Chemistry Reference
In-Depth Information
140
Air
120
100
80
60
40
20
0
Carbogen
140
120
100
80
60
40
20
0
Figure 16.7
PEDRI oxygen images obtained using Oxo63 probe (
0). The images
were obtained from a mouse bearing the SCC tumor (squamous cell carcinoma) under air and carbogen-
breathing conditions. The imaging times ranged between 4 and 8min. The tumor region is shown within the
square.(Reprintedwithpermissionfrom[33].Copyright2002NationalAcademyofSciences.)
H
pp
=
165mG at [O
2
]
=
16.5 EPR spectroscopy and imaging of pH using nitroxyl and trityl probes
The critical role of pH status in physiology and pathophysiology of living organisms is well recognized.
At the microscopic scale, local pH drastically affects the vital activities of cell, cellular organelles, and
enzymes. Therefore, spatially and temporarily addressed pH measurements
in vivo
are of considerable
clinical relevance. For
in vivo
pH measurements,
31
P-NMR has proven to be the most suitable non-
invasive approach. However, pH assessment using
31
P-NMR and inorganic phosphate (P
i
) has its own
limitations which are rarely discussed, including the lack of resolution (about 0.2 - 0.3 pH units and even
less at lower pH), the fact that P
i
concentrations vary with metabolism and ischemia, and the chemical shift
dependence on ionic strength.
93,94
Because of these problems, exogenous pH probes are being designed
for NMR spectroscopy to improve detection of myocardial acidosis
93
and extracellular pH in tumors.
95,96
Upon application of exogenous probes, low field EPR spectroscopy has higher sensitivity compared with
NMR for the same probe concentration, and reasonable depth of penetration in living tissues.
16.5.1 pH-sensitive nitroxyl radicals
The first pH effect on the EPR spectra of stable NRs was observed in very strong acids and was related to
protonation of the nitroxyl fragment itself.
97,98
Synthesized by Ullman and Osiecki in 1970,
99
2-imidazoline
NRs or imino nitroxyl radicals (INR, Scheme 16.4) were apparently the first reported stable pH-sensitive
NRs with pK
a
close to physiological range. A large pH effect on nitrogen hyperfine splitting (
a
N1
≈
2 G/pH unit) was observed upon protonation of the imino nitrogen of the INR1 and INR2. However, INRs
did not find applications as pH probes, probably due to the relative complexity of their EPR spectra and
extremely fast reduction to EPR silent products in biological fluids.
16,100,101
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