Mechanistic Insights into Chemical Exchange during the Signal Amplification by Reversible Exchange Sensitization of Pyruvate

Signal amplification by reversible exchange (SABRE) is a nuclear spin hyperpolarization technique in which the transient interaction of parahydrogen (pH2) and a target substrate with an iridium complex leads to polarization transfer to the sub-strate. Here, we use a parahydrogen-enhanced, spin-selective NMR method to investigate pyruvate binding, which is com-bined with exchange-model fitting and DFT calculations. Our study reveals several key findings that reshape the current understanding of SABRE: (a) intramolecular hydrogen exchange of the hydrides, occurring faster than pyruvate or H2 loss; (b) the discovery of a novel stable H2-IrImes-DMSO2-pyruvate complex; and (c) the crucial role of counterions, here Na⁺, in Ir-pyruvate binding. Previously unknown insights into complex kinetics and distributions as a function of temperature, [DMSO], [pyruvate], and hydrogen pressure are presented. The methods demonstrated here, exemplified by SABRE, pro-vide a framework that is expected to guide future research in the field.