the reaction
of oleate (n ) 1) and linoleate (n ) 2) radicals with oxygen
are 1 and 0.3 × 109 M-1 s-1, respectively, suggesting that
the rate constant for the reaction with oxygen decreases as
the number (n) of conjugated double bonds in the free radical
increases.1b
Figure 1. Structures of carotenoids used in this study.
carotenoids have been reported previously; however, the
influence of oxygen concentration on the decay kinetics of
the addition radicals has not been investigated.8
Laser photolysis (355 nm) of phenyl disulfide (PhS-SPh)
in benzene forms phenylthiyl radicals (PhS•), which exhibit
a broad absorption band in the 400-500 nm region and decay
by second-order kinetics (see Figures S1 and S2 and eq 1)
with a first half-life of ∼50 µs at room temperature under
our experimental conditions (i.e., for a laser energy of ∼1
mJ). The decay of PhS• is not influenced by oxygen9 in the
range of oxygen concentrations used in this study (up to
∼0.01 M).
Previously we studied the influence of oxygen on the decay
of carotenoid-derived carbon-centered neutral radicals gener-
ated by reaction of the carotenoid with acylperoxyl radicals.7a
However, the carotenoid-derived radicals formed in such
reactions undergo unimolecular SHi (epoxidation) processes
with rate constants around 4 × 103 s-1, and we were not
able to detect any influence of oxygen on the decay of such
radicals using oxygen concentrations up to ∼10-2 M,
suggesting that rate constants for reaction with oxygen do
not exceed 105 M-1 s-1 for these carotenoid addition radicals.
PhS-SPh {hν
\
} 2 PhS•
(1)
In this letter we report the results of laser flash photolysis
experiments in which we have studied the influence of
oxygen on the decay rates of carotenoid-derived carbon-
centered neutral radicals generated from reaction of the
carotenoids with phenylthiyl radicals (PhS•) in benzene. Such
carotenoid radicals do not undergo SHi decay and thus are
Laser photolysis (355 nm) of benzene solutions of ca.
10-3-10-2 M PhS-SPh in the presence of ∼2 × 10-5
M
77DH leads to the formation (on a microsecond time scale)
of an exceptionally intense transient visible absorption band
around 470 nm (Figure S3) attributed to PhS-77DH•, which
is spectrally similar to the transient absorption band observed
for addition radicals formed in the reaction of 77DH with
acylperoxyl radicals (465 nm in benzene),7a suggesting a
common site of attack for acylperoxyl and phenylthiyl
radicals. Although a number of addition sites are possible
for PhS• addition to carotenoids, terminal addition is likely
to be favored as a result of the extensive conjugation within
(4) (a) Pratt, D. A.; Mills, J. H.; Porter, N. A. J. Am. Chem. Soc. 2003,
125, 5801. (b) Yin, H. Y.; Havrilla, C. M.; Morrow, J. D.; Porter, N. A. J.
Am. Chem. Soc. 2002, 124, 7745. (c) Tallman, K. A.; Pratt, D. A.; Porter
N. A. J. Am. Chem. Soc. 2001, 123, 11827. (d) Lowe, J. R.; Porter, N. A.
J. Am. Chem. Soc. 1997, 119, 11534. (e) Porter, N. A.; Caldwell, S. E.;
Mills, K. A. Lipids 1995, 30, 277. (f) Porter N. A.; Mills, K. A.; Carter, R.
L. J. Am. Chem. Soc. 1994, 116, 6690. (g) Boyd, S. L.; Boyd, R. J.; Shi,
Z.; Barclay, L. R. C.; Porter, N. A. J. Am. Chem. Soc. 1993, 115, 687. (h)
Porter, N. A. Acc. Chem. Res. 1986, 19, 262.
(5) Burton, G. W.; Ingold, K. U. Science 1984, 224, 569.
(6) Doba, T.; Burton, G. W.; Ingold, K. U.; Matsuo, M. J. Chem. Soc.,
Chem. Commun. 1984, 461.
(7) (a) El-Agamey, A.; McGarvey, D. J. J. Am. Chem. Soc. 2003, 125,
3330. (b) Mortensen, A.; Skibsted, L. H.; Truscott, T. G. Arch. Biochem.
Biophys. 2001, 385, 13. (c) Edge, R.; McGarvey, D. J.; Truscott, T. G. J.
Photochem. Photobiol., B 1997, 41, 189. (d) Rice-Evans, C. A.; Sampson,
J.; Bramley, P. M.; Holloway, D. E. Free Radical Res. 1997, 26, 381. (e)
Mortensen, A.; Skibsted, L. H. Free Radical Res. 1996, 25, 515. (f)
Mortensen, A.; Skibsted, L. H. Free Radical Res. 1996, 25, 355. (g) Hill,
T. J.; Land, E. J.; McGarvey, D. J.; Schalch, W.; Tinkler, J. H.; Truscott,
T. G. J. Am. Chem. Soc. 1995, 117, 8322.
(8) (a) D’Aquino, M.; Dunster, C.; Willson, R. L. Biochem. Biophys.
Res. Commun. 1989, 161, 1199. (b) Everett, S. A.; Dennis, M. F.; Patel, K.
B.; Maddix, S.; Kundu, S. C.; Willson, R. L. J. Biol. Chem. 1996, 271,
3988. (c) Mortensen, A.; Skibsted, L. H.; Sampson, J.; Rice-Evans, C.;
Everett, S. A. FEBS Lett. 1997, 418, 91. (d) Mortensen, A. Asian Chem.
Lett. 2000, 4, 135.
(9) Ito, O. In S-Centered Radicals; Alfassi, Z. B., Ed., John Wiley &
Sons: Chichester, 1999; pp 193-224.
3958
Org. Lett., Vol. 7, No. 18, 2005

the resulting resonance-stabilized radical.10 No transient
absorption features were observed at longer wavelengths in
the visible or near-infrared regions. The rate constant for
the reaction of PhS• with 77DH was determined to be 1.32
( 0.06 × 109 M-1 s-1 (Figure S3 inset). Rate constants for
the addition of PhS• to alkenes and conjugated dienes ranging
from ∼103 to 107 M-1 s-1 have been reported previously.9
Under our experimental conditions, in the absence of
oxygen, the transient absorption signal at 470 nm decays
over hundreds of milliseconds by second-order kinetics, and
its decay is relatively insensitive to temperature (Figure S4).
In the presence of oxygen at room temperature, the rate of
decay of PhS-77DH• increases (Figure 2) and follows
decay is altered and changes from monoexponential to
biexponential (Figures 3 and S5). This behavior is interpreted
Figure 3. The influence of temperature on the normalized transient
profiles, at 470 nm, of PhS-77DH• following 355 nm laser
photolysis (laser energy ) 1.3 mJ) of PhS-SPh (∼4 × 10-3 M) in
the presence of 77DH (∼2 × 10-5 M), in air-saturated benzene
solution.
in terms of the system approaching equilibrium (the first
exponential) with the second exponential component repre-
senting the decay of the system at equilibrium (Scheme 1).
Figure 2. Kinetic absorption profiles for the decay of PhS-77DH•
at 470 nm in benzene at various oxygen concentrations (5%, 21%,
50%, 100%) formed following 355 nm laser photolysis (laser energy
∼1.2 mJ) of PhS-SPh (∼8 × 10-3 M) in the presence of 77DH
(∼2 × 10-5 M) in benzene. The amplitudes of the traces have been
normalized for clarity. The inset shows a plot of the room-
temperature pseudo-first-order rate constant (kobs) for the decay of
PhS-77DH• (at 470 nm) in benzene versus the oxygen concentration.
Scheme 1
The amplitude of the second exponential increases with
increasing temperature, and this is consistent with an
equilibrium that has an exothermic forward reaction (Table
S1) and thus indicates that the rate constant for the reverse
reaction has a larger activation energy than the forward
reaction and is therefore more sensitive to temperature. This
behavior has been observed previously for reversible oxygen
addition to pentadienyl radicals13 and 1-methylallyl radicals14
in the gas phase and interpreted along similar lines. The
reverse reaction is proposed to be a â-fragmentation. Such
reactions in lipid peroxyl radicals have been studied exten-
sively by Porter et al.,4,15-18 who has shown that oxygen off
rates (â-fragmentation) are much more rapid for peroxyl
The intercept of the plot is 1.3 ( 3.3 s-1
.
pseudo-first-order kinetics.
Stern-Volmer analysis of the variation of kobs with [O2]
leads to a rate constant (k1) for oxygen addition of 4.3 (
0.07 × 104 M-1 s-1 (Figure 2 inset), which is consistent with
the upper limit predicted from our previous studies of
addition radicals formed by reactions of carotenoids with
acylperoxyl radicals.7a
At room temperature there is no clear evidence that the
reaction with oxygen is reversible under our experimental
conditions, although a small residual baseline offset that is
oxygen-concentration-dependent is evident on close inspec-
tion (Figure 2).
(12) Fogg, P. G. T.; Gerrard, W. Solubility of Gases in Liquids: A Critical
EValuation of Gas/Liquid Systems in Theory and Practice; John Wiley &
Sons: New York, 1991; pp 294-295.
(13) Zils, R.; Inomata, S.; Imamura, T.; Miyoshi, A.; Washida, N. J.
Phys. Chem. A 2001, 105, 1277.
(14) Knyazev, V. D.; Slagle, I. R. J. Phys. Chem. A 1998, 102, 8932.
(15) Porter N. A.; Wujek, D. G. J. Am. Chem. Soc. 1984, 106, 2626.
(16) Khan, J. A.; Porter N. A. Angew. Chem., Int. Ed. Engl. 1982, 21,
217.
(17) Porter N. A.; Lehman, L. S.; Weber, B. A.; Smith, K. J. J. Am.
Chem. Soc. 1981, 103, 6447.
However, at elevated temperatures and constant oxygen
concentration,11,12 the kinetic profile of the addition radical
(10) Sykes, P. A Guidebook to Mechanism in Organic Chemistry, reprint
of 6th ed.; Longman Singapore Publishers Pte Ltd.: Singapore, 1991; pp
309-313.
(11) The small oxygen concentration change, in benzene solution, at
different temperatures was taken into consideration.
(18) Porter N. A.; Weber, B. A.; Weenen, H.; Khan, J. A. J. Am. Chem.
Soc. 1980, 102, 5597.
Org. Lett., Vol. 7, No. 18, 2005
3959

radicals derived from nonterminal oxygen addition than for
those derived from terminal oxygen addition. We speculate
that the reaction we observe results from terminal oxygen
addition at either end of the resonance-stabilized addition
radical (Figure S6).
equilibrium lying heavily on the side of the carbon-centered
radical under our experimental conditions. However, the
â-fragmentation rate constant we have estimated is several
orders of magnitude smaller than that predicted by this
empirical relationship.
Kinetic fitting of the absorption profiles at different
temperatures to a biexponential function (eq 2) yields
From an Arrhenius plot for the rate constant (k-1) of the
â-fragmentation reaction, the activation energy (Ea-1), was
calculated as 57.4 ( 4.0 kJ mol-1 (Figure S8). Therefore,
the activation energy for oxygen addition to PhS-77DH• (Ea1)
is determined to be 3.6 ( 4.3 kJ mol-1, and this small value
is consistent with the observed temperature insensitivity of
the forward reaction.
Laser (355 nm) photolysis of benzene solutions of PhS-
SPh in the presence of ∼3 × 10-5 M â-CAR leads to the
formation of a transient absorption band around 540 nm
(Figure S9), which is much weaker than that observed for
77DH and which decays on millisecond time scales under
our experimental conditions (laser intensities were used that
were higher than those used for 77DH studies because the
transient absorption band is weaker (Figures S9 and S10).
This absorption band is attributed to the addition radical PhS-
â-CAR•. The decay of PhS-â-CAR• is oxygen-concentration-
dependent, and the observed rate constant for the reaction
with oxygen, obtained in the same manner as for 77DH
(Figure S10 inset), is 0.64 ( 0.09 × 104 M-1 s-1, which is
smaller by a factor of ∼7 than the value for PhS-77DH•.
This may reflect the increased conjugated chain length in
PhS-â-CAR• compared with PhS-77DH•.
parameters A1, A2, kobs(1) and kobs(2)
:
obs(2)t
At ) A1 e-k t + A2 e-k
(2)
obs(1)
Assuming only PhS-77DH• absorbs at the monitoring
wavelength (470 nm), then A1 [PhS-77DH-OO•]e and A2
[PhS-77DH•]e. Thus the equilibrium constant K is given
by
•
[PhS - 77DH - O2 ]e
[PhS - 77DH•]e[O2]e
A1
k1
K )
)
)
(3)
k-1
A2[O2]e
The first exponential component represents the approach
to equilibrium where the observed rate constant (kobs(1)) has
been taken as
kobs(1) ) k1[O2] + k-1
(4)
The second exponential component represents the decay
of the system at equilibrium and reflects the slow intrinsic
decays of PhS-77DH-OO• and PhS-77DH•.
In conclusion, the results reported in this letter show, for
the first time, direct evidence for the reversible addition of
oxygen to carotenoid-derived carbon-centered neutral radi-
cals. For these systems, the rate constant for oxygen addition
is much lower than for shorter resonance-stabilized radicals
such as 1-methylallyl14 and pentadienyl.13 We are currently
extending these studies to the investigation of oxygen
addition to radicals derived from other carotenoids.
Using the van’t Hoff equation (eq 5), the standard molar
enthalpy change (∆H°) and the standard molar entropy
change (∆S°) for the reaction were calculated as -53.9 (
1.6 kJ and -99.6 ( 5.0 J K-1 mol-1, respectively (Figure
S7). Porter’s group4a suggest that C-OO BDEs approach a
limiting value of ∼44 kJ mol-1 for peroxyl radicals with
conjugated π systems and the value of ∆H° we obtain from
the van’t Hoff plot is consistent with this.
Acknowledgment. The authors are grateful to the Le-
verhulme Trust (Research Grant F/00130F) for financial
support and to DSM Nutritional Products for supplying the
carotenoids used in this work.
∆H° ∆S°
ln K ) -
+
(5)
RT
R
Supporting Information Available: Transient absorption
spectra for PhS• and PhS-â-CAR•. Kinetic absorption profiles
for PhS• and PhS-77DH• at different temperatures and for
PhS-â-CAR• at various oxygen concentrations. Arrhenius
plot for â-fragmentation of PhS-77DH-OO•. Reaction scheme
and experimental data for temperature-dependent measure-
ments. This material is available free of charge via the
Internet at http://pubs.acs.org.
From eqs 3 and 4, the values of k1 and k-1 can be
calculated, and these are given in Table S1 for the different
temperatures used.
Using the empirical relationship between log kâ and BDE
reported by Pratt et al.,4a the â-fragmentation rate constant
for peroxyl radicals bearing only extended π systems is
predicted to approach 105 s-1, suggesting a small equilibrium
constant (K ) k1/k-1 with k-1 g k1) with the position of
OL051436S
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Org. Lett., Vol. 7, No. 18, 2005