(15)N Hyperpolarization of Imidazole-(15)N2 for Magnetic Resonance pH Sensing via SABRE-SHEATH.
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(15)N nuclear spins of imidazole-(15)N2 were hyperpolarized using NMR signal amplification by reversible exchange in shield enables alignment transfer to heteronuclei (SABRE-SHEATH). A (15)N NMR signal enhancement of ∼2000-fold at 9.4 T is reported using parahydrogen gas (∼50% para-) and ∼0.1 M imidazole-(15)N2 in methanol:aqueous buffer (∼1:1). Proton binding to a (15)N site of imidazole occurs at physiological pH (pKa ∼ 7.0), and the binding event changes the (15)N isotropic chemical shift by ∼30 ppm. These properties are ideal for in vivo pH sensing. Additionally, imidazoles have low toxicity and are readily incorporated into a wide range of biomolecules. (15)N-Imidazole SABRE-SHEATH hyperpolarization potentially enables pH sensing on scales ranging from peptide and protein molecules to living organisms.