Accurate ¹H-¹⁴N distance measurements by phase-modulated RESPDOR at ultra-fast MAS

The combination of a phase-modulated (PM) saturation pulse and symmetry-based dipolar recoupling into a rotational-echo saturation-pulse double-resonance (RESPDOR) sequence has been employed to measure ¹H-¹⁴N distances. Such a measurement is challenging owing to the quadrupolar interaction of ¹⁴N nucleus and the intense ¹H-¹H homonuclear dipolar interactions. Thanks to the recent advances in probe technology, the homonuclear dipolar interaction can be sufficiently suppressed at a fast MAS frequency (ν_R ≥ 60 kHz). PM pulse is robust to large variations of parameters on quadrupolar spins, but it has not been demonstrated under very fast MAS conditions. On the other hand, the RESPDOR sequence is applicable to such condition when it employs symmetry-based pulses during the recoupling period, but a prior knowledge on the system is required. In this article, we demonstrated the PM-RESPDOR combination for providing accurate ¹H-¹⁴N distances at a very fast MAS frequency of 70 kHz on two samples, namely L-tyrosine⋅HCl and N-acetyl-L-alanine. This sequence, supported by simulations and experiments, has shown its feasibility at ν_R = 70 kHz as well as the robustness to the ¹⁴N quadrupolar interaction. It is applicable to a wide range of ¹H-¹⁴N dipolar coupling constants when a radio frequency field on the ¹⁴N channel is approximately 80 kHz or more, while the PM pulse length lasts 10 rotor periods. For the first time, multiple ¹H-¹⁴N heteronuclear dipolar couplings, thus multiple quantitative distances, are simultaneously and reliably extracted by fitting the experimental fraction curves with the analytical expression. The size of the ¹H-¹⁴N dipolar interaction is solely used as a fitting parameter. These determined distances are in excellent agreement with those derived from diffraction techniques.