Enhancing carbon-13 NMR signals in liquids

The power of nuclear magnetic resonance spectroscopy lies in its high resolution, while the technique’s weakness is its low sensitivity. One way to boost NMR signals is through dynamic nuclear polarization (DNP), in which microwave irradiation transfers spin polarization from electrons of a stable radical to the nuclear spins of interest. A research team has now demonstrated signal enhancement of two to three orders of magnitude for room-temperature carbon-13 NMR experiments (Nat. Chem. 2017, DOI: 10.1038/nchem.2723).

DNP has historically worked well in solid-state NMR experiments but struggled in liquids. Spectroscopists believed that molecular motions in liquids impeded the transfer of spin polarization at high magnetic fields. One work-around is to polarize the radical at low temperatures and then warm up the sample, but that approach limits the number of scans that can be taken for signal averaging.

The new work was conducted by a team at the Max Planck Institute for Biophysical Chemistry, led by Marina Bennati and Guoquan Liu, who is now at Peking University. The researchers found that one key to improving DNP-enhanced carbon-13 NMR of liquids at high magnetic fields was using a nitroxide radical as a polarizer.

After optimizing several parameters in their DNP-NMR protocol, including efficiently saturating the radical electron spin polarization, the team obtained significantly improved signals for molecules such as CCl₄ and CHCl₃, as well as for biologically relevant compounds such as pyruvate and ethyl acetoacetate. The experiments can be repeated within seconds for signal averaging.