Zero-Field Nuclear Magnetic Resonance of Chemically Exchanging Systems

Zero- and ultralow-field (ZULF) nuclear magnetic resonance (NMR) is an emerging tool for precision chemical analysis. Unlike conventional (high-field) NMR, which relies on chemical shifts for molecular identification, zero-field analog reports J-spectra that depend on the nuclear spin-spin coupling topology of molecules under investigation. While chemical shifts are usually a small fraction of the resonance frequencies, J-spectra for various spin systems are completely different from each other. In this work, we use zero-field NMR to study dynamic chemical processes and investigate the influence of chemical exchange on ZULF NMR spectra. We develop a computation approach that allows quantitative calculation of ZULF NMR spectra in the presence of chemical exchange and apply it to study aqueous solutions of [¹⁵N]ammonium as a model system. In this system, proton exchange rates span more than three orders of magnitude depending on acidity (pH), as monitored by high-field and ZULF NMR. We show that chemical exchange substantially affects the J-coupled NMR spectra and, in some cases, can lead to degradation and complete disappearance of the spectral features. To demonstrate potential applications of ZULF NMR for chemistry and biomedicine, we show a ZULF NMR spectrum of [2-¹³C]pyruvic acid hyperpolarized via dissolution dynamic nuclear polarization (dDNP). The metabolism of pyruvate provides valuable biochemical information and its monitoring by zero-field NMR could give spectral resolution that is hard to achieve at high magnetic fields. We foresee applications of affordable and scalable ZULF NMR coupled with hyperpolarization modalities to study chemical exchange phenomena in vivo and in situations where high-field NMR detection is not possible to implement.