SABRE is a nuclear spin hyperpolarization technique based on the reversible association of a substrate molecule and para-hydrogen (p-H2) to a metal complex. During the lifetime of such a complex, generally fractions of a second, the spin order of p-H2 is transferred to the nuclear spins of the substrate molecule via a transient scalar coupling network, resulting in strongly enhanced NMR signals. This technique is generally applied at relatively high concentrations (mM), in large excess of substrate with respect to metal complex. Dilution of substrate ligands below stoichiometry results in progressive decrease of signal enhancement, which precludes the direct application of SABRE to the NMR analysis of low concentration (μM) solutions. Here, we show that the efficiency of SABRE at low substrate concentrations can be restored by addition of a suitable coordinating ligand to the solution. The proposed method allowed NMR detection below 1 μM in a single scan.

Nuclear magnetic resonance is often the technique of choice in chemical analysis because of its sensitivity to molecular structure, quantitative character, and straightforward sample preparation. However, determination of trace analytes in complex mixtures is generally limited by low sensitivity and extensive signal overlap. Here, we present an approach for continuous hyperpolarization at high magnetic field that is based on signal amplification by reversible exchange (SABRE) and can be straightforwardly incorporated in multidimensional NMR experiments. This method was implemented in a 2D correlation experiment that allows detection and quantification of analytes at nanomolar concentration in complex solutions.

SABRE hyperpolarizes substrates by polarization transfer from para-hydrogen in a metal complex. We have measured the signal enhancement of pyridine and its exchange rate in various [Ir(NHC)(Py)3(H)2](+) complexes to gain insight into their dependence on the N-Heterocyclic Carbene (NHC) ligand's steric and electronic properties.

Signal amplification by reversible exchange (SABRE) is an emerging nuclear spin hyperpolarization technique that strongly enhances NMR signals of small molecules in solution. However, such signal enhancements have never been exploited for concentration determination, as the efficiency of SABRE can strongly vary between different substrates or even between nuclear spins in the same molecule. The first application of SABRE for the quantitative analysis of a complex mixture is now reported. Despite the inherent complexity of the system under investigation, which involves thousands of competing binding equilibria, analytes at concentrations in the low micromolar range could be quantified from single-scan SABRE spectra using a standard-addition approach.

Sensitivity enhancement by parahydrogen hyperpolarization allows NMR detection and quantification of hundreds of urinary metabolites at down to nanomolar concentrations.

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An experiment is presented that allows the quantitative measurement of the cross-correlation rate between 1HN CSA and 1HN−15N dipolar interaction in uniformly 15N-enriched samples. The CSA/DD cross-correlation rate is obtained from the intensity ratio of an experiment in which the CSA/DD cross-correlation is active for a fixed time, τ, with a reference experiment in which it is inactive. The CSA/DD cross-correlation rates of 75 residues of the HU protein from Bacillus stearothermophilus were obtained from the linear fits of CSA/DD to reference ratios recorded for five values of τ and at two different B o fields. After correction for the mobility of the 1H−15N bond vector the values of (σ∥ − σ⊥)(3 cos2(θ) − 1)/2, containing information about the chemical shielding anisotropy, were derived for individual amide protons. The average value of 13 ± 5 ppm compares well with the results from previous solid state NMR measurements. The data also show a dependence upon hydrogen bonding and secondary structure:  residues in α-helical conformation show values of 9 ± 4 ppm, whereas residues in β-sheet conformation show substantially higher values of 16 ± 6 ppm.

NMR signal amplification by reversible exchange (SABRE) has been observed for pyridine, methyl nicotinate, N-methylnicotinamide, and nicotinamide in D 2 Ow ith the new catalyst[ Ir(Cl)(IDEG)(COD)] (IDEG = 1,3-bis(3,4,5-tris(diethyleneglycol)benzyl)imidazole-2-ylidene). During the activation and hyperpolarization steps, exclusively D 2 Ow as used, resulting in the first fully biocompatible SABRE system. Hyperpolarized 1 Hs ubstrate signals wereo bserved at 42.5 MHz upon pressurizing the solutionw ith parahydrogen at close to the Earth's magnetic field, at concentrations yielding barely detectablet hermals ignals. Moreover,4 2-, 26-, 22-, and 9-fold enhancements wereo bserved for nicotinamide, pyridine, methyl nicotinate, and N-methylnicotinamide, respectively,i nc onventional 300 MHz studies. This research opens up new opportunities in af ield in which SABRE has hitherto primarilyb een conducted in CD 3 OD.T his system uses simple hardware, leaves the substrate unaltered, and shows that SABRE is potentially suitable for clinical purposes.