Rapid 13 C Hyperpolarization of the TCA Cycle Intermediate α-Ketoglutarate via SABRE-SHEATH.

α-Ketoglutarate is a key biomolecule involved in a number of metabolic pathways─most notably the TCA cycle. Abnormal α-ketoglutarate metabolism has also been linked with cancer. Here, isotopic labeling was employed to synthesize [1- 13 C,5- 12 C,D 4 ]α-ketoglutarate with the future goal of utilizing its [1- 13 C]-hyperpolarized state for real-time metabolic imaging of α-ketoglutarate analytes and its downstream metabolites in vivo . The signal amplification by reversible exchange in shield enables alignment transfer to heteronuclei (SABRE-SHEATH) hyperpolarization technique was used to create 9.7% [1- 13 C] polarization in 1 minute in this isotopologue. The efficient 13 C hyperpolarization, which utilizes parahydrogen as the source of nuclear spin order, is also supported by favorable relaxation dynamics at 0.4 μT field (the optimal polarization transfer field): the exponential 13 C polarization buildup constant T b is 11.0 ± 0.4 s whereas the 13 C polarization decay constant T 1 is 18.5 ± 0.7 s. An even higher 13 C polarization value of 17.3% was achieved using natural-abundance α-ketoglutarate disodium salt, with overall similar relaxation dynamics at 0.4 μT field, indicating that substrate deuteration leads only to a slight increase (∼1.2-fold) in the relaxation rates for 13 C nuclei separated by three chemical bonds. Instead, the gain in polarization (natural abundance versus [1- 13 C]-labeled) is rationalized through the smaller heat capacity of the "spin bath" comprising available 13 C spins that must be hyperpolarized by the same number of parahydrogen present in each sample, in line with previous 15 N SABRE-SHEATH studies. Remarkably, the C-2 carbon was not hyperpolarized in both α-ketoglutarate isotopologues studied; this observation is in sharp contrast with previously reported SABRE-SHEATH pyruvate studies, indicating that the catalyst-binding dynamics of C-2 in α-ketoglutarate differ from that in pyruvate. We also demonstrate that 13 C spectroscopic characterization of α-ketoglutarate and pyruvate analytes can be performed at natural 13 C abundance with an estimated detection limit of 80 micromolar concentration × *% P 13C . All in all, the fundamental studies reported here enable a wide range of research communities with a new hyperpolarized contrast agent potentially useful for metabolic imaging of brain function, cancer, and other metabolically challenging diseases.