Direct observation of hyperpolarization breaking through the spin diffusion barrier

Dynamic nuclear polarization (DNP) is a widely used tool for overcoming the low intrinsic sensitivity of nuclear magnetic resonance spectroscopy and imaging. Its practical applicability is typically bounded, however, by the so-called ‘spin diffusion barrier’, which relates to the poor efficiency of polarization transfer from highly polarized nuclei close to paramagnetic centers to bulk nuclei. A quantitative assessment of this barrier has been hindered so far by the lack of general methods for studying nuclear-polarization flow in the vicinity of paramagnetic centers. Here we fill this gap and introduce a general set of experiments based on microwave gating that are readily implemented. We demonstrate the versatility of our approach in experiments conducted between 1.2 – 4.2 K in static mode and at 100 K under magic angle spinning (MAS) — conditions typical for dissolution-DNP and MAS-DNP — and for the first time directly observe the dramatic dependence of polarization flow on temperature.

Methods
All data consist of NMR spectra. Data were collected using high field NMR instruments by Bruker using the software Topspin 3.5.7 and Topspin 3.6.2. They were exported to CSV files.