A R T I C L E S
Kim et al.
Figure 1. Structures of cyclic nitrone spin traps.
Electron paramagnetic resonance (EPR) spin trapping is a
commonly used technique for radical detection which involves
formation of radical adducts that exhibit a distinctive EPR
phosphoryl)-5-methylpyrroline N-oxide (DEPMPO),26 these ni-
•-
trones suffer from limitations such as slow reactivity with O2
and poor O2 adduct stability.27
•-
Almost four decades ago28 since the inception of nitrone spin
traps as probes for radical detection, hundreds of new spin traps
with improved properties have been synthesized,29 but these
spin traps still suffer from major limitations such as poor
reactivity with O2•-, a short adduct half-life, or nonspecificity
to cellular compartments, or they exhibit a spectrum that cannot
discern one radical adduct from another.30 Our laboratory uses
a computational approach for a rational design of spin traps
with improved spin trapping properties.31 By exploiting the
nucleophilic nature of O2•- addition to nitrones, the electrophi-
licity of the nitronyl C (the site of O2•- addition) was increased
through conjugation with an amide group at the C-5 position.
Furthermore, initial H-bond interaction of the amide substituent
•
spectrum. The direct or indirect observations of O2•-/HO2
formation have been achieved by spin trapping using melano-
somes,12 mitochondria,13 photosynthetic systems,14 nitric oxide
synthase (NOS),6,15 endothelial cells,16 human neutrophils,17 and
reperfused heart tissue.18,19 Spin trapping has also found a wide
range of applications in the fields of fuel cell research,20 nano-
technology,21 catalysis,22 environmental remediation,23 and pho-
todynamic therapy.24 In spite of the popularity of the cyclic nitrones,
such as 5,5-dimethylpyrroline N-oxide (DMPO)25 and 5-(diethoxy-
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Chem. 2006, 281, 13159–13168. Dugan, L. L.; Sensi, S. L.; Canzo-
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Gescheidt, G. J. Am. Chem. Soc. 2003, 125, 1376–1384.
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J. Biol. Chem. 1998, 273, 25804–25808.
•-
with O2 enhances its nucleophilicity to the nitrone via the
R-effect.32,33 Kinetic experimental studies showed that the
amide-substituted nitrone (AMPO) exhibited a rate constant for
•-
O2 trapping in DMF/H2O of k ) 135 M-1 s-1 compared to
those of DMPO and DEPMPO of k ) 2.0 and 0.7 M-1 s-1
,
respectively (in H2O, for AMPO, k ) 25 M-1 s-1, for DMPO,
k ) 2.0 M-1 s-1, and, for DEPMPO, k ) 4.0 M-1 s-1).31
The H-bond donor rich ꢀ-cyclodextrin-nitrone conjugates
CDNMPO and C12CDMPO, linked through an amide bond,
gave k2 values of 221 M-1 s-1 (DMF/H2O) and 72 M-1 s-1
(H2O), respectively.34,35 The role of H-bond donors in increasing
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•-
the O2 nucleophilicity was further demonstrated by Winter-
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O2•- adduct stability was also realized using CDNMPO through
H-bond stabilization of the radical adduct, giving a maximum
half-life in water of t1/2 ) 30 min compared to those of DMPO
and DEPMPO of t1/2 ) 1 and 14 min, respectively (see Figure
1 for the structures).34,35 However, the permethylated CD-
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