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Hyperpolarized multiple quantum coherences at ultra-low magnetic fields increase 15N parahydrogen-induced polarization
Andrey N. Pravdivtsev1, Nicolas Kempf2, Markus Plaumann3, Johannes Bernarding3, Klaus Scheffler2, Jan-Bernd Hövener1, and Kai Buckenmaier2
1SBMI, MOIN CC, UKSH, Kiel University, Kiel, Germany, 2High-Field Magnetic Resonance Center, Max Planck Institute for Biological Cybernetics, Tübingen, Germany, 3Institute for Biometrics and Medical Informatics, Otto-von-Guericke University, Magdeburg, Germany

Synopsis

We used signal amplification by reversible exchange of parahydrogen (SABRE) at low (~1 mT) and ultra-low (~1 μT, ULF) magnetic fields. We proposed and used ULF correlation spectroscopy (COSY) method to analyze PHIP spin order in real-time. Coherences up to the third-order were observed experimentally. Furthermore, we analyzed SABRE in alternating magnetic fields (alt-SABRE). We measured the evolution of 1H-15N zero-quantum coherences and have shown that they persist during field alternation and depend on the magnetic field strength. The resulting 15N-polarization in the alt-SABRE experiment was with magnetic was appoximately 30% higher.

Introduction

Parahydrogen-induced polarization (PHIP) is a fast developing, cost-efficient hyperpolarization method. Here, we study the parahydrogen (pH2) spin order distribution at low magnetic fields using a PHIP variant called signal amplification by reversible exchange (SABRE).[1] Our goal was to understand in more detail the spin distribution at low (~1mT) and ultra-low fields (~1μT, ULF). PHIP and SABRE already featured high 13C and 15N polarization levels above 20%. Ultimately we want to increase these levels making the method even more attractive for in vivo applications. We used a superconducting quantum interference device (SQUID) based ULF NMR system to observe hyperpolarization build-up and evolution for any nuclear spins in real-time.

Methods

pH2 generator. We used a liquid helium dipstick filled with iron-oxide to enrich the parahydrogen fraction of H2 close to 100%. The flow rate was set to 2–3L/h.[2] SABRE (Figure 1).[1] When the Ir-catalyst is activated in the presence of H2 and the substrate (here acetonitrile, MeCN, or 3-fluoropyridine, 3FP, Figure 1) and pH2 is used instead of normal H2, then the enriched singlet spin order of pH2 can be distributed in the active SABRE complex [Ir]. As a result, bounded substrates also get polarized. The constant exchange of labile ligands allows refreshment of spin order depleted H2 in [Ir] with fresh pH2 and exchange of polarized bounded substrate with a nonpolarized free substrate. Hence, the lifetime of [Ir] complex must be long enough for spin order distribution (polarization transfer). However, the lifetime should not be too long compared to T1 relaxation; otherwise, only a tiny amount of free substrates is polarized.[3] Therefore, the main challenge of SABRE is to enable efficient spin order transfer from pH2 to a substrate. By efficient, we usually mean a high amplitude and fast compared to the lifetime of [Ir]. SQUID NMR.[4] We used a SQUID-based NMR spectrometer that operates at B0≈54µT. The system is ideal for low and ultralow NMR spectroscopy and allows measurement of any heteronuclear simultaneously. During the entire experiment, pH2 is continuously supplied with a flow rate of ≈2.5L/h for convenience. Susceptibility effects on magnetic field homogeneity are negligible during pH2 bubbling, which is demonstrated by the spectral linewidth <0.5 Hz at B0≈54 µT. ULF COSY (Figure 2).[2] This is an adaptation of high-resolution correlation spectroscopy for low fields. It enables observation of hyperpolarized high-order quantum coherences. SABRE-SHEATH and alt-SABRE-SHEATH (Figure 3).[5] Usually, pH2 to 15N spin order transfer occurs at the magnetic field of 1uT (SABRE-SHEATH conditions). Here we used an alternating magnetic field (alt-SABRE-SHEATH).

Results

Multiple quantum coherences (MQF, Figure 4). We observed that high order spin states are generated in the low-field SABRE experiment with a polarization field Bp=5.2mT (Figure 4). Using the ULF COSY experiment, we were able first to measure high order coherences and differentiate them using a 4 step post-processing phase cycle technique. The four-step phase cycling allowed us to distinguish 2n, 2n+1, 2n+2, and 2n+3 coherences, with n an integer number. Homonuclear (TH or TF) and heteronuclear (THF) coherences from -3 to +3 were visible experimentally. 1H-15N spin order transfer (Figure 5). We alternated between two different magnetic fields: Blow~1μT and Bhigh~55μT with corresponding time intervals tlow and thigh. Polarization transfers from pH2 to 15N of a substrate was measured with the frequency vlow=119±1Hz and vhigh=2541±13Hz. Oscillations deemed at Blow on the time scale of 50ms. The resulting alt-SABRE-SHEATH 15N polarization was 30% higher than in the SABRE-SHEATH experiment with a constant magnetic field. Both experiments were reproduced using spin dynamics simulations.[2,5,6]

Discussion

Observation of multiple quantum coherences confirmed the hypothesis that high-order multi-spin states are populated at ULF. As a result, polarization is indeed distributed among many spins. The deuteration of untargeted protons was proposed.[7] This solves the problem of distributing the spin-order among numerous spins because protons and deuterons are weakly coupled at a field above 1μT. However, this strategy would not be as efficient for SABRE-SHEATH experiments, where almost all spins are strongly coupled. Traditionally SABRE-SHEATH is carried out in a constant magnetic field where all coherences are averaged out, and as a result, lower polarization for a specific nucleus is gained. Alternating magnetic fields make use of the fast oscillating coherences. Spin order starts at a lower field and then accelerates at a higher field. It is essential that the lifetime of the [Ir] complex is long compared to the polarization transfer rate. We are planning to add a temperature control unit and analyze the effect of temperature on alt-SABRE-SHEATH efficiency.

Conclusion

We were able to detect high order coherences and spins states up to third order. This is a clear demonstration of spin order distribution among coupled spins. We introduced the alt-SABRE-SHEATH approach and increased the 15N polarization of acetonitrile by 30% compared to conventional SABRE-SHEATH. This approach will be extended to other 15N labeled metabolites like nicotinamide and drugs like metronidazole. Their clinical imaging application is to be investigated; however, the lifetime of more than 1 minute at clinical 1-3T MRI and 15N-polarization above 20% looks very promising.[8]

Acknowledgements

We acknowledge funding from the German Federal Ministry of Education and Research (BMBF) within the framework of the e:Med research and funding concept (01ZX1915C), DFG (PR-1868/3-1, BU-2694/6-1, PL-576/6-1, HO-4602/2-2, HO-4602/3, GRK2154-2019, EXC2167, FOR5042, TRR287), Kiel University and the Faculty of Medicine. MOIN CC was founded by a grant from the European Regional Development Fund (ERDF) and the Zukunftsprogramm Wirtschaft of Schleswig-Holstein (Project no. 122-09-053).

References

[1] M. J. Cowley, R. W. Adams, K. D. Atkinson, M. C. R. Cockett, S. B. Duckett, G. G. R. Green, J. A. B. Lohman, R. Kerssebaum, D. Kilgour, R. E. Mewis, J. Am. Chem. Soc. 2011, 133, 6134–6137.

[2] K. Buckenmaier, K. Scheffler, M. Plaumann, P. Fehling, J. Bernarding, M. Rudolph, C. Back, D. Koelle, R. Kleiner, J.-B. Hövener, A. N. Pravdivtsev, ChemPhysChem 2019, 20, 2823–2829.

[3] D. A. Barskiy, A. N. Pravdivtsev, K. L. Ivanov, K. V. Kovtunov, I. V. Koptyug, Phys. Chem. Chem. Phys. 2016, 18, 89–93.

[4] K. Buckenmaier, M. Rudolph, P. Fehling, T. Steffen, C. Back, R. Bernard, R. Pohmann, J. Bernarding, R. Kleiner, D. Koelle, M. Plaumann, K. Scheffler, Rev. Sci. Instrum. 2018, 89, 125103.

[5] A. N. Pravdivtsev, N. Kempf, M. Plaumann, J. Bernarding, K. Scheffler, J.-B. Hövener, K. Buckenmaier, ChemPhysChem n.d., n/a, DOI 10.1002/cphc.202100543.

[6] A. N. Pravdivtsev, J.-B. Hövener, Chem. Eur. J. 2019, 25, 7659–7668.

[7] P. J. Rayner, M. J. Burns, A. M. Olaru, P. Norcott, M. Fekete, G. G. R. Green, L. A. R. Highton, R. E. Mewis, S. B. Duckett, PNAS 2017, 201620457.

[8] J. F. P. Colell, A. W. J. Logan, Z. Zhou, R. V. Shchepin, D. A. Barskiy, G. X. Ortiz, Q. Wang, S. J. Malcolmson, E. Y. Chekmenev, W. S. Warren, T. Theis, J. Phys. Chem. C 2017, 121, 6626–6634.

Figures

Figure 1. SABRE: activation and exchange. The precursor [IrIMesCODCl] (IMes = 1,3-bis(2,4,6-trimethylphenyl) imidazole-2-ylidene) of SABRE complex is activated after pH2 bubbling in the presence of substrate (here acetonitrile = MeCN). The product [IrIMesH2MeCN3]=[Ir] is an active SABRE complex. The unique feature of this kind of [Ir] is that it experiences constant H2 and MeCN3 exchanges and can gain polarization spin order from pH2, which reveals itself in signal enhancement.[1]

Figure 2. ULF COSY experiment (a) and the corresponding evolution of the quantum coherences, pn (b). We constantly add pH2 to the system. The coherence selection pathway starts from zero quantum coherences and multiplet spin orders with p0=0, a first 90o pulse converts these spin order into quantum coherences p1 for a period of time, t1, (→ frequency 1 domain) and after the second 90o pulse NMR signal with p2=-1 acquired (acq., → frequency 2 domain). Figure is adapted from Ref [2].

Figure 3. Sequence schematics. (a) Magnetic field pattern for alt-SABRE-SHEATH and (b) for SABRE-SHEATH. (c) A simple free induction decay (FID) readout consisting of a pulse excitation of 1H and 15N spins and signal acquisition at B0≈54µT. Alt-SABRE-SHEATH consists of a hyperpolarization stage of length thyp, where the magnetic field is alternated between Blow and Bhigh with duration tlow and thigh, respectively. This part is repeated n times. Figure is adapted from Ref [5].

Figure 4. Separation of quantum coherences (QCs) in the coherence selective ULF COSY experiment (Bp=5.2mT and B0=91μT): simulated (top) and experimental (bottom) amplitude spectra of 3FPy obtained using four four-step phase cycling: φ1=x,y,-x,-y, φ2= x, φrec= x, -y,-x,y (A), φrec= x, y,-x,-y (B), φrec= x, -x, x,-x (C), φrec= x, x, x, x (D). is a symbol of QCs, where b is the order of the QC and a indicates the involved nuclei. Figure is adapted from Ref [2].

Figure 5. Experimental alt-SABRE-SHEATH 15N and 1H spectra (a, b, d, e) and integral over the real part of the 15N signal (c, f) as a function of tlow at Blow = 2.6 µT (a,b,c) or as a function of thigh at Bhigh = 54 µT (d, e, f). Note the difference in the oscillation frequencies of both 15N and 1H signals at high (2541±13Hz) and low magnetic fields (119±1Hz). Figure is adapted from Ref [5].

Proc. Intl. Soc. Mag. Reson. Med. 30 (2022)
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DOI: https://doi.org/10.58530/2022/1883