Chemistry Reference
In-Depth Information
Notable among these widely studied radicals is
91
, which has been extensively derivatized for optimum
properties.
27b,29a
This reacts with superoxide (O
2
•
−
)
radicals with an apparent second order rate constant
10
3
M
−
1
s
−
1
, and this was used for analysis of the superoxide.
33a,b
Assay of superoxide by
monitoring the newly formed product absorption peak at 546 nm
33a
or by EPR
33b
gave good agreement
with the widely used cytochrome
c
method of superoxide detection.
33a
The identity of the species giving
rise to the observed absorption at 546 nm, which was stable for 12 hours, was not identified. Hydroperoxide
was detected by ferrous oxidation-xylenol orange (FOX) assay,
33b
but mass spectrometric analysis did not
detect a covalent adduct, and formation of an ion pair
92
of hydroperoxide anion and the trityl cation was
proposed (Equation 1.28).
33a
of 3.1
×
HO
HO
HO
HO
CO
2
H
HO
S
HO
CO
2
H
S
HO
HO
S
S
S
S
S
S
OH
S
S
OH
O
2
•
−
OH
S
S
OH
HO
2
C
C
•
+
HO
2
C
C
(1.28)
S
S
OH
OH
SS
SS
S
OH
S
OH
S
S
HO
HO
_
HO
S
HO
S
CO
2
H
OOH
CO
2
H
HO
HO
HO
HO
91
92
Dynamic nuclear polarization of the substrate by microwave irradiation of
91
permitted the measurement
of the kinetics of trypsin catalyzed hydrolysis of
N
α
-benzoyl-L-arginine ethyl ester by
13
C NMR using
stopped flow.
33c
Extension of this method to the study of other enzyme catalyzed reactions is anticipated.
The dynamic nuclear polarization (DNP) of the triphenylmethyl radical in water has been compared to
that of the 4-hydroxyTEMPO radical
93
and the latter showed considerably larger DNP enhancements up
to 100 (9 GHz) and
20 (94 GHz) using continuous microwave irradiation.
33d
−
HO
N
O
•
93
The degradation of tetrathiatriarylmethyl radicals
86
used as oximetry probes for EPR imaging applica-
tions due to reaction with oxygen was investigated experimentally and by using computational methods.
33e
Tricarboxylate salts bearing CO
2
Na groups were the most stable, and it was proposed that degradation by
oxygen attack on the TAMs preferentially occurred at unsubstitued 4-positions of the aryl rings leading to
hydroperoxide intermediates. Radical pathways with addition of O
2
or HO
2
•
to
86
forming
94
or
95
were
considered (Equation 1.29), as well as ionic routes, such as the addition of the anion HO
2
−
to the cation
of
74
forming
80
.
33e
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