Hyperpolarization of Pyridine ¹⁵N-Oxide Molecular Probes Enabled by Parahydrogen

Magnetic resonance (MR) is a powerful non-invasive technique for probing structural, functional, and metabolic processes with high spatial and temporal resolution. However, its inherently low sensitivity restricts broader applications. The use of hyperpolarized contrast agents has thus, emerged as an attractive approach to overcome this limitation and expand the capabilities. Among the available hyperpolarization techniques, parahydrogen-induced polarization (PHIP) provides a rapid and cost-efficient means to enhance magnetic resonance signals substantially. Yet, direct hyperpolarization of biomolecules, metabolites, or pharmaceuticals in vivo remains challenging, necessitating the development of versatile molecular tags and probes for hyperpolarized magnetic resonance (HP-MR). In particular, imparting specific sensing functions-such as pH responsiveness and enzyme activity detection-to these HP molecular tags is of growing importance. Herein, we introduce pyridine N-oxides as hyperpolarizable molecular tags and present [^{1 5}N, D]-labeled 2-alkenylpyridine N-oxides as highly efficient candidates for HP-MR with up to 47% ¹⁵N spin polarization. This performance opens pathways for broad potential in biomedical and preclinical HP-MR applications. The systems feature long ^{1 5}N spin-lattice relaxation times (up to T₁ = 477 s), broad functional-group compatibility, and excellent structural tunability. Their practical utility is exemplified by pH and H₂O₂ sensing and monitoring enzymatic reactions in water.