Adiabatic approach for heteronuclear SABRE hyperpolarization at high magnetic field

Signal Amplification By Reversible Exchange (SABRE) is a technique aimed at enhancing weak NMR signals of heteronuclei by utilizing the non-equilibrium spin order of parahydrogen. SABRE polarization transfer takes place by means of metalorganic complexes that interact with parahydrogen and the substrate to be polarized in a reversible manner. To achieve substrate hyperpolarization in the high magnetic field of an NMR magnet, radiofrequency (RF) excitation is required. There are two general options for the RF field amplitude: constant or modulated. To date, there has been limited optimization of the adiabatic SABRE conditions. In SABRE, the presence of chemical exchange significantly complicates the spin dynamics involved in polarization transfer and the optimization of adiabatic RF sweeps. We conducted a comprehensive analysis of high-field SABRE pulse sequences with RF sweeps on the heteronuclear channel, specifically ¹⁵N. We proposed a simple method for optimizing the amplitude modulation profile of the RF field, which is efficient for systems undergoing chemical exchange. Our approach involved utilizing the dependence of ¹⁵N polarization on the amplitude of the constant RF field on the ¹⁵N channel. By employing the "optimal" adiabatic RF profile, we achieved a 2.5-fold increase in ¹⁵N SABRE-derived polarization at high magnetic field compared to a linear sweep. We theoretically assessed the benefit of RF sweeps over constant RF fields for SABRE at high magnetic field. We demonstrated experimentally that at temperatures $-5^{\circ }$C - $+{10}^{\circ }$C RF sweeps are more efficient than constant RF field. Maximal increase in ¹⁵N polarization achieved was 1.7-fold for bound and 1.4-fold for free substrate. We attribute this increase in polarization to the adiabaticity of the polarization transfer process. This behavior was explained via numerical solution of SABRE master equation for different dissociation rate constants.