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., C-13 and N-15, are examined. Selective destruction of hyperpolarization on H-1, C-13, and N-15 sites is employed, followed by their rehyperpolarization from neighboring spins within the molecules of interest (pyridine for H-1 studies and metronidazole-N-15(2)-C-13(2), for C-13 and N-15 studies). We conclude that long-range sites can be efficiently hyperpolarized when a network of spin-1/2 nuclei enables relayed polarization transfer (i.e., via short-range interactions between sites). In the case of proton SABRE in the millitesla regime, a relay network consisting of protons only is sufficient. However, in case C-13 and N-15 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 N-15) enables a relay mechanism that is significantly more efficient than the direct transfer of spin order from para-H-2-derived hydrides.