Chemistry Reference
In-Depth Information
O
O
O
O
O
N
N
N
O
O
N
N
O
2
O
O
O
N
H
O
O
OH
106
108
107
Scheme 5.64
At high magnetic field in the solid state (90 K) and pumping at 260 GHz microwave frequency, Griffin
reported that using biradicals as polarizing agents, the Cross Effect (CE) mechanism leads to improved
polarization enhancements when comparing with Solid Effect (SE) and Thermal Mixing (TM) mechanisms.
The more efficient biradicals (Scheme 5.64) consist of two TEMPO nitroxides tethered by an ethylene
glycol chain (
106
),
302
a more water soluble TEMPO-based dinitroxide bearing a propan-2-ol tethering chain
(Totapol,
107
),
303
and a rigid binitroxide bearing orthogonal TEMPO moieties (bTbK,
108
).
304
Compared
with that obtained with TEMPO, the signal enhancement observed with biradical dinitroxides
106
,
107
and
108
increased by factors of about 4, 5.5 and 7.5 respectively.
Immobilized TEMPO-based nitroxides on agarose gel were prepared and studied as possible polarizing
medium for liquid DNP experiments at 0.35 T and room temperature for the preparation of radical-free
steady state injection solutions.
305
However, the signal enhancement ( - 38) obtained using the TEMPO
agarose gel is lower than with TEMPO ( - 110) dissolved in water at the same concentration. This effect
was attributed to a lower coupling factor for the agarose-bound TEMPO.
5.9 Nitroxides as pH-sensitive spin probes
Local pH value is one of the most important parameters in the biochemistry of living systems and pH
changes have been associated with many physiological and pathological processes. Current methods for
assessing local pH involve invasive procedures or transparent samples, and typically can assess pH values
from a limited numbers of locations.
In vitro
and
in vivo
measurements of pH in a non-invasive way
would provide useful information in clinical diagnostic applications. In the last two decades, imidazoline
and imidazolidine-based stable nitroxides
306 - 308
have shown to be interesting candidates as pH spin probes
due to the effect of pH changes on their EPR spectra (Scheme 5.65).
309
The change in A
N
between the
protonated and the unprotonated forms can reach values close to 0.1 mT.
Along with progress in low-frequency EPR, such as Longitudinally-Detected EPR (LODEPR), Proton
Electron Double-Resonance Imaging (PEDRI), and Field-Cycled Dynamic Nuclear Polarization (FC-DNP),
the design of efficient pH-sensitive nitroxide spin probes is a key goal in the successful development of
such diagnostic methods.
1. TsCl
Et
3
N
2.
HO
N
N
1. EtMgBr
2. MnO
2
CH
3
CH
2
CHO
Et
3
N
N
i-
PrONO
Et
3
N
O
N
NH
Et
N
N
H
Et
Et
N
Et
Et
Et
N
Et
Et
Et
Et
N
N
Et
NHOH
Et
Et
N
Et
Et
OH
O
O
O
pKa = 7.4
∆
A
N
=0.09mT
Scheme 5.65
Synthesis of a pH sensitive nitroxide
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