Hermkens, N.K.J., et al., High field hyperpolarization-EXSY experiment for fast determination of dissociation rates in SABRE complexes. J. Magn. Reson., 2017. 276: p. 122-127.
SABRE (Signal Amplification By Reversible Exchange) is a nuclear spin hyperpolarization technique based on the reversible concurrent binding of small molecules and para-hydrogen (p-H2) to an iridium metal complex in solution. At low magnetic field, spontaneous conversion of p-H2 spin order to enhanced longitudinal magnetization of the nuclear spins of the other ligands occurs. Subsequent complex dissociation results in hyperpolarized substrate molecules in solution. The lifetime of this complex plays a crucial role in attained SABRE NMR signal enhancements. Depending on the ligands, vastly different dissociation rates have been previously measured using EXSY or selective inversion experiments. However, both these approaches are generally time-consuming due to the long recycle delays (up to 2 min) necessary to reach thermal equilibrium for the nuclear spins of interest. In the cases of dilute solutions, signal averaging aggravates the problem, further extending the experimental time. Here, a new approach is proposed based on coherent hyperpolarization transfer to substrate protons in asymmetric complexes at high magnetic field. We have previously shown that such asymmetric complexes are important for application of SABRE to dilute substrates. Our results demonstrate that a series of high sensitivity EXSY spectra can be collected in a short experimental time thanks to the NMR signal enhancement and much shorter recycle delay.