J Coupling Constants of <1 Hz Enable 13 C Hyperpolarization of Pyruvate via Reversible Exchange of Parahydrogen.

Observing pyruvate metabolism in vivo has become a focal point of molecular magnetic resonance imaging. Signal amplification by reversible exchange (SABRE) has recently emerged as a versatile hyperpolarization technique. Tuning of the spin order transfer (SOT) in SABRE is challenging as the small 1 H- 13 C J couplings, in the 13 C-pyruvate case, result in SOT being not readily discernible. We demonstrate an experimental method using frequency-selective excitation of parahydrogen-derived polarization SOT sequence (SEPP-SPINEPT); its application led to up to 5700-fold 13 C signal gain. In this way, we estimated the lifetime of two Ir-pyruvate SABRE complexes alongside the individual probing of eight small 1 H- 13 C J couplings that connect the hydride protons in these complexes to 1- and 2- 13 C pyruvate spins, affording values between 0 and 2.69 Hz. Using electronic structure calculations, we define the low-energy structure of the corresponding complexes. Hence, this study demonstrates a novel approach to analyzing the spin topology of short-lived organometallic complexes.