Nitroxide Radicals: Properties, Synthesis and Applications - Stable Radicals: Fundamentals and Applied Aspects of Odd-Electron Compounds - page 214

bR 568

no DNP
317 MHz

with DNP
380 MHz

250 150 50

15 N Chemical Shift (ppm)

Figure 5.5 DNP improvement of the 1D 1 H decoupled 15 N MAS spectrum of light adapted - 15 N-Lys-bR. ζ

(Reprintedfrom[281],Copyright2001,withpermissionfromElsevier.)

thermal equilibrium after 65 hours of acquisition. After the injection of 13 C 1 -enriched pyruvate in a rat
(Figure 5.6, right), 13 C NMR spectra were acquired with a time interval of three seconds at 1.5 T on a
clinical MRI. This experiment showed the capability of in vivo monitoring the fate of pyruvate in a rat
using clinical MRI and opens up the way to clinical molecular imaging with MRI.

5.8.2 Nitroxides as polarizing agents for DNP
In 1953, Overhauser proposed an experiment where the large Boltzmann polarization of unpaired electrons
could be transferred to surrounding nuclei by saturating the corresponding electron paramagnetic resonance
(EPR) transition. 287 DNP is the extension of the original Overhauser effect from metals to frozen liquids 288
and liquids. A prerequisite for DNP is the presence of paramagnetic centers in the sample at concentration
low enough to limit the paramagnetic broadening of the NMR spectrum. On irradiation close to the
frequency of the EPR transition, the large polarization of the electron spin system can be transferred to
the nearby nuclei through the dipolar interaction between the electron and nuclear spins.
Since the early 1960s, the DNP process was first demonstrated experimentally and intensively studied at
low magnetic field. 289 - 294 The maximum theoretical enhancement achievable is given by the gyromagnetic

2600 for 13 C nuclei. However, extending DNP to higher
fields proved to be challenging. Hyperpolarization processes can be performed in the liquid or in the solid
states, at low or very low temperatures, and at 0.34 T or up to 5 T. Several different and even potentially
multiple processes can then be involved in DNP experiments. To date, this has prevented the complete
DNP mechanistic understanding for paramagnetic centers with broader EPR spectrum than the Larmor
frequency of the nuclei that are polarized.
Different kinds of paramagnetic species (chromium(V) complexes, char, BDPA, trityls, nitroxides) 295 - 298
have been employed, and it appears that TEMPO-like nitroxides are among the most promising polarizing
agents. 299,300 Nitroxides have been shown to have strong dipolar coupling to water, which can result in
large DNP enhancement. Nitroxides have the additional advantages of being easily available and non-toxic
at the concentration used for in vivo applications. It has been shown in

1 H nuclei and

ratio ( γ e /γ i ) ,being ∼ 660 for ∼

1 H-DNP experiments in aqueous

2 Hand 15 N isotope-labeled nitroxides

solution and pumping at 9.7 GHz microwave frequency, that using

the effective saturation factor can be increased. 301