Relaxation Dynamics of Nuclear Long-Lived Spin States in Propane and Propane-d6 Hyperpolarized by Parahydrogen

We report a systematic study of relaxation dynamics of hyperpolarized (HP) propane and HP propane- d 6 prepared by heterogeneous pairwise parahydrogen (pH 2 ) addition to propylene and propylene- d 6, respectively. Long-lived spin states (LLSs) created for these molecules at a low magnetic field of 0.0475 T were employed for this study. The parahydrogen-induced overpopulation of a HP propane LLS decays exponentially with a time constant ( T LLS ) approximately threefold greater than the corresponding T 1 values. Both T LLS and T 1 increase linearly with propane pressure in the range of 1 atm (the most biomedically relevant conditions for pulmonary MRI) to 5 atm. The T LLS value of HP propane gas at 1 atm is ∼3 s. Deuteration of the substrate (propylene- d 6 ) yields HP propane- d 6 gas with T LLS values approximately 20% shorter than those of HP fully protonated propane gas, indicating that deuteration does not benefit the lifetime of the LLS HP state. The use of pH 2 or Xe/N 2 buffering gas during heterogeneous hydrogenation reaction [leading to production of pure HP propane (100:0, i.e., no buffering gas) versus a 43:57 mixture of HP propane gas with buffering gas] results in (i) no significant changes in T 1, (ii) decrease of T LLS values (by 35 ± 7 and 8 ± 7%, respectively); and (iii) an increase of the polarization levels of HP propane gas with a propane concentration decrease (by 1.6 ± 0.1-fold and 1.4 ± 0.1-fold, respectively, despite the decrease in T LLS, which leads to disproportionately greater polarization losses during HP gas transport). Moreover, we demonstrate the feasibility of HP propane cryocollection (which can be potentially useful for preparing larger amounts of concentrated HP propane, when buffering gas is employed), and T LLS of liquefied HP propane reaches 14.7 s, which is greater than the T LLS value of HP propane gas at any pressure studied. Finally, we have explored the utility of using a partial spin-lock induced crossing (SLIC) radio-frequency (RF) pulse sequence for converting the overpopulated LLS into observable 1 H nuclear magnetization at a low magnetic field. We find that (i) the bulk of the overpopulated LLS is retained even when the optimal or near-optimal values of SLIC pulse duration are employed, and (ii) the overpopulated LLS of propane is also relatively immune to strong RF pulses, thereby indicating that LLS is highly suitable as a spin-polarization reservoir in the context of NMR/MRI detection applications. The presented findings may be useful for improving the levels of polarization of HP propane produced by HET-PHIP via the use of an inert buffer gas; increasing the lifetime of the HP state during preparation and storage; and developing efficient approaches for ultrafast MR imaging of HP propane in the context of biomedical applications of HP propane gas, including its potential use as an inhalable contrast agent.