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arranged in supramolecular equilateral triangles. In each triangle, the distances between the oxygen atoms
of the three nitroxides are in the range 8.4 - 9.0 A, with an average value of 8.7 A. When
105
@CB[8] crystals
were dissolved in pure water, an X-band EPR signal composed of the superposition of the expected three-
line spectrum from free
105
and a seven-line spectrum were observed. This additional spectrum exhibited
a 1 : 3 : 6 : 7 : 6 : 3 : 1 seven-line pattern with a hyperfine coupling constant (
a
N
)
of 5.13 G, corresponding
≈
.
to one-third of the value observed for an included TEMPO-like nitroxide (
a
N
5 G). This spectrum is
characteristic of a system composed of three identical nitroxide moieties coupled through spin exchange
with
J
12
,
J
23
,
J
13
15
{
}
a
N
in agreement with the “trinitroxide” supraradical
105
@CB[8]
3
crystal structure.
5.7.4 Interaction of nitroxides with micelles
Two dynamic processes take place in micellar solutions: the faster process is the association - dissociation
process involving the exchange of individual surfactant molecules between the micelles and the water
phase, while the slower process is identified as the rearrangement of the system involving the creation
and destruction of micelles.
258
When the micelles contain hydrophobic solutes, another type of dynamic
process takes place, namely, the exchange of solute molecules between the micellar and the water phases.
Monitoring of nitroxide probes with EPR has been used to obtain useful information on their interaction
in micellar environments.
259
Tordo
et al
.
260
investigated the interaction of a series of stable
-phosphorylated cyclic aminoxyl radicals
with SDS micelles by EPR spectroscopy. The hyperfine coupling constant with the phosphorus atom (a
P
)
proved to be very sensitive to the environment; as a result, two distinct spectra corresponding to the
nitroxide in bulk water and in micelles were observed (Scheme 5.60).
EPR investigation of the kinetics of the exit and re-entry processes in SDS micelles of
100
and various
para
-substituted benzyl
tert
-butyl nitroxides and
para
-substituted benzyl hydroxyalkyl nitroxides was also
reported.
261
Micellar aggregation of sulfonate surfactants and alkyltrimethylammonium bromide surfactants
[CH
3
(CH
2
)
n
−
1
N(CH
3
)
3
Br; n
β
6, 8, 12, 16] was investigated by EPR of TEMPO-choline and the sodium
salt of 3-carboxy-PROXYL, respectively.
262
There is limited information related to the participation of free radical processes and the antioxi-
dant mechanism involved in biological membranes. A prefluorescent hydrophobic TEMPO derivative
has been used to evaluate local reactivity of phenolic antioxidant in micellar systems
263
(Scheme 5.61).
An apparent rate constant that directly reflects the relevance of antioxidant hydrophobicity on the reac-
tion toward the nitroxide has been defined. Dramatic differences in the apparent rate constant were found
on moving from methanol to micellar media (90- and 230-fold enhancements for SDS and Triton X100
micelles respectively).
=
P(O)(OEt)
2
P(O)(OiPr)
2
Ph
P(O)(OiPr)
2
N
N
N
O
O
O
a
P
(water) = 47.99 G
a
N
(water) = 15.33 G
a
P
(micelles) = 45.10 G
a
N
(micelles) = 14.10 G
K = 95 M
−
1
a
P
(water) = 47.41 G
a
N
(water) = 15.38 G
a
P
(micelles) = 43.94 G
a
N
(micelles) = 15.00 G
K = 450 M
−
1
a
P
(water) = 54.52 G
a
N
(water) = 15.24 G
a
P
(micelles) = 53.05 G
a
N
(micelles) = 14.92 G
K = 813 M
−
1
Scheme 5.60
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