Zero to ultralow magnetic field NMR of [1−13C]pyruvate and [2−13C]pyruvate enabled by SQUID sensors and hyperpolarization

Abstract

Accurately characterizing magnetic resonance of molecules at zero to ultralow magnetic field (nTs-µTs) is challenging, due to vanishingly small sensitivity, which depends on the thermal equilibrium polarization of the nuclear spins and instrumentation. We overcome the former limitation with the parahydrogen-based hyperpolarization method SABRE-SHEATH (signal amplification by reversible exchange in shield enables alignment transfer to heteronuclei). This method allows for the continuous transfer of spin order from parahydrogen to a substrate via chemical exchange, reaching polarization levels of some percent (level equivalent to C13 polarization at 20 kT). We address the latter with our application of a superconducting quantum interference device (SQUID)-based detector setup that allows for broadband detection (dc-MHz) with exquisite sensitivity over its entire range. Here, we present the results of our comprehensive characterization of [1−13C]pyruvate and [2−13C]pyruvate, hyperpolarized via SABRE-SHEATH, from zero field to 100 µT. To this end, we show low-noise, high-resolution spectra for both molecules, detecting how the NMR spectrum changes from the -coupling dominated zero-field spectrum to the strongly coupled spectrum, and then finally to the conventional high-field, otherwise known Zeeman-dominated spectrum. We also analytically derive the evolution of product operators in arbitrary magnetic fields, which aid in the understanding of the differences between spin evolution and spin-coupling regimes. We predict and confirm that the absence of spin precession at zero field can result in observable oscillation of magnetization along one axis with a frequency of the -coupling constant, no observable spin evolution, or observing spin evolution that corresponds to “forbidden” transitions at high field. The zero-field spectra with their near-dc signals reveal different relaxation rates for the different spin states of hyperpolarized C13 pyruvates, demonstrating the utility of SQUID detectors and hyperpolarization for the characterization of magnetic resonance phenomena. Published by the American Physical Society 2024