Determination of NH proton chemical shift anisotropy with 14N-1H heteronuclear decoupling using ultrafast magic angle spinning solid-state NMR

The extraction of chemical shift anisotropy (CSA) tensors of protons either directly bonded to N-14 nuclei (I = 1) or lying in their vicinity using rotor-synchronous recoupling pulse sequence is always fraught with difficulty due to simultaneous recoupling of N-14-H-1 heteronuclear dipolar couplings and the lack of methods to efficiently decouple these interactions. This difficulty mainly arises from the presence of large N-14 quadrupolar interactions in comparison to the rf field that can practically be achieved. In the present work it is demonstrated that the application of on-resonance N-14-H-1 decoupling with rf field strength similar to 30 times weaker than the N-14 quadrupolar coupling during H-1 CSA recoupling under ultrafast MAS (90 kHz) results in CSA lineshapes that are free from any distortions from recoupled N-14-H-1 interactions. With the use of extensive numerical simulations we have shown the applicability of our proposed method on a naturally abundant r-Histidine HCl center dot H2O sample. (C) 2015 Elsevier Inc. All rights reserved.