MAS DNP is increasingly establishing itself as a powerful technique to boost sensitivity of NMR. It makes it possible to run, in minutes, experiments that would take weeks, or simply would not be possible otherwise because they are too insensitive. At 9.4 T (400 MHz) and 100 K, DNP enhancements of around 250 are now possible with several polarizing agents (PA) (AMUPol1, TEKPol2, and more recently(3) SPIROPOL,(4) PyPolPEG2OH,(5) bcTol,(6) AsymPolPOK,(7)…). Despite excellent performance at 9.4 T, these dinitroxide biradicals are much less efficient at 18.8 T, both in terms of enhancement and overall sensitivity gain. The need for more efficient PA stimulated the research of biradical with a narrow EPR line,(8) such as for trityl in TEMTriPOL.(9)
Here we show how either monoradical like BDPA, based on the Overhauser effect, or hybrid biradicals, based on the Cross-Effect mechanism and designed by coupling BDPA with a nitroxide unit, provide very high DNP enhancements and overall sensitivity gains at 18.8 T, with performance significantly exceeding dinitroxides at this field. Overhauser DNP with BDPA in OTP shows enhancements of over 100 at 18.8 T and 40 kHz MAS with 1.3 mm rotors at 100 K, with no depolarization or quenching effects.10 The enhancement also persists at higher temperature, with values of around 30 at –30 °C!(11)
Hybrid BDPA-nitroxide biradical show enhancements up to 185 at 18.8 T, 100K and 40 kHz MAS, which is so far the highest enhancement and sensitivity gain at high magnetic field.(12) We will discuss how the DNP performance of all these systems depends on a combination of several factors, from the magnetic properties of the polarizing agent to the role played by spin-diffusion. The potential of these polarizing agents for the characterization of pharmaceutical and functionalized material surfaces will be presented.