Heterogeneous Microtesla SABRE Enhancement of 15 N NMR Signals

Spin Relays Enable Efficient Long-Range Heteronuclear Signal Amplification By Reversible Exchange

A systematic experimental study is reported on the polarization transfer to distant spins, which do not directly bind to the polarization transfer complexes employed in Signal Amplification By Reversible Exchange (SABRE) experiments. Both, long-range transfer to protons and long-range transfer to heteronuclei i.e. 13C and 15N are examined. Selective destruction of hyperpolarization on 1H, 13C, and 15N sites is employed, followed by their re-hyperpolarization from neighboring spins within the molecules of interest (pyridine for 1H studies and metronidazole-15N2-13C2 for 13C and 15N studies). We conclude that long-range sites can be efficiently hyperpolarized when a network of spin-½ nuclei enables relayed polarization transfer (i.e. via short-range interactions between sites). In case of proton SABRE in the milli-Tesla regime, a relay network consisting of protons only is sufficient. However, in case 13C and 15N are targeted (i.e. via SABRE in SHield Enables Alignment Transfer to Heteronuclei or SABRE-SHEATH experiment), the presence of a heteronuclear network (e.g. consisting of 15N) enables a relay mechanism that is significantly more efficient than the direct transfer of spin order from para-H2-derived hydrides.

Imaging of Biomolecular NMR Signals Amplified by Reversible Exchange with Parahydrogen Inside an MRI Scanner

The Signal Amplification by Reversible Exchange (SABRE) technique employs exchange with singlet-state parahydrogen to efficiently generate high levels of nuclear spin polarization. Spontaneous SABRE has been shown previously to be efficient in the milli-Tesla and micro-Tesla regimes. We have recently demonstrated that high-field SABRE is also possible, where proton sites of molecules that are able to reversibly coordinate to a metal center can be hyperpolarized directly within high-field magnets, potentially offering the convenience of in situ hyperpolarization-based spectroscopy and imaging without sample shuttling. Here, we show efficient polarization transfer from parahydrogen (para-H₂) to the ¹⁵N atoms of imidazole-¹⁵N₂ and nicotinamide-¹⁵N achieved via high-field SABRE (HF-SABRE). Spontaneous transfer of spin order from the para-H₂ protons to ¹⁵N atoms at the high magnetic field of an MRI scanner allows one not only to record enhanced ¹⁵N NMR spectra of in situ hyperpolarized biomolecules, but also to perform imaging using conventional MRI sequences. 2D ¹⁵N MRI of high-field SABRE-hyperpolarized imidazole with spatial resolution of 0.3×0.3 mm² at 9.4 T magnetic field and a high signal-to-noise ratio (SNR) of ~99 was demonstrated. We show that ¹H MRI of in situ HF-SABRE hyperpolarized biomolecules (e.g. imidazole-¹⁵N₂) is also feasible. Taken together, these results show that heteronuclear (¹⁵N) and ¹H spectroscopic detection and imaging of high-field-SABRE-hyperpolarized molecules are promising tools for a number of emerging applications.