Kinetic studies of retinol addition radicals

Article abstract of DOI:10.1039/c0ob00799d

Retinol neutral radicals (RS-retinol), generated from the reaction of retinol with 4-pyridylthiyl and 2-pyridylthiyl radicals in argon-saturated methanol, undergo β-elimination, which can be monitored via the slow secondary absorption rise at 380 nm attributed to the rearrangement of the unstable retinol neutral addition radicals to the more stable addition radicals. Rate constants for the β-elimination reactions (k_β) of 4-PyrS-retinol were measured at different temperatures and the Arrhenius equation for the reaction is described by log (k_β/s ^-1) = (12.7 ± 0.2) - (54.3 ± 1.3)/, where = 2.3RT kJ mol^-1. The reactivities of retinol addition radicals (RS-retinol), generated from the reaction of retinol with various thiyl radicals, towards oxygen have also been investigated in methanol. In the presence of oxygen, the decay of RS-retinol fits to biexponential kinetics and both observed rate constants for the RS-retinol decay are oxygen-concentration dependent. This suggests that at least two thiyl addition radicals, formed from the reaction of RS with retinol, undergo oxygen addition reactions. In light of the estimated rate constants for oxygen addition to RS-retinol and RS-CAR (CAR: carotenoid), the antioxidant-prooxidant properties of retinol are discussed. The Royal Society of Chemistry 2011.

Full text of DOI:10.1039/c0ob00799d

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PAPER
Kinetic studies of retinol addition radicals†‡
Ali El-Agamey,*a,b,c Shunichi Fukuzumi,*a,d K. Razi Naqvi and David J. McGarvey
e
b
Received 29th September 2010, Accepted 22nd November 2010
DOI: 10.1039/c0ob00799d
∑
Retinol neutral radicals (RS-retinol ), generated from the reaction of retinol with 4-pyridylthiyl and
2
-pyridylthiyl radicals in argon-saturated methanol, undergo b-elimination, which can be monitored via
the slow secondary absorption rise at 380 nm attributed to the rearrangement of the unstable retinol
neutral addition radicals to the more stable addition radicals. Rate constants for the b-elimination
∑
reactions (k
b
) of 4-PyrS-retinol were measured at different temperatures and the Arrhenius equation
-
1
-1
for the reaction is described by log (k
The reactivities of retinol addition radicals (RS-retinol ), generated from the reaction of retinol with
b
/s ) = (12.7 ± 0.2) - (54.3 ± 1.3)/q, where q = 2.3RT kJ mol .
∑
various thiyl radicals, towards oxygen have also been investigated in methanol. In the presence of
∑
oxygen, the decay of RS-retinol fits to biexponential kinetics and both observed rate constants for the
∑
RS-retinol decay are oxygen-concentration dependent. This suggests that at least two thiyl addition
∑
radicals, formed from the reaction of RS with retinol, undergo oxygen addition reactions. In light of
∑
∑
the estimated rate constants for oxygen addition to RS-retinol and RS-CAR (CAR: carotenoid), the
antioxidant-prooxidant properties of retinol are discussed.
1
–3
Introduction
skin diseases and cosmetic formulations. In the literature, there
4–6
are many reports about the antioxidant activities of retinoids.
However, the prooxidant effects of retinoids have also been
observed. Moreover, there is considerable debate about whether
retinoids combat or promote photocarcinogenesis and heart
diseases.
Whilst there are many reports on the photochemistry of
Vitamin A and various natural retinoids are essential in many
biological processes such as vision, reproduction and normal cell
differentiation. They have been used in the treatment of some
7–9
1
1,10
a
Department of Material and Life Science, Graduate School of
11–20
retinoids,
there are comparatively few studies of time-
Engineering, Osaka University, Suita, Osaka 565-0871, Japan.
E-mail: a_el_agamey@yahoo.co.uk, fukuzumi@chem.eng.osaka-u.ac.jp;
Fax: +81-0668797370; Tel: +81-0668797369
16,19
resolved free radical reactions with them.
The reactions of
) with different retinoids,
in 2% Triton X-100, lead to the formation of retinoid radical
∑
trichloromethylperoxyl radical (CCl
3
O
2
b
School of Physical and Geographical Sciences, Keele University, Keele,
Staffordshire, ST5 5BG, UK
∑
+
c
cations (retinoid , lmax = 590 nm) and another unidentified
19
Chemistry Department, Faculty of Science, Mansoura University, New
Damietta, Damietta, Egypt
intermediate absorbing at shorter wavelength. Retinoids can be
d
∑+
Department of Bioinspired Science, Ewha Womans University, Seoul 120-
regenerated from retinoid via their efficient reactions (k ~1–7 ¥
0 M s ) with vitamin C. Moreover, glutathione thiyl radical
GS ) reacts with vitamin A (retinol) in aqueous methanolic (60%)
solution to form a transient species that absorbs strongly at 380 nm
7
50, Korea
8
-1 -1
19
1
(
e
Department of Physics, Norwegian University of Science and Technology
∑
(
NTNU), N-7491, Trondheim, Norway
†
This work is dedicated to the memory of Athel Beckwith, a teacher and
4
-1
-1 16
scientist from whom we learned how to study chemistry by example. His
pioneering advances in radical chemistry laid the foundation for much of
the current radical clock methodology.
(emax = 4 ¥ 10 M cm ). This transient was previously attributed
∑ ∑+
to either GS-retinol (addition radical) or retinol . However, since
∑+
14,19
retinol is known to absorb at ~590 nm,
the transient can be
‡
Electronic supplementary information (ESI) available: Transient profiles
∑
21
∑
∑
∑
∑
assigned to GS-retinol (Scheme 1).
(
380 nm) for 4-PyrS-retinol , 4-PyrS , 2-PyrS-retinol , 2-PyrS and PhS-
∑
retinol in argon-saturated methanol at 334 K, transient profiles (380 nm)
∑
for RS-retinol at different retinol concentrations, transient profiles for the
decay of PhS at 450 nm in air- and argon-saturated methanol, transient
∑
∑
∑
∑
∑
spectra of 2-PyrS , PhS , 2-PyrS-retinol and PhS-retinol in methanol and
transient spectra of 4-PyrS-retinol in cyclohexane, dependence of kobs for
the formation of RS-retinol on retinol concentration, dependence of kobs
∑
∑
∑
∑
for the decay of 4-PyrS-retinol and PhS-retinol on oxygen concentration,
data for the influence of the temperature on secondary growth rate
constants, Eyring and Arrhenius plots for the slow growth (at 380 nm)
∑
∑
generated from the reaction of 4-PyrS or 2-PyrS with retinol in argon-
∑
saturated methanol, and the influence of temperature on 2-PyrS-retinol
∑
and PhS-retinol in air-saturated methanol. See DOI: 10.1039/c0ob00799d
Scheme 1
Org. Biomol. Chem., 2011, 9, 1459–1465 | 1459
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∑
+
Therefore, retinol and vitamin A neutral addition radical are
the principal initial intermediates involved in the free radical re-
∑
+
actions with retinol. However, formation of retinol in biological
systems is less likely since lipid peroxyl radicals, involved in lipid
peroxidation, are much weaker electron accepting oxidizing agents
∑
19
than CCl
3
O
2
. On the other hand, formation of vitamin A neutral
addition radicals is favored due to the formation of a highly
16
resonance-stabilized neutral radical.
In general, thiyl addition radicals can undergo two important
22–26
reactions, namely b-elimination and oxygen addition reactions.
Firstly, b-elimination reactions of thiyl addition radicals are
the key steps in the thiyl radical catalyzed Z/E isomerization
2
3,24
reactions of unsaturated fatty acids (Scheme 2).
et al.
Chatgilialoglu
23,24,27–29
have investigated these reactions extensively, due
to the possible adverse effects on the functions of biological
membranes if these reactions occur in vivo. In these reactions,
thiyl radicals add to the double bond of the unsaturated fatty acid
to form thiyl addition radicals. These thiyl addition radicals can
undergo b-elimination of the thiyl radical to form either Z- or
E-isomers depending on the conformation of the thiyl addition
radicals. Trans-isomers are the major products of these reactions
Fig. 1 Structures of retinol and some carotenoids.
Results and discussion
Generation of thiyl radicals
Laser flash photolysis (LFP), with 266 nm excitation, of air-
saturated methanolic solutions of 4,4¢-dipyridyl disulfide leads to
the formation of a transient spectrum with two absorption bands at
420 nm and <350 nm (Fig. 2 and eqn (1)). The 420 nm absorption
band was observed previously following LFP of 4,4¢-dipyridyl
disulfide or N-hydroxypyridine-4-thione (eqn (2)) in THF and
23,24,27–29
because of their greater stability relative to cis-isomers.
In
22,30–34
addition, Ito et al.
studied extensively the addition reactions
of various arylthiyl radicals with non-conjugated and conjugated
olefins using the selective radical trapping flash photolysis method.
Using this method they estimated the rate constants for the
forward and backward reactions.
∑
30
was attributed to 4-pyridylthiyl radical (4-PyrS ); however, the
transient spectra in this work do not cover wavelengths shorter
∑
than 390 nm. The decay of 4-PyrS radical has been ascribed
to radical recombination, leading to the formation of the parent
30
compound.
Scheme 2
(
(
1)
2)
Secondly, oxygen addition reactions to various carotenoid thiyl
25,26,35
addition radicals have been investigated,
where it has been
shown that the oxygen addition rate constants generally decrease
as the number of conjugated double bonds increases in the parent
26
carotenoid. In addition, the reversibility of oxygen addition to
∑
∑
PhS-77DH , formed from the reaction of PhS with 7,7¢-dihydro-
b-carotene (77DH) in benzene, was observed (Scheme 3 and Fig.
25
1
). In 1984, Burton and Ingold proposed a reversible oxygen
addition to carotenoid peroxyl addition radicals to explain the
36
prooxidant-antioxidant behavior of carotenoids (Scheme 3).
Scheme 3
To the best of our knowledge, the chemistry of retinol thiyl
addition radicals is unexplored. We report herein studies on b-
elimination reactions of thiyl addition radicals of retinol and the
reactivities of these addition radicals towards oxygen. In addition,
the influence of the thiyl radical reactivity on b-elimination and
oxygen addition reactions has been examined.
∑
Fig. 2 Transient spectra of 4-PyrS obtained following LFP (266 nm)
-4
of 4,4¢-dipyridyl disulfide (~2 ¥ 10 M) in air-saturated methanol (laser
energy ~6.0 mJ). The inset shows the transient profile at 420 nm.
On the other hand, LFP of 2,2¢-dipyridyl disulfide in air-
saturated methanol gives a transient spectrum with lmax at 490 nm.
1
460 | Org. Biomol. Chem., 2011, 9, 1459–1465
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∑
The transient decay at 490 nm leads to the growth of a transient
(lmax < 410 nm, see Fig. S1‡). Similar observations have been
Table 1 Rate constants (k
r
) for the reactions of thiyl radicals (RS ) with
a
37
retinol in methanol
reported for LFP of 2,2¢-dipyridyl disulfide in THF (eqn (3))
or N-hydroxypyridine-2-thione in acetonitrile (eqn (4)) and the
transient spectrum was ascribed to 2-pyridylthiyl radicals (2-
∑
k
r
/10 M s-1
9
-1
RS
∑
4-PyrS
-PyrS
PhS
8.1 ± 0.5
5.3 ± 0.4
1.48 ± 0.1
∑
∑
31,38–41
2
PyrS ).
unreactive toward oxygen.
Both 4-pyridylthiyl and 2-pyridylthiyl radicals are
∑
22,30,31,39,40
a
The estimated rate constants represent the sum of the addition rate
constants leading to the formation of various thiyl addition radicals.
(
(
3)
4)
(
6)
It should be noted that no transient was detected at 380 nm
following 266 nm LFP of retinol alone in methanol, which
indicates that the transient observed at 380 nm is due to the
reaction of thiyl radicals with retinol.
Moreover, the electron-transfer reaction between RS and
retinol is excluded because of the absence of the absorption band
due to retinol , (lmax ~ 590 nm).
triplet retinol ( retinol, lmax ~ 400 nm) to the observed spectra has
been eliminated.
spectra (Fig. 3, S4 and S5‡) to RS-retinol-OO has been excluded
because of the similarity of the transient spectra for the reaction
of RS and retinol in air and argon-saturated solutions (Fig. 3 and
S7‡).
By varying the concentration of retinol and maintaining it
higher than that of the generated RS , the rate constants (k
the reaction of 4-PyrS , 2-PyrS and PhS radicals with retinol
were determined (see Table 1 and Fig. S8‡) from the plot of the
observed pseudo-first-order rate constant (kobs) for the growth of
LFP of phenyl disulfide in methanol forms phenylthiyl radical
∑
(
4
PhS ) (Fig. S2‡ and eqn (5)) with an absorption maximum at
∑
∑
50 nm. A similar transient spectrum has been reported for PhS
in benzene. It is known that PhS is unreactive toward oxygen
see Fig. S3‡).
25
∑
22
∑+
14,19
(
Similarly, contribution of
3
(
5)
47–49
Furthermore, the assignment of the transient
∑
Generation of retinol thiyl addition radicals
LFP (266 nm) of methanolic solutions of 4,4¢-dipyridyl disulfide,
∑
2
,2¢-dipyridyl disulfide or phenyl disulfide in the presence of
retinol leads to the formation of transient absorption features
max ~ 370 nm; see Fig. 3, S4 and S5,‡ respectively), similar
∑
r
) for
(
l
∑
∑
∑
to those observed for addition radicals formed in the reaction
∑
16
of retinol with GS radicals in aqueous methanolic solution.
∑
Therefore, these transients can be attributed to 4-PyrS-retinol ,
-PyrS-retinol and PhS-retinol , respectively. In addition, LFP of
,4¢-dipyridyl disulfide and retinol in cyclohexane gives a similar
∑
∑
∑
RS-retinol at 380 nm versus retinol concentration using eqn (7),
2
4
where k
0
(intercept) is the apparent rate constant for the decay of
∑
RS in the absence of retinol (Fig. S8–S11‡).
transient spectrum (lmax ~ 370 nm; see Fig. S6‡).
k
obs = k
o
+ k [retinol]
r
(7)
∑
The rate constants for the reaction of PyrS with retinol are
∑
comparable to that reported for the reaction of 2-PyrS with b-
9
-1
-1 39
carotene (7.3 ¥ 10 M s ). Furthermore, the rate constant
∑
for PhS is close to the reported rate constant for the reaction
of GS with retinol in aqueous methanol (1.4 ¥ 10 M s ).
The lower reactivity of PhS , compared to 2-PyrS and 4-PyrS ,
can be attributed to the higher electrophilicity of the S atom in
pyridylthiyl radicals than in phenylthiyl radical.
order of reactivity (4-PyrS ª 2-PyrS > PhS ) is the same as the
order of reactivities toward isoprene in THF (1.0, 1.2 and 0.047 ¥
0 M s , respectively).
∑
9
-1
-1 16
∑
∑
∑
22,30,31
Also, this
∑
∑
∑
9
-1 -1
22,31
1
50
b-Elimination of retinol-thiyl addition radical
∑
Fig. 3 Transient spectra of 4-PyrS-retinol obtained following 266 nm
At room temperature, LFP (266 nm) of argon-saturated methano-
lic solutions of either 4,4¢-dipyridyl disulfide or 2,2¢-dipyridyl
disulfide in the presence of retinol leads to a slow secondary
-4
laser photolysis of 4,4¢-dipyridyl disulfide (~2 ¥ 10 M) in the presence of
-5
45,46
retinol (~4.0 ¥ 10 M)
in air-saturated methanol at room temperature
(
laser energy ~4 mJ). The inset shows a kinetic absorption profile for the
3
3
-1
∑
absorption rise (kb = 1.4 ¥ 10 and 3.5 ¥ 10
s
at 296 K,
decay of 4-PyrS-retinol at 380 nm.
∑
respectively) after the fast formation of 4-PyrS-retinol or 2-
∑
∑
The formation of retinol neutral radical (retinol ), instead of
thiyl addition radicals, is ruled out due to the poor H-abstraction
PyrS-retinol . As the temperature increases, the rate of secondary
growth, which is monitored at 380 nm, increases (Fig. S12, S13,‡ 4
and 5). These observations can be attributed to the rearrangement
of the less stable addition radical to the more stable one via b-
elimination (Scheme 4).
22,26,31
ability of arylthiyl radicals (eqn (6)).
In addition, it has been
reported that thiyl radical reactions with various carotenoids give
25,26,42–44
rise to thiyl addition radicals.
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Scheme 4
Fig. 4 The influence of temperature on the transient profiles, at 380 nm, of
∑
4
-PyrS-retinol following 266 nm LFP (laser energy ~4 mJ) of 4,4¢-dipyridyl
addition radical will decrease the activation energy of the forward
reaction and increase that of the reverse reaction, i.e. the rate con-
stant for the forward reaction will increase while that for the reverse
-4
-5
disulfide (~2 ¥ 10 M) in the presence of retinol (~8.0 ¥ 10 M), in
argon-saturated methanol.
∑
reaction will decrease. For example, the rate constants for PhS
addition to 1,3-butadiene and non-conjugated olefins are 3.0 ¥
7
3
-1
-1
1
0 and 4.0 ¥ 10 M s , respectively, while those for the b-
elimination reactions of their corresponding addition radicals are
5
7
-1
54
>
3.0 ¥ 10 and 3.3 ¥ 10 s , respectively (Scheme 5).
Further elongation of the conjugated chain, as in retinol, is
expected to increase the rate constant for the forward reaction
and decrease the rate constant for b-elimination reactions. The
∑
∑
rate constants for the reactions of 4-PyrS and 2-PyrS radicals
9
9
-1
with retinol (k
r
= (8.1 ± 0.5) ¥ 10 and (5.3 ± 0.4) ¥ 10 M
-
1
s , respectively) and those for b-elimination reactions of 4-PyrS-
∑
∑
3
3
-1
retinol and 2-PyrS-retinol (k
b
= 1.4 ¥ 10 and 3.5 ¥ 10 s at
2
96 K, respectively) are in agreement with this trend. In addition,
for carotenoids with longer conjugated chains (e.g. b-carotene and
lycopene have 11 conjugated double bonds), the rate constants for
b-elimination reactions are expected to be much smaller than that
for retinol (5 conjugated double bonds). This could explain the
difficulty of observing the b-elimination reactions for these longer
Fig. 5 The influence of temperature on the transient profiles, at 380 nm, of
∑
2
-PyrS-retinol following 266 nm LFP (laser energy ~4 mJ) of 2,2¢-dipyridyl
-4
-5
disulfide (~3.0 ¥ 10 M) in the presence of retinol (~8.0 ¥ 10 M), in
argon-saturated methanol. The inset shows an Arrhenius plot for the slow
∑
growth (at 380 nm) generated from the reaction of 2-PyrS with retinol.
25,26
carotenoids.
‡
‡
It should be noted that LFP of an argon-saturated methanolic
solution of retinol shows no transient at 380 nm. Also, following
Activation enthalpies (DH ) and entropies (DS ) were deter-
mined from measurements of the secondary growth rate constants
for the different retinol addition radicals (4-PyrS-retinol and 2-
PyrS-retinol ) as a function of temperature (Tables 2, S1, S2 and
Fig. S14‡). In Table 2, the small negative activation entropies are
consistent with many b-elimination reactions, which require some
degree of organization to achieve the transition state. For example,
b-elimination of a-cumyloxyl radical and decarboxylation of
alkoxycarbonyl radicals have log A values in the range of 12–
∑
∑
‡
‡
Table 2 Activation enthalpies (DH ) and entropies (DS ) for the slow
∑
absorption rise following the fast formation of PyrS-retinol (at 380 nm)
from the reaction of retinol (~8 ¥ 10 M) with 4-PyrS or 2-PyrS in
argon-equilibrated methanol (laser energy ~4 mJ)
-5
∑
∑
‡
-1
DS /J K mol-1
‡
-1
Precursor
DH /kJ mol
28e,51–53
4,4¢-dipyridyl disulfide
51.7 ± 1.3
45.8 ± 0.9
-10.1 ± 4.1
-22.6 ± 2.7
1
3.
a
2
,2¢-dipyridyl disulfide
22
∑
Moreover, previous studies for the reactions of PhS with 1,3-
a
∑
Arrhenius equation for the b-elimination reaction of 2-PyrS-retinol is
butadiene and non-conjugated olefins indicate that as the number
of conjugated double bonds increases the resonance stabilization
of the addition radical formed increases. This rise in the stability of
-1
described by log (k /s ) = (12.1 ± 0.14) - (48.4 ± 0.9)/q, where q = 2.3 RT
b
-1
kJ mol .
Scheme 5
1
462 | Org. Biomol. Chem., 2011, 9, 1459–1465
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LFP of argon-saturated methanolic solutions of 2,2¢-dipyridyl
disulfide or 4,4¢-dipyridyl disulfide at 334 K, there is no slow
The rate constants for each step (fast and slow) can be calculated
by plotting kobs of each step versus the oxygen concentration (eqn
(8)). The slope equals the rate constant for oxygen addition of
∑
absorption rise at 380 nm after the fast formation of 2-PyrS
∑
and 4-PyrS (Fig. S15 and S16‡). Therefore, the observed slow
each step (k ) and the intercept equals the apparent first-order
2
∑
O
∑
∑
absorption rise is not due to thiyl radicals (2-PyrS or 4-PyrS ) or
rate constant for the decay of the RS-retinol (k
1
) in the absence
∑
∑
retinol; but it arises from the reaction of 2-PyrS or 4-PyrS with
retinol.
of oxygen.
k
obs = k
1
+ k
O
[O
2
]
(8)
2
There is no evidence for the b-elimination reaction following
LFP of phenyl disulfide and retinol in argon-saturated methanolic
solutions even at higher temperatures (Fig. S17‡). This can be
∑
The rate constants for oxygen addition to 4-PyrS-retinol of the
5
fast and slow steps were estimated as (20.4 ± 2.3) ¥ 10 and (2.1 ±
∑
5
-1 -1
attributed to the lower reactivity of PhS , which will lead to the
0.3) ¥ 10 M s , respectively (see inset of Fig. S18‡). Similarly,
∑
formation of either one addition radical or a mixture of addition
radicals with much higher stability than those obtained from the
for 2-PyrS-retinol , the rate constants were calculated as (19.5 ±
5
5
-1 -1
1.0) ¥ 10 and (1.7 ± 0.1) ¥ 10 M s , respectively (see inset of
Fig. 6).
∑
∑
reaction of retinol with 2-PyrS and 4-PyrS . In the latter case,
the extra stability of these addition radicals will slow down the
b-elimination reaction.
In addition, the influence of the temperature on 4-PyrS-
∑
∑
retinol and 2-PyrS-retinol transients has been investigated in air-
57,58
saturated methanol (Fig. 7 and S19‡).
At high temperature,
a small residual baseline offset was observed which may be
Oxygen addition reactions of retinol-thiyl addition radicals
attributed to the b-fragmentation of oxygen from 4-PyrS-retinol-
∑
∑
OO and 2-PyrS-retinol-OO , i.e. the reversibility of oxygen
Oxygen addition reactions play an essential role in determin-
ing the antioxidant–prooxidant properties of carotenoids and
∑
∑
addition to 4-PyrS-retinol and 2-PyrS-retinol at high temperature
see Scheme 3). Similar behavior has been observed for the
59
(
21,55
retinoids.
In Scheme 3, if the equilibrium associated with the
∑
oxygen addition to PhS-77DH , formed from the reaction of
oxygen addition reaction is in favor of the formation of R-CAR-
∑
∑
PhS with 7,7¢-dihydro-b-carotene (77DH) in benzene, at high
OO , a prooxidant effect may result. On the other hand, if the
25
∑
temperature.
equilibrium shifts toward the formation of stable R-CAR , the
35,36
antioxidant effect may prevail (Scheme 3).
The influence of oxygen concentration on the decay of PyrS-
∑
retinol following 266 nm LFP of 4,4¢-dipyridyl disulfide or 2,2¢-
dipyridyl disulfide in the presence of retinol has been investigated
∑
in methanol (Fig. S18‡ and 6). The decay of 4-PyrS-retinol and
∑
2
-PyrS-retinol fits to biexponential kinetics. Both the observed
∑
rate constants for PyrS-retinol decay are oxygen-concentration
dependent. This observation can be ascribed to the oxygen
addition to various thiyl addition radicals absorbing at 380 nm,
56
which have different thermodynamic stabilities. Moreover, this
result supports our earlier observation for b-elimination reactions,
which involve at least two addition radicals (vide supra).
Fig. 7 The influence of temperature on the normalized transient profiles,
∑
at 380 nm, of 4-PyrS-retinol following 266 nm LFP (laser energy ~4 mJ)
-4
of 4,4¢-dipyridyl disulfide (~2 ¥ 10 M) in the presence of retinol (~9.0 ¥
-5
1
0
M), in air-saturated methanol.
The influence of oxygen concentration on the decay of PhS-
∑
∑
retinol has also been investigated (Fig. S20‡). The PhS-retinol
decay fits to biexponential kinetics similar to those observed
∑
for PyrS-retinol , and is oxygen-concentration dependent. This
supports the assumption for the formation of a mixture of addition
radicals (vide supra). Using eqn (8), the rate constants for the fast
5
and slow steps were estimated as (3.5 ± 0.3) ¥ 10 and (1.1 ± 0.1) ¥
5
-1 -1
1
0 M s , respectively (Fig. S20‡).
The small difference between the oxygen addition rate constants
Fig.
6
Normalized kinetic absorption profiles for the decay of
for the fast and slow steps, compared with those obtained with
∑
2
-PyrS-retinol at 380 nm in methanol at various oxygen concentrations
∑
PyrS-retinol is consistent with a smaller difference in the stabilities
(
5, 21, 50, 100%) formed following 266 nm LFP (laser energy ~4.0 mJ) of
∑
∑
-4
of the PhS-retinol radicals formed compared to PyrS-retinol
2
1
,2¢-dipyridyl disulfide (~3.0 ¥ 10 M) in the presence of retinol (~8.0 ¥
5
-
radicals. Also, the rate constants for oxygen addition to PyrS-
0
M). The inset shows plots of pseudo-first-order rate constants (kobs
)
∑
∑
∑
for the fast and slow decay of 2-PyrS-retinol , at 380 nm, versus the oxygen
concentration.
retinol and PhS-retinol radicals indicate lower reactivity of PhS-
∑ ∑
60
retinol radicals compared with PyrS-retinol radicals. Therefore,
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the failure to observe b-elimination can be attributed to the
greater stabilities of thedifferentPhS-retinol radicals formed. This
Acknowledgements
∑
We thank Dr Thor Bernt Melø (Department of Physics, Nor-
wegian University of Science and Technology (NTNU), N-7491,
Trondheim, Norway) for his valuable discussions. Dr El-Agamey
is grateful to the Leverhulme Trust and JSPS (Japan Society
for the Promotion of Science) for financial support. We are
also grateful for financial support provided by Grants-in-Aids
(Nos. 20108010 and 21111501) and a Global COE program, “the
Global Education and Research Center for Bio-Environmental
Chemistry” from the Japan Society of Promotion of Science
stability, as indicated from their oxygen addition rate constants,
will significantly hinder the b-elimination reaction (vide supra).
Furthermore, the rate constants for oxygen addition to PhS-
∑
retinol are 1–2 orders of magnitude larger, depending on the
carotenoid, than those reported for oxygen addition to thiyl
∑
addition radicals (PhS-CAR ) generated from the reaction of
∑
25,26
PhS with long chain carotenoids.
The lower reactivity of PhS-
∑
CAR toward oxygen can be attributed to the greater stability, by
resonance, of the longer chain neutral addition radical formed.
(JSPS), and by KOSEF/MEST through WCU project (R31-2008-
10010-0).
Conclusions
b-Elimination and oxygen addition reactions have been observed
for the retinol–thiyl addition radicals, generated from the reaction
of various thiyl radicals with retinol. The observation of the b-
elimination reaction and of biexponential kinetics observed for
the oxygen addition reaction have been interpreted as evidence
for the formation of more than one addition radical. Moreover,
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(
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-1 -1 22
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∑
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3
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31,38
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3
3
4
4
4
4
over the temperature range of our experiments is very small. .
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1
59 LFP of phenyl disulfide and retinol in air-saturated methanol shows
∑
1
a small residual baseline offset for the PhS-retinol transient at high
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∑
of PhS-retinol-OO (i.e. the reverse of oxygen addition).
∑
∑
60 Since PyrS radicals are more reactive than PhS , their reactions with
∑
retinol will generate highly reactive PyrS-retinol .
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