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NO
R
R
S
S
R
S
S
R
NO
N
Fe
2+
N
N
Fe
2+
N
R
1
R
1
S
S
S
S
R
1
R
1
55
R=R
1
=CH
3
CH
2
56
R=CH
3
and R
1
=CH
2
(CHOH)
4
CH
2
OH
57
R=CH
3
and R
1
=CH
2
COO
−
58
R=R
1
=CH
2
CH
2
OH
Scheme 5.19
Structure of Fe
2+
-dithiocarbamate complexes
NO
O
O
N
N
N
N
1
COO
+
NO
COO
COO
COO
N
N
N
N
2
NO
2
O
O
O
O
INO
Figure5.3
TrappingofnitricoxidewithNNOandEPRsignaloftheresultingiminylnitroxide(INO).
Nevertheless, dithiocarbamate complexes are widely used, even though the high quantities of added Fe
2
+
and dithiocarbamate ligands can initiate unwanted reactions and a high toxicity.
Kalyanaraman
et al
. were the first to suggest that nitronyl nitroxides could be a viable alternative to
iron(II)-dithiocarbamate complexes
83
to characterize nitric oxide. They showed that 2-carboxyNNO reacts
specifically with nitric oxide giving rise to an imino nitroxide (INO) which shows a characteristic EPR
signal
83
0G
(Figure 5.3)). The rate constant of the reaction between NNOs and NO in aqueous solutions is in the
region of 10
4
M
−
1
s
−
1
.
84
AsshownbyPeng
et al
., who reported the synthesis of 30 different NNO labeled with amino acid
fragments,
85
the possibility to modulate NNO solubility and specificity makes possible the detection of
nitric oxide at different tissue sites. To improve nitric oxide rate trapping, Rosen
et al
. reported the first
synthesis of dendrimer linked NNOs (from two to eight units of NNO).
86
Unfortunately, the EPR spectrum
of the dendrimer-linked NNOs is broadened by spin exchange interactions, and the rate constant of trapping
is similar to that observed with nitronyl nitroxides.
Despite the easy access to various NNOs, their use for specific EPR nitric oxide detection is not without
limitation; it has been reported that NNOs can undergo fast reduction into EPR silent diamagnetic products.
Indeed, Blasig
et al
. reported that NNOs can react with superoxide anion radical (O
2
•
−
) with a rate constant
of 8
composed of 7 lines with an intensity close to 1 : 1 : 2 : 1 : 2 : 1 : 1 (a
N1
=
4
.
5 and a
N2
=
9
.
10
5
M
−
1
s
−
1
, which is more than two orders of magnitude higher than the value reported previously
for the NNO's reaction with nitric oxide.
87
.
×
8
5.4.3 Nitronyl nitroxides as building blocks for magnetic materials
Since the pioneering work of Cambi and coworkers on spin-crossover magnetic materials in the 1930s,
88
it has been a holy grail for chemists and physicists to create pure organic magnetic materials. In 1991,
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