Parahydrogen Induced Polarization
PHIP (Parahydrogen Induced Polarization) is a way to achieve hyperpolarization of specific spins inside a molecule via a chemical route. It makes use of the parahydrogen spin order (antiparallel spin orientation) that can be transferred to different molecules either via a hydrogenation reaction (hPHIP) or by temporary association of parahydrogen and a substrate on a transition-metal based catalyst (SABRE: Signal Amplification by Reversible Exchange). The hydrogenative PHIP (hPHIP) approach exploits the parahydrogen spin orientation very efficiently because the two protons are becoming part of the target molecule and thus results in very high enhancement factors. However, an unsaturated precursor of the target molecule is required to accomplish this approach. The SABRE approach allows for a more indirect way of transferring the spin order of parahydrogen into a molecule. The spin order is transferred via the J-coupling network in this case and allows for hyperpolarization of a different set of molecules without the need of using unsaturated precursors. Both methods allow for hyperpolarization of a variety of nuclei such as 1H, 13C, 15N, 19F and 31P. The shortcoming of the limited lifetime of the accomplished hyperpolarization can be alleviated by storing the hyperpolarization in slowly relaxing states (i.e. singlet states). This approach is easy to implement in PHIP as parahydrogen already possesses singlet symmetry.
In experiments several difficulties are tackled by our group:
- Acquisition of highyl resolved PHIP spectra in inhomogeneous magnetic fields. Typically antiphase signals are obtained from PHIP with a separation of the resonance lines of a few Hz. Detection of these signals by means of a CPMG pulse train allows to avoid partial peak cancellations.
- Storage of the hyperpolarized signal as a singlet state with subsequent conversion into observable magnetization inside the observation field. The required change of the effective magnetic field can be achieved by a special pulse sequence; therefore the sample can be left inside the NMR magnet. In this approach proton imaging can still be performed 3 minutes after hyperpolarization thus showing the long lifetime of the singlet state in specific molecules.
These findings might enable even medical MRI of proton-hyperpolarized molecules, and we continue our research to extend our knowledge about singlet states and imaging approaches to reach this goal