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 N-14 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 N-14 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 N-14 is replaced with N-15. The alternative strategy of using the abundant N-14 spins calls for methods enabling efficient polarization transfer between N-14 and its binding partners. This work demonstrates that the recently introduced (RESPIRATION) CP transfer method can be optimized to achieve efficient H-1 <-> N-14 polarization transfer under magic angle spinning conditions. The method is demonstrated numerically and experimentally on powder samples of NH4NO3 and L-alanine. (C) 2014 Elsevier Inc. All rights reserved.