Abstract
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 R(2) 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 R(2) 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, R(2) relaxation rates are indicative primarily of molecular properties. R(2) 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.