Optimisation of pyruvate hyperpolarisation using SABRE by tuning the active magnetisation transfer catalyst

Article abstract of DOI:10.1039/c9cy02498k

Hyperpolarisation techniques such as signal amplification by reversible exchange (SABRE) can deliver NMR signals several orders of magnitude larger than those derived under Boltzmann conditions. SABRE is able to catalytically transfer latent magnetisation from para-hydrogen to a substrate in reversible exchange via temporary associations with an iridium complex. SABRE has recently been applied to the hyperpolarisation of pyruvate, a substrate often used in many in vivo MRI studies. In this work, we seek to optimise the pyruvate-¹³C₂ signal gains delivered through SABRE by fine tuning the properties of the active polarisation transfer catalyst. We present a detailed study of the effects of varying the carbene and sulfoxide ligands on the formation and behaviour of the active [Ir(H)₂(η²-pyruvate)(sulfoxide)(NHC)] catalyst to produce a rationale for achieving high pyruvate signal gains in a cheap and refreshable manner. This optimisation approach allows us to achieve signal enhancements of 2140 and 2125-fold for the 1-¹³C and 2-¹³C sites respectively of sodium pyruvate-1,2-[¹³C₂].

Full text of DOI:10.1039/c9cy02498k

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Optimisation of pyruvate hyperpolarisation using
SABRE by tuning the active magnetisation transfer
catalyst†
Cite this: DOI: 10.1039/c9cy02498k
a
a
Ben. J. Tickner,
Olga Semenova, ‡a Wissam Iali, §a Peter J. Rayner,
Adrian C. Whitwood b and Simon B. Duckett
a
*
Hyperpolarisation techniques such as signal amplification by reversible exchange (SABRE) can deliver NMR
signals several orders of magnitude larger than those derived under Boltzmann conditions. SABRE is able to
catalytically transfer latent magnetisation from para-hydrogen to a substrate in reversible exchange via
temporary associations with an iridium complex. SABRE has recently been applied to the hyperpolarisation
of pyruvate, a substrate often used in many in vivo MRI studies. In this work, we seek to optimise the
pyruvate-13C2 signal gains delivered through SABRE by fine tuning the properties of the active polarisation
transfer catalyst. We present a detailed study of the effects of varying the carbene and sulfoxide ligands on
the formation and behaviour of the active [IrIJH)2IJη2-pyruvate)IJsulfoxide)IJNHC)] catalyst to produce a
rationale for achieving high pyruvate signal gains in a cheap and refreshable manner. This optimisation
approach allows us to achieve signal enhancements of 2140 and 2125-fold for the 1-13C and 2-13C sites
respectively of sodium pyruvate-1,2-[13C2].
Received 18th October 2019,
Accepted 10th December 2019
DOI: 10.1039/c9cy02498k
rsc.li/catalysis
frequency of the electron at very low temperatures (1–2 K).4–6
Rapid heating of such solids then generates materials that
Introduction
Nuclear magnetic resonance (NMR) and magnetic resonance
imaging (MRI) are some of the most widely used tools for the
characterisation of molecules and the clinical diagnosis of
disease. While these techniques are used widely in their
fields, they remain insensitive as their signal strengths are
derived from small Boltzmann population differences across
the nuclear spin energy levels they probe. Recently, a growing
number of researchers are turning their attention to
hyperpolarisation to help address this problem.1–3 For
example, dynamic nuclear polarisation (DNP) can achieve
yield MR signal enhancements in solution of up to 5 orders
of magnitude.5,6 This approach has been applied to the
production of hyperpolarised biomolecules such as
pyruvate,7–14 succinate,15,16 and fumarate17,18 which are then
injected and detected in vivo alongside their metabolic by-
products. Imaging the formation of such metabolites provides
a route to studying biochemical tissue function in real time
with obvious benefits for disease diagnosis.7–17
Para-Hydrogen induced polarisation (PHIP) methods are
potentially a faster and cheaper alternative to DNP.19–21 The
feedstock of PHIP is para-hydrogen (p-H2), which is the isomer
of dihydrogen that exists as a nuclear spin singlet. In the first
generation of PHIP studies, p-H2 was typically incorporated
1
polarisation levels of 92% and 70% for H and 13C signals in
times as short as 150 seconds and 20 minutes respectively.4,5
DNP transfers the inherent polarisation of an electron into
target nuclei when both are located in a frozen glass matrix
and subject to microwave irradiation at or near the resonance
into
a substrate via a
hydrogenation reaction.22,23 The
resulting product detection by NMR has since provided many
significant observations in the field of catalysis wherein
reaction intermediates are detected.24–26 The catalytic
production of hyperpolarised probes suitable for in vivo study
using this version of PHIP was therefore limited to
biomolecules that have facile access to their dehydro-
precursor.15,27,28 This limitation has been elegantly alleviated
using a variant of PHIP, termed para-hydrogen induced
polarisation by side arm hydrogenation (PHIP-SAH), which
can produce aqueous solutions of hyperpolarised pyruvate
and acetate.29 In the precursor, pyruvate is functionalised as
an ester with an unsaturated side arm which, after
a Centre for Hyperpolarization in Magnetic Resonance (CHyM), University of York,
Heslington, YO10 5NY, UK. E-mail: simon.ducket@york.ac.uk
b Department of Chemistry, University of York, Heslington, YO10 5DD, UK
† Electronic supplementary information (ESI) available. CCDC 1957542–1957543.
For ESI and crystallographic data in CIF or other electronic format see DOI:
10.1039/c9cy02498k
‡ Present Address: Dr. O. Semenova, Drug Discovery Unit, School of Life
Sciences, University of Dundee, Dundee, DD1 5EH, United Kingdom.
§ Present Address: Dr. W. Iali, Department of Chemistry, King Fahd University
of Petroleum and Minerals (KFUPM), Dhahran, 31261, Saudi Arabia.
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hydrogenation by p-H2 and a magnetic field cycling step to
transfer polarisation into the modified pyruvate, can be
rapidly released through simultaneous hydrolysis and phase
separation.29,30 The resulting pyruvate can then be detected
by MRI through a much stronger, hyperpolarised, response.
Hyperpolarised pyruvate prepared in this way is the result of a
one-shot, irreversible batch synthesis.
In contrast, signal amplification by reversible exchange
(SABRE) is an alternative non-hydrogenative PHIP based
method that involves the transfer of spin polarisation from p-H2
to a substrate when both are concurrently bound to an iridium
catalyst, as depicted in Fig. 1.31 As the ligands are in reversible
exchange, a pool of hyperpolarized substrate is readily created
in solution. Hence, the magnetisation transfer step is catalytic
in nature, occurring via the temporary J-coupled network within
the organometallic complex. Consequently, the process is
completed without chemical change and is continuous and
refreshable in nature.32 The identity of the ligands used in such
SABRE magnetisation transfer catalysts are important in
delivering high MR signal gains and controlling the type of
substrates that can be hyperpolarised.33–35 SABRE has had the
greatest success to date in polarising structures with
approach readily delivers 50-fold 13C enhancements.40
However, rapid in situ condensation of pyruvate with the
amine carrier forms products of the type [IrIJH)2IJη2-α-
carboxyimine)IJamine)IJNHC)] which deactivate the system to
further pyruvate polarisation.42
It has since been reported that by using appropriate
stabilising ligands, SABRE can hyperpolarise pyruvate in a
low cost, fast, and reversible fashion that does not involve
the technologically demanding equipment of DNP, or the
multiple steps of PHIP-SAH.39 This is possible due to the
formation of the polarisation transfer catalyst [IrIJH)2IJη2-
pyruvate)IJDMSO)IJIMes)] (where IMes = 1,3-bisIJ2,4,6-trimethyl-
phenyl)imidazol-2-ylidene) when solutions of [IrClIJCOD)-
IJIMes)] (1a) (where COD = cis,cis-1,5-cyclooctadiene), DMSO
and sodium pyruvate in methanol-d4 or 70 : 30 mixtures of
D2O and ethanol-d6 are activated with 3 bar of H2. These
sulfoxide based complexes exhibit significantly more
elaborate catalysis than more commonly observed in SABRE
with N-donor substrates as the Sub and H2 exchange
pathways are no longer localised within a single inorganic
species.43 This is because while [IrIJH)2IJη2-pyruvate)IJDMSO)-
IJIMes)] reflects the active polarisation transfer catalyst for 13C
pyruvate enhancement, it is [IrClIJH)2IJDMSO)2IJIMes)] (2) that
mediates the necessary H2 exchange processes.43 This
situation is complicated yet further by the fact [IrIJH)2IJη2-
pyruvate)IJDMSO)IJIMes)] (3) exists as two regioisomers that are
differentiated by the geometry of η2-pyruvate coordination, as
depicted in Fig. 1b. We have previously shown that the
regioisomer where pyruvate binds in the same plane as the
hydride ligands (3b) contains a spin topology that allows
active polarisation transfer of singlet order from p-H2 derived
hydride ligands to coordinated 13C pyruvate sites in the
catalyst.39,43
N-heterocyclic motifs which have
a simple and readily
understandable binding mode.34,36–38 In these cases,
polarisation transfer catalysts of the form [IrIJH)2IJNHC)IJSub)3]Cl
provide suitable substrate (Sub) and H2 exchange rates for
significant polarisation build up in solution.
Until recently α-keto acids, such as pyruvate, were
incompatible with SABRE because they were unable to form
stable complexes due to their weak ligation to iridium.39
A
related
technique,
SABRE-Relay,
has
allowed
the
hyperpolarisation of a wide range of non-ligating substrates
that contain functional groups which can receive
hyperpolarised protons through exchange from a suitable
carrier.40,41 When applied to sodium pyruvate-1-[13C] this
SABRE is dependent on the magnetic field experienced by
the sample during polarisation transfer because a suitable
matching condition for optimal polarisation transfer between
p-H2 derived hydride ligands and the ligated target substrate
must be achieved. For 13C-SABRE by complexes of this type,
optimal transfer typically occurs at mG fields if direct
transfer from the p-H2 derived hydride ligands into bound
13C sites is involved.39,43,44 For sodium pyruvate-1-[13C] and
sodium pyruvate-2-[13C] this necessitates fields of 9 and
3
mG respectively.39 Interestingly, when sodium pyruvate-1,2-
[13C2] is used, the resulting process leads to the spontaneous
creation of long lived 13C2 singlet order in the product being
detected whose decoherence lifetime exceeds that of T1.39 In
such states, the underlying magnetisation involves two
coupled spins and, in this case, its formation is independent
of magnetic field. In the context of this paper, it is important
to appreciate that p-H2 reflects another example of such a
singlet state which is now not only very long lived, but NMR
invisible. This singlet order becomes visible to NMR by a
symmetry breaking reaction, such as the oxidative addition of
p-H2 to the iridium centre.45 In contrast to p-H2, the two
coupled 13C spins of sodium pyruvate-1,2-[13C2] are already
magnetically distinct and consequently its singlet state is
Fig.
1
a) Traditionally, SABRE catalytically transfers magnetisation
temporary
from p-H2 to an N-donor substrate (NSub) through
a
J-coupled network when both p-H2 and NSub are in reversible
exchange with a complex such as [IrIJH)2IJIMes)IJNSub)Cl b) pyruvate
hyperpolarisation using SABRE can be achieved via [IrIJH2)IJη2-
pyruvate)IJDMSO)IJIMes)] where p-H2 exchange is now predominantly
mediated by [IrClIJH2)IJDMSO)2IJIMes)].
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immediately accessible by NMR and evolves more quickly
that than of p-H2.
In this paper we report on a series of rigorous catalytic
studies that investigate the role that the [IrIJH)2IJIMes)IJη2-
pyruvate)IJsulfoxide)] and [IrClIJH)2IJDMSO)2IJIMes)] type species
play in the 13C hyperpolarisation of pyruvate. Throughout
this work we use sodium pyruvate-1,2-[13C2] as the target
substrate because the longer lifetime of its hyperpolarised
singlet state may have future benefits in reaction monitoring
or medical imaging. It must be remembered that as we create
the hyperpolarised molecule remote to the final point of
observation there is a time delay between preparation and
detection. Thus increased magnetic state lifetimes extend the
timescale over which signal detection is possible and offer
significant potential benefits for tracer analysis. We show
here that optimisation of the hyperpolarisation level
delivered by the catalyst is complex, with factors such as
catalyst identity, concentration and temperature exhibiting
non-trivial behaviour due to the complex interplay that exists
between the roles of the different species present in solution.
Consequently, we explore the properties of the active catalyst
by varying the identity of both sulfoxide and NHC ligand to
produce a rationale for achieving high 13C pyruvate NMR
signal enhancements using SABRE.
Results and discussion
Formation of an active sulfoxide containing magnetisation
transfer catalyst, [IrIJH)2IJIMes)IJη2-pyruvate)IJDMSO)]
When sodium pyruvate-1-2-[13C2] (6 equivalents relative to
iridium) is used as the substrate and added to 1 in the
presence of dimethyl sulfoxide (DMSO) (4 equivalents) and 3
bar H2 in methanol-d4, an equilibrium mixture of
[IrClIJH)2IJsulfoxide)2IJNHC)] (2) and [IrIJH)2IJη2-pyruvate)-
IJsulfoxide)IJNHC)] (3) is formed (Fig. 1b).39,43 The regioisomer
containing ligated pyruvate which lies trans to both hydride,
and the NHC, is labelled 3a whereas the regioisomer where
pyruvate lies in the same plane as the two hydride ligands is
labelled 3b. Both of these structures are illustrated in Fig. 1b.
When examining these solutions with a signal averaged 32
scan 1H NMR measurement at 298 K, the main hydride
containing complex present is 3b. Resonances for 2 and 3a
could not be discerned under these conditions, although,
upon shaking this solution with 3 bar of p-H2 for 10 seconds
at 65 G, hyperpolarised hydride responses for 2, 3a and 3b
are immediately detected, as shown in Fig. 2a.39,43
Furthermore, upon shaking this sample for 10 seconds in a
mu metal shield (ca. 300-fold shielding), hyperpolarised 13C
resonances are observed, as shown in Fig. 2b. These
Fig. 2 NMR spectra of a) 1H hydride region and b) 13C carbonyl region
after shaking a solution of 1a, 6 equivalents sodium pyruvate-1,2-[13C2]
and 4 equivalents of DMSO with 3 bar p-H2 for 10 seconds a) at 65 G
or b) in a mu metal shield c) the averaged 13C signal gain across the
[1-13C] and [2-13C] sites and hyperpolarised hydride signal intensity of
3b can be monitored over time with fresh p-H2 shaking. Note that one
anomalous data point was omitted.
multi-scan thermally polarised 13C{1H} NMR measurement
confirms these assignments for ligated pyruvate in both 3a
and 3b in addition to those of the free material and its
hydrate. 2D NMR characterisation data for these complexes
has been previously reported.39 When we examine the signals
of the [1-13C] and [2-13C] sites more closely we observe ∼2 Hz
and ∼20 Hz resonance broadening upon pyruvate
coordination respectively. We note that the [2-13C] resonance
of the free material appears as a doublet of quartets with a
correspond to those of free pyruvate at δ 169 and δ 203 (JCC
=
62 Hz) and pyruvate bound in 3b at δ 168 and δ 207 (JCC = 60
Hz) and we quantify the 13C signal gains as 1215-fold and
910-fold for the [1-13C] and [2-13C] sites respectively in the free
material. Additional resonances corresponding to pyruvate
hydrate at δ 97 and δ 177 (JCC = 62 Hz) and pyruvate bound
within 3a at δ 166 and δ 196 (JCC = 63 Hz) are also visible. A
2
1JCC value of 62 Hz and a smaller JHC coupling between the
2
adjacent methyl group protons of 6 Hz. This smaller JHC is
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not visible for pyruvate bound in 3b due to broadening
effects. In order to explore how the efficiency of polarisation
transfer changes with reaction time, this sample was shaken
with fresh p-H2 at various time intervals after initial p-H2
addition. The hyperpolarised 13C and 1H signals that could
be detected for pyruvate and 3b respectively were found to
decrease with time, as shown in Fig. 2c, which is consistent
with catalyst decomposition.
binding and subsequent 13C signal gains and for this reason
we explore how its identity affects this process.
Effect of sulfoxide identity on pyruvate 13C2 signal
enhancement
Studies on the effect of sulfoxide identity on the formation of
[IrClIJH)2IJsulfoxide)2IJNHC)] (2) and [IrIJH)2IJη2-pyruvate)-
IJsulfoxide)IJNHC)] (3) and the subsequent 13C pyruvate
enhancement proved to be complex. For this work
[IrClIJCOD)IJIMes)] (1a) was activated in methanol-d4 with 3
bar H2 in the presence of 6 equivalents of sodium pyruvate-
1,2-[13C2] and 4 equivalents of one of the ten sulfoxides (I–X)
of Fig. 3. The pyruvate 13C signal enhancement was then
quantified after shaking the sample with fresh p-H2 several
times over a 90 minute time period following H2 addition. In
order to compare the performance efficiency as a function of
sulfoxide identity we define several parameters. The first,
εmax, describes the highest attained free 13C pyruvate signal
enhancement for either the 1-[13C] or 2-[13C] site relative to
the Boltzmann derived response. We observe that in most
cases the signal gain on the 1-[13C] and 2-[13C] sites are the
same within error, and we also quote averaged signal
enhancements across the two sites. This is due to creation of
13C2 singlet order which must be shared equally amongst the
two 13C sites. The second parameter, τ60, describes the
percentage decrease in 13C pyruvate signal enhancement at
the 60 minute reaction point when compared to the first
measurement. R3b is the ratio of the 3b type product at the
εmax point relative to the sum of all the hydride containing
species and this should illustrate the stability of the
sulfoxide–catalyst combination. The relative absolute
Variation of the [IrIJH)2IJIMes)IJη2-pyruvate)IJL)] co-ligand, L
Polarisation transfer catalysts of the form [IrIJH)2IJIMes)IJη2-
pyruvate)IJL)] containing η2-ligated pyruvate and p-H2 derived
hydride ligands are essential for catalytic polarisation
transfer into bound pyruvate 13C sites and ultimately free
pyruvate after ligand dissociation. A range of different co-
ligands, L, (5 equivalents relative to iridium) were screened
with [IrClIJCOD)IJIMes)] (1a) (5 mM), and sodium pyruvate-1,2-
[13C2] (6 equivalents), and 3 bar p-H2 in methanol-d4 to
identify if any other classes of co-ligands besides DMSO
could form analogous complexes to 2 and 3.
The use of 4-chlorobenzenemethanethiol as a co-ligand
did not initially yield any hydride containing species.
However, upon leaving the solution for a period of several
months at 278 K, the growth of single crystals was observed.
Upon examination by X-ray diffraction they were found to
correspond to [Ir2IJH)4IJκ2-SCH2C6H4Cl)2IJIMes)2] as detailed in
the ESI.† We, and others, have reported structures of similar
sulphur bridged iridium dimers26,46 and other products
resulting from SH bond functionalisation.47 The other tested
co-ligands,
ethylisothiocyanate and thiophene, all resulted in the
formation of hydride complexes within hour of H2
formaldehyde,
triphenylphosphine
(PPh3),
1
1
integrals of the enhanced hydride H NMR signals of 3b after
addition, but the corresponding solutions did not display
any PHIP enhanced hydride signals upon shaking with p-H2.
When a solution of 1a, PPh3 and sodium pyruvate-1,2-[13C2]
with 3 bar H2 in methanol-d4 is left at 278 K for several
months, the growth of single crystals was again observed.
X-ray diffraction studies identified the product as
[IrIJH)3IJPPh3)3], as detailed in the ESI.† In contrast, the use of
imidazole as a co-ligand did result in a hydride complex at δ
−22.3 that exhibited PHIP, as detailed in the ESI.†48,49
However, in each of these cases no additional 13C pyruvate
resonances were observed by NMR spectroscopy thereby
suggesting that pyruvate coordination to iridium in these
systems does not occur.
shaking at 65 G, S3b, was also determined during the reaction
period and was found to exhibit similar behaviour. These
values are presented for each of the sulfoxides I–X in Table 1.
For sulfoxides I–VIII, hyperpolarised 13C pyruvate
responses are observed immediately upon shaking the
sample with 3 bar p-H2 in a mu metal shield. Over the next
Of the co-ligands tested here, only sulfoxides supported
pyruvate ligation to iridium. We expect this to be related to
the optimum binding strength of the co-ligand which must
be similar to that of pyruvate if its binding is not to be
inhibited. For example, when the nitrogen based donor
imidazole is used it seems to out compete pyruvate
binding.31,34,50–52 In these cases [IrIJH)2IJIMes)IJNSub)3]Cl type
complexes form as revealed by a single hydride signal around
δ −22.3.47,48 The use of a sulfoxide based co-ligand is
therefore a suitable compromise that leads to pyruvate
Fig. 3 Structures of the sulfoxides I–X used in this work.
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Table 1 Comparison of εmax, τ60, S3b and R3b values (see text for definition) for methanol-d4 solutions of 1a, 6 equivalents of sodium pyruvate-1,2-[13C2]
and 4 equivalents of the specified sulfoxide I–X of Fig. 3 after shaking with 3 bar p-H2 for 10 seconds in a mu metal shield.
Sulfoxide
13C1 εmax/folda
13C2 εmax/folda
τ60/%
S3b/arb. units
R3b/%
I
II
1215 40
1090 35
1090 35
910 30
1035 30
1040 30
23
6
32
92
3
3
9
60
N/A
17
50
85
20
5
60
5
25
70
N/A
N/Ac
60
90
III
IV
V
VI
VII
VIII
IX
X
93
115
555 15
195
5
105
545 15
180
5
N/Ab
90
5
5
70
45
95
400 10
1150 35
0
385 10
1040 30
0
0
130
5
115
5
N/Ac
a
These reflect one shot measurements due to the change in signal over time. Errors are based on an average of three measurements for a
b
sample containing II where the observed enhancement is relatively constant thereby allowing repeat measurement. Rapid sample degradation
prevented recording R3b at a similar time point to εmax
c
.
No signals for a species analogous to 3b were evident.
90 minute time period, the resulting 13C pyruvate signal
enhancements all gradually decrease (see ESI†). In all cases,
the major dihydride complex present in solution proved to be
of type 3b. Similarly, the hydride signals corresponding to 3b
quantified for VIII, catalyst deactivation is extremely rapid.
Hence, it is clear that SABRE efficiency is linked to the
concentration of the 3b derivative in solution, which falls as
the reaction time increases.
type
products dominate the associated hyperpolarised
Sulfoxides VI and VII result in lower proportions of 3b (R3b
of 70% and 45% in solution respectively) being present in these
mixtures which will contribute to the lower pyruvate signal
enhancements that are observed. In contrast, sulfoxides II–V
commonly result in high proportions of 3b (R3b > 90%) and any
differences in pyruvate 13C signal enhancement between these
sulfoxides must now relate more closely to the efficiency of the
polarisation transfer catalysis rather than catalyst concentration.
For example, we note that I achieves a similar level of pyruvate
enhancement to II, III and VIII despite the much lower ratio
(60%) of 3b present in solution. In contrast, IV and V contain
similar R3b values to II and III (90%) yet yield much lower
pyruvate enhancements (<550-fold as compared to >1000 fold).
This suggests that when sulfoxide I is utilized, a more effective
catalytic system is created when compared to those derived
from co-ligands IV and V.
1
hydride region of these H NMR spectra, and the intensity of
their signals also decrease with increasing reaction time.
Increasing the structural complexity of the co-ligand through
the use of amino acid derived sulfoxide, IX, resulted in no
13C pyruvate signal enhancement, or detection of signals for
species of type 3. While a hydride containing complex forms,
which yields resonances at δ −12.3 and δ −27.3 that exhibit
weak PHIP enhancement upon shaking with p-H2 (see ESI†),
no evidence of pyruvate coordination was found. The poor
performance of this sulfoxide could relate to the ready
formation of an insoluble white precipitate, likely to be the
corresponding [pyruvate-COO−
+
+NH3-IX] salt. When the
protected co-ligand analogue, X, is used instead, pyruvate
coordination and subsequent enhancement is again
observed, but the resulting signal gains are just ∼120 fold.
This is consistent with the lack of visible hydride signals for
species of type 3. Hence, we link these low pyruvate
enhancements to a low concentration of what we prove later
to be the active magnetisation transfer catalyst.
We conclude from these data that the sulfoxide co-ligand
identity plays
a
significant role in determining the
concentration of the active SABRE catalyst in solution and
the efficiency of the polarisation transfer process. Thus
increasing the proportion of 3b present in solution is clearly
one requirement for optimal SABRE. Of the four sulfoxides
that gave pyruvate 13C NMR signal enhancements greater
than 1000-fold, II appeared to be most stable to catalyst
decomposition exhibiting only a 6% drop in signal intensity
after 1 hour compared to 23, 32 and 60% for I, III and VIII
respectively. These results show that H2 reaction time is also
an important parameter that must be considered when
optimising these pyruvate 13C NMR signal gains. This is often
neglected when polarising N-donor substrates using SABRE
as the associated magnetisation transfer catalysts are often
more stable over longer reaction times.
Sulfoxides I–III and VIII delivered the highest levels of 13C
signal enhancement for pyruvate across this series (εmax
>
1000 fold) while IV–VII produced hyperpolarised 13C pyruvate
signals of lower intensity (100–550 fold). These trends
broadly matched those seen for the levels of hydride
hyperpolarisation of the corresponding 3b derivatives. In the
case of IV, the 13C pyruvate response rapidly decayed to zero
as a consequence of hydrogenation of the original sulfoxide
ligand and subsequent catalyst decomposition which has
been observed in closely related systems.26 We have reported
that C–S bond activation products result from this process
which logically accounts for the low R3b and εmax values
achieved by IV.26 In fact, we suggest that similar sulfoxide
reactivity accounts for the loss of the 3b derivatives in all
Methylphenylsulfoxide, II, was identified as the best
performing sulfoxide of this series as the associated complex gave
some of the highest 13C pyruvate signal enhancements whilst also
samples. We highlight that despite
a high εmax being
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resulting in the highest hyperpolarised hydride ligand signal
intensities for the isomer of type 3b. The concentration of
methylphenylsulfoxide II was then varied to determine its effect
on pyruvate enhancement. Similar behaviour is seen for both the
1H and 13C NMR signal enhancements of 3b when compared to
those seen for free pyruvate. We find that using 10 equivalents of
sulfoxide II relative to 1a provides the highest 13C pyruvate
response, as shown in the ESI.† We expect that this is related to
optimal ligand exchange processes at these effective reagent
concentrations.
We have previously reported that complexes of the type
[IrClIJH)2IJNHC)IJsulfoxide)2] (2) exchange hydrogen rapidly and
are important in refreshing the p-H2 derived hydride ligands
within the catalytic system.43 It is therefore likely that the
rate of this process depends on sulfoxide identity and is
reflected in the differing hydride signal intensities for the 3b
isomer. The rate of H2 exchange within 2 was found to be
independent of sulfoxide concentration when the sulfoxide is
in excess (6–14 equivalents relative to iridium)43 in
accordance with the first step of this process being the
dissociative loss of sulfoxide. Therefore, we expect that
changing sulfoxide concentration must have a greater effect
on exchange between 2 and 3, however this process could not
be quantified using EXSY methods. It is clear that the
sulfoxide identity plays a role in p-H2 refreshment within 2
and likely the exchange between 2 and 3.
from 1000-fold to 920-fold. Further decreases to 715-fold and
570-fold are observed as the amount of NaCl is increased to 3
and 5 equivalents respectively. We note that greater chloride
concentrations also result in higher proportions of free
pyruvate signal relative to that seen for the associated bound
resonances within 3b. These changes are accompanied by an
increase in the size of the hydride signals seen for 2 relative
to those of 3b, as shown in Fig. 4. These changes are
therefore consistent with a shift in the equilibrium position
towards 2 and the resulting fall in pyruvate signal gain is
linked to a reduction in the amount of the active polarisation
transfer catalyst, 3b present in these solutions.
The effect of the reduction in active catalyst concentration
was tested explicitly by increasing the initial amount of 1a
and II so that a greater amount of 3b was present in solution.
The resulting mixture with 1a (10 mM), II (100 mM) and
sodium pyruvate-1,2-[13C2] in a 1 : 10 : 6 ratio yielded averaged
13C NMR signal enhancements of 705 and 255-fold for free
and bound pyruvate respectively. The corresponding
enhancements for the same solution containing 5 equivalents
NaCl in 5 μL D2O were now much closer for the free pyruvate
signal at 690-fold but the bound signal fell to 140 fold. These
results confirm that the elevated chloride concentrations
increase the proportion of free pyruvate enhancement relative
to its bound counterpart. In addition they show that at the 10
mM catalyst concentration the free pyruvate signal
enhancement remains comparable to that with the higher
NaCl concentration. When the metal concentration was 5
mM, a reduction of pyruvate signal gain upon salt addition
takes the averaged signal gain down from 1000-fold to 570-
fold. As expected, this difference suggests the greater flux
associated with improved efficiency in what would be a
bimolecular H2 addition step can help offset the effect of
increased chloride concentration.
Effect of chloride ions on pyruvate 13C2 signal enhancement
The rapid rate of H2 exchange in 2 in comparison to 3
indicates that 2 provides a clear route to refresh the p-H2
derived hydride ligands in 3. It is for this reason that the rate
of exchange between 2 and 3 is proposed to play a significant
role in determining the observed 13C pyruvate signal
enhancement. As 2 contains a chloride ligand that is released
The equilibrium between 2 and 3 is also expected to be
influenced by the concentration of pyruvate in solution.
Therefore, samples containing 1a, 10 equivalents of II and 3, 6
or 8.5 equivalents of sodium pyruvate-1,2-[13C2] were shaken
into solution when
3 is formed, the concentration of
available chloride might also be important in the formation
of 3. We have already reported that there is a large decrease
in the resulting pyruvate signal enhancement when chloride
is replaced by bromide or acetonitrile.43 Here, we investigate
the effect of changing the chloride concentration. To do this,
solutions of 1a (5 mM), 10 equivalents of sulfoxide I and 5
equivalents of sodium pyruvate-1,2-[13C2] in 0.6 mL methanol-
d4 containing 0–5 equivalents of NaCl in 5 μL of D2O were
prepared. The resulting SABRE solutions were then activated
with
3 bar p-H2 in methanol-d4. Lowering the pyruvate
concentration from 6 equivalents to 3 equivalents resulted in
the averaged pyruvate signal enhancement reducing from 1085-
fold and 515-fold for the free and bound pyruvate respectively
to just 770-fold and 365-fold respectively. An increase in
pyruvate concentration from
6 to 8.5 equivalents was
accompanied by a similar drop in averaged signal gain to 630-
fold and 180-fold respectively for the free and bound signals in
comparative runs. Interestingly, as the ratio of pyruvate to
iridium increases, the proportion of free pyruvate enhancement
relative to the bound counterpart in 3b also increases. This is
consistent with an increased likelihood of binding unpolarised
pyruvate during SABRE as its concentration increases.
with
3
bar H2 and their 13C NMR pyruvate signal
function of time. The
enhancements monitored as
a
associated signal intensity versus reaction time profiles are
given in the ESI† and they all show an initial increase in 13C
pyruvate signal enhancement over the first ∼30 minute
period followed by a subsequent decrease as the reaction
time increases. This change mirrors the associated change in
concentration of 3b based on changes in hyperpolarised
hydride resonance intensity. Upon increasing the chloride
concentration from 0 to 1 equivalents, we observe a decrease
in the average pyruvate enhancement across the two sites
Effect of catalyst identity on pyruvate 13C2 signal enhancement
The efficiency of traditional [IrIJH)2IJNHC)IJSub)3]Cl based
SABRE catalysts is also influenced by the identity of the NHC
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Fig. 4 Partial a) 13C and b) 1H hyperpolarised NMR spectra resulting from shaking a sample of [IrClIJCOD)IJIMes)], 5 equivalents of sodium pyruvate-
1,2-[13C2], and 10 equivalents of DMSO with varying amounts of NaCl in 0.6 mL methanol-d4 with 3 bar p-H2 for 10 seconds, a) in a mu metal
shield 35 minutes following the initial H2 addition step and b) at 65 G, 10 minutes following the initial H2 addition step.
ligand in the [IrClIJCOD)IJNHC)] precatalyst.34 Variation of this
ligand has been used as route to optimise signal
include symmetric N-heterocyclic carbenes with a range of
Tolman electronic parameters and % buried volumes.55
Additionally, asymmetric N-heterocyclic carbenes59 and
phosphine containing precatalysts,31,35 which have both been
used previously for SABRE, were also included. Samples were
a
enhancements by tuning substrate exchange rates.35,53–55
Changes to these ligands has also been used to synthesise
water soluble SABRE catalysts.56–58 The effect of catalyst
identity on the pyruvate signal enhancement was therefore
probed by investigating the behaviour of the iridium
precatalysts, 1a–h, of Fig. 5. These complexes were chosen to
prepared containing
6 equivalents of pyruvate and 10
equivalents of methylphenylsulfoxide (II) with 3 bar p-H2 in
0.6 mL methanol-d4. The 13C pyruvate signal enhancement,
1H hydride signal enhancement for the 3b type product, and
its relative proportion in solution as measured in a 32 scan
1
thermal H NMR spectrum, were monitored periodically over
the first 90 minutes of reaction. These values are shown in
Table 2 and are displayed graphically in the ESI.†
The identity of the precatalyst 1a–1f proved to have little
effect on the proportion of the 3b type product in solution
although there was an effect on 13C pyruvate signal
Table 2 Comparison of εmax, τ60, and R3b values (see text for definition)
measured after shaking a solution of 6 equivalents sodium-1,2-pyruvate-
[
13C2] and 10 equivalents II with the specified iridium precatalyst 1a–h of
Fig. 5 in methanol-d4 with 3 bar p-H2 for 10 seconds in a mu metal shield
Precatalyst
13C1 εmax/folda
13C2 εmax/folda
R3b/%
1a
1b
1c
1d
1e
1f
1085 35
915 25
905 30
980 30
650 20
870 25
1085 35
905 25
885 25
980 30
660 20
860 25
98
95
95
90
98
95
1g
1h
35
60
2
3
25
55
2
3
50
N/Ab
a
These data reflect one shot measurements due to the change in
signal over time. Errors are calculated based on an average of three
measurements for
a sample containing 1a where the observed
enhancement is relatively constant thereby allowing repeat
b
measurements. No signals for the form corresponding to 3b were
Fig. 5 Structures of the eight precatalysts used in this work.
discerned.
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enhancement. In all cases, a hyperpolarised 13C pyruvate
response was observed after shaking with 3 bar p-H2. In contrast
to 1a, precatalysts 1b–1h result in lower 13C pyruvate signal
enhancements immediately after H2 addition. For some of these
precatalysts, εmax, occurs at much longer reaction times. Hence
the differing precatalysts exhibit different activation periods
and subsequently, different time points where the maximum
concentration of 3b is reached. This demonstrates how reaction
time can be an important parameter that plays a large effect on
the observed signal gain. For example, 1a activates very rapidly
and the proportion of its 3b derivative is at a maximum shortly
after the initial H2 addition step. The corresponding 13C
pyruvate signal enhancement is also maximised at this time
point. In contrast, 1c has one of the slowest rates of 3b
derivative formation and hence the corresponding
hyperpolarised pyruvate signal increases after the initial H2
addition step as a function of the growth in 3b concentration
(see ESI†). Other precatalysts, such as 1b and 1d, also form 3b
derivatives which is reflected in an initial increase in pyruvate
enhancement followed by a slow decrease as the concentration
of 3 falls over a longer timescale. The resulting 13C pyruvate
signal enhancements can therefore be used as a route to
effectively track the concentrations of 3b type products in
solution and hence monitor the conversion of 1 to 3.
Fig. 6 Partial hyperpolarised a) 13C and b) 1H NMR responses after
samples of i) 1a and ii) 1h containing 6 equivalents of sodium pyruvate-
1,2-[13C2], and 10 equivalents of II are shaken in methanol-d4 with 3 bar
p-H2 for 10 seconds in a mu metal shield. SII and h in the structures
refers to II and ligand h as shown in Fig. 3 and 5 respectively.
The phosphine based precatalyst 1g yields just 50% of the
3b type product in solution and the resulting pyruvate signal
enhancements are now just ∼30-fold. In contrast, when
precatalyst 1h, containing an asymmetric N-heterocyclic
carbene ligand is used an isomer of type 3b no longer forms.
Here, we form larger amounts of 3a which we expect to be
due to reduced steric crowding associated with the smaller
carbene ligand. Now, the hyperpolarised pyruvate signal
methanol-d4 at three different temperatures. These
temperatures (278 K, 293 K and 323 K) were achieved by
placing the NMR tube in a thermostatically controlled water
bath for 60 seconds prior to shaking for 10 seconds under
p-H2 at room temperature. Care was taken to record the NMR
measurements at similar reaction times in order to compare
these data. The effect of temperature upon the observed
pyruvate 13C signal enhancement is shown in Fig. 7a. For I,
II, VI, and VII the resulting free pyruvate 13C signal
enhancement is maximised at room temperature while
shows
a
∼60-fold enhancement which is an order of
magnitude lower than that provided by the symmetric
carbene 1a. This is consistent with the fact isomer 3b is
essential for attaining high levels of pyruvate polarisation.
Interestingly, when 3a is the more dominant species, the
hyperpolarised 13C2 pyruvate profile no longer appears in the
typical pattern diagnostic of 13C2 singlet order as created with
catalysts 1a–1g. This is also consistent with previous
theoretical modelling studies which suggest that the spin
topology of 3a is incompatible with the easy retention of p-H2
derived singlet spin order in the product.39 It is clear that 3b,
which is formed when the precatalysts 1a–f are used, reflects
an active polarisation transfer catalysts with the necessary
spin topology to mediate efficient polarisation transfer into
bound 13C2 pyruvate. Some examples of representative NMR
spectra are shown in Fig. 6.
sulfoxides III and
V perform better at the elevated
temperature. Hence, whilst ligand exchange is slow and not
detectable on the EXSY timescale, there must be an optimum
rate for each complex as reported for other N-heterocyclic
substrates.60
When this study was expanded to include solutions
of the iridium precatalysts 1a–f of Fig. 5, 6 equivalents
of sodium pyruvate-1,2-[13C2], and 10 equivalents of
methylphenylsulfoxide (II), similar temperature effects were
seen as shown in Fig. 7b. For 1a, 293 K proved optimal with
warming clearly moving away from the required exchange
rate. For 1b and 1c, the proportion of bound substrate
polarisation is much higher relative to the free material when
compared to the other precatalysts which may suggest slower
exchange. This is expected for 1b on account of the chloride
substituent which decreases electron density on the metal
Effect of temperature on pyruvate 13C2 signal enhancement
Many studies have varied the temperature to achieve a
substrate exchange rate optimum for SABRE magnetisation
transfer.34,41,60 To this end, solutions of 1a, 6 equivalents of
sodium pyruvate-1,2-[13C2], and 4 equivalents of the sulfoxides
I–III, IV–VII, shown in Fig. 3, were shaken with 3 bar p-H2 in
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that forms in solution. Further optimisation of pyruvate
signal gain is possible by subtle variation of the electronic
effects of the active catalyst.
Effect of selective deuteration on pyruvate 13C2 signal
enhancement
Relaxation within the substrate when bound to the catalyst
has been shown to limit its degree of 1H hyperpolarisation
using SABRE. This effect has been reduced by the inclusion
of deuterium labels within the active polarisation transfer
catalyst whilst simultaneously reducing any polarisation
leakage into the catalyst.31,34,61,62 Therefore, we examined the
effect of deuterating the sulfoxide ligand and the IMes
backbone of the precatalyst.
Shaking a solution of 1a with 6 equivalents of sodium
pyruvate-1,2-[13C2], 4 equivalents of I and 3 bar p-H2 in
methanol-d4 for 10 seconds in a mu metal shield yielded
averaged free and bound pyruvate 13C signal enhancements of
1070 and 200-fold respectively. When this process was repeated
using the corresponding deuterium labelled sulfoxide I-d6 at the
same time point after H2 addition these enhancements
remained comparable at 1070 and 220-fold. This suggests that
there is little polarisation leakage into this sulfoxide and that
relaxation of hyperpolarised magnetisation via 1H sites in its 3b
derivative does not limit the efficiency of SABRE.
In contrast, when the results from shaking a solution of 1a
with 6 equivalents of sodium pyruvate-1,2-[13C2], and 10
equivalents of II under 3 bar p-H2 in methanol-d4 are
compared to those achieved with 1a-d22 an effect is seen; in
this case all the protons in the NHC except the two in the
Fig. 7 Averaged hyperpolarised free (lower, darker colour) and bound
(upper, lighter colour) 13C pyruvate NMR signal enhancement values
following shaking of methanol-d4 solutions under SABRE containing a)
4
equivalents of sulfoxide I–III, V–VII with 1a and b) the iridium
precatalysts 1a–1f with 6 equivalents of sodium pyruvate-1,2-[13C2] and
10 equivalents of II at 3 bar p-H2 in a mu metal shield after 10 seconds
exposure at the specified temperatures.
2
imidazole ring are labelled with H. This is reflected in a fall
in the averaged free 13C2 signal enhancement from 1085 to
875-fold although the bound pyruvate signals remain
comparable at 515-fold and 535-fold respectively. When this
measurement is repeated using 1a-d24 these signal gains
further decrease to 675 and 465-fold for free and bound
pyruvate respectively. This suggests that 2H labelling of the
NHC is now detrimental to SABRE: a finding which is in direct
contrast to commonly observed effects when [IrIJH)2IJNHC)-
relative to 1a and is therefore likely to result in stronger
pyruvate binding and slower ligand exchange. 1c contains a
sterically demanding NHC which might be expected to
promote exchange, although this is clearly not the case. In
contrast, the metal centre in 1d is electron rich thereby
increasing exchange which agrees with the lower retained
bound pyruvate polarisation level. Of the series, 1e is the
most electron deficient with high proportions of bound
pyruvate signal and low levels of pyruvate enhancement
which increase at higher temperatures. This is consistent
with slower pyruvate exchange in the 1e system.
IJNSub)3]Cl
polarisation
transfer
catalysts
are
deuterated.31,34,61,62 We suggest this could be due to the effects
of quadrupolar relaxation at the mG polarisation transfer field
caused by the introduction of deuterium, whereas previous
studies have typically employed G polarisation transfer fields.
We conclude that variation of the NHC ligand can have a
large effect on the attained pyruvate enhancements.
Understanding these effects is challenging and we expect that
both steric and electronic effects associated with the ligands
are important, as previously suggested for SABRE with
N-heterocyclic substrates.53,55 Here though, the ligand effects
are likely to be more complex as they will not only influence
the rate of pyruvate exchange within 3, but also the rate of
interconversion between 2 and 3 and the rate of H2 exchange
in 2. Steric effects are important in determining the
concentration of the active polarisation transfer catalyst, 3b,
Further optimisation of pyruvate 13C2 signal enhancement by
varying shaking time and hydrogen pressure
The effects of the p-H2 shaking time and hydrogen pressure
on the 13C pyruvate signal enhancements were also
investigated. This involved using a sample of 1a with two
equivalents of II at 1, 2 and 3 bar of p-H2. The results
revealed that there was
a
growth in averaged
hyperpolarisation level for free pyruvate from 510 to 730-fold
upon increasing the pressure from 1 bar to 3 bar, after which
point the signal gain plateaus (see ESI†). This suggests that,
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in this case, hydrogen exchange is rate limiting at pressures
lower than 3 bar but once this pressure is exceeded it is
ligand exchange and the associated relaxation processes
within the catalyst that become limiting. We have already
investigated how hydrogen exchange within [IrClIJH)2IJIMes)-
IJDMSO)2] increases as H2 pressure is increased from 0.5–2
bar.43 We expect that the increase in pyruvate signal at these
H2 pressures is related to increased p-H2 refreshment in the
2/3 system which does not become more efficient at
pressures higher than 3 bar.
gain. This averaged gain corresponds to signal enhancements
of 2140 and 2125-fold for the 1-13C and 2-13C sites
respectively. We can therefore increase pyruvate signal gains
by two orders of magnitude (from 30-fold for catalyst 1g,
Table 2, to 2135-fold) by careful optimisation of factors
including temperature, shaking time, catalyst, sulfoxide and
their concentrations. These optimum enhancements involved
a sample containing 1a (5 mM), 10 equivalents II and 6
equivalents sodium pyruvate-1,2-[13C2] in 0.6 mL methanol-d4
that was shaken with 3 bar p-H2 for 30 seconds in a mu metal
shield. The effect of these optimisation steps on improving
the signal gain is depicted in Fig. 8. We failed to see further
increases with deuterium labelling of the sulfoxide or the
NHC ligands or by shaking with 4.5 bar p-H2. When all 13C
species in this sample, including bound pyruvate and its
hydrated form, are included a net 13C polarisation of 2845-
fold (2.3% polarisation) is achieved which exceeds those
previously reported.39 We also note that the preparations
used here are stable for a greater time period after hydrogen
addition, thereby allowing for more repeat measurements
and improved sample examination.
Higher averaged 13C2 pyruvate signal enhancements result
(1050 compared to 425-fold) when the shaking time in the
mu metal shield is extended from 5 to 30 seconds in 5
second intervals, as detailed in the ESI.† This change allows
polarisation to build up more effectively on both the bound
and free 13C sites. The observed effect on the 1H signals of
the hydride resonances in 3b is the opposite, with the visible
signal gains decreasing. This implies more signal is
transferred to the 13C centres at longer shaking times. We
confirmed that these trends are also observed for a sample of
1a-d24 with ten equivalents of II.
Conclusions
Combining optimisation steps to achieve improved pyruvate
13C2 signal enhancement
Optimising pyruvate signal gains is important for a range of
applications that may include hyperpolarised reaction
monitoring. For example, we have already demonstrated how
the reaction of sodium pyruvate with hydrogen peroxide can
be monitored using hyperpolarised 13C NMR spectroscopy.39
We have presented a detailed study that optimises the
pyruvate signal gain by variation of sulfoxide and carbene
ligand within [IrIJH)2IJη2-pyruvate)IJDMSO)IJNHC)] polarisation
transfer catalysts. We have shown that sulfoxide co-ligands
are essential for the formation of these active species and
We were able to achieve a maximum averaged pyruvate 13C
signal enhancement of 2135-fold (1.7% polarisation) for
the free material alongside a 585-fold bound pyruvate signal
the highest pyruvate signals can be achieved using
a
[IrClIJCOD)IJIMes)] precatalyst in conjunction with 10
equivalents of phenylmethylsulfoxide. Sterically large
carbenes are required if the formation of the active isomer
3b is to be favoured, although electronic effects are
important in fine tuning the ligand exchange processes.
Hyperpolarised 13C2 pyruvate signal intensities are shown to
be closely linked to the amount of the 3b isomer present in
solution, although systems containing similar amounts but
different ligands can result in very different pyruvate
enhancements. These results highlight the tension between
many different factors that influence the efficiency of
polarisation transfer within this complex. In all cases we
observe a decrease in both hyperpolarised 13C2 pyruvate level
and the 1H hydride ligand signals of the 3b type isomer at
longer reaction times which we associate with catalyst
deactivation. By combining these effects we attained an
averaged pyruvate 13C signal enhancement level of 2135-fold
(1.7% polarisation) for free sodium pyruvate-1,2-[13C2].
Fig. 8 Partial hyperpolarised 13C NMR spectra for a) keto region and b)
carbonyl region recorded after samples of i) 1g (5 mM), 10 eq. II and 6
equivalents of sodium pyruvate-1,2-[13C2] ii) 1a (5 mM), 4 eq. I and 6
equivalents of sodium pyruvate-1,2-[13C2] and iii) 1a (5 mM), 10 eq. II and
6 equivalents of sodium pyruvate-1,2-[13C2] are shaken in methanol-d4
with 3 bar p-H2 for i) and ii) 10 or iii) 30 seconds in a mu metal shield.
For biomedical applications, attaining high signal
enhancements in aqueous, rather than methanolic solvents,
is of more importance. When a solution of 1a and 10 eq. II
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are preactivated with 3 bar H2 in ethanol-d6 before adding
sodium pyruvate-1,2-[13C2] in D2O and shaking in a mu metal
shield for 30 seconds 13C signal gains are approximately an
order of magnitude lower than those achieved in methanol-
d4. In these 70/30 D2O/ethanol-d6 mixtures, which might
reflect a system suitable for future in vivo studies, the
proportion of bound pyruvate enhancement is significantly
higher than those achieved in methanol-d4. It is therefore
clear that in non-methanolic solvents pyruvate exchange is
reduced which limits the attained polarisation levels. It is
expected that the application of a similar optimisation
approach would lead to improved pyruvate enhancements in
this biocompatible solvent mixture via catalysts that exhibit
faster exchange kinetics.
The shake & drop method was employed for recording
hyperpolarised NMR spectra. NMR tubes were filled with
p-H2 at 3 bar pressure and shaken vigorously for 10 seconds
unless otherwise stated in the 65 G stray field next to a 9.4 T
1
magnet for H polarisation or in a mu metal shield (ca. 300-
fold shielding) for 13C polarisation and placing inside a 9.4 T
spectrometer for NMR detection. Pyruvate 13C enhancements
were calculated by reference to a more concentrated thermal
sample as outlined in Shchepin et al.64 In cases where
averaged 13C enhancements are given across both 1-[13C] and
[2-13C] sites the sum of these integrals is referenced to the
sum of these integrals in the corresponding thermal
measurement. Data points usually reflect an average of three
shake and drop measurements while those that monitor
signal growth over time are single point measurements with
typical errors as determined from averaged data.
In summary, while SABRE provides a cheap, simple and
reversible route to hyperpolarise pyruvate with time and cost
advantages over alternative techniques such as DNP and
PHIP-SAH, there is a limitation associated with the lower Conflicts of interest
signal enhancements delivered here. Nevertheless, the
B. J. T., W. I., and S. B. D. (and others) are inventors on a
patent application filed by the University of York related to
this work (patent no. GB1818171.9, filed 7 November 2018).
formation and behaviour of these novel polarisation transfer
catalysts and their applications to hyperpolarise pyruvate
reflect an important step forward in para-hydrogen based
hyperpolarisation. Future work is directed at gaining greater
understanding of these catalyst effects, probing the ligand
exchange processes that govern the attained polarisation
levels and optimising them for use in conjunction with
biocompatible solvents.
Acknowledgements
We thank Robin Brabham and Mark Dowsett for providing
samples of sulfoxides IX and X. Financial support from the
Wellcome Trust (Grants 092506 and 098335), the MRC (MR/
M008991/1), the EPSRC (B. J. T. studentship and Impact
Accelerator Award G0025101) and the University of York is
gratefully acknowledged.
Experimental
All NMR measurements were carried out on a 400 MHz
Bruker Avance III spectrometer using solutions at room
temperature (298 K) unless otherwise stated. Para-Hydrogen
(p-H2) was produced by passing hydrogen gas over a spin-
exchange catalyst (Fe2O3) and used for all hyperpolarisation
experiments. This method produces constant p-H2 with ca.
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