Communications
DOI: 10.1002/anie.201000934
NMR Spectroscopy
Scavenging Free Radicals To Preserve Enhancement and Extend
Relaxation Times in NMR using Dynamic Nuclear Polarization**
Pascal Miꢀville, Puneet Ahuja, Riddhiman Sarkar, Sami Jannin,* Paul R. Vasos, Sandrine Gerber-
Lemaire, Mor Mishkovsky, Arnaud Comment, Rolf Gruetter, Olivier Ouari, Paul Tordo, and
Geoffrey Bodenhausen
Dynamic nuclear polarization (DNP) can enhance the
nuclear polarization, that is the difference between the
populations of the Zeeman levels j ai and j bi of spin I =
1/2, by up to four orders of magnitude with respect to their
Boltzmann distribution at room temperature.[1] This enhance-
ment arises from thermal mixing, which is brought about by
microwave saturation of the EPR transitions of stable radicals
that are mixed with the sample under investigation before
freezing. In dissolution DNP, the sample is usually polarized
at low temperatures and moderate magnetic fields (T= 1.2 K
and B0 = 3.35 or 5 T in our laboratory),[2] rapidly dissolved,[3]
and heated to ambient temperature by a burst of water vapor.
To minimize losses of nuclear spin polarization, the transfer
from the polarizer to the NMR spectrometer or MRI magnet,
including the settling of mechanical vibrations and convection
currents, and, if required, the infusion into living organisms,
must be completed within an interval T< T1. In our labo-
ratory, the interval T has recently been lowered to 4.5 s. The
radicals in the hyperpolarized solution lead to an increase of
the longitudinal relaxation rate R1 = 1/T1 of the solute, thus
limiting the timescales of the dynamic processes that can be
monitored with hyperpolarized nuclei. A concomitant en-
hancement of the transverse relaxation rates R2 = 1/T2 leads
to undesirable line-broadening. The relaxation rates RLLS
=
1/TLLS of the populations of long-lived states (LLS)[4] and the
decay rates RLLC = 1/TLLC of long-lived coherences (LLC)[5]
are even more sensitive to the presence of free radicals than
populations of eigenstates and single-quantum coherences.
Free radicals can be toxic, and hyperpolarized solutions
should not be infused into living organisms unless the radicals
are removed.
Herein, we demonstrate how N-oxide radicals that are
widely used for DNP, such as 4-hydroxy-2,2,6,6-tetramethyl-
piperidine-1-oxyl (TEMPOL), can be reduced by scavengers
like sodium ascorbate (vitamin C) during the dissolu-
tion process into 2,2,6,6-tetramethylpiperidine-1,4-diol
(TEMPOL-H; Scheme 1), thus extending transverse and
longitudinal relaxation times of solutes and slowing down
the decay of their polarization during and after transfer.
Scavenging free radicals with ascorbate merely leaves
ascorbyl radicals, which rapidly disproportionate,[6] in contrast
to scavenging with thiol-based (DTT) or phenolic (Vit-
amin E) antioxidants, so that no paramagnetic species are
present in the sample after dissolution and reduction.
[*] P. Miꢀville, P. Ahuja, Dr. R. Sarkar, Dr. S. Jannin, Dr. P. R. Vasos,
Dr. S. Gerber-Lemaire, Prof. G. Bodenhausen
Institut des Sciences et Ingꢀnierie Chimiques
Ecole Polytechnique Fꢀdꢀrale de Lausanne
EPFL, Batochime, 1015 Lausanne (Switzerland)
Fax: (+41)21-693-94 35
E-mail: sami.jannin@epfl.ch
Dr. M. Mishkovsky, Dr. A. Comment, Prof. R. Gruetter
Laboratory for Functional and Metabolic Imaging
EPFL, 1015 Lausanne (Switzerland)
Dr. O. Ouari, Prof. P. Tordo
Laboratoire LCP, Universitꢀ d’Aix-Marseille I, II, III et CNRS
Avenue Escadrille Normandie-Niemen
13397 Marseille Cedex 20 (France)
Prof. G. Bodenhausen
Dꢀpartement de Chimie, associꢀ au CNRS
Ecole Normale Supꢀrieure
24 rue Lhomond 75231, Paris Cedex 5 (France)
[**] We thank Prof. Jacques van der Klink of the EPFL, and Dr. Ben van
den Brandt, Dr. Jacobus Konter, and Dr. Patrick Hautle of the Paul
Scherrer Institute for the design and construction of the DNP
polarizer. We gratefully acknowledge Martial Rey for technical
assistance. This work was supported by the Swiss National Science
Foundation (grant 200020_124694 to G.B. and P.R.V. and grant
200020_124901 to A.C.), the Commission pour la Technologie et
l’Innovation (CTI), the Ecole Polytechnique Fꢀdꢀrale de Lausanne
(EPFL), and the French CNRS.
Scheme 1. The reduction of TEMPOL by sodium ascorbate, leading to
the formation of a delocalized sodium ascorbyl radical and diamag-
netic TEMPOL-H.
Supporting information for this article is available on the WWW
6182
ꢀ 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2010, 49, 6182 –6185