ESR analysis of the oxidation reactions of phosphorus-containing nitrone-type spin traps with gold(III) ion

Article abstract of DOI:10.1139/V10-033

Phosphorus-containing cyclic nitrones, such as DEPMPO, CYPMPO, and DPPMPO, were oxidized by hydrogen tetrachloroaurate(III) to DEPMPOX, CYPMPOX, and DPPMPOX with the precipitation of Au(0). The reaction was depressed by the addition of chloride or hydroxide ions. The peculiar pH dependency was observed in DEPMPOX, CYPMPOX, and DPPMPOX formation, which should be caused by the diethoxyphosphoryl group in DEPMPO, the 1,3- propoxy cyclophosphoryl group in CYPMPO, and the diphenylphosphinoyl group in DPPMPO. The oxidation of the nitrones proceeded through the ligand exchange of Cl- in AuCl₄ - with >N⁺-O^- in nitrone and the nucleophilic addition of the water molecule to the C-2 position in the nitrones, the stepwise intra-molecular transfer of three electrons from the nitrones to Au(III), and the release of the resulting Au(0). The phosphoryl group in the nitrones suppressed the first ligandexchange interaction by its electronegativity, while the group promoted the electron transfer from the nitrones to Au(III) by its inductive effect.

Full text of DOI:10.1139/V10-033

556
ESR analysis of the oxidation reactions of
phosphorus-containing nitrone-type spin traps
with gold(III) ion
Akira Nakajima, Emiko Matsuda, Yuto Ueda, and Kunihiko Tajima
Abstract: Phosphorus-containing cyclic nitrones, such as DEPMPO, CYPMPO, and DPPMPO, were oxidized by hydrogen
tetrachloroaurate(III) to DEPMPOX, CYPMPOX, and DPPMPOX with the precipitation of Au(0). The reaction was de-
pressed by the addition of chloride or hydroxide ions. The peculiar pH dependency was observed in DEPMPOX,
CYPMPOX, and DPPMPOX formation, which should be caused by the diethoxyphosphoryl group in DEPMPO, the 1,3-
propoxy cyclophosphoryl group in CYPMPO, and the diphenylphosphinoyl group in DPPMPO. The oxidation of the nitro-
nes proceeded through the ligand exchange of Cl– in AuCl4 with >N+–O– in nitrone and the nucleophilic addition of the
–
water molecule to the C-2 position in the nitrones, the stepwise intra-molecular transfer of three electrons from the nitro-
nes to Au(III), and the release of the resulting Au(0). The phosphoryl group in the nitrones suppressed the first ligand-
exchange interaction by its electronegativity, while the group promoted the electron transfer from the nitrones to Au(III)
by its inductive effect.
Key words: ESR, spin traps, DMPOX, DEPMPO, CYPMPO, DPPMPO, Au(III) ion.
´
´
´ ´
´
Resume : Phosphore-contenant les nitrones cycliques, tels que DEPMPO, CYPMPO, et DPPMPO, ont ete oxydes par le
`
`
`
`
´
`
´
tetrachloroaurate(III) d’hydrogene a DEPMPOX, a CYPMPOX, et a DPPMPOX avec la precipitation d’Au(0). La reaction
´ ´ ´ ´
´
´
´
a ete diminuee par l’addition des ions de chlorure ou d’hydroxyde. La dependance particuliere de pH a ete observee dans
ˆ
´
DEPMPOX, CYPMPOX, et la formation de DPPMPOX, qui devrait etre causee par le groupe de diethoxyphosphoryl dans
DEPMPO, le 1,3-propoxy groupe de cyclophosphoryl dans CYPMPO, et le groupe de diphenylphosphinoyl dans
–
–
´
´
DPPMPO. L’oxydation du nitrones a parcouru les etapes l’interaction d’echange de ligand de Cl dans d’AuCl4
+
–
´
avec >N –O dans nitrone et l’addition de nucleophilic de la molecule d’eau au C2 position dans le nitrones, le transfert
´
´
´
`
ˆ
´
dans-moleculaire dispose en gradins de trois electrons du nitrones a Au(III), et le relachement du resultant Au(0). Le
´
`
´
groupe de phosphoryl dans les nitrones a supprime la premiere interaction d’echange de ligand par son electronegativity,
´
`
`
alors que le groupe favorisait le transfert d’electron a partir des nitrones a l’Au(III) par son effet inductif.
´
`
`
Mots-cles : RPE, piege a spin, DMPOX, DEPMPO, CYPMPO, DPPMPO, ion Au(III).
of the oxidation reaction of these spin traps containing phospho-
rus atom.
Introduction
Phosphorus-containing nitrone-type spin traps should be
useful for the trapping of superoxides. First, a spin trap con-
taining phosphorus atom, 5-diethoxyphosphoryl-5-methyl-1-
pyrroline N-oxide (DEPMPO), was developed for superoxide
trapping.1 Recently, two spin traps containing phosphorus
atom, such as 5-(2,2-dimethyl-1,3-propoxycyclophosphoryl)-
5-methyl-1-pyrroline N-oxide (CYPMPO)2 and 2-(diphenyl-phos-
phinoyl)-2-methyl-3,4-dihydro-2H-pyrrole 1-oxide (DPPMPO),3
were developed and actively used for superoxide trapping.4–10
The fact that DEPMPO was oxidized to the 5-diethoxyphos-
phoryl-5-methyl-1-pyrrolid-2-one-N-oxyl (DEPMPOX) radical in
the biosystems11 in a manner similar to 5,5-dimethyl-1-pyrroline-
N-oxide (DMPO)12–18 required that we analyze the characteristics
Previously, we found that cyclic nitrones, such as 5,5-di-
methyl-1-pyrroline-N-oxide (DMPO), 4-phenyl-5,5-dimethyl-
1-pyrroline-N-oxide (PDMPO), and 3,3,5,5-tetramethyl-1-
pyrroline-N-oxide (M4PO), were oxidized by hydrogen tet-
rachloroaurate(III) (HAuCl4) to 5,5-dimethyl-1-pyrrolid-2-
one-N-oxyl (DMPOX), 4-phenyl-5,5-dimethyl-1-pyrrolid-2-
one-N-oxyl (PDMPOX), and 3,3,5,5-tetramethyl-1-pyrrolid-
2-one-N-oxyl (M4POX), respectively, through three elec-
tron-transfer reactions.19 As HAuCl4 oxidizes the nitrone to
DMPOX-type aminoxyl radicals20 directly without any other
radicals, the nitrone-HAuCl4 system is suitable for the anal-
ysis of the mechanism of nitrone oxidation.
Received 5 December 2009. Accepted 22 February 2010. Published on the NRC Research Press Web site at canjchem.nrc.ca on 13 May
2010.
A. Nakajima.1 Section of Obstetrics and Gynecology, Department of Reproductive and Developmental Medicine, Faculty of Medicine,
University of Miyazaki, Kiyotake, Miyazaki 889-1692, Japan.
E. Matsuda. Section of Chemistry, Department of Medical Science, Faculty of Medicine, University of Miyazaki, Kiyotake, Miyazaki
889-1692, Japan.
Y. Ueda. Department of Psychiatry, Faculty of Medicine, University of Miyazaki, Kiyotake, Miyazaki 889-1692, Japan.
K. Tajima. Department of Biomolecular Engineering, Kyoto Institute of Technology, Matsugasaki, Sakyo-ku, Kyoto 606-8585, Japan.
1Corresponding author (e-mail: akanaka@med.miyazaki-u.ac.jp).
Can. J. Chem. 88: 556–562 (2010)
doi:10.1139/V10-033
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Nakajima et al.
557
Fig. 1. Molecular structure of phosphorous-containing spin traps
In the present study, we examined the oxidation reaction
of spin traps containing phosphorus atoms, such as
DEPMPO, CYPMPO, and DPPMPO, with HAuCl4 and dis-
cussed the reaction profiles of the formation of DMPOX-
type aminoxyl radicals.
and the corresponding DMPOX-type aminoxyl radicals.
Experimental
Chemicals
Hydrogen tetrachloroaurate(III) (HAuCl4) dihydrate and
other chemicals were obtained from Nacalai Tesque, Kyoto,
Japan. DEPMPO was obtained from Alexis Biochemicals,
Enzo Life Sciences, Inc., USA; CYPMPO was obtained
from Radical Research, Inc., Tokyo, Japan; and DPPMPO
was obtained from DOJINDO, Kumamoto, Japan. The mo-
lecular structures of the nitrones used in this study are
shown in Fig. 1. All nitrones used were examined by ESR,
and no radical species were observed.
ESR measurements
ESR measurements were conducted using procedures
nearly identical to those described previously,19 namely,
40 mL of a HAuCl4 solution (1.25 mmol/L) and 40 mL of a
nitrone solution (1.25 mmol/L) were mixed at pH 4. As
DPPMPO formed far smaller amounts of DPPMPOX than
other nitrones, experiments were conducted in a solution
containing 6.25 mmol/L of HAuCl4 and DPPMPO. In the ni-
trone concentration experiments, the desired amounts of the
nitrones were mixed with HAuCl4 (1.25 mmol/L for
DEPMPO and CYPMPO and 6.25 mmol/L for DPPMPO).
After aspirating the mixture into a capillary tube, its ESR
spectrum was immediately recorded using an X-band ESR
spectrometer (JES TE-100, JEOL, Ltd., Tokyo, controlled
by a WIN-RAD ESR data analyzer, Radical Research, Inc.,
Tokyo) under the conditions described previously.19 For the
observation of the hyperfine structure of methyl protons, the
modulation width was set to 0.025 mT and the time was set
to 8 min. The solution pH was adjusted to the desired value
with 0.1 N HNO3 and 0.1 N NaOH solutions. To avoid the
photochemical reduction of Au(III), the reaction was con-
ducted in the darkness. The concentration of the DMPOX-
type radical was calibrated using TEMPOL as a standard
after double integration of the observed ESR signal. A com-
puter simulation of each ESR spectrum was also conducted
using a WIN-RAD ESR data analyzer.
Fig. 2. ESR spectra of DEPMPOX, CYPMPOX, and DPPMPOX
and their computer simulations.
Gold determination
The total amounts of the Au(III) species in the reaction
mixture and the resulting amounts of Au(0) were determined
as described previously.19
Results
Formation of aminoxyl radicals in phosphorus-
containing nitrone-HAuCl4 solutions
When solutions of phosphorus-containing cyclic nitrones,
such as DEPMPO, CYPMPO, and DPPMPO, were mixed
with a hydrogen tetrachloroaurate(III) (HAuCl4) solution at
pH 4, ESR signals indicating a large doublet with seven
lines appeared (Fig. 2). Two former spectra had some addi-
tional small hyperfine structures. Their hyperfine coupling
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Can. J. Chem. Vol. 88, 2010
Table 1. Hyperfine coupling constants of DMPOX-type aminoxyl radicals.
Hyperfine coupling constant (mT)
a
Aminoxyl radical
DMPOX
DEPMPOX
aN
a3H
aH–Me
aP
Reference
Ref. 19
Present
0.70
0.67
0.71
0.74
0.39
0.37
0.30
0.40
0.33
0.40
0.39
0.04
3.78
3.93
2.84
CYPMPOX
DPPMPOX
0.04
Present
Present
0.69
Note: As the C-5 atom in nitrones containing phosphorus atom are chiral, protons combined with C-3 atom
are not equivalent in the two isomers.
aHyperfine coupling constant for protons combined with the C-3 atom in nitrone.
constants, estimated from a computer simulation, are listed
in Table 1. Simulation spectra for the three are shown in
Fig. 2 in comparison with the ones observed. From these val-
ues, the resultant radicals were identified to be DMPOX-type
aminoxyl radicals, DEMPOX, CYPMPOX, and DPPMPOX.
The assumed structures of the DMPOX-type aminoxyl radi-
cals containing DMPOX are depicted in Fig. 1 with the cor-
responding nitrones.
No other free radical species, except DMPOX-type ones,
were observed in DEPMPO-Au(III) and CYPMPO-Au(III)
solutions for a period of 2 h after the reaction was started.
On the other hand, in the DPPMPO-Au(III) solution, an
ESR signal with seven lines appeared, superposed upon the
DPPMPOX signal. In the present work, we concentrated on
DPPMPOX after subtracting the effect of the signal.21
Fig. 3. Time course of the aminoxyl radical concentrations and
Au(III) concentration in the mixtures of nitrones (1.25 mmol/L and
6.25 mmol/L for DPPMPO) and HAuCl4 (1.25 mmol/L and
6.25 mmol/L for DPPMPO). The closed circle indicates the amount of
DEPMPOX; the closed triangle indicates the amount of CYPMPOX;
the closed diamond indicates the amount of DPPMPOX; and the
closed square indicates the amount of DMPOX. The open circle
indicates the Au(III) concentration in the DEPMPO solution; the
open triangle indicates the Au(III) concentration in the CYPMPO
solution; the open diamond indicates the Au(III) concentration in
the DPPMPO solution; and the open square indicates the Au(III)
concentration in the DMPO solution.
Aspects of DMPOX-type radical formation in cyclic
nitrone-HAuCl4 solutions
Time course of radical formation
The time course of DMPOX-type aminoxyl radical forma-
tion was examined using the mixture of nitrones and
HAuCl4 at pH 4. As shown in Fig. 3, the amounts of
DMPOX-type aminoxyl radicals increased with the passing
of time and reached a maximum value within 40 min for
DEMPOX, 50 min for CYPMPOX, and 15 min for
DPPMPOX. As the time courses of the amounts of
DMPOX-type aminoxyl radicals indicate the combination of
the sigmoid-type curves {1/[1 + exp(–a(t – b))]} and the ex-
ponential decay [exp(–ct)], the reaction mechanisms should
be very complicated. However, the decrease of the Au(III)
ion in the reaction mixtures obeyed the second-order ki-
netics, eq. [1], as shown previously19
POX, CYPMPOX, and DPPMPOX were examined using a
mixture of nitrones and HAuCl4. As shown in Fig. 4, the
amounts of DEPMPOX indicated a maximum at around
pH 4 and abruptly decreased above and below pH 4. The
amounts of CYPMPOX indicated a maximum at around
pH 3 and abruptly decreased above and below pH 3. The
amounts of DPPMPOX were far smaller than those of
DEPMPOX and CYPMPOX and indicated a maximum at
½1ꢀ
d½AuðIIIÞꢀ=dt ¼ C=½kCt þ 1ꢀ
The kinetic constant, k, for each nitrone is listed in Ta-
ble 2, as well as the amounts of the DMPOX-type aminoxyl
radicals at 60 min after the start of the reaction. As shown in
Table 2, the magnitudes of k are in the order of DMPO >
DEPMPO > CYPMPO >> DPPMPO. The amounts of
DMPOX-type aminoxyl radicals are in the order of CYPMPO >
DEPMPO >> DMPO >> DPPMPO.
–
pH 2. The ratios of the chemical species, AuCl4 ,
Au(OH)Cl3 , Au(OH)2Cl2 , Au(OH)3Cl–, and Au(OH)4 , in
the solution, depicted in Fig. 4, were calculated using the
parameters listed by Sillen and Martell.22 As the ratios of
–
–
–
Effect of solution pH
The effects of the solution pH on the formation of DEPM-
–
–
AuCl4 and Au(OH)4 in the reaction mixture increased, the
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Nakajima et al.
559
Table 2. Second-order rate constants and amounts of DMPOX-type aminoxyl radical formed.
Nitrone
DMPO
DEPMPO
CYPMPO
DPPMPO
k (M–1s–1)
0.043
0.016
0.0076
0.0017
Amounts of free radical at 60 min (mmol/L)a
Reference
Ref. 19
Present
Present
Present
0.014
0.069
0.098
0.011b
Note: [Au(III)] = C/[kCt + 1].
aThe initial concentration of each nitrone was 2.5 mmol/L and HAuCl4, 2.5 mmol/L.
bDPPMPO was 6.25 mmol/L and HAuCl4, 6.25 mmol/L.
Fig. 4. Effect of the solution pH on the formation of DMPOX-type
aminoxyl radicals in the mixtures of nitrones (1.25 mmol/L for
DEPMPO and CYPMPO and 6.25 mmol/L for DPPMPO) and
HAuCl4 (1.25 mmol/L for DEPMPO and CYPMPO and
Fig. 5. Effect of sodium salts on the formation of DMPOX-type
aminoxyl radicals in the mixtures of nitrones (1.25 mmol/L for
DEPMPO, CYPMPO, and DMPO and 6.25 mmol/L for DPPMPO),
HAuCl4 (1.25 mmol/L for DEPMPO, CYPMPO, and DMPO and
6.25 mmol/L for DPPMPO), and the desired amounts of sodium
salts for 1 h. In the NaCl solution, the closed circle indicates the
amount of DEPMPOX; the closed triangle indicates the amount of
CYPMPOX; and the closed diamond and closed square indicate the
amount of DMPOX. In the NaNO3 solution, the open circle indi-
cates DEPMPOX; the open triangle indicates CYPMPOX; the open
diamond indicates DPPMPOX; and the open square indicates
6.25 mmol/L for DPPMPO) at each pH for 1 h. The closed circle
indicates the amount of DEPMPOX; the open circle indicates the
amount of CYPMPOX; the closed triangle indicates the amount of
DPPMPOX; and the open triangle indicates the amount of
-
DMPOX. The dotted line indicates the ratios of AuCl4 and
–
Au(OH)4 calculated using the formation constants listed by Sillen
and Martell.22
-
DMPOX. The solid thick line indicates the ratio of [AuCl4 ] calcu-
lated as described in the legend of Fig. 4.
ESR signal intensities decreased. Figure 4 indicates that
DMPOX-type aminoxyl radical formations in DEPMPO and
CYPMPO solutions were so pH-sensitive that radicals only
formed in a very narrow pH range. In other words, the for-
mations of DEPMPOX and CYPMPOX were strongly af-
fected by AuCl4 and Au(OH)4 . These results were quite
different from those of DMPOX, which formed in a wide
pH range.
of the NaCl concentration than did DMPOX. These results
indicated that the formations of DEPMPOX and CYPMPOX
were strongly affected by AuCl4 , as described previously.
–
–
–
Effect of nitorne concentration
The amounts of DEPMPOX in the solution containing
1.25 mmol/L of DEPMPO increased gradually with time
and reached a maximum 40 min after the start of the reac-
tion (Fig. 6). However, in the 3.75 mmol/L DEPMPO solu-
tion, the amounts of DEPMPOX reached a maximum
20 min after the start of the reaction and then gradually de-
creased, in accordance with the decay curve of those in the
1.25 mmol/L DEPMPO solution. In 6.25 and 12.5 mmol/L
DEPMPO solutions, the amounts of DEPMPOX reached a
maximum 20 min after the start of the reaction and then
abruptly decreased to about 0. A similar anomalous nitrone
concentration effect was found in the DMPOX formation.19
Effect of co-existing salts
The addition of sodium chloride to the reaction mixture
markedly decreased the amounts of DEPMPOX, CYPMPOX,
and DPPMPOX, while no effect was observed by the addi-
–
tion of NaNO3 (Fig. 5). The amounts of AuCl4 in the solu-
tion, depicted in Fig. 5, were calculated using the formation
constants listed by Sillen and Martell.22 As shown in Fig. 5,
–
the formation of a stable complex anion, AuCl4 , suppressed
the redox reaction as described previously.19 DEMPOX and
CYPMPOX decreased much more heavily with the increase
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Can. J. Chem. Vol. 88, 2010
Fig. 6. Effect of the DEPMPO concentration on the formation of
DEPMPOX in the mixtures of desired amounts of DEPMPO and
1.25 mmol/L of HAuCl4. The closed circle indicates 1.25 mmol/L;
the open circle indicates 3.75 mmol/L; the closed triangle indicates
6.25 mmol/L; and the open triangle indicates 12.5 mmol/L
DEPMPO.
Fig. 7. Effect of the CYPMPO concentration on the formation of
CYPMPOX in the mixtures of CYPMPO (1.25–12.5 mmol/L) and
1.25 mmol/L of HAuCl4. The closed circle indicates 1.25 mmol/L;
the open circle indicates 2.5 mmol/L; the closed triangle indicates
6.25 mmol/L; and the open triangle indicates 12.5 mmol/L CYPMPO.
Fig. 8. Effect of the DPPMPO concentration on the formation of
DPPMPOX in the mixtures of DPPMPO (1.25–12.5 mmol/L) and
6.25 mmol/L of HAuCl4. The closed circle indicates 1.25 mmol/L;
the open circle indicates 2.5 mmol/L; the closed triangle indicates
6.25 mmol/L; and the open triangle indicates 12.5 mmol/L
DPPMPO.
The amounts of CYPMPOX in the solution containing
1.25 mmol/L of CYPMPO increased gradually with time
and reached a maximum 54 min after the start of the reac-
tion (Fig. 7). The time required to reach a maximum de-
creased with a higher CYPMPO concentration (48, 38–40,
36–38 min for 2.5, 6.25, and 12.5 mmol/L CYMPO, respec-
tively), while the maximum amounts became smaller (0.103,
0.095, and 0.062 mmol/L CYPMPOX for 2.5, 6.25, and
12.5 mmol/L CYPMPO, respectively). The amounts of
DPPMPOX in the solution containing 1.25–12.5 mmol/L of
DPPMPO increased gradually and reached a maximum
18–20 min after the start of the reaction (Fig. 8). The
amounts of DPPMPOX linearly increased with the increase
of DPPMPO.
Discussion
The hyperfine coupling constants of phosphorus in
DEPMPOX and CYPMPOX were larger than those of
DPPMPOX, while those of nitrogen and b-hydrogen were
almost identical among three nitrones. As phosphorus in
DPPMPOX belongs to the phosphine oxide group containing
only one oxygen atom, the inductive effect of the group is
smaller than that of the phosphonic acid group with three
oxygen atoms found in DEPMPOX and CYPMPOX. Fur-
thermore, the resonance effect caused by the diphenyl-phos-
phinoyl group in DPPMPOX also decreases the electron
density of the phosphorus atom. The hyperfine coupling con-
stant of phosphorus in DPPMPOX, therefore, becomes smaller
than that of the other two, DEPMPOX and CYPMPOX.
In previous work, we showed that the formation of
DMPOX-type aminoxyl radicals should proceed through the
scheme (Fig. 9) of a ligand exchange of Cl– in AuCl4
–
with >N+–O– in nitrone, a nucleophilic addition of a water
molecule to nitrone, and a stepwise intramolecular transfer
of three electrons from nitrone to Au(III).19 Previously, we
showed that substituent groups added to DMPO affected the
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Nakajima et al.
561
Fig. 9. Overall reaction scheme of cyclic nitrones and HAuCl4.
reaction aspects. The present results indicated that the sub-
stituents had drastic effects.
the redox reaction, as described previously.19 As DEPMPOX
and CYPMPOX were formed in a very narrow pH range,
their formations were strongly affected by AuCl4 , as ob-
–
The first step of the reaction is a ligand exchange interac-
tion of Cl– in AuCl4 with >N+–O– in nitrone. As DEPMPO
served for the salt effects. As described above, the initial li-
–
gand exchange of Cl– in AuCl4 with >N+–O– in nitrone was
–
and CYPMPO have large bulky substituents, such as dieth-
oxyphosphoryl and 2,2-dimethyl-1,3-propoxycyclophos-
phoryl, the steric effect of these substituents hindered the
ligand exchange. Furthermore, the inductive effects of these
substituents also weakened the ligand exchange. These ef-
fects caused the retardation of DMPOX-type radical forma-
tion. Therefore, the magnitudes of k are in the order of
DMPO > DEPMPO > CYPMPO >> DPPMPO.
heavily affected by the addition of a phosphonyl group.
Large amounts of DEPMPOX (0.14–0.17 mmol/L)
formed rapidly in the solution containing a concentration of
DEPMPO higher than 1.25 mmol/L, and a slower increase
and decrease were observed with 1.25 mmol/L of DEPMPO,
as for the formation of DMPOX.19 For CYPMPOX forma-
tion, a similar rapid increase and decrease were only ob-
served in 1.25 mmol/L of CYPMPO. However, DPPMPOX
indicated a slower increase and decrease in the DPPMPO
concentration range of 1.25 to 12.5 mmol/L. Previously, we
proposed two possible reaction schemes: (i) the elimination
of DMPOX by excess amounts of DMPO described by Hill
and Thornalley23 and (ii) the dimerization of DMPOX. Only
small amounts of DPPMPOX were formed at a very high
DPPMPO concentration; for such a case, scheme (i) was
not applicable, and scheme (ii) became more predominant.
Once the ligand exchange was established, the inductive
effect of the phosphonyl group promoted the nucleophilic
addition of the water molecule, which caused the formation
of large amounts of DEPMPOX and CYPMPOX. On the
other hand, DPPMPO, being so hydrophobic and having a
large resonance effect by the phosphine group, donated elec-
trons to >N–O against the weak inductive effect, which
caused the rapid but smaller formation of DPPMPOX. These
discussions successfully explained the facts that the amounts
of DMPOX-type aminoxyl radicals are in the order of
CYPMPO > DEPMPO >> DMPO >> DPPMPO.
Conclusion
–
–
As the ratios of AuCl4 and Au(OH)4 in the reaction mix-
ture increased, the amounts of DMPOX-type radical de-
creased. These results suggest that the formation of stable
The formation of DMPOX-type aminoxyl radicals in
phosphorus-containing cyclic nitrones should proceed through
–
–
–
complex anions, such as AuCl4 and Au(OH)4 , suppresses
the scheme of the ligand exchange between AuCl4
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Can. J. Chem. Vol. 88, 2010
and >N+–O– in nitrone, the nucleophilic addition of a water
molecule to nitrone, and the stepwise intramolecular transfer
of three electrons from nitrone to Au(III). Although the
phosphonyl groups in DEPMPO and CYPMPO retard the in-
itial step of the reaction, they promote the nucleophilic addi-
tion of the water molecule, and, finally, large amounts of
DEMPOX and CYPMPOX are formed. DPPMPO, being hy-
drophobic, formed only small amounts of DPPMPOX. Care
should be taken with the oxidation reaction and the forma-
tion of DMPOX-type aminoxyl radical in the use of
DEPMPO and CYPMPO as spin traps in biosystems. On
the other hand, the phosphine group resists the oxidation re-
action, which becomes a merit of DPPMPO as a spin trap.
Our former and present investigations on the oxidation of
cyclic nitrones with HAuCl4 are summarized as follows:
From the results of the oxidation reaction of DMPO and
HDMPN, a precursor of DMPOX, by HAuCl4, we proposed
a reaction scheme containing the ligand exchange between
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Cl– in AuCl4 and >N+–O– in nitrone, the nucleophilic addi-
–
tion of a water molecule to nitrone, the stepwise intramolec-
ular transfer of three electrons from nitrone to Au(III), and
the production of DMPOX-type aminoxyl radical after the
release of Au(0). Taking electronic properties, such as the
steric and the inductive effects, and hydrophobicity of sub-
stituents in cyclic nitrones into consideration, the reaction
scheme proposed can explain the behavior for DMPOX-
type aminoxyl radical formation with HAuCl4 not only for
cyclic nitrones having 3,3-dimethyl group, such as PDMPO
and M4PO, but also for phosphorus-containing cyclic nitro-
nes, such as DEPMPO, CYPMPO, and DPPMPO. These
facts, therefore, give us important information on side reac-
tion of spin trapping.
(11) Clement, J.-L.; Gilbert, B. C.; Rockenbauer, A.; Tordo, P. J.
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Acta 1983, 759 (1-2), 16. PMID:6309246.
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90068-T. PMID:2160410.
(15) Shi, X.; Dalal, N. S.; Kasprzak, K. S. Arch. Biochem. Bio-
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(16) Makino, K.; Hagi, A.; Ide, H.; Murakami, A.; Nishi, M. Can.
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(20) Hereafter, DMPOX derivatives are referred to as DMPOX-
type amioxyl radicals.
(21) As the concentration of the free radical with seven lines is
about 10% of DPPMPOX, it should be a byproduct of the
oxidation reaction of DPPMPO by HAuCl4 rather than an
impurity of DPPMPO synthesis. Precise information regard-
ing the radical will be provided in subsequent research.
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Acknowledgements
This work was supported by the Cooperative Grant for In-
novative Technology and Advanced Research in Evolutional
Areas from the Ministry of Education, Science, Sports, and
Culture of Japan, and it was also supported by the Research
for Promoting Technological Seeds, No. 1223, Japan Sci-
ence and Technology Agency.
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