High static pressure alters spin trapping rates in solution. Dependence on the structure of nitrone spin traps

Article abstract of DOI:10.1039/b515682c

Using a competitive spin trapping method, relative spin trapping rates were quantified for various short-lived radicals (methyl, ethyl, and phenyl radicals). High static pressure was applied to the competitive spin-trapping system by employing high-pressure electron spin resonance (ESR) equipment. Under high pressure (490 bar), spin trapping rate constants for alkyl and phenyl radicals increased by 10 to 40%, and the increase was dependent on the structure of nitrone spin traps. A maximum increase was obtained when tert-butyl(4-pyridinylmethylene)amine N-oxide (4-POBN) was used as a spin trap. Activation volumes (ΔΔV^?) for the two spin trapping reactions were calculated to be -17-(-9) cm³ mol ^-1 for the 4-POBN system. The Royal Society of Chemistry 2006.

Full text of DOI:10.1039/b515682c

PAPER
www.rsc.org/obc | Organic & Biomolecular Chemistry
High static pressure alters spin trapping rates in solution. Dependence on
the structure of nitrone spin traps
Yoshimi Sueishi,*a Daisuke Yoshioka,a Chiharu Yoshioka,a Shunzo Yamamotoa and Yashige Kotakeb,c
Received 4th November 2005, Accepted 9th January 2006
First published as an Advance Article on the web 24th January 2006
DOI: 10.1039/b515682c
Using a competitive spin trapping method, relative spin trapping rates were quantified for various
short-lived radicals (methyl, ethyl, and phenyl radicals). High static pressure was applied to the
competitive spin-trapping system by employing high-pressure electron spin resonance (ESR)
equipment. Under high pressure (490 bar), spin trapping rate constants for alkyl and phenyl radicals
increased by 10 to 40%, and the increase was dependent on the structure of nitrone spin traps. A
maximum increase was obtained when tert-butyl(4-pyridinylmethylene)amine N-oxide (4-POBN) was
used as a spin trap. Activation volumes (DDV‡) for the two spin trapping reactions were calculated to be
−17–(−9) cm3 mol−1 for the 4-POBN system.
Introduction
Results and discussion
The spin trapping technique has been widely used to understand
reaction kinetics and mechanisms in chemical and biological free
radical reactions.1–4 The improvement of this technique may be
accomplished by developing new spin traps that can produce spin
adducts with longer half-lives.3,5 However, our approach has been
to change the thermodynamic parameters to alter reaction kinetics
and equilibria.6
Determination of spin trapping rates
We employed a competitive spin-trapping method using two dif-
ferent spin traps.4 Fig. 1a–b show the typical ESR spectra obtained
from UV-irradiated solution of (a) trimethyllead acetate (methyl
External pressure is a thermodynamic parameter that could
control spin-trapping rates. Using a free radical probe and
high-pressure ESR spectroscopy, we have demonstrated that
high static pressure is a useful means for the manipulation of
chemical equilibria.7,8 We have also shown that spin-trapping rates
were altered under high pressure.6 In the present high-pressure
study, we utilized various PBN (a-phenyl-N-tert-butylnitrone or
benzylidene(tert-butyl)amine-N-oxide) type compounds as spin
traps. Thus, three components, i.e., the selected spin trap, the
competing spin trap 2,2-dimethyl-3,4-dihydro-2H-pyrrole-1-oxide
(DMPO), and free radical generating compound were mixed, and
ESR spectrum was recorded under high pressure. Relative spin
trapping rates were calculated by quantifying the yield of free
radical adducts. The effects of substituents in PBN- and DMPO-
analogs on alkyl and phenyl radical trapping rate constants were
determined. We report that spin-trapping rates by various PBN-
and DMPO-type traps are pressure dependent, and that the
magnitude of pressure dependence is a function of the spin-trap
structure.
Fig. 1 ESR spectra obtained in aqueous solution immediately after
UV-irradiation (irradiation time 1 s, sweep time 60 s) in the presence of
DMPO and PBN. Broken lines in the spectra are computer simulated
spectra: (a) methyl radical trapping; ᭺: DMPO-CH3 and ᭹: PBN-CH3.
[PBN]0 : [DMPO]0 (ratio of initial trap concentrations) was 3.6. (b)
Ethyl radical trapping; ᭺: DMPO–C2H5 and ᭹: PBN–C2H5. [PBN]0/
[DMPO]0 = 5.2. (c) The ratio of methyl (᭺) and ethyl-adduct (᭹) formation
rates for PBN and DMPO, plotted as a function of the ratio of initial
concentration of spin traps ([PBN]0/[DMPO]0).
aDepartment of Chemistry, Faculty of Science, Okayama University, 3-
1-1 Tsushima Naka, Okayama, 700-8530, Japan. E-mail: ysueishi@cc.
okayama-u.ac.jp
bFree Radical Biology and Aging Research Program, Oklahoma Medical
Research Foundation, Oklahoma City, OK, 73104, USA
cKyoto University International Innovation Center, Uji, Kyoto, 611-0011,
Japan
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radical source) and (b) triethyllead acetate (ethyl radical source)
in the presence of the spin traps, DMPO and PBN. Two clearly
identifiable spin adducts were obtained and peak assignments
were made according to the reported ESR hyperfine splitting
constants.9 In Fig. 1, spectral lines marked with open circles are
from DMPO spin-adducts (DMPO–CH3) and closed circles are
from PBN spin-adducts (PBN–CH3). These ESR spectra were
readily reproduced by computer spectrum simulations (Fig. 1).
The relative abundance of the two spin adducts was calculated
usingthesimulatedspectra, fromwhichrelativespin-trappingrates
were calculated (see the following paragraph). Thus, we carried
out competitive spin-trapping for various PBN- and DMPO-type
spin traps (Scheme 1) with DMPO as a reference spin trap. ESR
parameters for alkyl radical spin-adducts trapped with PBN- and
DMPO-analogs are listed in Table 1.
scheme for spin trapping in the presence of two different traps (for
example, PBN and DMPO) is:
hv
(R) PbOAc
R•
−−−→
3
k1
DMPO + R•
PNB + R•
DMPO-R
−−−→
k2
PBN-R
−−−→
The ratio of DMPO-R and PBN-R formation rates is expressed
as follows:
RPBN
RDMPO
d[PBN-R]/dt
d[DMPO-R]/dt
k2 [PBN]0
k1 [DMPO]0
=
=
(1)
where [PBN]0 and [DMPO]0 denote the initial concentration of
the spin traps. The relative rate constant (k2/k1) can be calculated
from the relative rates of spin-adduct formation and the initial
concentration of the spin-traps. Fig. 1c shows a typical plot for
the PBN/DMPO system. In Fig. 1c, as predicted by eqn (1),
the plot of RPBN/RDMPO (the relative abundance of spin adducts)
against [PBN]0/[DMPO]0 gives a straight line with the slope k2/k1
which passes through the origin, suggesting that the calculation
of relative spin trapping rate constants (k2/k1) using eqn (1) is
justifiable. The ratio of spin-trapping rate constants of methyl and
ethyl radicals for various DMPO- and PBN-type traps are listed
in Table 1.
Effect of spin-trap structure on spin-trapping rates (atmospheric
pressure)
We determined the relative spin-trapping rate constants (k2/k1) of
methyl and ethyl radicals for five PBN- and four DMPO-type spin
traps as illustrated in Scheme 1. To demonstrate spin-trap structure
dependence for methyl and ethyl radical trapping, we evaluated
spin-trapping rate constants (k2). Using time-resolved ESR spec-
troscopy, Taniguchi and Madden have determined the DMPO
trapping rate constants in aqueous solution for various alkyl
radicals produced with hydrogen abstraction by radiolytically-
produced hydroxyl radicals, yielding k1 = 1.4 × 107 and 1.6 ×
107 dm3 mol−1 s−1 for methyl and ethyl radicals, respectively.10
Scheme 1
The theoretical basis of the calculation of relative spin trapping
rates from competitive spin trapping is as follows. The reaction
Table 1 Hyperfine coupling constants (hfcc) and methyl and trapping rate constants for methyl and ethyl radicals at 298 K in aqueous solution
Methyl adduct
hfcc/mT
Ethyl adduct
hfcc/mT
OH adductd
a
a
a
Spin trap
aH
aN
aP
k2/k1
10−7 k2
aH
aN
aP
k2/k1
10−7 k2
10−9 k2
DEPMPO
DMPO
4-NO2-PBN
4-POBN
2-SO3-PBN
TMPO
PBN
2.260 1.542 4.820 2.15 0.08
3.0
2.260 1.542 4.880 2.35 0.05
4.0
6.16e
3.60
3.06
5.33
4.82
2.48e
2.56
0.90
1.58
2.380 1.650
0.653 1.667
0.297 1.635
0.360 1.670
2.720 1.680
0.340 1.660
0.330 1.646
1.600
1
1.4b
2.370 1.640
0.310 1.628
0.280 1.590
0.520 1.597
2.420 1.640
0.330 1.640
0.320 1.647
1.565
1
1.6b
0.395 0.017
0.193 0.004
0.0712 0.0008
0.977 0.001c
0.0682 0.0004
0.0589 0.0055
0.0178 0.0005
0.55
0.27
0.099
0.098
0.095
0.082
0.025
0.462 0.009
0.566 0.002
0.269 0.002
2.82 0.01c
0.111 0.014
0.0632 0.0015
0.0133 0.0005
0.74
0.91
0.43
0.063
0.18
0.10
0.021
4-HO-PBN
2-Ph-DMPO
a k2 (dm3 mol−1 s−1). b Cited from ref. 10. c Ratio of trapping rate constants for PBN (k2) and DMPO (k1). d Cited from ref. 4. e Estimated using k2/k1 for
the TMPO/PBN and DEPMPO/DMPO systems.
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These DMPO trapping rate constants were utilized to calculate
spin-trapping rate constants for the selected spin traps, and the
results are listed in Table 1.
and phenyl radical trapping.4 Table 1 also shows that spin-
trapping rate constants by TMPO and 2-Ph-DMPO are extremely
small compared with those of DMPO and DEPMPO. Such a
decrease cannot be attributed to the electron-inductive effect on
spin trapping, but rather to the steric hindrance by methyl and
phenyl groups around the trapping site (nitrone group). This is
in agreement with the results obtained for hydroxyl and phenyl
radical trapping by TMPO and 2-Ph-DMPO.4
In our previous paper, we proposed that the nucleophilic attack
=
of hydroxyl and phenyl radicals may occur against the C N
double bond in nitrone traps.4 To evaluate the effects of the spin-
trap structure on the trapping rate constants of alkyl radicals, the
Hammett plot may be useful. However, substituent constants for
the 2-subtituent in 2-SO3-PBN and N→O group in the phenyl
ring of 4-POBN are not available. Therefore, alternatively we
plotted the spin trapping rate constants of alkyl radicals against
those of the hydroxyl radical (Fig. 2). Fig. 2 clearly shows the
tendency that an electron-withdrawing group tends to increase
PBN(-type) trapping rate constants of alkyl radicals, suggesting
that there was nucleophilic addition similar to the case of hydroxyl
Pressure effects on spin-trapping reaction
Effects on reaction rates. Under a pressure of 490 bar, methyl
or ethyl radicals were produced in a UV-irradiated solution of
trimethyllead acetate or triethyllead acetate in the presence of
DMPO and 4-POBN. The relative rates of methyl radical trapping
clearly increased at 490 bar, as indicated by the change in the
intensity ratio between the lines marked with arrows in Fig. 3a.
The relative rate constants (k2/k1) at 490 bar are quantified
according to eqn (1), and the results are listed in Table 2. Percent
changes in relative trapping rate constants (= 100((k2/k1)490
−
(k2/k1)1)/(k2/k1)1) induced by 490 bar are listed in Table 2. The
percentage induction in the 4-POBN/DMPO system is larger than
others.
Activation volumes and transition states. Based on the pressure
dependence on the relative rate constants (Table 1), we evaluated
the difference in activation volumes (DDV‡) for the two trapping
activation processes according to the following equations:
ln(k2/k1) = aP + b,
(2)
ꢀ
ꢁ
d ln(k2/k1)
dP
DDV‡ = –RT
= DDV‡PBN−type–DDV‡
(3)
DMPO−type
T
The calculated values for DDV‡ are given in the right hand side
column in Table 2.
As indicated in Table 2, in alkyl and phenyl radical trapping,
DDV‡ values are negative. It is noted that the |DDV‡| values
(= 9–17 cm3 mol−1) for alkyl and phenyl radical trapping in
the 4-POBN/DMPO system are obviously larger than others.
Fig. 2 Plots of spin-trapping rate constants of (a) methyl and (b) ethyl
radicals against trapping rate constants of hydroxyl radical; ᭺: PBN- and
᭹: DMPO-type traps.
Table 2 External pressure dependence of relative spin trapping rate constants and activation volumes at 298 K
k2/k1
DDV‡
Trapsa
Radical
Solventb
1 bar
98 bar
245 bar
490 barc
Ratiod
cm3 mol−1
4-POBN/DMPO
4-POBN/DMPO
4-POBN/2-Ph-DMPO
4-POBN/DMPO
4-POBN/DMPO
PBN/DMPO
Phenyle
Methyl
Phenyl
Ethyl
B
W
B
0.64 0.02
0.193 0.004
1.21 0.01
0.566 0.002
1.45 0.16
0.0682 0.0004
0.111 0.014
0.0178 0.0005
0.0133 0.0005
0.59 0.03
0.69
0.208
1.23
0.581
1.39
0.0711
0.112
0.0185
0.0140
0.63
0.77
0.234
1.36
0.625
1.44
0.0761
0.115
0.0193
0.0144
0.65
0.89
0.259
1.44
0.675
1.43
0.0870
0.118
0.0212
0.0157
0.69
39
34
19
19
1
28
6
19
18
17
4
19
13
26
5
−17
−16
1
1
−9.3 1.2
−8.9 0.5
0.17 0.05
W
W
W
W
W
W
B
W
W
W
B
Hydroxyle
Methyl
Ethyl
−12
1
PBN/DMPO
−4.1 0.2
−8.8 0.3
−8.0 0.5
−7.8 0.3
−2.3 0.3
−9.2 0.4
−6.9 0.6
2-Ph-DMPO/DMPO
2-Ph-DMPO/DMPO
2-Ph-DMPO/DMPO
2-Ph-DMPO/DMPO
2-SO3-PBN/DMPO
2-SO3-PBN/DMPO
4-NO2-PBN/DMPO
PBN/TMPO
Methyl
Ethyl
Phenyle
Hydroxyle
Methyl
Ethyl
0.51 0.01
0.52
0.53
0.53
0.0712 0.0008
0.269 0.002
1.76 0.02
0.977 0.001
2.82 0.01
0.0739
0.279
1.94
1.01
2.74
0.0787
0.292
2.02
1.02
2.63
0.0849
0.305
2.21
1.03
2.49
Phenyl
Methyl
Ethyl
−11
1
W
W
−2.2 0.1
PBN/TMPO
12
6.5 0.8
a The solubility of 4-NO2-PBN in water and 2-SO3-PBN in benzene is low, thus some combinations are missing from this list. b W: water; B: benzene. c
1
bar = 1 × 105 Pa d Ratio = 100((k2/k1)490 − (k2/k1)1)/(k2/k1)1. e Cited from ref. 6.
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Scheme 2
addition to 2- or 4-position of t-butylbenzene, the product ratio
(2-position addition : 4-position addition) increased with rising
pressure. Furthermore, the DDV‡ value for the additions to the
two positions was reported to be DDV‡ = −8 cm3 mol−1.13 This is
in good agreement with our present observation for competitive
trapping of alkyl- and phenyl-radicals in the PBN analogs and
2-Ph-DMPO systems (4–12 cm3 mol−1), suggesting that the
trapping proceeds through crowded transition states. It should be
emphasized that |DDV‡| values for the 4-POBN/DMPO system
are large as compared with the others (Table 2). We calculated
the difference in the activation volumes for trapping processes
by 4-POBN and PBN using the DDV‡ values in Table 2 and the
following equation.
Fig. 3 ESR spectra obtained in aqueous solution immediately af-
ter UV-irradiation (irradiation time 1 s, sweep time 60 s) in the
presence of 4-POBN and DMPO at
1 and 490 bar. (a) Methyl
radical trapping; ᭺: DMPO-CH3 and ᭹: 4-POBN-CH3. [4-POBN]0/
[DMPO]0 = 3.0. [4-POBN-CH3] : [DMPO-CH3] was 0.579 and 0.777 at
1 and 490 bar, respectively. (b) Ethyl radical trapping; ᭺: DMPO-C2H5
and ᭹: 4-POBN-C2H5. [4-POBN]0/DMPO]0 = 1.3. [4-POBN-C2H5] :
[DMPO-C2H5] was 0.736 and 0.878 at 1 and 490 bar, respectively.
|DDV‡| values in the other PBN analogs are in the range of 4
to 12 cm3 mol−1 for alkyl and phenyl radicals, but the pressure
dependence of hydroxyl radical trapping rates was negligibly
small6 (Table 2).
DDV‡
= DV‡
− DV‡
4-POBN
PBN
4-POBN/PBN = DDV‡
− DDV‡
.
(6)
4-POBN/DMPO
PBN/DMPO
The activation volume for chemical reactions is conventionally
divided into two terms: intrinsic (DV‡int) and solvational (DVs‡ol
part, i.e.
)
The DDV‡
values for methyl- and ethyl-trappings are
4-POBN/PBN
calculated to be −4.0 and −4.8 cm3 mol−1, respectively. These
values suggest that the spin-trapping reaction with 4-POBN
involves the presence of extra volume constriction as compared
with PBN. We consider that the N→O group in the POBN
DV‡ = DV‡int + DVs‡ol
(4)
Usually, the intrinsic volume change (DV‡int) for the addition
reaction of neutral radical molecules is a negative value, and the
volume change (DV‡sol) due to solvation is negligible. From the
pressure dependence on the relative trapping rates, the difference
in activation volumes for two trapping processes (DDV‡) can be
expressed as follows:
phenyl ring contributes to the negative DDV‡
value (ca.
4-POBN/PBN
−5 cm3 mol−1). Scheme 3 shows the canonical resonance structures
of 4-POBN with formal charges on the nitrogen and oxygen
atoms, resulting in the delocalization of the p-electrons. Upon
DDV‡ = DDV‡int + DDVs‡ol
(5)
In chemical reaction in solution, steric crowding causes lower a
partial volume than non-crowded species because the reaction
that proceeds through sterically-hindered transition state could
result in a substantial loss of freedom of internal rotation of
the mutually-repelling groups of atoms.11,12 Such difference could
cause substantial decrease in the free volume.
Negative DDV‡ values in Table 2 suggest that the degree of
steric crowding in the transition state for PBN-analog or 2-Ph-
DMPO spin-trapping is higher than that in DMPO spin-trapping.
Scheme 2 illustrates how steric crowding influences the magnitude
of |DDV‡|. This is also supported from the visual inspection of
the Corey–Pauling–Koltun (CPK) space-filling model (this figure
is not shown). Gonikberg et al.13,14 have shown that phenyl radical
Scheme 3
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radical-trapping with 4-POBN, the twist of the phenyl moiety
causes a reduction in the resonance contribution, and the polarity
of the transition state will increase. Thus, the negative value of
were obtained from Wako Pure Chemicals (Osaka, Japan), and
were used as sources of methyl, ethyl, phenyl and hydroxyl radicals.
Benzene and water were purified by distillation.
DDV‡
can be explained in terms of the change in volume
4-POBN/PBN
(DDV‡sol) due to solvation for the polar activation complex of 4-
ESR measurements of free radical (spin) adducts
POBN spin trapping.
A competitive spin trapping method was used to determine relative
spin trapping rates. The competitor spin trap was DMPO except
when the counterpart was TMPO. A JEOL-FE3XG spectrometer
equipped with a 100 kHz field modulator was used for the
ESR measurements. Methyl and ethyl radicals were generated
with UV-irradiation (200 W mercury arc RUVF-203S, Radical
Research Inc., Hino, Japan). For instance, trimethyllead acetate
(5 × 10−3 mol dm−3) and spin traps (5–10 × 10−3 mol dm−3)
were mixed in water and loaded in the ESR flat cell. The
sample solution was set inside the ESR cavity. ESR signals
were recorded immediately after UV irradiation: irradiation time
1 s, sweep time 60 s, time constant 0.3 s, microwave power 5
mW. ESR spectra of two different spin-adducts were computer-
simulated with the aid of an attached computer program (WIN-
RAD computer system, Radical Research Inc., Hino, Japan) by
adjusting the relative intensity of the two radical adducts. The
relative abundance (concentration) of the two components was
calculated as follows: 1) obtain a best-fit simulated spectrum,
and 2) using simulated component spectra, calculate the relative
abundance of the two components with a computer-mediated
double-integration routine of the first-derivative signal of each
component. The plot of the relative abundance of spin-adducts
against initial concentrations of spin-traps gives a line with the
slope k2/k1. In case of TMPO, because the ESR signal of TMPO
spin-adduct overlapped with DMPO adduct, we used PBN as a
competitor trap instead of DMPO.
In PBN analogs, |DDV‡| for methyl radical trapping is larger
•
•
than those for ethyl radical, i.e., |DDV‡(CH3 ) − DDV‡(C2H5 )| =
7.1, 7.9, and 2.3 cm3 mol−1 for 4-POBN/DMPO, PBN/DMPO,
and 2-SO3-PBN/DMPO, respectively. From Table 1, we notice
that methyl-radical trapping rate constants by DMPO analogs
are similar to those of ethyl radical. Taniguchi and Madden,10
in their kinetic spin trapping study, suggested that spin trapping
rate constants are usually influenced by both electronic and steric
factors, but DMPO did not exhibit the strong electronic character
in their study. In the present study, methyl radical trapping rate
constants by PBN analogs are smaller than ethyl radical (Table 1),
indicating that the electronic factor is operative in the case of
PBN analogs. The difference in the methyl and ethyl radical
trapping rate constants by PBN analogs may be attributed to
the difference in the activation energy for the trapping reaction.
According to the Hammond postulate,15 the transition state for
the reaction with a large activation energy should be product-
like in terms of energy and geometry. Therefore, we may assume
that the methyl and ethyl trapping reactions by DMPO analogs
have similar transition-state structures, while the transition state
for the methyl radical trapping by PBN analogs lies closer to
the product as compared with that of ethyl radical trapping.
This could give rise to the larger negative DV‡ value for methyl
radical trapping by PBN. In contrast, for 2-Ph-DMPO/DMPO
and TMPO/DMPO( = DDV‡
− DDV‡PBN/TMPO) systems,
PBN/DMPO
the DDV‡ values for methyl radicals are comparable to ethyl
radical. Again, this supports our interpretation.
In order to confirm that spin-adduct decay would not influence
the trapping rate measurement, we monitored the decay rate of the
spin adducts. Time-dependent decrease of ESR peak heights was
measured for PBN-CH3, PBN-C2H5, DMPO-CH3, and DMPO-
C2H5 in aqueous solution at 298 K. The results indicated that all
the decay was first-order, and half-lives are 3.1, 1.8, 1.2 and 1.2 ×
103 s, respectively. Other PBN-type adducts showed longer half-
lives than PBN-CH3. Because spin-trapping rates are much higher
In summary, we show the large effect of external pressure
on the spin-trapping reactions of methyl, ethyl, and phenyl
radicals. Pressure-induced acceleration in trapping rates is more
pronounced in systems that can form sterically-hindered trapping
sites. We believe that the external pressure could be a useful factor
that can effectively control the rate of spin-trapping reactions.
Experimental
Materials
Spin traps used are shown in Scheme 1: benzylidene(tert-butyl)-
amine-N-oxide (PBN), sodium tert-butyl(2-sulfonatobenzylidene)-
amine-N-oxide (2-SO3-PBN), tert-butyl(4-nitorobenzylidene)-
amine-N-oxide (4-NO2-PBN), tert-butyl(4-pyridinylmethylene)-
amine-N-oxide (4-POBN), tert-butyl(4-hydroxybenzylidene)amine-
N-oxide (4-HO-PBN), 2,2-dimethyl-3,4-dihydro-2H-pyrrole-1-
oxide (DMPO), 2,2,4,4-tetramethyl-3,4-dihydro-2H-pyrrole-1-
oxide (TMPO), 2,2-dimethyl-5-phenyl-3,4-dihydro-2H-pyrrole-
1-oxide (2-Ph-DMPO), and 2-(diethoxyphosphoryl)-2-methyl-
3,4-dihdro-2H-pyrrole-1-oxide (DEPMPO). DEPMPO was pur-
chased from Radical Research Inc. (Hino, Japan), and 4-HO-
PBN was synthesized in the OMRF laboratory. Other PBN-
and DMPO-type traps were obtained from Aldrich Chemical
Company, Inc. (Milwaukee WI, USA). Trimethyllead acetate,
triethyllead acetate, tetraphenyllead and hydrogen peroxide (30%)
Fig. 4 High-pressure cell for ESR measurement.
900 | Org. Biomol. Chem., 2006, 4, 896–901
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©

than the spin-adducts’ decay rates,10 we concluded that the decay
did not interfere with spin-trapping rate measurements.
2 F. A. Villamena, C. M. Hadad and J. L. Zweier, J. Am. Chem. Soc.,
2004, 126, 1816.
3 F. Chalier and P. Tordo, J. Chem. Soc., Perkin Trans. 2, 2002,
EPR signals from all described spin-trapping systems were
recorded at 1 bar (atmospheric pressure) and high pressure (ca.
490 bar). Procedures for ESR measurements at high pressure
were the same as those described elsewhere.7 A diagram of the
high-pressure ESR system is shown in Fig. 4. A thick-wall quartz
capillary tubing (i.d. 1 mm and o.d. 6 mm) was used as a sample
cell, and the sample tube packed with chemicals was fitted to a
manual static pressure generator (Model KP3W, Hikari Kouatsu
Inc., Hiroshima, Japan) through a Heise–Bourdon gauge and
a small stop-valve. After applying pressure, the stop valve was
closed and the cell was disconnected from the high-pressure pump
system and set into the ESR cavity. UV light was illuminated
to the pressurized sample in situ and ESR signals were recorded
immediately after the UV illumination.
2110.
4 Y. Sueishi, C. Yoshioka, C. Olea-Azar, L. A. Reinke and Y. Kotake,
Bull. Chem. Soc. Jpn., 2002, 75, 2043.
5 V. Roubaud, H. Dozol, C. Rizzi, R. Lauricella, J. C. Bouteiller and B.
Tuccio, J. Chem. Soc., Perkin Trans. 2, 2002, 958.
6 C. Yoshioka, Y. Kotake and Y. Sueishi, Chem. Phys. Lett., 2004, 385,
189.
7 Y. Sueishi, H. Tobisako and Y. Kotake, J. Phys. Chem. B, 2004, 108,
12623.
8 Y. Sueishi, H. Inoue, T. Oka, H. Tsukube and S. Yamamoto, Bull.
Chem. Soc. Jpn., 1998, 71, 817.
9 G. R. Buettner, J. Free Radicals Biol. Med., 1987, 3, 259.
10 H. Taniguchi and K. P. Madden, J. Am. Chem. Soc., 1999, 121, 11875.
11 N. S. Isaacs, Liquid Phase High Pressure Chemistry, John Wiley & Sons,
New York, 1981.
12 A. Drljaca, C. D. Hubbard, R. van Eldik, T. Asano, M. V. Basilevsky
and W. J. le Noble, Chem. Rev., 1998, 98, 2167.
13 M. G. Gonikberg, High Pressure Chemistry, Nikkan Kogyo Shimbun,
Tokyo, 1972.
14 M. G. Gonikberg, N. I. Prokhorova and E. F. Litvin, Dokl. Akad. Nauk
References
SSSR, 1963, 148, 105.
1 E. G. Janzen, Acc. Chem. Res., 1977, 4, 31.
15 G. S. Hammond, J. Am. Chem. Soc., 1955, 77, 334.
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