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

ABSTRACT

SABRE (Signal Amplification By Reversible Exchange) is a technique for enhancement of NMR (Nuclear Magnetic Resonance) signals, which utilises parahydrogen (pH₂, the H₂ molecule in its nuclear singlet spin state) as a source of non-thermal spin order. In SABRE experiments, pH₂ 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 H₂ in SABRE experiments performed at high magnetic fields is not necessarily the singlet order but rather anti-phase polarisation, ${\text{S}}_{1\text{z}}{\text{S}}_{2\text{z}}$. Although SABRE exploits pH₂, i.e. the starting spin order of H₂ is supposed to be the singlet order, in solution, S−T₀ conversion becomes efficient once pH₂ binds to a complex. Such a variation of the spin order, which becomes ${\text{S}}_{1\text{z}}{\text{S}}_{2\text{z}}$, 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 ${\text{S}}_{1\text{z}}{\text{S}}_{2\text{z}}$ order; however, a simple modification makes them efficient again.

GRAPHICAL ABSTRACT

KEYWORDS:

• Spin hyperpolarisation
• parahydrogen
• SABRE method
• spin order
• polarisation transfer

Acknowledgements

We are thankful to FASO of RF (project 0333-2017-0002) for providing access to NMR facilities. We acknowledge Prof. Warren S. Warren (Duke University) for useful discussions.

Disclosure statement

No potential conflict of interest was reported by the authors.

Notes

1. An additional selective pulse can be used to recover any remaining transversal polarisation of the bound substrate but this was not attempted in the study presented here.

Additional information

Funding

This work has been supported by the Russian Science Foundation (grant No. 14-13-01053); S.K. acknowledges Deutscher Akademischer Austauschdienst (DAAD) for a fellowship for coming to the ITC (Novosibirsk, Russia) and the Emmy Noether program of the Deutsche Forschungsgemeinschaft (DFG) (HO 4604/2-1) for funding his PhD studies when in Germany.