Nitroxide Radicals: Properties, Synthesis and Applications - Stable Radicals: Fundamentals and Applied Aspects of Odd-Electron Compounds - page 213

O

O

O

N

N

N H

N

a) Na 2 SO 4 /
1,2-dichlorobenzene
180

O

H 3 C

CH 3

C
b) m-CPBA / toluene
RT

°

+ +

C 60

H 2 N CO 2 H

H 3 C CH 3

Scheme 5.63 A fulleropyrrolidine nitroxide and a fulleropyrrolidine dinitroxide

The inner environment of single-walled carbon nanotubes has been investigated by EPR using a fullerene
nitroxide probe that was inserted using supercritical carbon dioxide. 270

5.8 Nitroxides for dynamic nuclear polarization (DNP) enhanced NMR

NMR spectroscopy is a widely used analytical technique, and Magnetic Resonance Imaging (MRI) is
one of the most powerful clinical imaging methods, due to a number of advantages including excellent
resolution, versatility, non-invasive nature, and the use of low-energy frequency irradiation. However, the
inherently low sensitivity of NMR is a major limitation and hampers the extension of its applications. The
lack of sensitivity results from the low magnetic energy of nuclear spins compared to thermal energy at
room temperature.
Improvements in instrumentation by the development of magnets operating at very high fields and of
cryogenic probes have led to significant but still limited enhancement of sensitivity. In recent years, large
NMR signal enhancements were obtained through different approaches based on spin polarization transfer.
Among these approaches, experiments involving hyperpolarized noble gases, 271,272 parahydrogen-induced
polarization (PHIP), 273,274 photochemical reaction centers (photoCIDNP), 275,276 or stable free radicals
(Dynamic Nuclear Polarization, DNP) 277 are very appealing. From this set of methods, DNP appears
to be the most versatile and permits a large number of applications both in solid and liquid states.

5.8.1 DNP for biological NMR and real-time metabolic imaging
In the last decade, the achievements of Griffin's group gave a strong emphasis to the development of DNP
for enhancing the spin polarization in high field MAS NMR experiments, 278 - 280 and signal enhancement

values up to 330 were reported.
Figure 5.5 shows the clear improvement in signal-to-noise ratio obtained using DNP SSNMR at 90 K
and 5 T in the active site of bacteriorhodopsin (bR), a membrane protein acting as a light-driven ion
pump. 281 The obtained enhancement of 50 provided sufficient resolution and shorter data acquisition to
gain 2-D spectra with high quality.
More recently, the generation of hyperpolarized spin systems in the solid state at very low temperatures,
followed by a rapid dissolution process and sample transfer to the NMR/MRI apparatus, has been developed
by the Ardenkjær-Larsen and Golman groups. This technique gained a tremendous interest in the MRI field
allowing real-time metabolic studies and improved spatial distribution of hyperpolarized molecules. 282 - 284
In Figure 5.6 (left part) are shown two 13 C NMR spectra of urea at 9.4 T, in water at room temperature.
Spectrum (a) (1 scan), was obtained after DNP (20 % polarization), spectrum (b) was obtained at the