2046 Bull. Chem. Soc. Jpn., 75, No. 9 (2002)
Trapping Rates of PBN-Type Spin Traps
Table 2. Hyperfine Coupling Constants (hfcc) and Rate Constants for Phenyl Radical Spin
Trapping at 298 K
Spin Trap
hfsc/mT
k2/k1
10−7 k2/dm3 mol−1 s−1
AN
AH
4-NO2-PBN
PBN
3-HO-PBN
DMPO
1.41
1.43
1.44
1.39
1.54
0.210
0.220
0.231
1.93
1.75 0.09
1.56 0.01
1.13 0.04
1
1.35
1.2
0.87
0.77
0.46
2-HO-PBN
0.179
0.60 0.03
·
·
AN = 1.43 mT and AH = 0.220 mT assigned to ·PBN-Ph (phen-
yl radical adduct of PBN).14,15 The relative spin trapping rate
constants of phenyl radical for 4 monosubstituted PBN spin
traps are listed in Table 2. Because 2-SO3- and 4-HO-PBNs
are insoluble in benzene, the rates were not measured. The ab-
solute trapping rate constant of phenyl radical by PBN was re-
ported as 1.2 × 107 dm3 mol−1 s−1 in methanol,16 and this val-
ue was used to calculate the rate constants (k2) for PBN-type
spin traps (Table 2).
where O-PBN and O-PBN(·)-OH denote phenoxyl radicals
formed by hydrogen abstraction from spin trap (HO-PBN) and
spin adduct (HO-PBN(·)-OH), respectively. The reactions
shown above consume originally added spin trap as well as
·OH radicals, and could lead to diminishing apparent trapping
efficiency by hydroxy-substituted PBN.
In a rat model, Reinke et al.18 investigated the metabolic fate
of PBN in vivo and showed that the phenyl group in PBN is
hydroxylated to form 2-, 3-, and 4-hydroxy-PBNs. A majority
(80%) of hydroxyl radical attack occurs on the aromatic ring of
PBN rather than on the nitrone group trapping. This result was
surprising, because it was thought that the hydroxyl radical at-
tack occurs primarily at the double bond in the nitrone group.
Nevertheless, hydroxy-substituted PBN should still retain free
radical-trapping capabilities and phenolic group should have
scavenging capability against hydroxyl radical. Since hydrox-
yl radicals show high reactivity with spin traps at the sites oth-
er than nitrone group sites, it is possible to speculate that free
radical trapping capability may not be a unique determinant for
the pharmacologic activity.
In conclusion, we determined spin trapping rates in various
substituted PBNs for hydroxyl radical (in water) and phenyl
radical (in benzene) using a competitive trapping method with
DMPO. We show a reason why hydroxy-substituted PBN ex-
hibits apparent low spin trapping rates. Although no correla-
tion was found between the pharmacologic activities and trap-
ping rate constants, this study may provide helpful models for
the interpretation of biological data in the future.
It is noted that, in phenyl radical spin trapping, the low trap-
ping efficiency that was seen in hydroxyl radical spin trapping
by hydroxylated PBNs was absent. Hammett-type plot for
phenyl radical trapping in benzene is shown in Fig. 3(2). The
Hammett σ-constant for 2-HO-group is not available, and thus
the σ-constant at 4-position is tentatively used. By this reason,
we excluded 2-HO-PBN from the calculation of the reaction
constant ρ, and so the slope of the Hammett plot is slightly
positive, i.e., ρ = 0.14. It is noted that the HO-trapping rates
are about 100 times faster than the Ph-trapping rates. In a ki-
netic study of superoxide and hydroxyl radicals, Finkelstein et
al.3 estimated the rate ratio for DMPO hydroxyl radical spin
·
trapping (kDMPO) and OH radical hydrogen abstraction from
ethanol (kEtOH) as kDMPO/kEtOH = 1.91. Using the kDMPO value,10
we calculate the kEtOH value as 1.89 × 109 dm3 mol−1 s−1,
·
which is comparable to the OH radical spin trapping rate of
PBN (kPBN) shown in Table 1. The ratio is calculated to be
kPBN/kEtOH = 1.35.
·OH Radical Reaction with Hydroxylated PBN. There
are many reports on hydrogen abstraction by free radicals from
OH group in phenol compounds. Hogg et al.17 studied the hy-
drogen abstraction by DPPH from phenols, and have shown
that ·OH radicals cause 1) hydrogen abstraction reaction from
the OH group in hydroxy-substituted PBNs, and 2) ·OH radi-
cal addition to the traps. Thus, after UV irradiation, the
possible ·OH radical reactions may be shown as fol-
lows:
We thank Dr. Edward G. Janzen (Belwood, Ontario Canada)
for the gift of spin traps used in this study. Claudio Olea-Azar
is grateful to the research fellowship provided by the American
Chemical Society to visit the University of Oklahoma Health
Sciences Center.
References
k1
HO· + DMPO → ·DMPO - OH
1
a) E. G. Janzen, Acc. Chem. Res., 4, 31 (1971). b) T. Doba,
k2
HO· + HO - PBN → HO - PBN(·)- OH
T. Ichikawa, and H. Yoshida, Bull. Chem. Soc. Jpn., 50, 3158
(1977). c) N. Nishimura, T. Nakamura, Y. Sueishi, and S.
Yamamoto, Bull. Chem. Soc. Jpn., 67, 165 (1994). d) Y. Sueishi
and Y. Miyake, Bull. Chem. Soc. Jpn., 70, 397 (1997).
k3
HO· + HO - PBN → ·O - PBN
k4
·O - PBN + HO - PBN → HO - PBN(·)- O - PBN
fast
2
(1973).
3
E. G. Janzen and C. A. Evans, J. Am. Chem. Soc., 95, 8205
→ Decomposition
k5
·O - PBN + HO - PBN(·)- OH → ·O - PBN(·)- OH
E. Finkelstein, G. M. Rosen, and E. J. Rauckman, J. Am.
Chem. Soc., 102, 4994 (1980).
Y. Kotake and E. G. Janzen, J. Am. Chem. Soc., 113, 9503
fast
→ Decomposition
k6
·
·O - PBN + HO - PBN → HO - PBN + O - PBN,
4