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Parahydrogen-enhanced nuclear magnetic resonance enables sensitivity enhancement of bio-relevant molecular targets

Book - Dissertation

The inherently low signal amplitude is probably the major weakness of nuclear magnetic resonance (NMR) technology. The external magnetic field required to align the nuclear spins of the magnetically active nuclei remains a very inefficient route to generate nuclear spin polarization and hence signal amplitude. Traditionally, spin polarization is enhanced by increasing the magnetic field strength. Yet, every linear increase in field strength represents an exponential increase in cost. The concept of molecular hyperpolarization provides an opportunity to achieve high signal amplitude without relying on large external magnetic fields. Parahydrogen (p-H2), a storable and inexpensive spin-isomer of molecular hydrogen, can provide a source of hyperpolarization to molecules. Hyperpolarization based on p-H2 raises the nuclear spin polarization of molecular targets to out-of-equilibrium levels in a matter of seconds and is considered a very attractive route to rapidly increase signal amplitude by several orders of magnitude in NMR applications. The key scientific challenge in using p-H2 as a general source of polarization, remains the chemical specificity associated with the hyperpolarization technology. An external magnetic field universally generates polarization in molecules, while the production of hyperpolarization via p-H2 is limited to a selection of molecular targets. For target molecules to become hyperpolarized, they are required to interact with transition metal catalysts at specified magnetic fields, therefore limiting the substrate scope and thus the application potential. Tackling the chemical specificity associated with current parahydrogen induced polarization (PHIP) technology is therefore the major objective of this thesis. Three routes were implemented to circumvent the chemical specificity: (i) hyperpolarization of exchangeable protons to redistribute polarization without chemical restrictions, (ii) rational design of hyperpolarization catalysts to achieve sensitivity enhancement of larger bio-relevant substrates and, (iii) straightforward identification of hyperpolarization catalysts by unlocking hyperpolarization at any magnetic field. First of all, generating a pool of hyperpolarized exchangeable protons proposes a clever solution to circumvent the chemical specificity. Instead of directly fueling targets with non-equilibrium magnetization at the catalyst center, polarization is relayed via 'relay agents' and concentrated in labile protons able to redistribute the spin polarization further. A new approach to efficiently hyperpolarize exchangeable protons at room temperature was discovered combining aminosilanes with an active Ir-catalyst. This method was inspired by the signal amplification by reversible exchange or SABRE hyperpolarization method that is able to continuously produce hyperpolarized agents. Aminosilanes were both capable of fueling labile protons with hyperpolarization (the amino functionality), while also providing a potential grafting site for future heterogenization onto solid supports (the silane functionality). The ability to control association of (co)ligands on the transition metal center provides another opportunity for PHIP technology to become sterically compatible with a larger substrate scope. Via gradual pH assisted release of deuterated ammonia using ammonium buffers, the amount of associating ammonia, and therefore also the associating behavior of molecular targets, could be elegantly controlled. Simultaneously, the non-equivalent hydride positions in the mixed-ligand catalyst enabled unlocking spin order from p-H2 at high magnetic field, while otherwise only a low-magnetic field regime could accomplish this. This privilege provides a wider range of options to redistribute hyperpolarization. Besides enhancing sensitivity, also general characterization of transition metal catalysts is becoming increasingly important. An NMR tool for straightforward elucidation of mixed-ligand hyperpolarization catalysts was therefore developed concurrently and termed isotopological fingerprinting. The identification method is based on zero-quantum coherences that can measure spontaneous isotopological proton-deuterium redistribution of coordinating molecular targets to identify all active catalyst complexes in solution. Finally, continuous high-field hyperpolarization of 15N nuclei in amino acids was accomplished. Amino acids are regarded unique metabolic markers in magnetic resonance applications which were hitherto incompatible with the state-of-the-art PHIP techniques based on reversible interactions. The hyperpolarization of the heteronuclei was achieved inside an in-house designed automated pressure enhanced hyperpolarization of H- and X-nuclei (APEHX) bubbling setup operating at high-field. A mixed-ligand Ir-catalyst with ammonia as coligand reduced the naturally occurring strong bidentate chelating effect between Ir and the amino acid. The use of ammonia favored monodentate ligation of amino acids using solely their amine functionality in the equatorial plane of the complex. Combined with the fact that heteronuclei such as 15N exhibit longer hyperpolarization lifetimes than protons, brings the scientific desire to develop portable ultrasensitive NMR systems, operating in a biomedical, fine chemical or pharmaceutical industrial context, closer to reality than ever before.
Publication year:2022