15N4-1,2,4,5-tetrazines as potential molecular tags: Integrating bioorthogonal chemistry with hyperpolarization and unearthing para-N2.
Hyperpolarized magnetic resonance (HP-MR) is a powerful, sensitive, and noninvasive approach to visualize molecular structure, function, and dynamics in vitro and in vivo. Current applications of HP-MR mostly rely on hyperpolarization of target compounds in dedicated hyperpolarizers because biomolecules can typically not be hyperpolarized directly in vivo. The injected hyperpolarized probes often undergo multiple metabolic pathways in living systems, and it remains challenging to localize and identify specific targets with high chemical selectivity. To address these current limitations in HP-MR, we report a novel hyperpolarization tagging strategy that integrates bioorthogonal chemistry and hyperpolarization to achieve the specific hyperpolarization of targets. This strategy is demonstrated by studies of hyperpolarized 15N4-1,2,4,5-tetrazines, which undergo rapid and selective cycloaddition with cyclooctyne to provide hyperpolarized 15N2-containing cycloaddition products and hyperpolarized 15N2 gas. This work not only suggests great potential of 15N4-1,2,4,5-tetrazines as molecular tags in HP-MR imaging (HP-MRI) but also supports the production of hyperpolarized para-15N2 gas, a biologically and medically innocuous gas with great potential for HP-MRI. This bioorthogonal reaction-based hyperpolarization tagging strategy enables a new class of in vitro and in vivo applications.