Improved SABRE hyperpolarisation using pulse sequences to reduce effective coupling

Abstract

Hyperpolarisation using Signal Amplification By Reversible Exchange (SABRE) is a convenient method for high repeatability studies. The core of this technique is polarisation transfer to the target substrate during an on-going chemical exchange process. Typically, polarisation transfer is achieved as fast as possible. In this study we employ NMR sequences that on contrary slow down the polarisation transfer and yet demonstrate improved performance. Simulations confirm that such methods can lead to high polarisation yield in SABRE system that exhibit higher magnetic inequivalence and lower chemical exchange rate.

Figures (6)

• Figure 1: The ${}^{15}\mathrm{N}$ spectra of hyperpolarised ${}^{15}\mathrm{N}$-acetonitrile recorded after four different SABRE protocols. Polarisation levels of the free substrate species are provided above each spectra. Deviation is estimated by repeating experiments four times. See subsection \ref{['Sec:protocols']} for more experimental details.
• Figure 2: General scheme of the SABRE protocol. First, the sample is moved from the NMR magnet to a low magnetic field. Then, parahydrogen bubbling is initiated, followed by the polarisation transfer pulse sequence. Once sequence is finished, bubbling is stopped and sample is moved back to the magnet for recording of the NMR spectra. The inset shows the active SABRE complex between parahydrogen and ${}^{15}\mathrm{N}$-acetonitrile. Relevant J-couplings are indicated in grey.mewisStrategiesHyperpolarizationAcetonitrile2015 Letters "L" indicate other possible competing ligands, such as pyridine.
• Figure 3: Low-field polarisation transfer sequences applied as depicted in Fig. \ref{['fgr:SABRE_intro']} for hyperpolarisation of ${}^{15}\mathrm{N}$ compounds at low magnetic field. a) SHEATH involves ramping the magnetic field down to approximately 400 nT for the duration of the bubbling. b) Spin-Lock Induced Crossing (SLIC) is performed at fixed magnetic field at 98 $\mu$T by applying transverse field $B_1$ on ${}^{15}\mathrm{N}$ resonance with amplitude of $\sim$16 Hz. Final adiabatic pulse is applied to flip ${}^{15}\mathrm{N}$ magnetization along the bias field axis before sample transportation. c) PulsePol method is performed at 1 mT bias field. Filled and unfilled shapes represent 90$^\circ$ and 180$^\circ$ shaped pulses, respectively with Gaussian shape truncated at 10%. Number of repetitions ($N$) is set to match bubbling time $T_{\mathrm{SABRE}}$. Duration $\tau$ and phase $\varphi$ was adjusted for optimal polarisation transfer. See more details in the text. d) Double-radio-frequency SLIC (DRF-SLIC) protocol uses on-resonance transverse field for ${}^{1}\mathrm{H}$ and off-resonant field for ${}^{15}\mathrm{N}$ nuclei. The resonance mismatch is adjusted for optimal polarization transfer. Final adiabatic pulse is applied to flip ${}^{15}\mathrm{N}$ magnetisation along the bias field axis before sample transportation.
• Figure 4: Numerical simulation of different singlet-to-magnetisation transfer methods in the active ${}^{15}\mathrm{N}$-acetonitrile SABRE complex (depicted in Fig. \ref{['fgr:SABRE_intro']}. a) Simulation of SHEATH transfer at 200 nT and 400 nT magnetic fields are given in grey and black, respectively. b) Polarisation transfer using ${}^{15}\mathrm{N}$ on-resonance field depicted in grey and black for amplitudes 7.7 Hz and 16 Hz. c) Numerical simulation of ${}^{15}\mathrm{N}$ magnetisation for each PulsePol cycle at different phase $\varphi$ and duration $\tau$ settings and pulse amplitude set to 600 Hz. Sinusoidal curve is provided as a guide. d) Simulation of polarisation transfer trajectories during DRF-SLIC method are calculated according the mathematical relations given below. Simulations are given for three different effective angles $\theta$.
• Figure 5: Numerically optimised SABRE performance for hyperpolarisation of ${}^{15}\mathrm{N}$-acetonitrile as a function of exchange rate using different polarisation transfer methods: SHEATH (red line), SLIC (orange line), PulsePol (black line) and DRF-SLIC (blue line). Simulation assumes a rapid parahydrogen exchange and slow dissociation of a single ${}^{15}\mathrm{N}$-molecule. Polarisation is evaluated after 20 s of its build-up.