RF-SABRE: A Way to Continuous Spin Hyperpolarization at High

Article pubs.acs.org/JPCB

RF-SABRE: A Way to Continuous Spin Hyperpolarization at High Magnetic Fields Andrey N. Pravdivtsev,*,†,‡ Alexandra V. Yurkovskaya,†,‡ Hans-Martin Vieth,§ and Konstantin L. Ivanov*,†,‡ †

International Tomography Center, Siberian Branch of the Russian Academy of Science, Institutskaya 3a, Novosibirsk, 630090, Russia Novosibirsk State University, Pirogova 2, Novosibirsk, 630090, Russia § Institut für Experimentalphysik, Freie Universität of Berlin, Arnimallee 14, Berlin, 14195, Germany ‡

S Supporting Information *

ABSTRACT: A new technique is developed that allows one to carry out the signal amplification by reversible exchange (SABRE) experiments at high magnetic field. SABRE is a hyperpolarization method, which utilizes transfer of spin order from parahydrogen to the spins of a substrate in transient iridium complexes. Previously, it has been thought that such a transfer of spin order is only efficient at low magnetic fields, notably, at level anti-crossing (LAC) regions. Here it is demonstrated that LAC conditions can also be fulfilled at high fields under the action of a RF field. The highfield RF-SABRE experiment can be implemented using commercially available nuclear magnetic resonance (NMR) and magnetic resonance imaging (MRI) machines and does not require technically demanding field-cycling. The achievable NMR enhancements are around 100 for several substrates as compared to their NMR signals at thermal equilibrium conditions at 4.7 T. The frequency dependence of RFSABRE is comprised of well pronounced peaks and dips, whose position and amplitude are conditioned solely by the magnetic resonance parameters such as chemical shifts and scalar coupling of the spin system involved in the polarization transfer and by the amplitude of the RF field. Thus, the proposed method can serve as a new sensitive tool for probing transient complexes. Simulations of the dependence of magnetization transfer (i.e., NMR signal amplifications) on the frequency and amplitude of the RF field are in good agreement with the developed theoretical approach. Furthermore, the method enables continuous re-hyperpolarization of the SABRE substrate over a long period of time, giving a straightforward way to repetitive NMR experiments.

I. INTRODUCTION Signal amplification by reversible exchange (SABRE) is a relatively new and rapidly developing hyperpolarization method.1−14 In a SABRE experiment, a substrate molecule is spin-polarized in a transient complex with para-hydrogen, p-H2, which is the H2 molecule in its singlet spin state, due to a process of spin order transfer; see Scheme 1. The SABRE technique belongs to the family of hyperpolarization methods aimed at increasing the low inherent sensitivity of nuclear magnetic resonance (NMR). In general, NMR techniques, despite their versatility and usefulness, suffer from notoriously low sensitivity, which curbs the range of potential applications. The sensitivity issue is coming from low polarization of nuclear spins at thermal equilibrium, which is usually of the order of 10−4; thus, an efficient method to tackle this problem is using hyperpolarized nuclear spins, i.e., spins shifted far off thermal equilibrium. SABRE is a hyperpolarization technique, which enables strong spin polarization of various molecules and NMR signal enhancements up to several thousands. A further advantage of the SABRE method, recently demonstrated by Hövener et al.,15,16 is that this technique allows continuous rehyperpolarization of the target substrate molecule: after the © 2015 American Chemical Society

strong nonthermal spin order is destroyed by an NMR measurement, under constant supply of p-H2, it is quickly restored within a few seconds. Moreover, successive measurements do not exhibit any significant loss of signal over a few hundreds of acquisitions. This property makes feasible measurements with signal accumulation; this is in contrast with the standard para-hydrogen induced polarization (PHIP) experiment,17 in which the substrate is chemically modified and cannot be repolarized. It has been shown7−11 that the polarization transfer process among spins, which is in the heart of the SABRE method, is efficient at low fields where the nuclear spins are coupled strongly, meaning that the difference in their Zeeman interactions with the field is smaller than the coupling strength. Specifically, it is established7−11 that the SABRE method works efficiently (i.e., produced enhancements from a few 100 up to several 1000 as compared with thermal polarization at a high Special Issue: Wolfgang Lubitz Festschrift Received: March 30, 2015 Revised: May 13, 2015 Published: May 13, 2015 13619

DOI: 10.1021/acs.jpcb.5b03032 J. Phys. Chem. B 2015, 119, 13619−13629

Article

The Journal of Physical Chemistry B

is an important step in developing high-field SABRE, are also discussed, as well as factors limiting the presently achieved NMR signal enhancement.

Scheme 1. SABRE Formation in the Complex of the Ir Based Catalyst with para-Hydrogen and Pyridine (Py)a

II. METHODS A. Sample Preparation. As SABRE substrates, we used pyridine (Py), 4,4′-bipyridine, 2,2′-bipyrazine, and 3-methyl1H-pyrazole; see the structures of the substrates in Chart 1. We Chart 1. Chemical Structures of Substrates Useda

a

Here, L = IMes (1,3-bis(2,4,6-trimethylphenyl imidazole-2-ylidene)) or L = PCy3 (tricyclohexyl phosphine). Transfer of the initial singlet spin order (represented by two anti-parallel arrows) of the parahydrogen molecule entering the complex results in polarization of Py (represented by “down” arrows near the ortho protons); at the same time, the H2 molecule goes to its ortho-state (shown by “up” arrows). Here, for simplicity, polarization transfer to the equatorial Py and exchange of the equatorial Py with free Py in solution are schematically depicted; in real SABRE complexes, the axial Py ligand is also involved in polarization transfer and SABRE formation.

field of 1−10 T) only at low magnetic fields, specifically,5,11 only within the regions of nuclear spin energy level anticrossings (LACs) of the SABRE complex. Typically, such LACs occur at low magnetic fields, below 20 mT;8,9,18 in contrast, spontaneous high-field SABRE19 is based on polarization transfer by cross-relaxation, which is by far less efficient. Since in modern NMR signal detection is usually done at high magnetic field where spectral resolution is the best, the standard SABRE experiment requires field-cycling between a low polarization field and the high detection field. During the field-cycling stage, depending on the lifetime of hyperpolarization, there is an inevitable loss of polarization; furthermore, doing experiments with field-cycling is technically demanding and requires using additional devices for switching the external magnetic field. However, in recent works, it has been demonstrated that one can use RF fields to fulfill LAC conditions at a high magnetic field of an NMR spectrometer or magnetic resonance imaging (MRI) scanner.20,21 Moreover, in a very recent work,4 we have shown that such high-field LACs enable SABRE experiments at high magnetic fields. This version of the SABRE technique, RF-SABRE, enables fast transfer of spin order (typically, within 1 s) and provides significant NMR signal enhancements. The high-field SABRE technique is based on using a RF field with carefully set amplitude and frequency: this enables modeling the low-field conditions at high magnetic fields. Thus, RF-SABRE4 gives a way to circumvent problems originating from the need of doing field-cycling by performing SABRE experiments directly at the high magnetic field of an NMR spectrometer. After this first showcasing example, we are going to describe here the method in more detail and demonstrate that it works efficiently for a wide range of SABRE substrates. In the article, we (i) explain in detail the physical principles behind RF-SABRE, (ii) present experimental results for highfield SABRE under LAC conditions for four different substrates and two iridium-based SABRE complexes, and (iii) demonstrate that RF-SABRE also allows one to generate hyperpolarization in a continuous way with high efficiency. Advantages and potential applications of the method, which

a

Assignments of the proton positions and absolute values of the achieved NMR signal enhancements in comparison with the NMR signal at 4.7 T are also specified. The structure of pre-catalyst ligands, IMes and PCy3, is also shown.

used two types of SABRE precatalysts: IrIMesCODCl (IMes = 1,3-bis(2,4,6-trimethylphenyl) imidazole-2-ylidene, COD = cyclooctadiene)9 and Crabtree’s catalyst, IrP(C6H11)3PyCOD]PF6.22 In experiments with Py, two different samples were used. Sample 1 contained 70 mM Py and 4 mM IrIMes methanol-d4 (MeOD). Sample 2 contained 70 mM Py and 4 mM Crabtree’s catalyst in MeOD. In experiments with the other substrates, we always used the IrIMesCODCl precatalyst and prepared the following samples: 20 mM 4,4′-bipyridine and 4 mM IrIMesCODCl in MeOD; 6 mM 2,2′-bipyrazine and 4 mM IrIMesCODCl in a mixture of 80% MeOD and 20% DMSO; 60 mM 3-methyl-1H-pyrazole and 4 mM IrIMesCODCl in MeOD. Crabtree’s catalyst was purchased from ABCR GmbH; IrIMesCODCl was synthesized using a procedure described before;23 methanol-d4 was purchased from Deutero GmbH; all other compounds were purchased from SigmaAldrich. Samples were prepared without additional purification. para-Hydrogen was enriched using a Bruker para-hydrogen generator yielding a content of 92% p-H2 and stored in a gasbottle prior to the experiments. 13620

DOI: 10.1021/acs.jpcb.5b03032 J. Phys. Chem. B 2015, 119, 13619−13629

Article

The Journal of Physical Chemistry B

Experiments were carried out at three different RF field amplitudes equal to 28.6, 20.3, and 11.4 kHz for the two iridium complexes and Py as substrate. For the three other substrates, we used the highest amplitude of v1 = 28.6 kHz. Here, only the parameters of the RF field were optimized to get the highest enhancement factors, but we did not optimize the concentration of substrates and iridium complexes nor the temperature to reach higher enhancement factors. NMR enhancement factors were measured in the following way. Thermal polarization values were obtained from NMR spectra measured prior to bubbling the sample by para-H2. Then, the integrated lines of the substrate signals in RF-SABRE were divided by the corresponding thermal signals. To obtain the enhancement factor for dissolved dihydrogen and dihydride at the active complex, we ran the SABRE experiments as described above; then, after a sufficiently long waiting period in order to let the spins relax to equilibrium, we acquired a thermal NMR spectrum. After that, the enhancement factor for dihydrogen was obtained as the ratio of the integrated signals in the RF-SABRE spectrum and in the thermal NMR spectrum. The highest enhancement factors achieved for the substrate protons are shown in Chart 1. Hereafter we also use the following abbreviations for different species: aPy and ePy for the axial and equatorial pyridine ligands, respectively, of the active complex; fPy for free pyridine in the solution; H2 is dihydrogen in the solvent bulk; cH2 is dihydride at the active complex.

B. Experiments. Experiments were run at a 200 MHz NMR spectrometer (magnetic field B0 = 4.7 T) at 23 °C. To observe the high-field SABRE effects, we used a protocol (shown in Scheme 2) with the following stages: Stage 1: waiting period of Scheme 2. Scheme of a Continuous RF-SABRE Experimenta

a (stage 1) Waiting during the time τ1 to allow all hyperpolarization that remains from the previous RF-SABRE cycle to relax; (stage 2) bubbling by p-H2 during the time period τb; (stage 3) application RF pulse with amplitude v1 and frequency vrf during the time period τrf; (stage 4) measuring the free induction decay (FID). The total time of one RF-SABRE experiment is τe. The experiment can be repeated several hundreds of times for a single sample.

a duration of τ1 (usually 20 s) required for the spins to relax to thermal equilibrium at the field B0. Stage 2: bubbling the H2 gas enriched in its para-component (92% p-H2) through the solution. During this period, the dihydrogen gas is efficiently dissolved in the sample. After the bubbling (duration 9.5 s), we stop the process and remove all remaining bubbles from the sample, as it is important for the homogeneity of B0. Altogether, the stage takes the time τb (here, 10 s). Stage 3: applying RF irradiation for a variable time τrf (varied in the range 0.1−7 s). Stage 4: detection of the transverse magnetization formed due to the high-field SABRE effect immediately after switching off the RF field (i.e., without applying any detecting pulses). The total duration of the experiment is τe. The experiment can be repeated several hundreds of times for a single sample (see below). The duration of RF excitation (the spin-locking period) is long enough that complex formation and dissociation can frequently occur; τrf is smaller than the relaxation time of the substrate protons. We checked that the polarization level gets saturated for our substrates at τrf ≈ 5 s (while for the relaxationbased mechanism the saturation occurs only after about 100 s of bubbling19). To bubble the sample by p-H2, we used a thin plastic capillary that is inserted in the NMR tube allowing us to bubble the sample in the NMR spectrometer without loss of homogeneity. For stopping the bubbling, we (1) close the pH2 source by a digitally controlled gas valve and then (2) open another valve to ambient pressure to allow p-H2 overpressure in the gas line to vanish. The gas system used allows us to control the experiment by a computer and to start the NMR measurement 0.5 s after the stop of p-H2 supply. In order to optimize the polarization of dihydrogen, which has a shorter relaxation time than the substrates, it is required to use a short duration of the spin-locking period (τrf = 0.7 s). When the RF field amplitude ν1 and frequency νrf are set properly (see the Theory section), the substrate and dihydrogen acquire observable hyperpolarization. Transverse spin magnetization (free induction) is observable immediately after switching off the RF field. By taking the Fourier transform of the free induction decay, we obtained NMR spectra with enhanced lines. If instead of transverse magnetization longitudinal magnetization is desired, it is simple to generate it by applying a proper pulse 90° out of phase with respect to the spin-locking RF field.

III. THEORY A. Calculation Method. To model the RF-SABRE hyperpolarization on the quantitative level, we used a theoretical approach, similar to the one developed earlier.21 We describe polarization transfer in a system, which consists of two spins originating from para-H2 and N protons of the substrate: N = 3 for 2,2′-bipyrazine, N = 4 for 4,4′-bipyridine, and N = 5 for pyridine and 3-methyl-1H-pyrazole. In the case of Crabtree’s catalyst, also the spin of the 31P nucleus was included in the calculation. Thus, the total number of spins that we treat is N + 2 in the case of IrIMes and N + 3 in the case of Crabtree’s catalyst. The spin system at high magnetic field, B0, in the presence of the RF field is described by the following Hamiltonian in the rotating reference frame: N+2

N+2

Ĥ rf = − ∑ (να − νrf )Iα̂ z + α=1

N+2

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DOI: 10.1021/acs.jpcb.5b03032 J. Phys. Chem. B 2015, 119, 13619−13629