Effects of substituent and external pressure on spin trapping rates of carbon dioxide anion, sulfur trioxide anion, hydroxyl, and ethyl radicals with various PBN- and DMPO-type spin traps

Article abstract of DOI:10.1515/zpch-2014-0538

Using a competitive trapping method employing two different traps, the trapping rates of anionic radicals (CO₂^-· and SO₃^-·), hydroxyl, and ethyl were quantified for various PBN- and DMPO-type traps. We have examined the characteristic effects of substituent and external pressure on the spin trapping of anionic radicals and compared with those of neutral radicals such as ethyl and hydroxyl radicals. In DMPO-type traps, the effects of substituent on trapping rates of two anionic radicals were small compared with that of C₂H₅·. In PBN-type traps, the large effects of substituent on trapping rates of CO₂^-· and C₂H₅· were observed. Further, the activation volumes determined from the pressure dependence experiments for competitive reactions show a difference in pressure behavior between the two trapping reactions, and enable us to speculate on the reaction mechanism of substituted PBN and DMPO traps for anionic radicals.

Full text of DOI:10.1515/zpch-2014-0538

Z. Phys. Chem. 2014; 228(9): 927–938
Yoshimi Sueishi*, Keitaro Miyazono, and Kazuki Kozai
Effects of Substituent and External Pressure
on Spin Trapping Rates of Carbon Dioxide
Anion, Sulfur Trioxide Anion, Hydroxyl, and
Ethyl Radicals with Various PBN- and
DMPO-Type Spin Traps
Abstract: Using a competitive trapping method employing two different traps,
−
−
the trapping rates of anionic radicals (CO2 ⋅ and SO3 ⋅), hydroxyl, and ethyl were
quantified for various PBN- and DMPO-type traps. We have examined the charac-
teristic effects of substituent and external pressure on the spin trapping of anionic
radicals and compared with those of neutral radicals such as ethyl and hydroxyl
radicals. In DMPO-type traps, the effects of substituent on trapping rates of two
anionic radicals were small compared with that of C2H5⋅. In PBN-type traps, the
−
large effects of substituent on trapping rates of CO2 ⋅ and C2H5⋅ were observed.
Further, the activation volumes determined from the pressure dependence exper-
iments for competitive reactions show a difference in pressure behavior between
the two trapping reactions, and enable us to speculate on the reaction mechanism
of substituted PBN and DMPO traps for anionic radicals.
Keywords: High-Pressure ESR, Spin Trap, Substituent Effect, Trapping Rate.
DOI 10.1515/zpch-2014-0538
Received May 14, 2014; accepted July 20, 2014
1 Introduction
Spin trapping involves the trapping of reactive free radicals by an addition reac-
tion to produce more stable radicals, detectable by electron spin resonance (ESR)
spectroscopy, and has become a valuable tool in the study of transient free rad-
*Corresponding author: Yoshimi Sueishi, Department of Chemistry, Faculty of Science,
Okayama University, 3-1-1 Tsushima Naka, Kita-ku, Okayama 700-8530, Japan,
e-mail: ysueishi@cc.okayama-u.ac.jp
Keitaro Miyazono, Kazuki Kozai: Department of Chemistry, Faculty of Science, Okayama
University, 3-1-1 Tsushima Naka, Kita-ku, Okayama 700-8530, Japan
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Y. Sueishi et al.
icals in chemical and biological systems [1–3]. Nitrone compounds such as 5,5-
dimethyl- pyrroline-1-oxide (DMPO) and ꢀ-phenyl-ꢁ-tert-butylnitrone (PBN) have
been most widely used as spin trapping compounds. It is recognized that nitrone
traps form stable radical adducts with neutral radicals such as hydroxyl and alkyl
radicals. However, the basic information for the trapping rates of various nitrone
traps against ionic radicals has not been well established because of the short-
lived ionic radical adducts.
Various sulfites are used as sanitizing agents in the food industry and are rec-
−
ognized as safe. However, the sulfur-center trioxide anion radical (SO3 ⋅), which
is formed during sulfite metabolism via one-electron oxidation, is proposed as
contributing to the mechanisms of toxicity [4]. Therefore, it is important to collect
basic data regarding the spin trapping of various nitrone traps against anionic
−
SO3 ⋅ radical.
−
−
In this study, since the SO3 ⋅ and carbon dioxide anion (CO2 ⋅) radicals form
relatively stable spin adducts with nitrone traps [5, 6], we have determined the
−
−
anionic SO3 ⋅ and CO2 ⋅ radical spin trapping rates for various PBN- and DMPO-
type traps, using a competitive trapping method. We then compared those rates
with the spin trapping rates for neutral radicals such as hydroxyl and ethyl radi-
cals. Further, we investigated the effects of substituent and external pressure on
the spin trapping rates for various PBN- and DMPO-type traps. Examination of
the pressure effect on chemical reactions is useful in characterizing the reaction
mechanism. Based on the experimental results, we have performed substituent
and volumetric studies on the spin trapping reactions.
2 Experimental
2.1 Material
The spin traps illustrated in Figure 1 were used in the present study: 5,5-dimethyl
pyrroline-1-oxide (DMPO), 2,2-dimethyl-5-phenyl-3,4-dihydro-2ꢂ-pyrrole 1-oxide
(2-Ph-DMPO), 2,2,4,4-tetramethyl-3,4-dihydro-2ꢂ-pyrrole 1-oxide (TMPO), 5-di-
phenylphosphoryl-5-methyl-3,4-dihdro-2ꢂ-pyrrole 1-oxide (DPPMPO), 2-(di-
ethoxyphosphoryl)-2-methyl-3,4-dihdro-2ꢂ-pyrrole 1-oxide (DEPMPO), 5-(2,2-
dimethyl-1,3-propoxyl cyclophosphoryl)-5-methyl-1-pyrroline ꢁ-oxide (CYPMPO),
benzylidene(tert-butyl)amine ꢁ-oxide (ꢀ-phenyl-ꢁ-tert-butylnitrone (PBN)),
tert-butyl(4-pyridinylmethylene)amine ꢁ-oxide (4-POBN), tert-butyl(4- hydrox-
ybenzylidene)amine ꢁ-oxide (4-HO-PBN), tert-butyl(2-sulfonatobenzylidene)-
amine ꢁ-oxide (2-SO3-PBN), tert-butyl(4-nitorobenzylidene)amine ꢁ-oxide
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Effects of Substituent and Pressure on Trapping Rates
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Figure 1: Structures of DMPO- and PBN-type spin traps.
(4-NO2-PBN). DEPMPO and CYPMPO were purchased from Radical Research
Inc. (Hino, Japan). 4-OH-PBN and 2-Ph-DMPO were synthesized at the OMRF
Laboratory (Oklahoma City, OK, USA). Other PBN- and DMPO-type traps were
obtained from Aldrich Chemical Company, Inc. (Milwaukee, WI, USA) and
Wako Pure Chemicals (Osaka, Japan). Spin traps were used as received. Sodium
sulfite and sodium formate (+ hydrogen peroxide) were used as a source of
−
−
sulfite anion trioxide radical (SO3 ⋅) and carbon dioxide anion radical (CO2 ⋅),
respectively [5–7]. The solvent used was distilled water.
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2.2 ESR measurements
−
The CO2 ⋅ radical was generated with UV irradiation of the phosphate buffer
(pH =7.4) containing NaHCO2 (100 mM) and H2O2 (100 ꢃM) [7]: 5 s irradiation
with 200 W mercury arc (RUVF-203S, Radical Research Inc. (Hino, Japan)). The
−
SO3 ⋅ radical was formed in KHCO3/NaOH buffer (pH =9.5) from the photolysis
of Na2SO3 [5, 8]. The hydroxyl and ethyl radicals were formed from the photolysis
of hydrogen peroxide and triethyl lead acetate, respectively [9].
In order to determine spin trapping rates, we used a competitive trapping
method, where two spin traps compete with each other to trap free radicals [3].
The sample solution was loaded in an ESR flat cell; ESR signals were immedi-
ately recorded with JEOL JES-FE3XG after UV irradiation: Sweep time 30 s, time
constant 0.1 s, microwave power 3–5 mW, modulation amplitude 0.063 mT at
100 kHz. ESR spectra observed were computer-simulated with an attached com-
puter program (WIN-RAD system, Radical Research Inc.) by superimposing the
ESR spectra of two different radicaladducts. The relative abundance of the adduct
components was calculated with a computer-mediated double-integration rou-
tine of the first-derivative signal of each component.
The high-pressure ESR system used has been described elsewhere [10]. The
sample solution was loaded into thick-walled quartz capillary tubing (ID 1 mm,
OD 6 mm). After pressure was applied, the ESR signals of radical adducts gener-
ated with UV irradiation were recorded with an ESR spectrometer.
3 Results and discussion
3.1 Determination of spin trapping rates
Competitive spin trapping between two distinctive spin traps was used to deter-
−
−
mine the rate constants for the SO3 ⋅ and CO2 ⋅ trapping [3]. Figure 2 shows the
typical ESR spectra obtained in the UV-irradiated solution of NaHCO3 and H2O2
in the presence of two traps: (a) the PBN-DMPO and (b) CYPMPO-DMPO systems.
Two spin adducts species are visible in these spectra; the hyperfine coupling con-
stant (hfcc) values of spin adducts are listed in Table 1. The hfcc of the ESR spectra
−
was assigned to the CO2 ⋅ radical adduct [7]. The ESR spectra of two different spin
adducts were computer-simulated with an attached system (Figure 2a and b). The
ESR spectra can be reproduced well by superimposing two components. The rel-
ative abundance of the adduct components was calculated using the simulation
spectra.
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Effects of Substituent and Pressure on Trapping Rates
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Figure 2: ESR spectra of carbon dioxide anion radical adducts obtained after UV-irradiation
in (a) the PBN/DMPO and (b) CYPMPO/DMPO systems at 298 K, and simulated spectra:
(a) [PBN] /[DMPO] = 36 and (b) [CYPMPO] /[DMPO] = 0.19. (c) Determination of ꢀ /ꢀ
0
0
0
0
PBN DMPO
according to Eq. (1) in the PBN/DMPO system.
−
The reaction scheme for spin trapping of CO2 ⋅ radical in the presence of PBN
and DMPO is given by [7]:
+
−
NaHCO2 → Na + HCO2
H2O2 → 2HO⋅(ℎꢄ)
−
−
HO⋅ + HCO2 → H2O + CO2 ⋅
−
−
DMPO + CO2 ⋅ → DMPO-CO2 (rate constant ꢅ1)
−
−
PBN + CO2 ⋅ → PBN-CO (rate constant ꢅ2)
2 ꢅ2 [PBN]0
−
−
The relative formation rates for DMPO-CO2 and PBN-CO2 adducts can be given
as follows [10].
−
d[PBN-CO2 ]/dꢆ
=
(1)
−
d[DMPO-CO2 ]/dꢆ ꢅ1 [DMPO]0
where the symbol [ ] denotes the initial concentration of traps. A typical plot of
0
the concentration ratio of spin adducts against the initial concentration ratio of
spin traps is shown in Figure 2c with a straight line passing through the origin,
suggesting that the calculation process of relative trapping rate constants (ꢅ2/ꢅ1)
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−
−
Table 1: Hyperfine coupling constants (hfcc) and rate constants for CO2 ⋅ and SO3 ⋅ at 298 K.
Spin Trap
Radical
hfcc (mT)
ꢀ2/ꢀ1(DMPO)
10−7 ꢀ2
3
−1 −1
ꢁH
ꢁN
ꢁP
(dm mol
s
)
−
CYPMPO
DPPMPO
DEPMPO
DMPO
CO2 ⋅
1.666 1.461 5.343
1.790 1.398 3.931
1.735 1.448 5.167
1.871 1.572
7.70 0.2
4.31 0.06
5.44 0.2
50.8
28.4
35.9
6.6a)
1.48
2.34
11.6
8.58
0.325
0.302
0.18
2.41
−
CO2 ⋅
−
CO2 ⋅
−
CO2 ⋅
1
−
TMPO
CO2 ⋅
1.981 1.581
0.204 0.002b)
0.355 0.003
1.76 0.01
−
2-Ph-DMPO
4-POBN
CO2 ⋅
1.528
−
CO2 ⋅
0.348 1.550
−
4-NO2-PBN
2-SO3-PBN
PBN
CO2 ⋅
0.414 1.576
1.30 0.01
−
CO2 ⋅
0.620 1.580
0.0493 0.0002
0.0458 0.0006
0.0272 0.0002
2.01 0.06
−
CO2 ⋅
0.470 1.590
−
4-HO-PBN
CYPMPOꢀ
CO2 ⋅
0.486 1.599
−
SO3 ⋅
1.499 1.328 5.100
1.640d) 1.550d) 3.080d)
1.497 1.294 3.812
1.521 1.358 5.002
1.615 1.453
−
DPPMPOc)
DEPMPOc)
DMPO
SO3 ⋅
1.21 0.01
1.46 0.02
1.45
1.75
−
SO3 ⋅
−
SO3 ⋅
1
1.2a)
−
TMPO
SO3 ⋅
1.300 1.410
0.55 0.02b)
0.034 0.003
0.0036 0.0002
0.0295 0.0010
0.064
0.041
−
4-POBN
2-SO3-PBN
PBNꢀ
SO3 ⋅
0.160 1.480
−
SO3 ⋅
0.459 1.586
0.0043
0.0354
−
SO3 ⋅
0.197 1.498
a) Cited from ref. [12].
b) The relative rate constants in the PBN/TMPO system.
c) Cited from ref. [6].
d) Diastereomeric spin adduct of CYPMPO against SO3 ⋅.
−
using Equation (1) is justifiable. We determined the relative trapping rate con-
−
−
stants of the SO3 ⋅ and CO2 ⋅ radicals for various PBN- and DMPO-type spin traps
(Table 1). Further, in two new DMPO-type traps (CYPMPO and DPPMPO), the rel-
ative trapping rates for hydroxyl and ethyl radicals were determined according to
Equation (1) (Table 2).
Determination of the reaction rate constants enables us to compare the trap-
ping rates of various spin traps. In the present experiment, we determined the
−
−
rate constants of SO3 ⋅, CO2 ⋅, C2H5⋅, and HO⋅ for various traps by using previ-
ously published DMPO trapping rate constants. The rate constants of DMPO for
−
−
7
7
7
SO ⋅, CO ⋅, C2H5⋅, and HO⋅ were ꢅ
= 1.2 × 10 , 6.6 × 10 , 1.6 × 10 , and
DMPO
3
2
9
−3
−1 −1
3.6 × 10 dm mol s , respectively [11, 12]. The rate constants data for various
traps are listed in Tables 1 and 2. The results show that, as a whole, the spin trap-
ping rate constants of anionic radicals by DMPO-type traps are higher compared
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Effects of Substituent and Pressure on Trapping Rates
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Table 2: Hyperfine coupling constants (hfcc) and rate constants for C2H5⋅ and HO⋅ at 298 K.
Spin Trap
CYPMPO
Radical
hfcc (mT)
ꢀ2/ꢀ1(DMPO)
10−7 ꢀ2
3
−1 −1
ꢁH
ꢁN
ꢁP
(dm mol
s
)
C2H5⋅
2.120 1.487 5.135
13.3 0.1
21.3
DPPMPO
DMPO
C2H5⋅
C2H5⋅
HO⋅
2.273 1.487 3.635
2.370 1.640
2.74 0.02
1
4.38
1.6a)
2800
CYPMPO
1.407 1.407 5.160
7.67 0.02
1.263b) 1.386b) 4.970b)
DPPMPO
DMPO
HO⋅
1.370 1.370 3.550
1.530 1.530
2.36
850
HO⋅
1
360c)
a) Cited from ref. [12].
b) Diastereomeric spin adduct of CYPMPO against HO⋅.
c) Cited from ref. [11].
with those of PBN-type traps. In particular, CYPMPO and DEPMPO are effective
−
−
for scavenging the anionic SO3 ⋅ and CO2 ⋅ radicals.
3.2 Substituent effects on spin trapping
The Hammett plot may be useful for a discussion on substituent effects on the spin
trapping reaction. However, substituent constants for various traps are not avail-
able. In a previous paper, we determined the ⋅OH radical trapping rate constants
for various traps [9]. The ⋅OH radical is recognized as a non-selective oxidant and
readily undergoes addition reaction to the phenyl groups or double bonds [13].
Thus, we believe that the ⋅OH radical trapping rate constants reflect the reactiv-
ity of the traps, while the effects of the substituent are small. Using the trapping
rate constants in Table 2 and the previously observed trapping data for ⋅OH radi-
−
−
cal [9], alternatively, we plotted the spin trapping rate constants of SO3 ⋅ and CO2 ⋅
against those of ⋅OH, together with those of C2H5⋅ (Figure 3). Figure 3a clearly
−
suggests that PBN-type trapping rate constants for CO2 ⋅ tend to increase with an
electron-withdrawing group in the traps (slope in Figure 3a =2.4) compared with
−
that of C2H5⋅ (slope =1.3). However, the trapping rate constants for SO3 ⋅ are in-
dependent of the substituents in the PBN traps (slope =0.20), which is ascribed
to the difference in the contribution to the resonance system in the PBN traps. In
the DMPO type spin trapping, as shown in Figure 3b, the effects of substituent on
the trapping rates of two anionic radicals are small compared with those of the
C2H5⋅ radical, which is responsible for the small electron-inductive effect of the
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−
−
Figure 3: Relationship of trapping rate constants of CO2 ⋅, SO3 ⋅, and C2H5⋅ for those of HO⋅:
(a) PBN- and (b) DMPO-type traps. The available trapping data of C2H5⋅ and HO⋅ were cited from
ref. [9].
−
−
−
SO3 and CO2 groups: the slopes in Figure 3b were calculated to be 0.82 (CO2 ⋅),
−
0.31 (SO3 ⋅), and 1.2 (C2H5⋅).
2-Ph-DMPO and TMPO exhibit a downward shift in the trapping rates as com-
pared with those of the DMPO-type traps in Figure 3b. These may be attributed to
the steric hindrance caused by the phenyl- and methyl-substituent groups.
3.3 Pressure effects on spin trapping
Using the pressure dependence of the relative rate constants (Table 3), we eval-
‡
uated the difference in the activation volumes (ꢇꢇꢈ ) for the two trapping pro-
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Effects of Substituent and Pressure on Trapping Rates
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Table 3: External pressure dependence of relative trapping rate constants and activation
volumes.
‡
Traps
Radical
ꢀ2/ꢀ1
ꢂꢂꢃ
3
−1
1
98
245
490 (bar) (cm mol
)
−
4-POBN/DMPO CO2 ⋅
1.76 0.08
1.96
1.99
2.28
0.0307
0.682
−13
−
4-POBN/DMPO SO3 ⋅
0.0340 0.0010 0.0332 0.0327
0.570 0.05 0.580 0.620
0.0458 0.0012 0.0483 0.05927
4.9
−8.9
−27
4-POBN/DMPO C2H5⋅
−
PBN/DMPO
CO2 ⋅
0.0788
−
PBN/DMPO
SO3 ⋅
0.0295 0.0009 0.0316 0.0365
0.111 0.014 0.112 0.115
0.0434
0.118
10.1
−19
−4.1
−13
PBN/DMPOa)
C2H5⋅
−
CYPMPO/DMPO CO2 ⋅
7.70 0.15
7.79
8.83
−
CYPMPO/DMPO SO3 ⋅
2.01 0.09
13.3 0.05
5.44 0.14
2.14
14.2
5.68
2.24
15.0
6.15
2.55
16.7
6.62
1.82
3.07
0.265
−12
−12
−10
CYPMPO/DMPO C2H5⋅
−
DEPMPO/DMPO CO2 ⋅
−
DEPMPO/DMPO SO3 ⋅
1.46 0.09
2.53 0.08
0.204 0.011 0.212 0.236
0.550 0.08 0.576 0.594
2.82 0.05 2.74 2.63
1.54
2.68
1.58
2.84
−10
−10
−13
DEPMPO/DMPO C2H5⋅
−
PBN/TMPO
CO2 ⋅
−
PBN/TMPO
SO3 ⋅
0.621
2.49
−6.4
6.5
PBN/TMPOa)
C2H5⋅
a) Cited from ref. [9].
‡
‡
cesses (ꢇꢈ and ꢇꢈ ) according to the following equations:
1
2
ln(ꢅ2/ꢅ1) = ꢉꢊ + ꢋ
(2)
(3)
‡
‡
‡
ꢇꢇꢈ = −RT(ꢌ ln(ꢅ2/ꢅ1)/ꢌꢊ)ꢄ = ꢇꢈ − ꢇꢈ )
2
1
The slope of the best-fit straight line in a plot for ln(ꢅ2/ꢅ1) against pressure
‡
was used to estimate ꢇꢇꢈ for two trapping processes (Figure 4) and the re-
‡
sults are listed in Table 3. An inspection of Table 3 suggests that the ꢇꢇꢈ values
3
−1
(=−27–4.9 cm mol ) for PBN and 4-POBN against DMPO are dependent on the
3
−1
kinds of trapped radicals, while those (=−13–−10 cm mol ) of CYPMPO and
DEPMPO against DMPO are independent of the trapped radicals. In a previous
paper, Sueishi et al. suggested that PBN-type traps have the steric crowded trap-
‡
ping site as compared with those of DMPO, resulting in the negative ꢇꢇꢈ values
for the PBN/DMPO system.
It is instructive to examine the trapping reactions of the 4-POBN/PBN and
TMPO/DMPO systems for the discussion on reaction mechanisms. In radicaltrap-
ping by the PBN- and DMPO-type traps, we can calculate the difference in acti-
‡
vation volumes for the various trapping processes by using the ꢇꢇꢈ values in
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Y. Sueishi et al.
Figure 4: Plots of relative trapping rate constants against external pressures in the PBN/DMPO
−
−
system: ( ) CO ⋅, ( ) SO ⋅, and (ꢍ) C H ⋅ radical trapping. The available trapping data of C H ⋅
∘
∙
2
5
2
5
2
3
were cited from ref. [9].
Table 3:‡
‡
‡
‡
‡
ꢇꢇꢈ
= ꢇꢈ
− ꢇꢈ = ꢇꢇꢈ
− ꢇꢇꢈ
(4)
(5)
4− POBN/PBN
4−POBN
PBN
4−POBN/DMPO
PBN/DMPO
‡
‡
‡
‡
‡
ꢇꢇꢈ
= ꢇꢈ
− ꢇꢈ
= ꢇꢇꢈ
− ꢇꢇꢈ
TMPO/DMPO
TMPO
DMPO
PBN/DMPO
PBN/TMPO
‡
‡
3
−1
The ꢇꢇꢈ
and ꢇꢇꢈ
values (cm mol ) were calculated to
4−POBN/PBN
TMPD/DMPO
−
−
be 14 (CO2 ⋅), 24 (SO3 ⋅), and −4.8 (C2H5⋅) for the 4-POBN/PBN system and −13
−
−
(CO2 ⋅), −13 (SO3 ⋅), and −11 (C2H5⋅) for the TMPO/DMPO system. The nega-
tive value for C2H5⋅-trapping in the 4-POBN/PBN system can be explained in
terms of the localization of the ꢎ-electrons upon trapping with 4-POBN [9]. The
large positive values for anionic radicals trapping in 4-POBN/PBN is ascribed
to desolvation for the less polar activation complex of ionic radicals upon trap-
‡
ping by 4-POBN having the electron-withdrawing group. Negative ꢇꢇꢈ val-
3
−1
ues (ca. −10 cm mol ) for the substituted-DMPO/DMPO system (DEPMPO and
CYPMPO) indicate that the degree of steric crowding in the transition state for
DEPMPO and CYPMPO spin trapping is higher than that in DMPO spin trapping. In
‡
the spin trapping of the TMPO/DMPO system, the negative ꢇꢇꢈ value for radical
trapping can be explained in terms of the steric hindrance around the TMPO trap-
ping site [9, 14], suggesting that the steric hindrance in the DEPMPO and CYPMPO
traps having bulky substituent groups is comparable to that by methyl groups
‡
around the TMPO trapping site. Using the ꢇꢇꢈ values for the DMPO-trapping
−
reaction of C2H5⋅ and CO2 ⋅ (Table 3), the volume profile for the trapping reac-
tions is shown in Figure 5. It is noted that PBN-type traps show the characteristic
effect of external pressure, depending on the nature of the trapped radicals.
In conclusion, the spin trapping rate constants of various substituted-PBN
−
−
and -DMPO traps for hydroxyl, ethyl, and anionic radicals (CO2 ⋅ and SO3 ⋅) were
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Effects of Substituent and Pressure on Trapping Rates
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‡
Figure 5: Volume profile (ꢂꢂꢃ ) of the trapping reactions with various traps for DMPO-trapping:
−
(a) C2H5⋅ and (b) CO2 ⋅.
determined using a competitive ESR trapping method. The influences of sub-
stituent and external pressure on the spin trapping rates of various traps for an-
ionic radicals were shown. A steric crowed trapping site of DMPO-type traps such
as CYPMPO and DEPMPO is responsible for the pressure-induced acceleration in
‡
trapping rates (the negative ꢇꢈ values). High-pressure studies provided useful
insight into the spin trapping reaction mechanism.
Acknowledgement: We thank Dr. Yashige Kotake for helpful discussion of the
manuscript.
References
1. E. G. Janzen, Accounts Chem. Res. 4 (1971) 31.
2. G. R. Buettner, Free Radical Bio. Med. 3 (1987) 259.
3. Y. Sueishi, C. Yoshioka, C. Claudio-Azar, L. A. Reinke, and Y. Kotake, B. Chem. Soc. Jpn. 75
(2002) 2043.
4. H. D. Rabinowitch, G. M. Rosen, and I. Fridovich, Free Radical Bio. Med. 6 (1989) 45.
5. C. Mottley and R. P. Mason, Arch. Biochem. Biophys. 267 (1985) 681.
6. Y. Sueishi, A. Miyata, D. Yoshioka, M. Kamibayashi, and Y. Kotake, J. Incl. Phenom. Macro.
66 (2010) 357.
7. F. A. Villamena, E. J. Locigno, A. Rockenbauer, C. M. Hadad, and J. L. Zweier, J. Phys.
Chem. A 110 (2006) 13253.
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938
| Y. Sueishi et al.
8. C. Mottley, R. P. Mason, C. F. Chignell, K. Sivarajah, and T. E. Eling, J. Biol. Chem. 257 (1982)
5050.
9. Y. Sueishi, D. Yoshioka, C. Yoshioka, S. Yamamoto, and Y. Kotake, Org. Biomol. Chem. 4
(2006) 896.
10. Y. Sueishi, S. Yamamoto, and N. Nishimura, Meas. Sci. Technol. 4 (1993) 1171.
11. R. Sridhar, P. C. Beaumont, and E. L. Powers, J. Radioanal. Nucl. Ch. 101 (1986), 227.
12. H. Taniguchi and K. P. Madden, J. Am. Chem. Soc. 121 (1999) 11875.
13. K. Hata, M. Lin, Y. Katsumura, Y. Muroya, H. Fu, S. Yamashita, and H. Nakagawa, J. Radiat.
Res. 52 (2011) 15.
14. M. G. Gonikberg, High Pressure Chemistry, Nikkan Kogyo Shinbun, Tokyo, 1972.
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