Order‐Unity ¹³C Nuclear Polarization of [1‐¹³C]Pyruvate in Seconds and the Interplay of Water and SABRE Enhancement

Abstract

Abstract Signal Amplification By Reversible Exchange in SHield Enabled Alignment Transfer (SABRE‐SHEATH) is investigated to achieve rapid hyperpolarization of 13 C 1 spins of [1‐ 13 C]pyruvate, using parahydrogen as the source of nuclear spin order. Pyruvate exchange with an iridium polarization transfer complex can be modulated via a sensitive interplay between temperature and co‐ligation of DMSO and H 2 O. Order‐unity 13 C (>50 %) polarization of catalyst‐bound [1‐ 13 C]pyruvate is achieved in less than 30 s by restricting the chemical exchange of [1‐ 13 C]pyruvate at lower temperatures. On the catalyst bound pyruvate, 39 % polarization is measured using a 1.4 T NMR spectrometer, and extrapolated to >50 % at the end of build‐up in situ. The highest measured polarization of a 30‐mM pyruvate sample, including free and bound pyruvate is 13 % when using 20 mM DMSO and 0.5 M water in CD 3 OD. Efficient 13 C polarization is also enabled by favorable relaxation dynamics in sub‐microtesla magnetic fields, as indicated by fast polarization buildup rates compared to the T 1 spin‐relaxation rates (e. g., ∼0.2 s −1 versus ∼0.1 s −1 , respectively, for a 6 mM catalyst‐[1‐ 13 C]pyruvate sample). Finally, the catalyst‐bound hyperpolarized [1‐ 13 C]pyruvate can be released rapidly by cycling the temperature and/or by optimizing the amount of water, paving the way to future biomedical applications of hyperpolarized [1‐ 13 C]pyruvate produced via comparatively fast and simple SABRE‐SHEATH‐based approaches.