R₂ Relaxometry of SABRE-Hyperpolarized Substrates at a Low Magnetic Field

Nuclear magnetic resonance (NMR) relaxometry at a low magnetic field, in the milli-Tesla range or less, is enabled by signal enhancements through hyperpolarization. The parahydrogen-based method of signal amplification by reversible exchange (SABRE) provides large signals in a dilute liquid for the measurement of R₂ relaxation using a single-scan Carr-Purcell-Meiboom-Gill (CPMG) experiment. A comparison of relaxation rates obtained at high and low fields indicates that an otherwise dominant contribution from chemical exchange is excluded in this low-field range. The SABRE process itself is based on exchange between the free and polarization transfer catalyst-bound forms of the substrate. At a high magnetic field of 9.4 T, typical conditions for producing hyperpolarization including 5 mM 5-fluoropyridine-3-carboximidamide as a substrate and 0.5 mM chloro(1,5-cyclooctadiene)[4,5-dimethyl-1,3-bis(2,4,6-trimethylphenyl)imidazol-2-ylidene]iridium(I) as a polarization transfer catalyst precursor resulted in an R₂ relaxation rate as high as 3.38 s^-1. This relaxation was reduced to 1.19 s^-1 at 0.85 mT. A quantitative analysis of relaxation rates and line shapes indicates that milli-Tesla or lower magnetic fields are required to eliminate the exchange contribution. At this magnetic field strength, R₂ relaxation rates are indicative primarily of molecular properties. R₂ relaxometry may be used for investigating molecular interactions and dynamics. The SABRE hyperpolarization, which provides signal enhancements without requiring a high magnetic field or large instrumentation, is ideally suited to enable these applications.