Nuclear spin hyperpolarization of the solvent using signal amplification by reversible exchange (SABRE)

Here we report the polarization of the solvent OH protons by SABRE using standard iridium-based catalysts under slightly acidic conditions. Solvent polarization was observed in the presence of a variety of structurally similar N-donor substrates while no solvent enhancement was observed in the absence of substrate or para-hydrogen (p-H-2). Solvent polarization was sensitive to the polarizing field and catalyst: substrate ratio in a manner similar to that of substrate protons. SABRE experiments with pyridine-d(5) suggest a mechanism where hyperpolarization is transferred from the free substrate to the solvent by chemical exchange while measured hyperpolarization decay times suggest a complimentary mechanism which occurs by direct coordination of the solvent to the catalytic complex. We found the solvent hyperpolarization to decay nearly 3 times more slowly than its characteristic spin-lattice relaxation time suggesting that the hyperpolarized state of the solvent may be sufficiently long lived (similar to 20 s) to hyperpolarize biomolecules having exchangeable protons. This route may offer future opportunities for SABRE to impact metabolic imaging. (C) 2015 Elsevier Inc. All rights reserved.