Reversible Interactions with para-Hydrogen Enhance NMR Sensitivity by Polarization Transfer

TLDR: It is shown here that a metal complex can facilitate the reversible interaction of para-hydrogen with a suitable organic substrate such that up to an 800-fold increase in proton, carbon, and nitrogen signal strengths are seen for the substrate without its hydrogenation.

Abstract: The sensitivity of both nuclear magnetic resonance spectroscopy and magnetic resonance imaging is very low because the detected signal strength depends on the small population difference between spin states even in high magnetic fields. Hyperpolarization methods can be used to increase this difference and thereby enhance signal strength. This has been achieved previously by incorporating the molecular spin singlet para-hydrogen into hydrogenation reaction products. We show here that a metal complex can facilitate the reversible interaction of para-hydrogen with a suitable organic substrate such that up to an 800-fold increase in proton, carbon, and nitrogen signal strengths are seen for the substrate without its hydrogenation. These polarized signals can be selectively detected when combined with methods that suppress background signals.

Figures

Figure 1. Single scan NMR spectra of samples containing a templating medium, the indicated substrate, and para-H2 at 295 K in d4-methanol where the polarisation transfer step was achieved in a 2 x 10 -2 T field. (A) 1 H control trace (upper) of 6 nanomoles of pyridine with 128 fold vertical expansion relative to the lower 1 H trace

Figure 3. Single average 1 H True-FISP MRI images of an 8 mm sample tube containing glass cylinders with 1 mm internal diameter showing: (A) signals from polarized pyridine in d4-methanol within a 0.5 mm slice; (B) signals from the same sample after the decay of polarisation for a 20 mm slice thickness.

Figure 2. (A) Unlocked 1 H NMR spectrum recorded in protio-CH2Cl2 using the OPSY filtration sequence to suppress background signals illustrating the detection of 60 μmoles of polarized pyridine in a single scan. (B) 1 H NMR spectrum recorded on the same sample using the same pulse sequence but recorded 150 s after the low field polarisation step.

Full text

Edinburgh Research Explorer
Reversible interactions with para-hydrogen enhance NMR
sensitivity by polarization transfer
Citation for published version:
Adams, RW, Aguilar, JA, Atkinson, KD, Cowley, MJ, Elliott, PIP, Duckett, SB, Green, GGR, Khazal, IG,
Lopez-serrano, J & Williamson, DC 2009, 'Reversible interactions with para-hydrogen enhance NMR
sensitivity by polarization transfer', Science, vol. 323, no. 5922, pp. 1708-1711.
https://doi.org/10.1126/science.1168877
Digital Object Identifier (DOI):
10.1126/science.1168877
Link:
Link to publication record in Edinburgh Research Explorer
Document Version:
Peer reviewed version
Published In:
Science
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Download date: 10. Aug. 2022

Reversible Interactions with para-Hydrogen Enhance NMR Sensitivity
by Polarization Transfer**
Ralph W. Adams,
1
Juan A. Aguilar,
1
Kevin D. Atkinson,
1
Michael J. Cowley,
1,‡
Paul I. P. Elliott,
1,†
Simon B. Duckett,
1,
* Gary G. R. Green,
2
Iman G. Khazal,
1
Joaquín López-Serrano
1
and David C. Williamson
1
[1]
Department of Chemistry, University of York, Heslington, York, YO10 5DD, UK.
[2]
York Neuroimaging Centre, The Biocentre York Science Park, University of York, Heslington, York, YO10
5DG, UK.
[
*
]
Corresponding author; e-mail: sbd3@york.ac.uk
[
**
]
This work was supported by the University of York, the White-Rose Health Innovation Partnership, the
Engineering and Physical Sciences Research Council, the Medical Research Council, Biotechnology and
Biological Sciences Research Council, the Spanish Ministry of Education and Science [Project Consolider
ORFEO (CSD 2007-00006)], and Bruker Bio-Spin. We thank Bruker Bio-Spin, R. Perutz, P. Walton, M.
Hymers, S. Johnson, M. Mortimer, and K. Armour for advice and helpful discussions. We have filed a patent
application, P115707GB, based on this work.
[
†
]
Current address: Department of Chemical and Biological Sciences, University of Huddersfield, Queensgate,
Huddersfield, HD1 3DH, UK.
[
‡
]
Current address:
EaStCHEM, School of Chemistry, Joseph Black Building, University of Edinburgh, West
Mains Road, Edinburgh, EH9 3JJ, UK.
Supporting information:
Supporting Online Material including Materials and Methods; Figs. S1 and S2; References; Movies S1 and S2
is available at www.sciencemag.org/cgi/content/full/323/5922/1705/DC1
This is the author's version of the work. It is posted here by permission of the AAAS for personal
use, not for redistribution. The definitive version was published in Science, 323(5922), doi:
http://dx.doi.org/10.1126/science.1168877
Cite as:
Adams, R. W., Aguilar, J. A., Atkinson, K. D., Cowley, M. J., Elliott, P. I. P., Duckett, S. B., Green,
G. G. R., Khazal, I. G., Lopez-serrano, J., & Williamson, D. C. (2009). Reversible interactions with
para-hydrogen enhance NMR sensitivity by polarization transfer. Science, 323(5922), 1708-1711.
Manuscript received: 24/11/2008; Accepted: 09/02/2009; Article published: 27/03/2009

Page 1 of 8
Abstract
The sensitivity of both nuclear magnetic resonance (NMR) spectroscopy and magnetic resonance imaging
(MRI) is very low because the detected signal strength depends upon the small population difference between
spin states even in high magnetic fields. Hyperpolarization methods can be used to increase this difference and
thereby enhance signal strength. This has been achieved previously by incorporating the molecular spin
singlet para-hydrogen into hydrogenation reaction products. We show here that a metal complex can facilitate
the reversible interaction of para-hydrogen with a suitable organic substrate such that up to an 800 fold
increase in proton, carbon, and nitrogen signal strengths are seen for the substrate without its hydrogenation.
These polarized signals can be selectively detected when combined with methods that suppress background
signals.
Main text
The wide variety of applications of nuclear magnetic resonance (NMR) (1-3) are limited by the technique’s
extremely low inherent sensitivity. Here we describe a new approach that uses hyperpolarized spins derived
from para-hydrogen (para-H
2
) (4) to sensitize the NMR experiment without actually incorporating para-H
2
into the molecule that is to be probed. Specifically, we show that high-resolution NMR spectra can be
collected for a range of molecules and nuclei where the detected signal strengths are up to 800 times greater
than would be normally achievable with an unpolarized sample. This improvement facilitates the collection of
diagnostic high resolution
1
H,
13
C,
15
N,
19
F NMR spectra and magnetic resonance images of selected signals in
a fraction of the time that would normally be necessary. When optimised, this route is predicted to increase
proton sensitivity by up to four orders of magnitude (5) such that the routine single shot characterization of
materials, even at picomole levels, will become possible (6).
The relative weakness of NMR signals exhibited by nuclei with a non-zero magnetic moment results from the
way the original energy levels split in a magnetic field (7). The bulk magnetic moment for an ensemble of
such nuclei is determined by the Boltzmann population of each energy level. In general, the difference in the
energy between these levels is so small that almost equal spin populations exist across them. For example, in a
magnetic field of 9.4 T such as that found in routine high-resolution NMR spectrometers, the difference in
spin population will only be around 1 in 32,000 for
1
H. Unfortunately,
1
H nuclei are the most sensitive and for
19
F,
31
P,
13
C, and
15
N, the next most common nuclei to be studied, the sensitivity problem is even more acute,
with the associated signal decreasing by factors of 1.2, 15, 64 and 10
4
respectively. The problem is further
exacerbated when the natural abundance of
13
C (1.108%) and
15
N (0.37%) isotopes are taken into account,
meaning the effective differences in sensitivity scale from 1 in 32,000 for
1
H to 1 in 120 million and 1 in 8.7
billion in these nuclei respectively. As a result of this, general routine human imaging experiments are

Page 2 of 8
restricted to measuring the
1
H signals coming from water and lipid in tissues. Furthermore the direct detection
of non-proton signals can require many hours of measurement even in NMR.
A number of “hyperpolarization” methods have been developed (8-18) to enhance signal strength by
transferring non-equilibrium nuclear spin polarizations. The method of dynamic nuclear polarization (DNP),
as reviewed in (10), creates a non-Boltzmann spin population by transfer of polarisation from an unpaired
electron. Recently this approach has been demonstrated to usefully enhance
13
C and
15
N signals by factors that
exceed 10,000 (11, 12, 13, 14). Currently, however, this method requires long polarization times (often over 6
hours), normally employs water and methanol as solvents and is unable to detect enhanced proton signals
routinely.
Here we tackle the sensitivity problem in a different way by using para-H
2
as the source of polarization. Para-
H
2
has the advantage that it can be prepared easily and stored at room temperature for months. Previously,
studies with para-H
2
have been limited to those involving the formation of hydrogenation products containing
para-H
2
derived protons (15). For example, Pines et al. have recently used it to image heterogeneous
hydrogenation reactions through the detection of polarized protons of propane (16). More usually, however, it
is the imaging of
13
C-based magnetization that is targeted because such nuclei can be polarized by para-H
2
based hydrogenation reactions in low magnetic field, as demonstrated by Bargon et al. (17) and exploited by
Golman et al. (12). In these hydrogenative processes the newly formed reaction products contain protons
originating from a single para-H
2
molecule and they can produce strongly enhanced NMR signals in the
reaction product provided the reaction does not change the magnetic arrangement of these coupled atoms (15).
The need for a suitable hydrogen-acceptor, however, reflects a significant limitation of the existing approach.
Nonetheless, the ability to increase proton signal strengths in such products by 32,000 with para-H
2
and hence
detect pico-moles of material in a single scan has been established (6, 18). In order to generalize the use of
para-H
2
as a source of polarization, a method for the transfer of polarization without the direct hydrogenation
of materials is needed. Here we show that the temporary association of a substrate and para-H
2
via a transition
metal center in low magnetic field can achieve just this. Thus, NMR spectral amplification by reversible
exchange (NMR-SABRE) is achieved without any chemical modification of the hyperpolarized material. As
an example, we use the labile complex [Ir(H)
2
(PCy
3
)(substrate)
3
][BF
4
], which is formed by the reaction of
[Ir(COD)(PCy
3
)(MeCN)][BF
4
] (where Cy is cyclohexyl and COD is cyclooctadiene) with para-H
2
and an
excess of the substrate to be polarized (19, 5). Notably, the same observations can be made on a range of
materials and metal templates according to the concept illustrated in Scheme 1.
We first illustrate this effect using pyridine as the substrate where the iridium dihydride complex
[Ir(H)
2
(PCy
3
)(pyridine)
3
][BF
4
] is formed. The single scan
1
H NMR spectrum shown in Fig. 1A was recorded
after the sample was first polarized in a magnetic field of around 2 x 10
-2
T and contains signals with
enhanced intensity for the three proton sites of the pyridine substrate. All that is necessary to achieve this
result is to shake the sample in low magnetic field (Movie S1). This dissolves fresh para-H
2
from the head

Page 3 of 8
space above the solvent, allowing it to associate with the metal complex, thereby activating the polarisation
transfer process in the solution. Specifically, the signals for free pyridine at 7.84, 8.54, and 7.43 appear in
the downward direction that is most simply described as emission. After the sample is then left in the NMR
spectrometer for ten minutes, the resulting NMR spectrum demonstrates that the pyridines magnetic states
have returned to their more usual Boltzmann arrangement distribution through relaxation. We note that the
resonances illustrated for free pyridine in Fig. 1A can be described as ‘hyperpolarized’.
Scheme 1. Schematic representation of magnetization transfer process.
Figure 1. Single scan NMR spectra of samples containing a templating medium, the indicated substrate, and
para-H
2
at 295 K in d
4
-methanol where the polarisation transfer step was achieved in a 2 x 10
-2
T field. (A)
1
H
control trace (upper) of 6 nanomoles of pyridine with 128 fold vertical expansion relative to the lower
1
H trace

Frequently asked questions

Q1. What is the role of para-H2 in the hydrogenation process?

In these hydrogenative processes the newly formed reaction products contain protons originating from a single para-H2 molecule and they can produce strongly enhanced NMR signals in the reaction product provided the reaction does not change the magnetic arrangement of these coupled atoms (15). 

Q2. What is the effect of the polarization transfer on free pyridine?

substrate exchange with that bound in the host-ligand template during this period leads to the build-up of hyperpolarization in free pyridine. 

Q3. How long has the pyridine signal enhancement been estimated?

The time saved through this 823-fold signal enhancement has been estimated to exceed 3 months assuming that the individual measurements are separated by a 20 second recovery delay and use 90 o observation pulses. 

Q4. How can a metal complex increase the signal strength of a sample?

the authors show that high-resolution NMR spectra can be collected for a range of molecules and nuclei where the detected signal strengths are up to 800 times greater than would be normally achievable with an unpolarized sample. 

Q5. What do the authors find in the spectra of nicotinamide?

The authors note that 3-fluoropyridine, nicotine, pyridazine, quinoline, quinazoline, quinoxaline and dibenzothiophene also show enhancement and that 19 F and 31 P signals can also be detected (5). 

Q6. What is the reversible interaction of para-hydrogen with a suitable organic substrate?

The authors show here that a metal complex can facilitate the reversible interaction of para-hydrogen with a suitable organic substrate such that up to an 800 fold increase in proton, carbon, and nitrogen signal strengths are seen for the substrate without its hydrogenation. 

Q7. How is the proton sensitivity of the nmr route predicted to be increased?

When optimised, this route is predicted to increase proton sensitivity by up to four orders of magnitude (5) such that the routine single shot characterization of materials, even at picomole levels, will become possible (6). 

Q8. How many proton signal strengths can be detected in a single scan?

the ability to increase proton signal strengths in such products by 32,000 with para-H2 and hence detect pico-moles of material in a single scan has been established (6, 18). 

Q9. How many folds of sensitivity would be apparent?

If the NMR spectra of 100% 13 C enriched materials were compared to those obtained from para-H2 enhancement of the un-enriched material, an eight fold gain in sensitivity would still be apparent. 

Q10. What is the direction of the signals for free pyridine?

the signals for free pyridine at 7.84, 8.54, and 7.43 appear in the downward direction that is most simply described as emission. 

Q11. What is the effect of the polarization transfer?

The authors conclude therefore that whilst at low field, spontaneous polarization transfer occurs between para-H2 and the pyridine substrate that is in temporary association with the metal template. 

Q12. What is the way to use a proton based MRI?

The method can be used on routine proton based MRI instruments without the need to exploit other magnetically active nuclei that provide much weaker signals. 

Q13. How does this improve the sensitivity of NMR?

This improvement facilitates the collection of diagnostic high resolution 1 H, 13 C, 15 N, 19 F NMR spectra and magnetic resonance images of selected signals in a fraction of the time that would normally be necessary. 

Q14. What is the effect of polarization on free pyridine?

This enhancement effect is not just limited to proton signals since the corresponding 13 C (Fig. 1B) and 15 N resonances (Fig. 1C) of pyridine are also polarized. 

Q15. How can the pyridine signal be regenerated?

these effects can be regenerated by simply removing the sample from the spectrometer and bringing it into contact with fresh para-H2 in low magnetic field. 

Q16. How many different isotopes are used in the NMR spectra?

The problem is further exacerbated when the natural abundance of 13 C (1.108%) and 15 N (0.37%) isotopes are taken into account, meaning the effective differences in sensitivity scale from 1 in 32,000 for 1 H to 1 in 120 million and 1 in 8.7 billion in these nuclei respectively.