Exchange facilitated indirect detection of hyperpolarized 15ND2-amido-glutamine

Hyperpolarization greatly enhances opportunities to observe in vivo metabolic processes in real time. Accessible timescales are, however, limited by nuclear spin relaxation times, and sensitivity is limited by magnetogyric ratios of observed nuclei. The majority of applications to date have involved direct ¹³C observation of metabolites with non-protonated carbons at sites of interest (¹³C enriched carbonyls, for example), a choice that extends relaxation times and yields moderate sensitivity. Interest in ¹⁵N containing metabolites is equally high but non-protonated sites are rare and direct ¹⁵N observation insensitive. Here an approach is demonstrated that extends applications to protonated ¹⁵N sites with high sensitivity. The normally short relaxation times are lengthened by initially replacing protons (H) with deuterons (D) and low sensitivity detection of ¹⁵N is avoided by indirect detection through protons reintroduced by H/D exchange. A pulse sequence is presented that periodically samples ¹⁵N polarization at newly protonated sites by INEPT transfer to protons while returning ¹⁵N magnetization of deuterated sites to the +Z axis to preserve polarization for subsequent samplings. Applications to ¹⁵ND₂-amido-glutamine are chosen for illustration. Glutamine is an important regulator and a direct donor of nitrogen in cellular metabolism. Potential application to in vivo observation is discussed.