Efficient conversion of anti-phase spin order of protons into ¹⁵N magnetisation using SLIC-SABRE

SABRE (Signal Amplification By Reversible Exchange) is a technique for enhancement of NMR (Nuclear Magnetic Resonance) signals, which utilises parahydrogen (pH ₂, the H ₂ molecule in its nuclear singlet spin state) as a source of non-thermal spin order. In SABRE experiments, pH ₂ binds transiently to an organometallic complex with a to-be-polarised substrate; subsequently, spin order transfer takes place and the substrate acquires non-thermal spin polarisation resulting in strong NMR signal enhancement. In this work, we argue that the spin order of H ₂ in SABRE experiments performed at high magnetic fields is not necessarily the singlet order but rather anti-phase polarisation, S1zS2z. Although SABRE exploits pH ₂, i.e. the starting spin order of H ₂ is supposed to be the singlet order, in solution, S−T ₀ conversion becomes efficient once pH ₂ binds to a complex. Such a variation of the spin order, which becomes S1zS2z, has an important consequence: NMR methods used for transferring SABRE polarisation need to be modified. Here we demonstrate that methods proposed for the initial singlet order may not work for the S1zS2z order; however, a simple modification makes them efficient again.