Efficient polarization transfer between spin-1/2 and ¹⁴N nuclei in solid-state MAS NMR spectroscopy.

Polarization transfer between spin-1/2 nuclei and quadrupolar spin-1 nuclei such as 14N in solid-state NMR is severely challenged by the typical presence of large quadrupole coupling interactions. This has effectively prevented the use of the abundant 14N spin as a probe to structural information and its use as an element in multi-dimensional solid-state NMR correlation experiments for assignment and structural characterization. In turn, this has been a contributing factor to the extensive use of isotope labeling in biological solid-state NMR, where 14N is replaced with 15N. The alternative strategy of using the abundant 14N spins calls for methods enabling efficient polarization transfer between 14N and its binding partners. This work demonstrates that the recently introduced RESPIRATIONCP transfer method can be optimized to achieve efficient 1H ↔ 14N polarization transfer under magic angle spinning conditions. The method is demonstrated numerically and experimentally on powder samples of NH4NO3 and l-alanine.